Using the GNU Compiler Collection (GCC) This file documents the use of the GNU compilers. Copyright © 1988-2019 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being “Funding Free Software”, the Front-Cover Texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled “GNU Free Documentation License”. (a) The FSF's Front-Cover Text is: A GNU Manual (b) The FSF's Back-Cover Text is: You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development. Short Table of Contents * 1 Programming Languages Supported by GCC * 2 Language Standards Supported by GCC * 3 GCC Command Options * 4 C Implementation-Defined Behavior * 5 C++ Implementation-Defined Behavior * 6 Extensions to the C Language Family * 7 Extensions to the C++ Language * 8 GNU Objective-C Features * 9 Binary Compatibility * 10 gcov—a Test Coverage Program * 11 gcov-tool—an Offline Gcda Profile Processing Tool * 12 gcov-dump—an Offline Gcda and Gcno Profile Dump Tool * 13 Known Causes of Trouble with GCC * 14 Reporting Bugs * 15 How To Get Help with GCC * 16 Contributing to GCC Development * Funding Free Software * The GNU Project and GNU/Linux * GNU General Public License * GNU Free Documentation License * Contributors to GCC * Option Index * Keyword Index Table of Contents * 1 Programming Languages Supported by GCC * 2 Language Standards Supported by GCC * 2.1 C Language * 2.2 C++ Language * 2.3 Objective-C and Objective-C++ Languages * 2.4 Go Language * 2.5 HSA Intermediate Language (HSAIL) * 2.6 D language * 2.7 References for Other Languages * 3 GCC Command Options * 3.1 Option Summary * 3.2 Options Controlling the Kind of Output * 3.3 Compiling C++ Programs * 3.4 Options Controlling C Dialect * 3.5 Options Controlling C++ Dialect * 3.6 Options Controlling Objective-C and Objective-C++ Dialects * 3.7 Options to Control Diagnostic Messages Formatting * 3.8 Options to Request or Suppress Warnings * 3.9 Options for Debugging Your Program * 3.10 Options That Control Optimization * 3.11 Program Instrumentation Options * 3.12 Options Controlling the Preprocessor * 3.13 Passing Options to the Assembler * 3.14 Options for Linking * 3.15 Options for Directory Search * 3.16 Options for Code Generation Conventions * 3.17 GCC Developer Options * 3.18 Machine-Dependent Options * 3.18.1 AArch64 Options * 3.18.1.1 -march and -mcpu Feature Modifiers * 3.18.2 Adapteva Epiphany Options * 3.18.3 AMD GCN Options * 3.18.4 ARC Options * 3.18.5 ARM Options * 3.18.6 AVR Options * 3.18.6.1 EIND and Devices with More Than 128 Ki Bytes of Flash * 3.18.6.2 Handling of the RAMPD, RAMPX, RAMPY and RAMPZ Special Function Registers * 3.18.6.3 AVR Built-in Macros * 3.18.7 Blackfin Options * 3.18.8 C6X Options * 3.18.9 CRIS Options * 3.18.10 CR16 Options * 3.18.11 C-SKY Options * 3.18.12 Darwin Options * 3.18.13 DEC Alpha Options * 3.18.14 FR30 Options * 3.18.15 FT32 Options * 3.18.16 FRV Options * 3.18.17 GNU/Linux Options * 3.18.18 H8/300 Options * 3.18.19 HPPA Options * 3.18.20 IA-64 Options * 3.18.21 LM32 Options * 3.18.22 M32C Options * 3.18.23 M32R/D Options * 3.18.24 M680x0 Options * 3.18.25 MCore Options * 3.18.26 MeP Options * 3.18.27 MicroBlaze Options * 3.18.28 MIPS Options * 3.18.29 MMIX Options * 3.18.30 MN10300 Options * 3.18.31 Moxie Options * 3.18.32 MSP430 Options * 3.18.33 NDS32 Options * 3.18.34 Nios II Options * 3.18.35 Nvidia PTX Options * 3.18.36 OpenRISC Options * 3.18.37 PDP-11 Options * 3.18.38 picoChip Options * 3.18.39 PowerPC Options * 3.18.40 RISC-V Options * 3.18.41 RL78 Options * 3.18.42 IBM RS/6000 and PowerPC Options * 3.18.43 RX Options * 3.18.44 S/390 and zSeries Options * 3.18.45 Score Options * 3.18.46 SH Options * 3.18.47 Solaris 2 Options * 3.18.48 SPARC Options * 3.18.49 SPU Options * 3.18.50 Options for System V * 3.18.51 TILE-Gx Options * 3.18.52 TILEPro Options * 3.18.53 V850 Options * 3.18.54 VAX Options * 3.18.55 Visium Options * 3.18.56 VMS Options * 3.18.57 VxWorks Options * 3.18.58 x86 Options * 3.18.59 x86 Windows Options * 3.18.60 Xstormy16 Options * 3.18.61 Xtensa Options * 3.18.62 zSeries Options * 3.19 Specifying Subprocesses and the Switches to Pass to Them * 3.20 Environment Variables Affecting GCC * 3.21 Using Precompiled Headers * 4 C Implementation-Defined Behavior * 4.1 Translation * 4.2 Environment * 4.3 Identifiers * 4.4 Characters * 4.5 Integers * 4.6 Floating Point * 4.7 Arrays and Pointers * 4.8 Hints * 4.9 Structures, Unions, Enumerations, and Bit-Fields * 4.10 Qualifiers * 4.11 Declarators * 4.12 Statements * 4.13 Preprocessing Directives * 4.14 Library Functions * 4.15 Architecture * 4.16 Locale-Specific Behavior * 5 C++ Implementation-Defined Behavior * 5.1 Conditionally-Supported Behavior * 5.2 Exception Handling * 6 Extensions to the C Language Family * 6.1 Statements and Declarations in Expressions * 6.2 Locally Declared Labels * 6.3 Labels as Values * 6.4 Nested Functions * 6.5 Nonlocal Gotos * 6.6 Constructing Function Calls * 6.7 Referring to a Type with typeof * 6.8 Conditionals with Omitted Operands * 6.9 128-bit Integers * 6.10 Double-Word Integers * 6.11 Complex Numbers * 6.12 Additional Floating Types * 6.13 Half-Precision Floating Point * 6.14 Decimal Floating Types * 6.15 Hex Floats * 6.16 Fixed-Point Types * 6.17 Named Address Spaces * 6.17.1 AVR Named Address Spaces * 6.17.2 M32C Named Address Spaces * 6.17.3 RL78 Named Address Spaces * 6.17.4 SPU Named Address Spaces * 6.17.5 x86 Named Address Spaces * 6.18 Arrays of Length Zero * 6.19 Structures with No Members * 6.20 Arrays of Variable Length * 6.21 Macros with a Variable Number of Arguments. * 6.22 Slightly Looser Rules for Escaped Newlines * 6.23 Non-Lvalue Arrays May Have Subscripts * 6.24 Arithmetic on void- and Function-Pointers * 6.25 Pointer Arguments in Variadic Functions * 6.26 Pointers to Arrays with Qualifiers Work as Expected * 6.27 Non-Constant Initializers * 6.28 Compound Literals * 6.29 Designated Initializers * 6.30 Case Ranges * 6.31 Cast to a Union Type * 6.32 Mixed Declarations and Code * 6.33 Declaring Attributes of Functions * 6.33.1 Common Function Attributes * 6.33.2 AArch64 Function Attributes * 6.33.2.1 Inlining rules * 6.33.3 AMD GCN Function Attributes * 6.33.4 ARC Function Attributes * 6.33.5 ARM Function Attributes * 6.33.6 AVR Function Attributes * 6.33.7 Blackfin Function Attributes * 6.33.8 CR16 Function Attributes * 6.33.9 C-SKY Function Attributes * 6.33.10 Epiphany Function Attributes * 6.33.11 H8/300 Function Attributes * 6.33.12 IA-64 Function Attributes * 6.33.13 M32C Function Attributes * 6.33.14 M32R/D Function Attributes * 6.33.15 m68k Function Attributes * 6.33.16 MCORE Function Attributes * 6.33.17 MeP Function Attributes * 6.33.18 MicroBlaze Function Attributes * 6.33.19 Microsoft Windows Function Attributes * 6.33.20 MIPS Function Attributes * 6.33.21 MSP430 Function Attributes * 6.33.22 NDS32 Function Attributes * 6.33.23 Nios II Function Attributes * 6.33.24 Nvidia PTX Function Attributes * 6.33.25 PowerPC Function Attributes * 6.33.26 RISC-V Function Attributes * 6.33.27 RL78 Function Attributes * 6.33.28 RX Function Attributes * 6.33.29 S/390 Function Attributes * 6.33.30 SH Function Attributes * 6.33.31 SPU Function Attributes * 6.33.32 Symbian OS Function Attributes * 6.33.33 V850 Function Attributes * 6.33.34 Visium Function Attributes * 6.33.35 x86 Function Attributes * 6.33.36 Xstormy16 Function Attributes * 6.34 Specifying Attributes of Variables * 6.34.1 Common Variable Attributes * 6.34.2 ARC Variable Attributes * 6.34.3 AVR Variable Attributes * 6.34.4 Blackfin Variable Attributes * 6.34.5 H8/300 Variable Attributes * 6.34.6 IA-64 Variable Attributes * 6.34.7 M32R/D Variable Attributes * 6.34.8 MeP Variable Attributes * 6.34.9 Microsoft Windows Variable Attributes * 6.34.10 MSP430 Variable Attributes * 6.34.11 Nvidia PTX Variable Attributes * 6.34.12 PowerPC Variable Attributes * 6.34.13 RL78 Variable Attributes * 6.34.14 SPU Variable Attributes * 6.34.15 V850 Variable Attributes * 6.34.16 x86 Variable Attributes * 6.34.17 Xstormy16 Variable Attributes * 6.35 Specifying Attributes of Types * 6.35.1 Common Type Attributes * 6.35.2 ARC Type Attributes * 6.35.3 ARM Type Attributes * 6.35.4 MeP Type Attributes * 6.35.5 PowerPC Type Attributes * 6.35.6 SPU Type Attributes * 6.35.7 x86 Type Attributes * 6.36 Label Attributes * 6.37 Enumerator Attributes * 6.38 Statement Attributes * 6.39 Attribute Syntax * 6.40 Prototypes and Old-Style Function Definitions * 6.41 C++ Style Comments * 6.42 Dollar Signs in Identifier Names * 6.43 The Character ESC in Constants * 6.44 Determining the Alignment of Functions, Types or Variables * 6.45 An Inline Function is As Fast As a Macro * 6.46 When is a Volatile Object Accessed? * 6.47 How to Use Inline Assembly Language in C Code * 6.47.1 Basic Asm — Assembler Instructions Without Operands * 6.47.2 Extended Asm - Assembler Instructions with C Expression Operands * 6.47.2.1 Volatile * 6.47.2.2 Assembler Template * 6.47.2.3 Output Operands * 6.47.2.4 Flag Output Operands * 6.47.2.5 Input Operands * 6.47.2.6 Clobbers and Scratch Registers * 6.47.2.7 Goto Labels * 6.47.2.8 x86 Operand Modifiers * 6.47.2.9 x86 Floating-Point asm Operands * 6.47.3 Constraints for asm Operands * 6.47.3.1 Simple Constraints * 6.47.3.2 Multiple Alternative Constraints * 6.47.3.3 Constraint Modifier Characters * 6.47.3.4 Constraints for Particular Machines * 6.47.4 Controlling Names Used in Assembler Code * 6.47.5 Variables in Specified Registers * 6.47.5.1 Defining Global Register Variables * 6.47.5.2 Specifying Registers for Local Variables * 6.47.6 Size of an asm * 6.48 Alternate Keywords * 6.49 Incomplete enum Types * 6.50 Function Names as Strings * 6.51 Getting the Return or Frame Address of a Function * 6.52 Using Vector Instructions through Built-in Functions * 6.53 Support for offsetof * 6.54 Legacy __sync Built-in Functions for Atomic Memory Access * 6.55 Built-in Functions for Memory Model Aware Atomic Operations * 6.56 Built-in Functions to Perform Arithmetic with Overflow Checking * 6.57 x86-Specific Memory Model Extensions for Transactional Memory * 6.58 Object Size Checking Built-in Functions * 6.59 Other Built-in Functions Provided by GCC * 6.60 Built-in Functions Specific to Particular Target Machines * 6.60.1 AArch64 Built-in Functions * 6.60.2 Alpha Built-in Functions * 6.60.3 Altera Nios II Built-in Functions * 6.60.4 ARC Built-in Functions * 6.60.5 ARC SIMD Built-in Functions * 6.60.6 ARM iWMMXt Built-in Functions * 6.60.7 ARM C Language Extensions (ACLE) * 6.60.8 ARM Floating Point Status and Control Intrinsics * 6.60.9 ARM ARMv8-M Security Extensions * 6.60.10 AVR Built-in Functions * 6.60.11 Blackfin Built-in Functions * 6.60.12 FR-V Built-in Functions * 6.60.12.1 Argument Types * 6.60.12.2 Directly-Mapped Integer Functions * 6.60.12.3 Directly-Mapped Media Functions * 6.60.12.4 Raw Read/Write Functions * 6.60.12.5 Other Built-in Functions * 6.60.13 MIPS DSP Built-in Functions * 6.60.14 MIPS Paired-Single Support * 6.60.15 MIPS Loongson Built-in Functions * 6.60.15.1 Paired-Single Arithmetic * 6.60.15.2 Paired-Single Built-in Functions * 6.60.15.3 MIPS-3D Built-in Functions * 6.60.16 MIPS SIMD Architecture (MSA) Support * 6.60.16.1 MIPS SIMD Architecture Built-in Functions * 6.60.17 Other MIPS Built-in Functions * 6.60.18 MSP430 Built-in Functions * 6.60.19 NDS32 Built-in Functions * 6.60.20 picoChip Built-in Functions * 6.60.21 Basic PowerPC Built-in Functions * 6.60.21.1 Basic PowerPC Built-in Functions Available on all Configurations * 6.60.21.2 Basic PowerPC Built-in Functions Available on ISA 2.05 * 6.60.21.3 Basic PowerPC Built-in Functions Available on ISA 2.06 * 6.60.21.4 Basic PowerPC Built-in Functions Available on ISA 2.07 * 6.60.21.5 Basic PowerPC Built-in Functions Available on ISA 3.0 * 6.60.22 PowerPC AltiVec/VSX Built-in Functions * 6.60.22.1 PowerPC AltiVec Built-in Functions on ISA 2.05 * 6.60.22.2 PowerPC AltiVec Built-in Functions Available on ISA 2.06 * 6.60.22.3 PowerPC AltiVec Built-in Functions Available on ISA 2.07 * 6.60.22.4 PowerPC AltiVec Built-in Functions Available on ISA 3.0 * 6.60.23 PowerPC Hardware Transactional Memory Built-in Functions * 6.60.23.1 PowerPC HTM Low Level Built-in Functions * 6.60.23.2 PowerPC HTM High Level Inline Functions * 6.60.24 PowerPC Atomic Memory Operation Functions * 6.60.25 RX Built-in Functions * 6.60.26 S/390 System z Built-in Functions * 6.60.27 SH Built-in Functions * 6.60.28 SPARC VIS Built-in Functions * 6.60.29 SPU Built-in Functions * 6.60.30 TI C6X Built-in Functions * 6.60.31 TILE-Gx Built-in Functions * 6.60.32 TILEPro Built-in Functions * 6.60.33 x86 Built-in Functions * 6.60.34 x86 Transactional Memory Intrinsics * 6.60.35 x86 Control-Flow Protection Intrinsics * 6.61 Format Checks Specific to Particular Target Machines * 6.61.1 Solaris Format Checks * 6.61.2 Darwin Format Checks * 6.62 Pragmas Accepted by GCC * 6.62.1 AArch64 Pragmas * 6.62.2 ARM Pragmas * 6.62.3 M32C Pragmas * 6.62.4 MeP Pragmas * 6.62.5 RS/6000 and PowerPC Pragmas * 6.62.6 S/390 Pragmas * 6.62.7 Darwin Pragmas * 6.62.8 Solaris Pragmas * 6.62.9 Symbol-Renaming Pragmas * 6.62.10 Structure-Layout Pragmas * 6.62.11 Weak Pragmas * 6.62.12 Diagnostic Pragmas * 6.62.13 Visibility Pragmas * 6.62.14 Push/Pop Macro Pragmas * 6.62.15 Function Specific Option Pragmas * 6.62.16 Loop-Specific Pragmas * 6.63 Unnamed Structure and Union Fields * 6.64 Thread-Local Storage * 6.64.1 ISO/IEC 9899:1999 Edits for Thread-Local Storage * 6.64.2 ISO/IEC 14882:1998 Edits for Thread-Local Storage * 6.65 Binary Constants using the '0b' Prefix * 7 Extensions to the C++ Language * 7.1 When is a Volatile C++ Object Accessed? * 7.2 Restricting Pointer Aliasing * 7.3 Vague Linkage * 7.4 C++ Interface and Implementation Pragmas * 7.5 Where's the Template? * 7.6 Extracting the Function Pointer from a Bound Pointer to Member Function * 7.7 C++-Specific Variable, Function, and Type Attributes * 7.8 Function Multiversioning * 7.9 Type Traits * 7.10 C++ Concepts * 7.11 Deprecated Features * 7.12 Backwards Compatibility * 8 GNU Objective-C Features * 8.1 GNU Objective-C Runtime API * 8.1.1 Modern GNU Objective-C Runtime API * 8.1.2 Traditional GNU Objective-C Runtime API * 8.2 +load: Executing Code before main * 8.2.1 What You Can and Cannot Do in +load * 8.3 Type Encoding * 8.3.1 Legacy Type Encoding * 8.3.2 @encode * 8.3.3 Method Signatures * 8.4 Garbage Collection * 8.5 Constant String Objects * 8.6 compatibility_alias * 8.7 Exceptions * 8.8 Synchronization * 8.9 Fast Enumeration * 8.9.1 Using Fast Enumeration * 8.9.2 C99-Like Fast Enumeration Syntax * 8.9.3 Fast Enumeration Details * 8.9.4 Fast Enumeration Protocol * 8.10 Messaging with the GNU Objective-C Runtime * 8.10.1 Dynamically Registering Methods * 8.10.2 Forwarding Hook * 9 Binary Compatibility * 10 gcov—a Test Coverage Program * 10.1 Introduction to gcov * 10.2 Invoking gcov * 10.3 Using gcov with GCC Optimization * 10.4 Brief Description of gcov Data Files * 10.5 Data File Relocation to Support Cross-Profiling * 11 gcov-tool—an Offline Gcda Profile Processing Tool * 11.1 Introduction to gcov-tool * 11.2 Invoking gcov-tool * 12 gcov-dump—an Offline Gcda and Gcno Profile Dump Tool * 12.1 Introduction to gcov-dump * 12.2 Invoking gcov-dump * 13 Known Causes of Trouble with GCC * 13.1 Actual Bugs We Haven't Fixed Yet * 13.2 Interoperation * 13.3 Incompatibilities of GCC * 13.4 Fixed Header Files * 13.5 Standard Libraries * 13.6 Disappointments and Misunderstandings * 13.7 Common Misunderstandings with GNU C++ * 13.7.1 Declare and Define Static Members * 13.7.2 Name Lookup, Templates, and Accessing Members of Base Classes * 13.7.3 Temporaries May Vanish Before You Expect * 13.7.4 Implicit Copy-Assignment for Virtual Bases * 13.8 Certain Changes We Don't Want to Make * 13.9 Warning Messages and Error Messages * 14 Reporting Bugs * 14.1 Have You Found a Bug? * 14.2 How and Where to Report Bugs * 15 How To Get Help with GCC * 16 Contributing to GCC Development * Funding Free Software * The GNU Project and GNU/Linux * GNU General Public License * GNU Free Documentation License * ADDENDUM: How to use this License for your documents * Contributors to GCC * Option Index * Keyword Index ────────────────────────────────────────────────────────────────────────────── Introduction This manual documents how to use the GNU compilers, as well as their features and incompatibilities, and how to report bugs. It corresponds to the compilers (GCC) version 9.3.0. The internals of the GNU compilers, including how to port them to new targets and some information about how to write front ends for new languages, are documented in a separate manual. See Introduction in GNU Compiler Collection (GCC) Internals. ────────────────────────────────────────────────────────────────────────────── 1 Programming Languages Supported by GCC GCC stands for “GNU Compiler Collection”. GCC is an integrated distribution of compilers for several major programming languages. These languages currently include C, C++, Objective-C, Objective-C++, Fortran, Ada, D, Go, and BRIG (HSAIL). The abbreviation GCC has multiple meanings in common use. The current official meaning is “GNU Compiler Collection”, which refers generically to the complete suite of tools. The name historically stood for “GNU C Compiler”, and this usage is still common when the emphasis is on compiling C programs. Finally, the name is also used when speaking of the language-independent component of GCC: code shared among the compilers for all supported languages. The language-independent component of GCC includes the majority of the optimizers, as well as the “back ends” that generate machine code for various processors. The part of a compiler that is specific to a particular language is called the “front end”. In addition to the front ends that are integrated components of GCC, there are several other front ends that are maintained separately. These support languages such as Mercury, and COBOL. To use these, they must be built together with GCC proper. Most of the compilers for languages other than C have their own names. The C++ compiler is G++, the Ada compiler is GNAT, and so on. When we talk about compiling one of those languages, we might refer to that compiler by its own name, or as GCC. Either is correct. Historically, compilers for many languages, including C++ and Fortran, have been implemented as “preprocessors” which emit another high level language such as C. None of the compilers included in GCC are implemented this way; they all generate machine code directly. This sort of preprocessor should not be confused with the C preprocessor, which is an integral feature of the C, C++, Objective-C and Objective-C++ languages. ────────────────────────────────────────────────────────────────────────────── 2 Language Standards Supported by GCC For each language compiled by GCC for which there is a standard, GCC attempts to follow one or more versions of that standard, possibly with some exceptions, and possibly with some extensions. 2.1 C Language The original ANSI C standard (X3.159-1989) was ratified in 1989 and published in 1990. This standard was ratified as an ISO standard (ISO/IEC 9899:1990) later in 1990. There were no technical differences between these publications, although the sections of the ANSI standard were renumbered and became clauses in the ISO standard. The ANSI standard, but not the ISO standard, also came with a Rationale document. This standard, in both its forms, is commonly known as C89, or occasionally as C90, from the dates of ratification. To select this standard in GCC, use one of the options -ansi, -std=c90 or -std=iso9899:1990; to obtain all the diagnostics required by the standard, you should also specify -pedantic (or -pedantic-errors if you want them to be errors rather than warnings). See Options Controlling C Dialect. Errors in the 1990 ISO C standard were corrected in two Technical Corrigenda published in 1994 and 1996. GCC does not support the uncorrected version. An amendment to the 1990 standard was published in 1995. This amendment added digraphs and __STDC_VERSION__ to the language, but otherwise concerned the library. This amendment is commonly known as AMD1; the amended standard is sometimes known as C94 or C95. To select this standard in GCC, use the option -std=iso9899:199409 (with, as for other standard versions, -pedantic to receive all required diagnostics). A new edition of the ISO C standard was published in 1999 as ISO/IEC 9899:1999, and is commonly known as C99. (While in development, drafts of this standard version were referred to as C9X.) GCC has substantially complete support for this standard version; see http://gcc.gnu.org/c99status.html for details. To select this standard, use -std=c99 or -std=iso9899:1999. Errors in the 1999 ISO C standard were corrected in three Technical Corrigenda published in 2001, 2004 and 2007. GCC does not support the uncorrected version. A fourth version of the C standard, known as C11, was published in 2011 as ISO/IEC 9899:2011. (While in development, drafts of this standard version were referred to as C1X.) GCC has substantially complete support for this standard, enabled with -std=c11 or -std=iso9899:2011. A version with corrections integrated was prepared in 2017 and published in 2018 as ISO/IEC 9899:2018; it is known as C17 and is supported with -std=c17 or -std=iso9899:2017; the corrections are also applied with -std=c11, and the only difference between the options is the value of __STDC_VERSION__. A further version of the C standard, known as C2X, is under development; experimental and incomplete support for this is enabled with -std=c2x. By default, GCC provides some extensions to the C language that, on rare occasions conflict with the C standard. See Extensions to the C Language Family. Some features that are part of the C99 standard are accepted as extensions in C90 mode, and some features that are part of the C11 standard are accepted as extensions in C90 and C99 modes. Use of the -std options listed above disables these extensions where they conflict with the C standard version selected. You may also select an extended version of the C language explicitly with -std=gnu90 (for C90 with GNU extensions), -std=gnu99 (for C99 with GNU extensions) or -std=gnu11 (for C11 with GNU extensions). The default, if no C language dialect options are given, is -std=gnu11. The ISO C standard defines (in clause 4) two classes of conforming implementation. A conforming hosted implementation supports the whole standard including all the library facilities; a conforming freestanding implementation is only required to provide certain library facilities: those in , , , and ; since AMD1, also those in ; since C99, also those in and ; and since C11, also those in and . In addition, complex types, added in C99, are not required for freestanding implementations. The standard also defines two environments for programs, a freestanding environment, required of all implementations and which may not have library facilities beyond those required of freestanding implementations, where the handling of program startup and termination are implementation-defined; and a hosted environment, which is not required, in which all the library facilities are provided and startup is through a function int main (void) or int main (int, char *[]). An OS kernel is an example of a program running in a freestanding environment; a program using the facilities of an operating system is an example of a program running in a hosted environment. GCC aims towards being usable as a conforming freestanding implementation, or as the compiler for a conforming hosted implementation. By default, it acts as the compiler for a hosted implementation, defining __STDC_HOSTED__ as 1 and presuming that when the names of ISO C functions are used, they have the semantics defined in the standard. To make it act as a conforming freestanding implementation for a freestanding environment, use the option -ffreestanding; it then defines __STDC_HOSTED__ to 0 and does not make assumptions about the meanings of function names from the standard library, with exceptions noted below. To build an OS kernel, you may well still need to make your own arrangements for linking and startup. See Options Controlling C Dialect. GCC does not provide the library facilities required only of hosted implementations, nor yet all the facilities required by C99 of freestanding implementations on all platforms. To use the facilities of a hosted environment, you need to find them elsewhere (for example, in the GNU C library). See Standard Libraries. Most of the compiler support routines used by GCC are present in libgcc, but there are a few exceptions. GCC requires the freestanding environment provide memcpy, memmove, memset and memcmp. Finally, if __builtin_trap is used, and the target does not implement the trap pattern, then GCC emits a call to abort. For references to Technical Corrigenda, Rationale documents and information concerning the history of C that is available online, see http://gcc.gnu.org/readings.html 2.2 C++ Language GCC supports the original ISO C++ standard published in 1998, and the 2011 and 2014 revisions. The original ISO C++ standard was published as the ISO standard (ISO/IEC 14882:1998) and amended by a Technical Corrigenda published in 2003 (ISO/IEC 14882:2003). These standards are referred to as C++98 and C++03, respectively. GCC implements the majority of C++98 (export is a notable exception) and most of the changes in C++03. To select this standard in GCC, use one of the options -ansi, -std=c++98, or -std=c++03; to obtain all the diagnostics required by the standard, you should also specify -pedantic (or -pedantic-errors if you want them to be errors rather than warnings). A revised ISO C++ standard was published in 2011 as ISO/IEC 14882:2011, and is referred to as C++11; before its publication it was commonly referred to as C++0x. C++11 contains several changes to the C++ language, all of which have been implemented in GCC. For details see https://gcc.gnu.org/projects/cxx-status.html#cxx11. To select this standard in GCC, use the option -std=c++11. Another revised ISO C++ standard was published in 2014 as ISO/IEC 14882:2014, and is referred to as C++14; before its publication it was sometimes referred to as C++1y. C++14 contains several further changes to the C++ language, all of which have been implemented in GCC. For details see https://gcc.gnu.org/projects/cxx-status.html#cxx14. To select this standard in GCC, use the option -std=c++14. The C++ language was further revised in 2017 and ISO/IEC 14882:2017 was published. This is referred to as C++17, and before publication was often referred to as C++1z. GCC supports all the changes in the new specification. For further details see https://gcc.gnu.org/projects/cxx-status.html#cxx1z. Use the option -std=c++17 to select this variant of C++. More information about the C++ standards is available on the ISO C++ committee's web site at http://www.open-std.org/jtc1/sc22/wg21/. To obtain all the diagnostics required by any of the standard versions described above you should specify -pedantic or -pedantic-errors, otherwise GCC will allow some non-ISO C++ features as extensions. See Warning Options. By default, GCC also provides some additional extensions to the C++ language that on rare occasions conflict with the C++ standard. See Options Controlling C++ Dialect. Use of the -std options listed above disables these extensions where they they conflict with the C++ standard version selected. You may also select an extended version of the C++ language explicitly with -std=gnu++98 (for C++98 with GNU extensions), or -std=gnu++11 (for C++11 with GNU extensions), or -std=gnu++14 (for C++14 with GNU extensions), or -std=gnu++17 (for C++17 with GNU extensions). The default, if no C++ language dialect options are given, is -std=gnu++14. 2.3 Objective-C and Objective-C++ Languages GCC supports “traditional” Objective-C (also known as “Objective-C 1.0”) and contains support for the Objective-C exception and synchronization syntax. It has also support for a number of “Objective-C 2.0” language extensions, including properties, fast enumeration (only for Objective-C), method attributes and the @optional and @required keywords in protocols. GCC supports Objective-C++ and features available in Objective-C are also available in Objective-C++. GCC by default uses the GNU Objective-C runtime library, which is part of GCC and is not the same as the Apple/NeXT Objective-C runtime library used on Apple systems. There are a number of differences documented in this manual. The options -fgnu-runtime and -fnext-runtime allow you to switch between producing output that works with the GNU Objective-C runtime library and output that works with the Apple/NeXT Objective-C runtime library. There is no formal written standard for Objective-C or Objective-C++. The authoritative manual on traditional Objective-C (1.0) is “Object-Oriented Programming and the Objective-C Language”: http://www.gnustep.org/resources/documentation/ObjectivCBook.pdf is the original NeXTstep document. The Objective-C exception and synchronization syntax (that is, the keywords @try, @throw, @catch, @finally and @synchronized) is supported by GCC and is enabled with the option -fobjc-exceptions. The syntax is briefly documented in this manual and in the Objective-C 2.0 manuals from Apple. The Objective-C 2.0 language extensions and features are automatically enabled; they include properties (via the @property, @synthesize and @dynamic keywords), fast enumeration (not available in Objective-C++), attributes for methods (such as deprecated, noreturn, sentinel, format), the unused attribute for method arguments, the @package keyword for instance variables and the @optional and @required keywords in protocols. You can disable all these Objective-C 2.0 language extensions with the option -fobjc-std=objc1, which causes the compiler to recognize the same Objective-C language syntax recognized by GCC 4.0, and to produce an error if one of the new features is used. GCC has currently no support for non-fragile instance variables. The authoritative manual on Objective-C 2.0 is available from Apple: * https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/Introduction/Introduction.html For more information concerning the history of Objective-C that is available online, see http://gcc.gnu.org/readings.html 2.4 Go Language As of the GCC 4.7.1 release, GCC supports the Go 1 language standard, described at https://golang.org/doc/go1. 2.5 HSA Intermediate Language (HSAIL) GCC can compile the binary representation (BRIG) of the HSAIL text format as described in HSA Programmer's Reference Manual version 1.0.1. This capability is typically utilized to implement the HSA runtime API's HSAIL finalization extension for a gcc supported processor. HSA standards are freely available at http://www.hsafoundation.com/standards/. 2.6 D language GCC supports the D 2.0 programming language. The D language itself is currently defined by its reference implementation and supporting language specification, described at https://dlang.org/spec/spec.html. 2.7 References for Other Languages See About This Guide in GNAT Reference Manual, for information on standard conformance and compatibility of the Ada compiler. See Standards in The GNU Fortran Compiler, for details of standards supported by GNU Fortran. ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Top Link: next: Option Summary Link: prev: Standards ────────────────────────────────────────────────────────────────────────────── 3 GCC Command Options When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The “overall options” allow you to stop this process at an intermediate stage. For example, the -c option says not to run the linker. Then the output consists of object files output by the assembler. See Options Controlling the Kind of Output. Other options are passed on to one or more stages of processing. Some options control the preprocessor and others the compiler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them. Most of the command-line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly. If the description for a particular option does not mention a source language, you can use that option with all supported languages. The usual way to run GCC is to run the executable called gcc, or machine-gcc when cross-compiling, or machine-gcc-version to run a specific version of GCC. When you compile C++ programs, you should invoke GCC as g++ instead. See Compiling C++ Programs, for information about the differences in behavior between gcc and g++ when compiling C++ programs. The gcc program accepts options and file names as operands. Many options have multi-letter names; therefore multiple single-letter options may not be grouped: -dv is very different from '-d -v'. You can mix options and other arguments. For the most part, the order you use doesn't matter. Order does matter when you use several options of the same kind; for example, if you specify -L more than once, the directories are searched in the order specified. Also, the placement of the -l option is significant. Many options have long names starting with '-f' or with '-W'—for example, -fmove-loop-invariants, -Wformat and so on. Most of these have both positive and negative forms; the negative form of -ffoo is -fno-foo. This manual documents only one of these two forms, whichever one is not the default. Some options take one or more arguments typically separated either by a space or by the equals sign ('=') from the option name. Unless documented otherwise, an argument can be either numeric or a string. Numeric arguments must typically be small unsigned decimal or hexadecimal integers. Hexadecimal arguments must begin with the '0x' prefix. Arguments to options that specify a size threshold of some sort may be arbitrarily large decimal or hexadecimal integers followed by a byte size suffix designating a multiple of bytes such as kB and KiB for kilobyte and kibibyte, respectively, MB and MiB for megabyte and mebibyte, GB and GiB for gigabyte and gigibyte, and so on. Such arguments are designated by byte-size in the following text. Refer to the NIST, IEC, and other relevant national and international standards for the full listing and explanation of the binary and decimal byte size prefixes. See Option Index, for an index to GCC's options. • Option Summary: Brief list of all options, without explanations. • Overall Options: Controlling the kind of output: an executable, object files, assembler files, or preprocessed source. • Invoking G++: Compiling C++ programs. • C Dialect Options: Controlling the variant of C language compiled. • C++ Dialect Options: Variations on C++. • Objective-C and Variations on Objective-C and Objective-C++ Dialect Options: Objective-C++. • Diagnostic Message Controlling how diagnostics should be Formatting Options: formatted. • Warning Options: How picky should the compiler be? • Debugging Options: Producing debuggable code. • Optimize Options: How much optimization? • Instrumentation Options: Enabling profiling and extra run-time error checking. • Preprocessor Options: Controlling header files and macro definitions. Also, getting dependency information for Make. • Assembler Options: Passing options to the assembler. • Link Options: Specifying libraries and so on. • Directory Options: Where to find header files and libraries. Where to find the compiler executable files. • Code Gen Options: Specifying conventions for function calls, data layout and register usage. • Developer Options: Printing GCC configuration info, statistics, and debugging dumps. • Submodel Options: Target-specific options, such as compiling for a specific processor variant. • Spec Files: How to pass switches to sub-processes. • Environment Variables: Env vars that affect GCC. • Precompiled Headers: Compiling a header once, and using it many times. ────────────────────────────────────────────────────────────────────────────── 3.1 Option Summary Here is a summary of all the options, grouped by type. Explanations are in the following sections. Overall Options See Options Controlling the Kind of Output. -c -S -E -o file -x language -v -### --help[=class[,…]] --target-help --version -pass-exit-codes -pipe -specs=file -wrapper @file -ffile-prefix-map=old=new -fplugin=file -fplugin-arg-name=arg -fdump-ada-spec[-slim] -fada-spec-parent=unit -fdump-go-spec=file C Language Options See Options Controlling C Dialect. -ansi -std=standard -fgnu89-inline -fpermitted-flt-eval-methods=standard -aux-info filename -fallow-parameterless-variadic-functions -fno-asm -fno-builtin -fno-builtin-function -fgimple -fhosted -ffreestanding -fopenacc -fopenacc-dim=geom -fopenmp -fopenmp-simd -fms-extensions -fplan9-extensions -fsso-struct=endianness -fallow-single-precision -fcond-mismatch -flax-vector-conversions -fsigned-bitfields -fsigned-char -funsigned-bitfields -funsigned-char C++ Language Options See Options Controlling C++ Dialect. -fabi-version=n -fno-access-control -faligned-new=n -fargs-in-order=n -fchar8_t -fcheck-new -fconstexpr-depth=n -fconstexpr-loop-limit=n -fconstexpr-ops-limit=n -fno-elide-constructors -fno-enforce-eh-specs -fno-gnu-keywords -fno-implicit-templates -fno-implicit-inline-templates -fno-implement-inlines -fms-extensions -fnew-inheriting-ctors -fnew-ttp-matching -fno-nonansi-builtins -fnothrow-opt -fno-operator-names -fno-optional-diags -fpermissive -fno-pretty-templates -frepo -fno-rtti -fsized-deallocation -ftemplate-backtrace-limit=n -ftemplate-depth=n -fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ -fvisibility-inlines-hidden -fvisibility-ms-compat -fext-numeric-literals -Wabi=n -Wabi-tag -Wconversion-null -Wctor-dtor-privacy -Wdelete-non-virtual-dtor -Wdeprecated-copy -Wdeprecated-copy-dtor -Wliteral-suffix -Wmultiple-inheritance -Wno-init-list-lifetime -Wnamespaces -Wnarrowing -Wpessimizing-move -Wredundant-move -Wnoexcept -Wnoexcept-type -Wclass-memaccess -Wnon-virtual-dtor -Wreorder -Wregister -Weffc++ -Wstrict-null-sentinel -Wtemplates -Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions -Wno-class-conversion -Wno-terminate -Wsign-promo -Wvirtual-inheritance Objective-C and Objective-C++ Language Options See Options Controlling Objective-C and Objective-C++ Dialects. -fconstant-string-class=class-name -fgnu-runtime -fnext-runtime -fno-nil-receivers -fobjc-abi-version=n -fobjc-call-cxx-cdtors -fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc -fobjc-nilcheck -fobjc-std=objc1 -fno-local-ivars -fivar-visibility=[public|protected|private|package] -freplace-objc-classes -fzero-link -gen-decls -Wassign-intercept -Wno-protocol -Wselector -Wstrict-selector-match -Wundeclared-selector Diagnostic Message Formatting Options See Options to Control Diagnostic Messages Formatting. -fmessage-length=n -fdiagnostics-show-location=[once|every-line] -fdiagnostics-color=[auto|never|always] -fdiagnostics-format=[text|json] -fno-diagnostics-show-option -fno-diagnostics-show-caret -fno-diagnostics-show-labels -fno-diagnostics-show-line-numbers -fdiagnostics-minimum-margin-width=width -fdiagnostics-parseable-fixits -fdiagnostics-generate-patch -fdiagnostics-show-template-tree -fno-elide-type -fno-show-column Warning Options See Options to Request or Suppress Warnings. -fsyntax-only -fmax-errors=n -Wpedantic -pedantic-errors -w -Wextra -Wall -Waddress -Waddress-of-packed-member -Waggregate-return -Waligned-new -Walloc-zero -Walloc-size-larger-than=byte-size -Walloca -Walloca-larger-than=byte-size -Wno-aggressive-loop-optimizations -Warray-bounds -Warray-bounds=n -Wno-attributes -Wattribute-alias=n -Wbool-compare -Wbool-operation -Wno-builtin-declaration-mismatch -Wno-builtin-macro-redefined -Wc90-c99-compat -Wc99-c11-compat -Wc11-c2x-compat -Wc++-compat -Wc++11-compat -Wc++14-compat -Wc++17-compat -Wcast-align -Wcast-align=strict -Wcast-function-type -Wcast-qual -Wchar-subscripts -Wcatch-value -Wcatch-value=n -Wclobbered -Wcomment -Wconditionally-supported -Wconversion -Wcoverage-mismatch -Wno-cpp -Wdangling-else -Wdate-time -Wdelete-incomplete -Wno-attribute-warning -Wno-deprecated -Wno-deprecated-declarations -Wno-designated-init -Wdisabled-optimization -Wno-discarded-qualifiers -Wno-discarded-array-qualifiers -Wno-div-by-zero -Wdouble-promotion -Wduplicated-branches -Wduplicated-cond -Wempty-body -Wenum-compare -Wno-endif-labels -Wexpansion-to-defined -Werror -Werror=* -Wextra-semi -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 -Wno-format-contains-nul -Wno-format-extra-args -Wformat-nonliteral -Wformat-overflow=n -Wformat-security -Wformat-signedness -Wformat-truncation=n -Wformat-y2k -Wframe-address -Wframe-larger-than=byte-size -Wno-free-nonheap-object -Wjump-misses-init -Whsa -Wif-not-aligned -Wignored-qualifiers -Wignored-attributes -Wincompatible-pointer-types -Wimplicit -Wimplicit-fallthrough -Wimplicit-fallthrough=n -Wimplicit-function-declaration -Wimplicit-int -Winit-self -Winline -Wno-int-conversion -Wint-in-bool-context -Wno-int-to-pointer-cast -Winvalid-memory-model -Wno-invalid-offsetof -Winvalid-pch -Wlarger-than=byte-size -Wlogical-op -Wlogical-not-parentheses -Wlong-long -Wmain -Wmaybe-uninitialized -Wmemset-elt-size -Wmemset-transposed-args -Wmisleading-indentation -Wmissing-attributes -Wmissing-braces -Wmissing-field-initializers -Wmissing-format-attribute -Wmissing-include-dirs -Wmissing-noreturn -Wmissing-profile -Wno-multichar -Wmultistatement-macros -Wnonnull -Wnonnull-compare -Wnormalized=[none|id|nfc|nfkc] -Wnull-dereference -Wodr -Wno-overflow -Wopenmp-simd -Woverride-init-side-effects -Woverlength-strings -Wpacked -Wpacked-bitfield-compat -Wpacked-not-aligned -Wpadded -Wparentheses -Wno-pedantic-ms-format -Wplacement-new -Wplacement-new=n -Wpointer-arith -Wpointer-compare -Wno-pointer-to-int-cast -Wno-pragmas -Wno-prio-ctor-dtor -Wredundant-decls -Wrestrict -Wno-return-local-addr -Wreturn-type -Wsequence-point -Wshadow -Wno-shadow-ivar -Wshadow=global, -Wshadow=local, -Wshadow=compatible-local -Wshift-overflow -Wshift-overflow=n -Wshift-count-negative -Wshift-count-overflow -Wshift-negative-value -Wsign-compare -Wsign-conversion -Wfloat-conversion -Wno-scalar-storage-order -Wsizeof-pointer-div -Wsizeof-pointer-memaccess -Wsizeof-array-argument -Wstack-protector -Wstack-usage=byte-size -Wstrict-aliasing -Wstrict-aliasing=n -Wstrict-overflow -Wstrict-overflow=n -Wstringop-overflow=n -Wstringop-truncation -Wsubobject-linkage -Wsuggest-attribute=[pure|const|noreturn|format|malloc] -Wsuggest-final-types -Wsuggest-final-methods -Wsuggest-override -Wswitch -Wswitch-bool -Wswitch-default -Wswitch-enum -Wswitch-unreachable -Wsync-nand -Wsystem-headers -Wtautological-compare -Wtrampolines -Wtrigraphs -Wtype-limits -Wundef -Wuninitialized -Wunknown-pragmas -Wunsuffixed-float-constants -Wunused -Wunused-function -Wunused-label -Wunused-local-typedefs -Wunused-macros -Wunused-parameter -Wno-unused-result -Wunused-value -Wunused-variable -Wunused-const-variable -Wunused-const-variable=n -Wunused-but-set-parameter -Wunused-but-set-variable -Wuseless-cast -Wvariadic-macros -Wvector-operation-performance -Wvla -Wvla-larger-than=byte-size -Wvolatile-register-var -Wwrite-strings -Wzero-as-null-pointer-constant C and Objective-C-only Warning Options -Wbad-function-cast -Wmissing-declarations -Wmissing-parameter-type -Wmissing-prototypes -Wnested-externs -Wold-style-declaration -Wold-style-definition -Wstrict-prototypes -Wtraditional -Wtraditional-conversion -Wdeclaration-after-statement -Wpointer-sign Debugging Options See Options for Debugging Your Program. -g -glevel -gdwarf -gdwarf-version -ggdb -grecord-gcc-switches -gno-record-gcc-switches -gstabs -gstabs+ -gstrict-dwarf -gno-strict-dwarf -gas-loc-support -gno-as-loc-support -gas-locview-support -gno-as-locview-support -gcolumn-info -gno-column-info -gstatement-frontiers -gno-statement-frontiers -gvariable-location-views -gno-variable-location-views -ginternal-reset-location-views -gno-internal-reset-location-views -ginline-points -gno-inline-points -gvms -gxcoff -gxcoff+ -gz[=type] -gsplit-dwarf -gdescribe-dies -gno-describe-dies -fdebug-prefix-map=old=new -fdebug-types-section -fno-eliminate-unused-debug-types -femit-struct-debug-baseonly -femit-struct-debug-reduced -femit-struct-debug-detailed[=spec-list] -feliminate-unused-debug-symbols -femit-class-debug-always -fno-merge-debug-strings -fno-dwarf2-cfi-asm -fvar-tracking -fvar-tracking-assignments Optimization Options See Options that Control Optimization. -faggressive-loop-optimizations -falign-functions[=n[:m:[n2[:m2]]]] -falign-jumps[=n[:m:[n2[:m2]]]] -falign-labels[=n[:m:[n2[:m2]]]] -falign-loops[=n[:m:[n2[:m2]]]] -fassociative-math -fauto-profile -fauto-profile[=path] -fauto-inc-dec -fbranch-probabilities -fbranch-target-load-optimize -fbranch-target-load-optimize2 -fbtr-bb-exclusive -fcaller-saves -fcombine-stack-adjustments -fconserve-stack -fcompare-elim -fcprop-registers -fcrossjumping -fcse-follow-jumps -fcse-skip-blocks -fcx-fortran-rules -fcx-limited-range -fdata-sections -fdce -fdelayed-branch -fdelete-null-pointer-checks -fdevirtualize -fdevirtualize-speculatively -fdevirtualize-at-ltrans -fdse -fearly-inlining -fipa-sra -fexpensive-optimizations -ffat-lto-objects -ffast-math -ffinite-math-only -ffloat-store -fexcess-precision=style -fforward-propagate -ffp-contract=style -ffunction-sections -fgcse -fgcse-after-reload -fgcse-las -fgcse-lm -fgraphite-identity -fgcse-sm -fhoist-adjacent-loads -fif-conversion -fif-conversion2 -findirect-inlining -finline-functions -finline-functions-called-once -finline-limit=n -finline-small-functions -fipa-cp -fipa-cp-clone -fipa-bit-cp -fipa-vrp -fipa-pta -fipa-profile -fipa-pure-const -fipa-reference -fipa-reference-addressable -fipa-stack-alignment -fipa-icf -fira-algorithm=algorithm -flive-patching=level -fira-region=region -fira-hoist-pressure -fira-loop-pressure -fno-ira-share-save-slots -fno-ira-share-spill-slots -fisolate-erroneous-paths-dereference -fisolate-erroneous-paths-attribute -fivopts -fkeep-inline-functions -fkeep-static-functions -fkeep-static-consts -flimit-function-alignment -flive-range-shrinkage -floop-block -floop-interchange -floop-strip-mine -floop-unroll-and-jam -floop-nest-optimize -floop-parallelize-all -flra-remat -flto -flto-compression-level -flto-partition=alg -fmerge-all-constants -fmerge-constants -fmodulo-sched -fmodulo-sched-allow-regmoves -fmove-loop-invariants -fno-branch-count-reg -fno-defer-pop -fno-fp-int-builtin-inexact -fno-function-cse -fno-guess-branch-probability -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 -fno-printf-return-value -fno-sched-interblock -fno-sched-spec -fno-signed-zeros -fno-toplevel-reorder -fno-trapping-math -fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-sibling-calls -fpartial-inlining -fpeel-loops -fpredictive-commoning -fprefetch-loop-arrays -fprofile-correction -fprofile-use -fprofile-use=path -fprofile-values -fprofile-reorder-functions -freciprocal-math -free -frename-registers -freorder-blocks -freorder-blocks-algorithm=algorithm -freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop -freschedule-modulo-scheduled-loops -frounding-math -fsave-optimization-record -fsched2-use-superblocks -fsched-pressure -fsched-spec-load -fsched-spec-load-dangerous -fsched-stalled-insns-dep[=n] -fsched-stalled-insns[=n] -fsched-group-heuristic -fsched-critical-path-heuristic -fsched-spec-insn-heuristic -fsched-rank-heuristic -fsched-last-insn-heuristic -fsched-dep-count-heuristic -fschedule-fusion -fschedule-insns -fschedule-insns2 -fsection-anchors -fselective-scheduling -fselective-scheduling2 -fsel-sched-pipelining -fsel-sched-pipelining-outer-loops -fsemantic-interposition -fshrink-wrap -fshrink-wrap-separate -fsignaling-nans -fsingle-precision-constant -fsplit-ivs-in-unroller -fsplit-loops -fsplit-paths -fsplit-wide-types -fssa-backprop -fssa-phiopt -fstdarg-opt -fstore-merging -fstrict-aliasing -fthread-jumps -ftracer -ftree-bit-ccp -ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -fcode-hoisting -ftree-loop-if-convert -ftree-loop-im -ftree-phiprop -ftree-loop-distribution -ftree-loop-distribute-patterns -ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize -ftree-loop-vectorize -ftree-parallelize-loops=n -ftree-pre -ftree-partial-pre -ftree-pta -ftree-reassoc -ftree-scev-cprop -ftree-sink -ftree-slsr -ftree-sra -ftree-switch-conversion -ftree-tail-merge -ftree-ter -ftree-vectorize -ftree-vrp -funconstrained-commons -funit-at-a-time -funroll-all-loops -funroll-loops -funsafe-math-optimizations -funswitch-loops -fipa-ra -fvariable-expansion-in-unroller -fvect-cost-model -fvpt -fweb -fwhole-program -fwpa -fuse-linker-plugin --param name=value -O -O0 -O1 -O2 -O3 -Os -Ofast -Og Program Instrumentation Options See Program Instrumentation Options. -p -pg -fprofile-arcs --coverage -ftest-coverage -fprofile-abs-path -fprofile-dir=path -fprofile-generate -fprofile-generate=path -fprofile-update=method -fprofile-filter-files=regex -fprofile-exclude-files=regex -fsanitize=style -fsanitize-recover -fsanitize-recover=style -fasan-shadow-offset=number -fsanitize-sections=s1,s2,... -fsanitize-undefined-trap-on-error -fbounds-check -fcf-protection=[full|branch|return|none] -fstack-protector -fstack-protector-all -fstack-protector-strong -fstack-protector-explicit -fstack-check -fstack-limit-register=reg -fstack-limit-symbol=sym -fno-stack-limit -fsplit-stack -fvtable-verify=[std|preinit|none] -fvtv-counts -fvtv-debug -finstrument-functions -finstrument-functions-exclude-function-list=sym,sym,… -finstrument-functions-exclude-file-list=file,file,… Preprocessor Options See Options Controlling the Preprocessor. -Aquestion=answer -A-question[=answer] -C -CC -Dmacro[=defn] -dD -dI -dM -dN -dU -fdebug-cpp -fdirectives-only -fdollars-in-identifiers -fexec-charset=charset -fextended-identifiers -finput-charset=charset -fmacro-prefix-map=old=new -fno-canonical-system-headers -fpch-deps -fpch-preprocess -fpreprocessed -ftabstop=width -ftrack-macro-expansion -fwide-exec-charset=charset -fworking-directory -H -imacros file -include file -M -MD -MF -MG -MM -MMD -MP -MQ -MT -no-integrated-cpp -P -pthread -remap -traditional -traditional-cpp -trigraphs -Umacro -undef -Wp,option -Xpreprocessor option Assembler Options See Passing Options to the Assembler. -Wa,option -Xassembler option Linker Options See Options for Linking. object-file-name -fuse-ld=linker -llibrary -nostartfiles -nodefaultlibs -nolibc -nostdlib -e entry --entry=entry -pie -pthread -r -rdynamic -s -static -static-pie -static-libgcc -static-libstdc++ -static-libasan -static-libtsan -static-liblsan -static-libubsan -shared -shared-libgcc -symbolic -T script -Wl,option -Xlinker option -u symbol -z keyword Directory Options See Options for Directory Search. -Bprefix -Idir -I- -idirafter dir -imacros file -imultilib dir -iplugindir=dir -iprefix file -iquote dir -isysroot dir -isystem dir -iwithprefix dir -iwithprefixbefore dir -Ldir -no-canonical-prefixes --no-sysroot-suffix -nostdinc -nostdinc++ --sysroot=dir Code Generation Options See Options for Code Generation Conventions. -fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions -fnon-call-exceptions -fdelete-dead-exceptions -funwind-tables -fasynchronous-unwind-tables -fno-gnu-unique -finhibit-size-directive -fno-common -fno-ident -fpcc-struct-return -fpic -fPIC -fpie -fPIE -fno-plt -fno-jump-tables -frecord-gcc-switches -freg-struct-return -fshort-enums -fshort-wchar -fverbose-asm -fpack-struct[=n] -fleading-underscore -ftls-model=model -fstack-reuse=reuse_level -ftrampolines -ftrapv -fwrapv -fvisibility=[default|internal|hidden|protected] -fstrict-volatile-bitfields -fsync-libcalls Developer Options See GCC Developer Options. -dletters -dumpspecs -dumpmachine -dumpversion -dumpfullversion -fchecking -fchecking=n -fdbg-cnt-list -fdbg-cnt=counter-value-list -fdisable-ipa-pass_name -fdisable-rtl-pass_name -fdisable-rtl-pass-name=range-list -fdisable-tree-pass_name -fdisable-tree-pass-name=range-list -fdump-debug -fdump-earlydebug -fdump-noaddr -fdump-unnumbered -fdump-unnumbered-links -fdump-final-insns[=file] -fdump-ipa-all -fdump-ipa-cgraph -fdump-ipa-inline -fdump-lang-all -fdump-lang-switch -fdump-lang-switch-options -fdump-lang-switch-options=filename -fdump-passes -fdump-rtl-pass -fdump-rtl-pass=filename -fdump-statistics -fdump-tree-all -fdump-tree-switch -fdump-tree-switch-options -fdump-tree-switch-options=filename -fcompare-debug[=opts] -fcompare-debug-second -fenable-kind-pass -fenable-kind-pass=range-list -fira-verbose=n -flto-report -flto-report-wpa -fmem-report-wpa -fmem-report -fpre-ipa-mem-report -fpost-ipa-mem-report -fopt-info -fopt-info-options[=file] -fprofile-report -frandom-seed=string -fsched-verbose=n -fsel-sched-verbose -fsel-sched-dump-cfg -fsel-sched-pipelining-verbose -fstats -fstack-usage -ftime-report -ftime-report-details -fvar-tracking-assignments-toggle -gtoggle -print-file-name=library -print-libgcc-file-name -print-multi-directory -print-multi-lib -print-multi-os-directory -print-prog-name=program -print-search-dirs -Q -print-sysroot -print-sysroot-headers-suffix -save-temps -save-temps=cwd -save-temps=obj -time[=file] Machine-Dependent Options See Machine-Dependent Options. AArch64 Options -mabi=name -mbig-endian -mlittle-endian -mgeneral-regs-only -mcmodel=tiny -mcmodel=small -mcmodel=large -mstrict-align -mno-strict-align -momit-leaf-frame-pointer -mtls-dialect=desc -mtls-dialect=traditional -mtls-size=size -mfix-cortex-a53-835769 -mfix-cortex-a53-843419 -mlow-precision-recip-sqrt -mlow-precision-sqrt -mlow-precision-div -mpc-relative-literal-loads -msign-return-address=scope -mbranch-protection=none|standard|pac-ret[+leaf]|bti -march=name -mcpu=name -mtune=name -moverride=string -mverbose-cost-dump -mstack-protector-guard=guard -mstack-protector-guard-reg=sysreg -mstack-protector-guard-offset=offset -mtrack-speculation Adapteva Epiphany Options -mhalf-reg-file -mprefer-short-insn-regs -mbranch-cost=num -mcmove -mnops=num -msoft-cmpsf -msplit-lohi -mpost-inc -mpost-modify -mstack-offset=num -mround-nearest -mlong-calls -mshort-calls -msmall16 -mfp-mode=mode -mvect-double -max-vect-align=num -msplit-vecmove-early -m1reg-reg AMD GCN Options -march=gpu -mtune=gpu -mstack-size=bytes ARC Options -mbarrel-shifter -mjli-always -mcpu=cpu -mA6 -mARC600 -mA7 -mARC700 -mdpfp -mdpfp-compact -mdpfp-fast -mno-dpfp-lrsr -mea -mno-mpy -mmul32x16 -mmul64 -matomic -mnorm -mspfp -mspfp-compact -mspfp-fast -msimd -msoft-float -mswap -mcrc -mdsp-packa -mdvbf -mlock -mmac-d16 -mmac-24 -mrtsc -mswape -mtelephony -mxy -misize -mannotate-align -marclinux -marclinux_prof -mlong-calls -mmedium-calls -msdata -mirq-ctrl-saved -mrgf-banked-regs -mlpc-width=width -G num -mvolatile-cache -mtp-regno=regno -malign-call -mauto-modify-reg -mbbit-peephole -mno-brcc -mcase-vector-pcrel -mcompact-casesi -mno-cond-exec -mearly-cbranchsi -mexpand-adddi -mindexed-loads -mlra -mlra-priority-none -mlra-priority-compact mlra-priority-noncompact -mmillicode -mmixed-code -mq-class -mRcq -mRcw -msize-level=level -mtune=cpu -mmultcost=num -mcode-density-frame -munalign-prob-threshold=probability -mmpy-option=multo -mdiv-rem -mcode-density -mll64 -mfpu=fpu -mrf16 -mbranch-index ARM Options -mapcs-frame -mno-apcs-frame -mabi=name -mapcs-stack-check -mno-apcs-stack-check -mapcs-reentrant -mno-apcs-reentrant -mgeneral-regs-only -msched-prolog -mno-sched-prolog -mlittle-endian -mbig-endian -mbe8 -mbe32 -mfloat-abi=name -mfp16-format=name -mthumb-interwork -mno-thumb-interwork -mcpu=name -march=name -mfpu=name -mtune=name -mprint-tune-info -mstructure-size-boundary=n -mabort-on-noreturn -mlong-calls -mno-long-calls -msingle-pic-base -mno-single-pic-base -mpic-register=reg -mnop-fun-dllimport -mpoke-function-name -mthumb -marm -mflip-thumb -mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking -mcallee-super-interworking -mtp=name -mtls-dialect=dialect -mword-relocations -mfix-cortex-m3-ldrd -munaligned-access -mneon-for-64bits -mslow-flash-data -masm-syntax-unified -mrestrict-it -mverbose-cost-dump -mpure-code -mcmse AVR Options -mmcu=mcu -mabsdata -maccumulate-args -mbranch-cost=cost -mcall-prologues -mgas-isr-prologues -mint8 -mn_flash=size -mno-interrupts -mmain-is-OS_task -mrelax -mrmw -mstrict-X -mtiny-stack -mfract-convert-truncate -mshort-calls -nodevicelib -nodevicespecs -Waddr-space-convert -Wmisspelled-isr Blackfin Options -mcpu=cpu[-sirevision] -msim -momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer -mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly -mno-csync-anomaly -mlow-64k -mno-low64k -mstack-check-l1 -mid-shared-library -mno-id-shared-library -mshared-library-id=n -mleaf-id-shared-library -mno-leaf-id-shared-library -msep-data -mno-sep-data -mlong-calls -mno-long-calls -mfast-fp -minline-plt -mmulticore -mcorea -mcoreb -msdram -micplb C6X Options -mbig-endian -mlittle-endian -march=cpu -msim -msdata=sdata-type CRIS Options -mcpu=cpu -march=cpu -mtune=cpu -mmax-stack-frame=n -melinux-stacksize=n -metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects -mstack-align -mdata-align -mconst-align -m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt -melf -maout -melinux -mlinux -sim -sim2 -mmul-bug-workaround -mno-mul-bug-workaround CR16 Options -mmac -mcr16cplus -mcr16c -msim -mint32 -mbit-ops -mdata-model=model C-SKY Options -march=arch -mcpu=cpu -mbig-endian -EB -mlittle-endian -EL -mhard-float -msoft-float -mfpu=fpu -mdouble-float -mfdivdu -melrw -mistack -mmp -mcp -mcache -msecurity -mtrust -mdsp -medsp -mvdsp -mdiv -msmart -mhigh-registers -manchor -mpushpop -mmultiple-stld -mconstpool -mstack-size -mccrt -mbranch-cost=n -mcse-cc -msched-prolog Darwin Options -all_load -allowable_client -arch -arch_errors_fatal -arch_only -bind_at_load -bundle -bundle_loader -client_name -compatibility_version -current_version -dead_strip -dependency-file -dylib_file -dylinker_install_name -dynamic -dynamiclib -exported_symbols_list -filelist -flat_namespace -force_cpusubtype_ALL -force_flat_namespace -headerpad_max_install_names -iframework -image_base -init -install_name -keep_private_externs -multi_module -multiply_defined -multiply_defined_unused -noall_load -no_dead_strip_inits_and_terms -nofixprebinding -nomultidefs -noprebind -noseglinkedit -pagezero_size -prebind -prebind_all_twolevel_modules -private_bundle -read_only_relocs -sectalign -sectobjectsymbols -whyload -seg1addr -sectcreate -sectobjectsymbols -sectorder -segaddr -segs_read_only_addr -segs_read_write_addr -seg_addr_table -seg_addr_table_filename -seglinkedit -segprot -segs_read_only_addr -segs_read_write_addr -single_module -static -sub_library -sub_umbrella -twolevel_namespace -umbrella -undefined -unexported_symbols_list -weak_reference_mismatches -whatsloaded -F -gused -gfull -mmacosx-version-min=version -mkernel -mone-byte-bool DEC Alpha Options -mno-fp-regs -msoft-float -mieee -mieee-with-inexact -mieee-conformant -mfp-trap-mode=mode -mfp-rounding-mode=mode -mtrap-precision=mode -mbuild-constants -mcpu=cpu-type -mtune=cpu-type -mbwx -mmax -mfix -mcix -mfloat-vax -mfloat-ieee -mexplicit-relocs -msmall-data -mlarge-data -msmall-text -mlarge-text -mmemory-latency=time FR30 Options -msmall-model -mno-lsim FT32 Options -msim -mlra -mnodiv -mft32b -mcompress -mnopm FRV Options -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 -mhard-float -msoft-float -malloc-cc -mfixed-cc -mdword -mno-dword -mdouble -mno-double -mmedia -mno-media -mmuladd -mno-muladd -mfdpic -minline-plt -mgprel-ro -multilib-library-pic -mlinked-fp -mlong-calls -malign-labels -mlibrary-pic -macc-4 -macc-8 -mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move -moptimize-membar -mno-optimize-membar -mscc -mno-scc -mcond-exec -mno-cond-exec -mvliw-branch -mno-vliw-branch -mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec -mno-nested-cond-exec -mtomcat-stats -mTLS -mtls -mcpu=cpu GNU/Linux Options -mglibc -muclibc -mmusl -mbionic -mandroid -tno-android-cc -tno-android-ld H8/300 Options -mrelax -mh -ms -mn -mexr -mno-exr -mint32 -malign-300 HPPA Options -march=architecture-type -mcaller-copies -mdisable-fpregs -mdisable-indexing -mfast-indirect-calls -mgas -mgnu-ld -mhp-ld -mfixed-range=register-range -mjump-in-delay -mlinker-opt -mlong-calls -mlong-load-store -mno-disable-fpregs -mno-disable-indexing -mno-fast-indirect-calls -mno-gas -mno-jump-in-delay -mno-long-load-store -mno-portable-runtime -mno-soft-float -mno-space-regs -msoft-float -mpa-risc-1-0 -mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime -mschedule=cpu-type -mspace-regs -msio -mwsio -munix=unix-std -nolibdld -static -threads IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic -mvolatile-asm-stop -mregister-names -msdata -mno-sdata -mconstant-gp -mauto-pic -mfused-madd -minline-float-divide-min-latency -minline-float-divide-max-throughput -mno-inline-float-divide -minline-int-divide-min-latency -minline-int-divide-max-throughput -mno-inline-int-divide -minline-sqrt-min-latency -minline-sqrt-max-throughput -mno-inline-sqrt -mdwarf2-asm -mearly-stop-bits -mfixed-range=register-range -mtls-size=tls-size -mtune=cpu-type -milp32 -mlp64 -msched-br-data-spec -msched-ar-data-spec -msched-control-spec -msched-br-in-data-spec -msched-ar-in-data-spec -msched-in-control-spec -msched-spec-ldc -msched-spec-control-ldc -msched-prefer-non-data-spec-insns -msched-prefer-non-control-spec-insns -msched-stop-bits-after-every-cycle -msched-count-spec-in-critical-path -msel-sched-dont-check-control-spec -msched-fp-mem-deps-zero-cost -msched-max-memory-insns-hard-limit -msched-max-memory-insns=max-insns LM32 Options -mbarrel-shift-enabled -mdivide-enabled -mmultiply-enabled -msign-extend-enabled -muser-enabled M32R/D Options -m32r2 -m32rx -m32r -mdebug -malign-loops -mno-align-loops -missue-rate=number -mbranch-cost=number -mmodel=code-size-model-type -msdata=sdata-type -mno-flush-func -mflush-func=name -mno-flush-trap -mflush-trap=number -G num M32C Options -mcpu=cpu -msim -memregs=number M680x0 Options -march=arch -mcpu=cpu -mtune=tune -m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 -m68060 -mcpu32 -m5200 -m5206e -m528x -m5307 -m5407 -mcfv4e -mbitfield -mno-bitfield -mc68000 -mc68020 -mnobitfield -mrtd -mno-rtd -mdiv -mno-div -mshort -mno-short -mhard-float -m68881 -msoft-float -mpcrel -malign-int -mstrict-align -msep-data -mno-sep-data -mshared-library-id=n -mid-shared-library -mno-id-shared-library -mxgot -mno-xgot -mlong-jump-table-offsets MCore Options -mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates -mno-relax-immediates -mwide-bitfields -mno-wide-bitfields -m4byte-functions -mno-4byte-functions -mcallgraph-data -mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim -mlittle-endian -mbig-endian -m210 -m340 -mstack-increment MeP Options -mabsdiff -mall-opts -maverage -mbased=n -mbitops -mc=n -mclip -mconfig=name -mcop -mcop32 -mcop64 -mivc2 -mdc -mdiv -meb -mel -mio-volatile -ml -mleadz -mm -mminmax -mmult -mno-opts -mrepeat -ms -msatur -msdram -msim -msimnovec -mtf -mtiny=n MicroBlaze Options -msoft-float -mhard-float -msmall-divides -mcpu=cpu -mmemcpy -mxl-soft-mul -mxl-soft-div -mxl-barrel-shift -mxl-pattern-compare -mxl-stack-check -mxl-gp-opt -mno-clearbss -mxl-multiply-high -mxl-float-convert -mxl-float-sqrt -mbig-endian -mlittle-endian -mxl-reorder -mxl-mode-app-model -mpic-data-is-text-relative MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips32r3 -mips32r5 -mips32r6 -mips64 -mips64r2 -mips64r3 -mips64r5 -mips64r6 -mips16 -mno-mips16 -mflip-mips16 -minterlink-compressed -mno-interlink-compressed -minterlink-mips16 -mno-interlink-mips16 -mabi=abi -mabicalls -mno-abicalls -mshared -mno-shared -mplt -mno-plt -mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfpxx -mfp64 -mhard-float -msoft-float -mno-float -msingle-float -mdouble-float -modd-spreg -mno-odd-spreg -mabs=mode -mnan=encoding -mdsp -mno-dsp -mdspr2 -mno-dspr2 -mmcu -mmno-mcu -meva -mno-eva -mvirt -mno-virt -mxpa -mno-xpa -mcrc -mno-crc -mginv -mno-ginv -mmicromips -mno-micromips -mmsa -mno-msa -mloongson-mmi -mno-loongson-mmi -mloongson-ext -mno-loongson-ext -mloongson-ext2 -mno-loongson-ext2 -mfpu=fpu-type -msmartmips -mno-smartmips -mpaired-single -mno-paired-single -mdmx -mno-mdmx -mips3d -mno-mips3d -mmt -mno-mt -mllsc -mno-llsc -mlong64 -mlong32 -msym32 -mno-sym32 -Gnum -mlocal-sdata -mno-local-sdata -mextern-sdata -mno-extern-sdata -mgpopt -mno-gopt -membedded-data -mno-embedded-data -muninit-const-in-rodata -mno-uninit-const-in-rodata -mcode-readable=setting -msplit-addresses -mno-split-addresses -mexplicit-relocs -mno-explicit-relocs -mcheck-zero-division -mno-check-zero-division -mdivide-traps -mdivide-breaks -mload-store-pairs -mno-load-store-pairs -mmemcpy -mno-memcpy -mlong-calls -mno-long-calls -mmad -mno-mad -mimadd -mno-imadd -mfused-madd -mno-fused-madd -nocpp -mfix-24k -mno-fix-24k -mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400 -mfix-r5900 -mno-fix-r5900 -mfix-r10000 -mno-fix-r10000 -mfix-rm7000 -mno-fix-rm7000 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130 -mno-fix-vr4130 -mfix-sb1 -mno-fix-sb1 -mflush-func=func -mno-flush-func -mbranch-cost=num -mbranch-likely -mno-branch-likely -mcompact-branches=policy -mfp-exceptions -mno-fp-exceptions -mvr4130-align -mno-vr4130-align -msynci -mno-synci -mlxc1-sxc1 -mno-lxc1-sxc1 -mmadd4 -mno-madd4 -mrelax-pic-calls -mno-relax-pic-calls -mmcount-ra-address -mframe-header-opt -mno-frame-header-opt MMIX Options -mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu -mabi=mmixware -mzero-extend -mknuthdiv -mtoplevel-symbols -melf -mbranch-predict -mno-branch-predict -mbase-addresses -mno-base-addresses -msingle-exit -mno-single-exit MN10300 Options -mmult-bug -mno-mult-bug -mno-am33 -mam33 -mam33-2 -mam34 -mtune=cpu-type -mreturn-pointer-on-d0 -mno-crt0 -mrelax -mliw -msetlb Moxie Options -meb -mel -mmul.x -mno-crt0 MSP430 Options -msim -masm-hex -mmcu= -mcpu= -mlarge -msmall -mrelax -mwarn-mcu -mcode-region= -mdata-region= -msilicon-errata= -msilicon-errata-warn= -mhwmult= -minrt NDS32 Options -mbig-endian -mlittle-endian -mreduced-regs -mfull-regs -mcmov -mno-cmov -mext-perf -mno-ext-perf -mext-perf2 -mno-ext-perf2 -mext-string -mno-ext-string -mv3push -mno-v3push -m16bit -mno-16bit -misr-vector-size=num -mcache-block-size=num -march=arch -mcmodel=code-model -mctor-dtor -mrelax Nios II Options -G num -mgpopt=option -mgpopt -mno-gpopt -mgprel-sec=regexp -mr0rel-sec=regexp -mel -meb -mno-bypass-cache -mbypass-cache -mno-cache-volatile -mcache-volatile -mno-fast-sw-div -mfast-sw-div -mhw-mul -mno-hw-mul -mhw-mulx -mno-hw-mulx -mno-hw-div -mhw-div -mcustom-insn=N -mno-custom-insn -mcustom-fpu-cfg=name -mhal -msmallc -msys-crt0=name -msys-lib=name -march=arch -mbmx -mno-bmx -mcdx -mno-cdx Nvidia PTX Options -m32 -m64 -mmainkernel -moptimize OpenRISC Options -mboard=name -mnewlib -mhard-mul -mhard-div -msoft-mul -msoft-div -mcmov -mror -msext -msfimm -mshftimm PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 -mint32 -mno-int16 -mint16 -mno-int32 -msplit -munix-asm -mdec-asm -mgnu-asm -mlra picoChip Options -mae=ae_type -mvliw-lookahead=N -msymbol-as-address -mno-inefficient-warnings PowerPC Options See RS/6000 and PowerPC Options. RISC-V Options -mbranch-cost=N-instruction -mplt -mno-plt -mabi=ABI-string -mfdiv -mno-fdiv -mdiv -mno-div -march=ISA-string -mtune=processor-string -mpreferred-stack-boundary=num -msmall-data-limit=N-bytes -msave-restore -mno-save-restore -mstrict-align -mno-strict-align -mcmodel=medlow -mcmodel=medany -mexplicit-relocs -mno-explicit-relocs -mrelax -mno-relax -mriscv-attribute -mmo-riscv-attribute RL78 Options -msim -mmul=none -mmul=g13 -mmul=g14 -mallregs -mcpu=g10 -mcpu=g13 -mcpu=g14 -mg10 -mg13 -mg14 -m64bit-doubles -m32bit-doubles -msave-mduc-in-interrupts RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -mpowerpc64 -maltivec -mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt -mno-powerpc-gfxopt -mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb -mpopcntd -mno-popcntd -mfprnd -mno-fprnd -mcmpb -mno-cmpb -mmfpgpr -mno-mfpgpr -mhard-dfp -mno-hard-dfp -mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc -m64 -m32 -mxl-compat -mno-xl-compat -mpe -malign-power -malign-natural -msoft-float -mhard-float -mmultiple -mno-multiple -mupdate -mno-update -mavoid-indexed-addresses -mno-avoid-indexed-addresses -mfused-madd -mno-fused-madd -mbit-align -mno-bit-align -mstrict-align -mno-strict-align -mrelocatable -mno-relocatable -mrelocatable-lib -mno-relocatable-lib -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian -mdynamic-no-pic -mswdiv -msingle-pic-base -mprioritize-restricted-insns=priority -msched-costly-dep=dependence_type -minsert-sched-nops=scheme -mcall-aixdesc -mcall-eabi -mcall-freebsd -mcall-linux -mcall-netbsd -mcall-openbsd -mcall-sysv -mcall-sysv-eabi -mcall-sysv-noeabi -mtraceback=traceback_type -maix-struct-return -msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt -mlongcall -mno-longcall -mpltseq -mno-pltseq -mblock-move-inline-limit=num -mblock-compare-inline-limit=num -mblock-compare-inline-loop-limit=num -mstring-compare-inline-limit=num -misel -mno-isel -mvrsave -mno-vrsave -mmulhw -mno-mulhw -mdlmzb -mno-dlmzb -mprototype -mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata -msdata=opt -mreadonly-in-sdata -mvxworks -G num -mrecip -mrecip=opt -mno-recip -mrecip-precision -mno-recip-precision -mveclibabi=type -mfriz -mno-friz -mpointers-to-nested-functions -mno-pointers-to-nested-functions -msave-toc-indirect -mno-save-toc-indirect -mpower8-fusion -mno-mpower8-fusion -mpower8-vector -mno-power8-vector -mcrypto -mno-crypto -mhtm -mno-htm -mquad-memory -mno-quad-memory -mquad-memory-atomic -mno-quad-memory-atomic -mcompat-align-parm -mno-compat-align-parm -mfloat128 -mno-float128 -mfloat128-hardware -mno-float128-hardware -mgnu-attribute -mno-gnu-attribute -mstack-protector-guard=guard -mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset RX Options -m64bit-doubles -m32bit-doubles -fpu -nofpu -mcpu= -mbig-endian-data -mlittle-endian-data -msmall-data -msim -mno-sim -mas100-syntax -mno-as100-syntax -mrelax -mmax-constant-size= -mint-register= -mpid -mallow-string-insns -mno-allow-string-insns -mjsr -mno-warn-multiple-fast-interrupts -msave-acc-in-interrupts S/390 and zSeries Options -mtune=cpu-type -march=cpu-type -mhard-float -msoft-float -mhard-dfp -mno-hard-dfp -mlong-double-64 -mlong-double-128 -mbackchain -mno-backchain -mpacked-stack -mno-packed-stack -msmall-exec -mno-small-exec -mmvcle -mno-mvcle -m64 -m31 -mdebug -mno-debug -mesa -mzarch -mhtm -mvx -mzvector -mtpf-trace -mno-tpf-trace -mfused-madd -mno-fused-madd -mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard -mhotpatch=halfwords,halfwords Score Options -meb -mel -mnhwloop -muls -mmac -mscore5 -mscore5u -mscore7 -mscore7d SH Options -m1 -m2 -m2e -m2a-nofpu -m2a-single-only -m2a-single -m2a -m3 -m3e -m4-nofpu -m4-single-only -m4-single -m4 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al -mb -ml -mdalign -mrelax -mbigtable -mfmovd -mrenesas -mno-renesas -mnomacsave -mieee -mno-ieee -mbitops -misize -minline-ic_invalidate -mpadstruct -mprefergot -musermode -multcost=number -mdiv=strategy -mdivsi3_libfunc=name -mfixed-range=register-range -maccumulate-outgoing-args -matomic-model=atomic-model -mbranch-cost=num -mzdcbranch -mno-zdcbranch -mcbranch-force-delay-slot -mfused-madd -mno-fused-madd -mfsca -mno-fsca -mfsrra -mno-fsrra -mpretend-cmove -mtas Solaris 2 Options -mclear-hwcap -mno-clear-hwcap -mimpure-text -mno-impure-text -pthreads SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -mmemory-model=mem-model -m32 -m64 -mapp-regs -mno-app-regs -mfaster-structs -mno-faster-structs -mflat -mno-flat -mfpu -mno-fpu -mhard-float -msoft-float -mhard-quad-float -msoft-quad-float -mstack-bias -mno-stack-bias -mstd-struct-return -mno-std-struct-return -munaligned-doubles -mno-unaligned-doubles -muser-mode -mno-user-mode -mv8plus -mno-v8plus -mvis -mno-vis -mvis2 -mno-vis2 -mvis3 -mno-vis3 -mvis4 -mno-vis4 -mvis4b -mno-vis4b -mcbcond -mno-cbcond -mfmaf -mno-fmaf -mfsmuld -mno-fsmuld -mpopc -mno-popc -msubxc -mno-subxc -mfix-at697f -mfix-ut699 -mfix-ut700 -mfix-gr712rc -mlra -mno-lra SPU Options -mwarn-reloc -merror-reloc -msafe-dma -munsafe-dma -mbranch-hints -msmall-mem -mlarge-mem -mstdmain -mfixed-range=register-range -mea32 -mea64 -maddress-space-conversion -mno-address-space-conversion -mcache-size=cache-size -matomic-updates -mno-atomic-updates System V Options -Qy -Qn -YP,paths -Ym,dir TILE-Gx Options -mcpu=CPU -m32 -m64 -mbig-endian -mlittle-endian -mcmodel=code-model TILEPro Options -mcpu=cpu -m32 V850 Options -mlong-calls -mno-long-calls -mep -mno-ep -mprolog-function -mno-prolog-function -mspace -mtda=n -msda=n -mzda=n -mapp-regs -mno-app-regs -mdisable-callt -mno-disable-callt -mv850e2v3 -mv850e2 -mv850e1 -mv850es -mv850e -mv850 -mv850e3v5 -mloop -mrelax -mlong-jumps -msoft-float -mhard-float -mgcc-abi -mrh850-abi -mbig-switch VAX Options -mg -mgnu -munix Visium Options -mdebug -msim -mfpu -mno-fpu -mhard-float -msoft-float -mcpu=cpu-type -mtune=cpu-type -msv-mode -muser-mode VMS Options -mvms-return-codes -mdebug-main=prefix -mmalloc64 -mpointer-size=size VxWorks Options -mrtp -non-static -Bstatic -Bdynamic -Xbind-lazy -Xbind-now x86 Options -mtune=cpu-type -march=cpu-type -mtune-ctrl=feature-list -mdump-tune-features -mno-default -mfpmath=unit -masm=dialect -mno-fancy-math-387 -mno-fp-ret-in-387 -m80387 -mhard-float -msoft-float -mno-wide-multiply -mrtd -malign-double -mpreferred-stack-boundary=num -mincoming-stack-boundary=num -mcld -mcx16 -msahf -mmovbe -mcrc32 -mrecip -mrecip=opt -mvzeroupper -mprefer-avx128 -mprefer-vector-width=opt -mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx -mavx2 -mavx512f -mavx512pf -mavx512er -mavx512cd -mavx512vl -mavx512bw -mavx512dq -mavx512ifma -mavx512vbmi -msha -maes -mpclmul -mfsgsbase -mrdrnd -mf16c -mfma -mpconfig -mwbnoinvd -mptwrite -mprefetchwt1 -mclflushopt -mclwb -mxsavec -mxsaves -msse4a -m3dnow -m3dnowa -mpopcnt -mabm -mbmi -mtbm -mfma4 -mxop -madx -mlzcnt -mbmi2 -mfxsr -mxsave -mxsaveopt -mrtm -mhle -mlwp -mmwaitx -mclzero -mpku -mthreads -mgfni -mvaes -mwaitpkg -mshstk -mmanual-endbr -mforce-indirect-call -mavx512vbmi2 -mvpclmulqdq -mavx512bitalg -mmovdiri -mmovdir64b -mavx512vpopcntdq -mavx5124fmaps -mavx512vnni -mavx5124vnniw -mprfchw -mrdpid -mrdseed -msgx -mcldemote -mms-bitfields -mno-align-stringops -minline-all-stringops -minline-stringops-dynamically -mstringop-strategy=alg -mmemcpy-strategy=strategy -mmemset-strategy=strategy -mpush-args -maccumulate-outgoing-args -m128bit-long-double -m96bit-long-double -mlong-double-64 -mlong-double-80 -mlong-double-128 -mregparm=num -msseregparm -mveclibabi=type -mvect8-ret-in-mem -mpc32 -mpc64 -mpc80 -mstackrealign -momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs -mcmodel=code-model -mabi=name -maddress-mode=mode -m32 -m64 -mx32 -m16 -miamcu -mlarge-data-threshold=num -msse2avx -mfentry -mrecord-mcount -mnop-mcount -m8bit-idiv -minstrument-return=type -mfentry-name=name -mfentry-section=name -mavx256-split-unaligned-load -mavx256-split-unaligned-store -malign-data=type -mstack-protector-guard=guard -mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset -mstack-protector-guard-symbol=symbol -mgeneral-regs-only -mcall-ms2sysv-xlogues -mindirect-branch=choice -mfunction-return=choice -mindirect-branch-register x86 Windows Options -mconsole -mcygwin -mno-cygwin -mdll -mnop-fun-dllimport -mthread -municode -mwin32 -mwindows -fno-set-stack-executable Xstormy16 Options -msim Xtensa Options -mconst16 -mno-const16 -mfused-madd -mno-fused-madd -mforce-no-pic -mserialize-volatile -mno-serialize-volatile -mtext-section-literals -mno-text-section-literals -mauto-litpools -mno-auto-litpools -mtarget-align -mno-target-align -mlongcalls -mno-longcalls zSeries Options See S/390 and zSeries Options. ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Invoking GCC Link: next: Invoking G++ Link: prev: Option Summary Next: Invoking G++, Previous: Option Summary, Up: Invoking GCC ────────────────────────────────────────────────────────────────────────────── 3.2 Options Controlling the Kind of Output Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in that order. GCC is capable of preprocessing and compiling several files either into several assembler input files, or into one assembler input file; then each assembler input file produces an object file, and linking combines all the object files (those newly compiled, and those specified as input) into an executable file. For any given input file, the file name suffix determines what kind of compilation is done: file.c C source code that must be preprocessed. file.i C source code that should not be preprocessed. file.ii C++ source code that should not be preprocessed. file.m Objective-C source code. Note that you must link with the libobjc library to make an Objective-C program work. file.mi Objective-C source code that should not be preprocessed. file.mm file.M Objective-C++ source code. Note that you must link with the libobjc library to make an Objective-C++ program work. Note that '.M' refers to a literal capital M. file.mii Objective-C++ source code that should not be preprocessed. file.h C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (default), or C, C++ header file to be turned into an Ada spec (via the -fdump-ada-spec switch). file.cc file.cp file.cxx file.cpp file.CPP file.c++ file.C C++ source code that must be preprocessed. Note that in '.cxx', the last two letters must both be literally 'x'. Likewise, '.C' refers to a literal capital C. file.mm file.M Objective-C++ source code that must be preprocessed. file.mii Objective-C++ source code that should not be preprocessed. file.hh file.H file.hp file.hxx file.hpp file.HPP file.h++ file.tcc C++ header file to be turned into a precompiled header or Ada spec. file.f file.for file.ftn Fixed form Fortran source code that should not be preprocessed. file.F file.FOR file.fpp file.FPP file.FTN Fixed form Fortran source code that must be preprocessed (with the traditional preprocessor). file.f90 file.f95 file.f03 file.f08 Free form Fortran source code that should not be preprocessed. file.F90 file.F95 file.F03 file.F08 Free form Fortran source code that must be preprocessed (with the traditional preprocessor). file.go Go source code. file.brig BRIG files (binary representation of HSAIL). file.d D source code. file.di D interface file. file.dd D documentation code (Ddoc). file.ads Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). Such files are also called specs. file.adb Ada source code file containing a library unit body (a subprogram or package body). Such files are also called bodies. file.s Assembler code. file.S file.sx Assembler code that must be preprocessed. other An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way. You can specify the input language explicitly with the -x option: -x language Specify explicitly the language for the following input files (rather than letting the compiler choose a default based on the file name suffix). This option applies to all following input files until the next -x option. Possible values for language are: c c-header cpp-output c++ c++-header c++-cpp-output objective-c objective-c-header objective-c-cpp-output objective-c++ objective-c++-header objective-c++-cpp-output assembler assembler-with-cpp ada d f77 f77-cpp-input f95 f95-cpp-input go brig -x none Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as they are if -x has not been used at all). If you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc where to start, and one of the options -c, -S, or -E to say where gcc is to stop. Note that some combinations (for example, '-x cpp-output -E') instruct gcc to do nothing at all. -c Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in the form of an object file for each source file. By default, the object file name for a source file is made by replacing the suffix '.c', '.i', '.s', etc., with '.o'. Unrecognized input files, not requiring compilation or assembly, are ignored. -S Stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler code file for each non-assembler input file specified. By default, the assembler file name for a source file is made by replacing the suffix '.c', '.i', etc., with '.s'. Input files that don't require compilation are ignored. -E Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output. Input files that don't require preprocessing are ignored. -o file Place output in file file. This applies to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code. If -o is not specified, the default is to put an executable file in a.out, the object file for source.suffix in source.o, its assembler file in source.s, a precompiled header file in source.suffix.gch, and all preprocessed C source on standard output. -v Print (on standard error output) the commands executed to run the stages of compilation. Also print the version number of the compiler driver program and of the preprocessor and the compiler proper. -### Like -v except the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or ./-_. This is useful for shell scripts to capture the driver-generated command lines. --help Print (on the standard output) a description of the command-line options understood by gcc. If the -v option is also specified then --help is also passed on to the various processes invoked by gcc, so that they can display the command-line options they accept. If the -Wextra option has also been specified (prior to the --help option), then command-line options that have no documentation associated with them are also displayed. --target-help Print (on the standard output) a description of target-specific command-line options for each tool. For some targets extra target-specific information may also be printed. --help={class|[^]qualifier}[,…] Print (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers. These are the supported classes: 'optimizers' Display all of the optimization options supported by the compiler. 'warnings' Display all of the options controlling warning messages produced by the compiler. 'target' Display target-specific options. Unlike the --target-help option however, target-specific options of the linker and assembler are not displayed. This is because those tools do not currently support the extended --help= syntax. 'params' Display the values recognized by the --param option. language Display the options supported for language, where language is the name of one of the languages supported in this version of GCC. 'common' Display the options that are common to all languages. These are the supported qualifiers: 'undocumented' Display only those options that are undocumented. 'joined' Display options taking an argument that appears after an equal sign in the same continuous piece of text, such as: '--help=target'. 'separate' Display options taking an argument that appears as a separate word following the original option, such as: '-o output-file'. Thus for example to display all the undocumented target-specific switches supported by the compiler, use: --help=target,undocumented The sense of a qualifier can be inverted by prefixing it with the '^' character, so for example to display all binary warning options (i.e., ones that are either on or off and that do not take an argument) that have a description, use: --help=warnings,^joined,^undocumented The argument to --help= should not consist solely of inverted qualifiers. Combining several classes is possible, although this usually restricts the output so much that there is nothing to display. One case where it does work, however, is when one of the classes is target. For example, to display all the target-specific optimization options, use: --help=target,optimizers The --help= option can be repeated on the command line. Each successive use displays its requested class of options, skipping those that have already been displayed. If --help is also specified anywhere on the command line then this takes precedence over any --help= option. If the -Q option appears on the command line before the --help= option, then the descriptive text displayed by --help= is changed. Instead of describing the displayed options, an indication is given as to whether the option is enabled, disabled or set to a specific value (assuming that the compiler knows this at the point where the --help= option is used). Here is a truncated example from the ARM port of gcc: % gcc -Q -mabi=2 --help=target -c The following options are target specific: -mabi= 2 -mabort-on-noreturn [disabled] -mapcs [disabled] The output is sensitive to the effects of previous command-line options, so for example it is possible to find out which optimizations are enabled at -O2 by using: -Q -O2 --help=optimizers Alternatively you can discover which binary optimizations are enabled by -O3 by using: gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts diff /tmp/O2-opts /tmp/O3-opts | grep enabled --version Display the version number and copyrights of the invoked GCC. -pass-exit-codes Normally the gcc program exits with the code of 1 if any phase of the compiler returns a non-success return code. If you specify -pass-exit-codes, the gcc program instead returns with the numerically highest error produced by any phase returning an error indication. The C, C++, and Fortran front ends return 4 if an internal compiler error is encountered. -pipe Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble. -specs=file Process file after the compiler reads in the standard specs file, in order to override the defaults which the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc. More than one -specs=file can be specified on the command line, and they are processed in order, from left to right. See Spec Files, for information about the format of the file. -wrapper Invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma separated list. gcc -c t.c -wrapper gdb,--args This invokes all subprograms of gcc under 'gdb --args', thus the invocation of cc1 is 'gdb --args cc1 …'. -ffile-prefix-map=old=new When compiling files residing in directory old, record any references to them in the result of the compilation as if the files resided in directory new instead. Specifying this option is equivalent to specifying all the individual -f*-prefix-map options. This can be used to make reproducible builds that are location independent. See also -fmacro-prefix-map and -fdebug-prefix-map. -fplugin=name.so Load the plugin code in file name.so, assumed to be a shared object to be dlopen'd by the compiler. The base name of the shared object file is used to identify the plugin for the purposes of argument parsing (See -fplugin-arg-name-key=value below). Each plugin should define the callback functions specified in the Plugins API. -fplugin-arg-name-key=value Define an argument called key with a value of value for the plugin called name. -fdump-ada-spec[-slim] For C and C++ source and include files, generate corresponding Ada specs. See Generating Ada Bindings for C and C++ headers in GNAT User's Guide, which provides detailed documentation on this feature. -fada-spec-parent=unit In conjunction with -fdump-ada-spec[-slim] above, generate Ada specs as child units of parent unit. -fdump-go-spec=file For input files in any language, generate corresponding Go declarations in file. This generates Go const, type, var, and func declarations which may be a useful way to start writing a Go interface to code written in some other language. @file Read command-line options from file. The options read are inserted in place of the original @file option. If file does not exist, or cannot be read, then the option will be treated literally, and not removed. Options in file are separated by whitespace. A whitespace character may be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) may be included by prefixing the character to be included with a backslash. The file may itself contain additional @file options; any such options will be processed recursively. ────────────────────────────────────────────────────────────────────────────── Next: Invoking G++, Previous: Option Summary, Up: Invoking GCC Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Invoking GCC Link: next: C Dialect Options Link: prev: Overall Options Next: C Dialect Options, Previous: Overall Options, Up: Invoking GCC ────────────────────────────────────────────────────────────────────────────── 3.3 Compiling C++ Programs C++ source files conventionally use one of the suffixes '.C', '.cc', '.cpp', '.CPP', '.c++', '.cp', or '.cxx'; C++ header files often use '.hh', '.hpp', '.H', or (for shared template code) '.tcc'; and preprocessed C++ files use the suffix '.ii'. GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name gcc). However, the use of gcc does not add the C++ library. g++ is a program that calls GCC and automatically specifies linking against the C++ library. It treats '.c', '.h' and '.i' files as C++ source files instead of C source files unless -x is used. This program is also useful when precompiling a C header file with a '.h' extension for use in C++ compilations. On many systems, g++ is also installed with the name c++. When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs. See Options Controlling C Dialect, for explanations of options for languages related to C. See Options Controlling C++ Dialect, for explanations of options that are meaningful only for C++ programs. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Invoking GCC Link: next: C++ Dialect Options Link: prev: Invoking G++ Next: C++ Dialect Options, Previous: Invoking G++, Up: Invoking GCC ────────────────────────────────────────────────────────────────────────────── 3.4 Options Controlling C Dialect The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that the compiler accepts: -ansi In C mode, this is equivalent to -std=c90. In C++ mode, it is equivalent to -std=c++98. This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of standard C++ (when compiling C++ code), such as the asm and typeof keywords, and predefined macros such as unix and vax that identify the type of system you are using. It also enables the undesirable and rarely used ISO trigraph feature. For the C compiler, it disables recognition of C++ style '//' comments as well as the inline keyword. The alternate keywords __asm__, __extension__, __inline__ and __typeof__ continue to work despite -ansi. You would not want to use them in an ISO C program, of course, but it is useful to put them in header files that might be included in compilations done with -ansi. Alternate predefined macros such as __unix__ and __vax__ are also available, with or without -ansi. The -ansi option does not cause non-ISO programs to be rejected gratuitously. For that, -Wpedantic is required in addition to -ansi. See Warning Options. The macro __STRICT_ANSI__ is predefined when the -ansi option is used. Some header files may notice this macro and refrain from declaring certain functions or defining certain macros that the ISO standard doesn't call for; this is to avoid interfering with any programs that might use these names for other things. Functions that are normally built in but do not have semantics defined by ISO C (such as alloca and ffs) are not built-in functions when -ansi is used. See Other built-in functions provided by GCC, for details of the functions affected. -std= Determine the language standard. See Language Standards Supported by GCC, for details of these standard versions. This option is currently only supported when compiling C or C++. The compiler can accept several base standards, such as 'c90' or 'c++98', and GNU dialects of those standards, such as 'gnu90' or 'gnu++98'. When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that do not contradict it. For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the asm and typeof keywords, but not other GNU extensions that do not have a meaning in ISO C90, such as omitting the middle term of a ?: expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard. As a result, some strict-conforming programs may be rejected. The particular standard is used by -Wpedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style '//' comments, while -std=gnu99 -Wpedantic does not. A value for this option must be provided; possible values are 'c90' 'c89' 'iso9899:1990' Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code. 'iso9899:199409' ISO C90 as modified in amendment 1. 'c99' 'c9x' 'iso9899:1999' 'iso9899:199x' ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See http://gcc.gnu.org/c99status.html for more information. The names 'c9x' and 'iso9899:199x' are deprecated. 'c11' 'c1x' 'iso9899:2011' ISO C11, the 2011 revision of the ISO C standard. This standard is substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The name 'c1x' is deprecated. 'c17' 'c18' 'iso9899:2017' 'iso9899:2018' ISO C17, the 2017 revision of the ISO C standard (published in 2018). This standard is same as C11 except for corrections of defects (all of which are also applied with -std=c11) and a new value of __STDC_VERSION__, and so is supported to the same extent as C11. 'c2x' The next version of the ISO C standard, still under development. The support for this version is experimental and incomplete. 'gnu90' 'gnu89' GNU dialect of ISO C90 (including some C99 features). 'gnu99' 'gnu9x' GNU dialect of ISO C99. The name 'gnu9x' is deprecated. 'gnu11' 'gnu1x' GNU dialect of ISO C11. The name 'gnu1x' is deprecated. 'gnu17' 'gnu18' GNU dialect of ISO C17. This is the default for C code. 'gnu2x' The next version of the ISO C standard, still under development, plus GNU extensions. The support for this version is experimental and incomplete. 'c++98' 'c++03' The 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as -ansi for C++ code. 'gnu++98' 'gnu++03' GNU dialect of -std=c++98. 'c++11' 'c++0x' The 2011 ISO C++ standard plus amendments. The name 'c++0x' is deprecated. 'gnu++11' 'gnu++0x' GNU dialect of -std=c++11. The name 'gnu++0x' is deprecated. 'c++14' 'c++1y' The 2014 ISO C++ standard plus amendments. The name 'c++1y' is deprecated. 'gnu++14' 'gnu++1y' GNU dialect of -std=c++14. This is the default for C++ code. The name 'gnu++1y' is deprecated. 'c++17' 'c++1z' The 2017 ISO C++ standard plus amendments. The name 'c++1z' is deprecated. 'gnu++17' 'gnu++1z' GNU dialect of -std=c++17. The name 'gnu++1z' is deprecated. 'c++2a' The next revision of the ISO C++ standard, tentatively planned for 2020. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. 'gnu++2a' GNU dialect of -std=c++2a. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. -fgnu89-inline The option -fgnu89-inline tells GCC to use the traditional GNU semantics for inline functions when in C99 mode. See An Inline Function is As Fast As a Macro. Using this option is roughly equivalent to adding the gnu_inline function attribute to all inline functions (see Function Attributes). The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for inline when in C99 or gnu99 mode (i.e., it specifies the default behavior). This option is not supported in -std=c90 or -std=gnu90 mode. The preprocessor macros __GNUC_GNU_INLINE__ and __GNUC_STDC_INLINE__ may be used to check which semantics are in effect for inline functions. See Common Predefined Macros in The C Preprocessor. -fpermitted-flt-eval-methods=style ISO/IEC TS 18661-3 defines new permissible values for FLT_EVAL_METHOD that indicate that operations and constants with a semantic type that is an interchange or extended format should be evaluated to the precision and range of that type. These new values are a superset of those permitted under C99/C11, which does not specify the meaning of other positive values of FLT_EVAL_METHOD. As such, code conforming to C11 may not have been written expecting the possibility of the new values. -fpermitted-flt-eval-methods specifies whether the compiler should allow only the values of FLT_EVAL_METHOD specified in C99/C11, or the extended set of values specified in ISO/IEC TS 18661-3. style is either c11 or ts-18661-3 as appropriate. The default when in a standards compliant mode (-std=c11 or similar) is -fpermitted-flt-eval-methods=c11. The default when in a GNU dialect (-std=gnu11 or similar) is -fpermitted-flt-eval-methods=ts-18661-3. -aux-info filename Output to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files. This option is silently ignored in any language other than C. Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped ('I', 'N' for new or 'O' for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition ('C' or 'F', respectively, in the following character). In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration. -fallow-parameterless-variadic-functions Accept variadic functions without named parameters. Although it is possible to define such a function, this is not very useful as it is not possible to read the arguments. This is only supported for C as this construct is allowed by C++. -fno-asm Do not recognize asm, inline or typeof as a keyword, so that code can use these words as identifiers. You can use the keywords __asm__, __inline__ and __typeof__ instead. -ansi implies -fno-asm. In C++, this switch only affects the typeof keyword, since asm and inline are standard keywords. You may want to use the -fno-gnu-keywords flag instead, which has the same effect. In C99 mode (-std=c99 or -std=gnu99), this switch only affects the asm and typeof keywords, since inline is a standard keyword in ISO C99. -fno-builtin -fno-builtin-function Don't recognize built-in functions that do not begin with '__builtin_' as prefix. See Other built-in functions provided by GCC, for details of the functions affected, including those which are not built-in functions when -ansi or -std options for strict ISO C conformance are used because they do not have an ISO standard meaning. GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to alloca may become single instructions which adjust the stack directly, and calls to memcpy may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library. In addition, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function. For example, warnings are given with -Wformat for bad calls to printf when printf is built in and strlen is known not to modify global memory. With the -fno-builtin-function option only the built-in function function is disabled. function must not begin with '__builtin_'. If a function is named that is not built-in in this version of GCC, this option is ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as: #define abs(n) __builtin_abs ((n)) #define strcpy(d, s) __builtin_strcpy ((d), (s)) -fgimple Enable parsing of function definitions marked with __GIMPLE. This is an experimental feature that allows unit testing of GIMPLE passes. -fhosted Assert that compilation targets a hosted environment. This implies -fbuiltin. A hosted environment is one in which the entire standard library is available, and in which main has a return type of int. Examples are nearly everything except a kernel. This is equivalent to -fno-freestanding. -ffreestanding Assert that compilation targets a freestanding environment. This implies -fno-builtin. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at main. The most obvious example is an OS kernel. This is equivalent to -fno-hosted. See Language Standards Supported by GCC, for details of freestanding and hosted environments. -fopenacc Enable handling of OpenACC directives #pragma acc in C/C++ and !$acc in Fortran. When -fopenacc is specified, the compiler generates accelerated code according to the OpenACC Application Programming Interface v2.0 https://www.openacc.org. This option implies -pthread, and thus is only supported on targets that have support for -pthread. -fopenacc-dim=geom Specify default compute dimensions for parallel offload regions that do not explicitly specify. The geom value is a triple of ':'-separated sizes, in order 'gang', 'worker' and, 'vector'. A size can be omitted, to use a target-specific default value. -fopenmp Enable handling of OpenMP directives #pragma omp in C/C++ and !$omp in Fortran. When -fopenmp is specified, the compiler generates parallel code according to the OpenMP Application Program Interface v4.5 https://www.openmp.org. This option implies -pthread, and thus is only supported on targets that have support for -pthread. -fopenmp implies -fopenmp-simd. -fopenmp-simd Enable handling of OpenMP's SIMD directives with #pragma omp in C/C++ and !$omp in Fortran. Other OpenMP directives are ignored. -fgnu-tm When the option -fgnu-tm is specified, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature. For more information on GCC's support for transactional memory, See The GNU Transactional Memory Library in GNU Transactional Memory Library. Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions). -fms-extensions Accept some non-standard constructs used in Microsoft header files. In C++ code, this allows member names in structures to be similar to previous types declarations. typedef int UOW; struct ABC { UOW UOW; }; Some cases of unnamed fields in structures and unions are only accepted with this option. See Unnamed struct/union fields within structs/unions, for details. Note that this option is off for all targets but x86 targets using ms-abi. -fplan9-extensions Accept some non-standard constructs used in Plan 9 code. This enables -fms-extensions, permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field, and permits referring to anonymous fields declared using a typedef. See Unnamed struct/union fields within structs/unions, for details. This is only supported for C, not C++. -fcond-mismatch Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. This option is not supported for C++. -flax-vector-conversions Allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types. This option should not be used for new code. -funsigned-char Let the type char be unsigned, like unsigned char. Each kind of machine has a default for what char should be. It is either like unsigned char by default or like signed char by default. Ideally, a portable program should always use signed char or unsigned char when it depends on the signedness of an object. But many programs have been written to use plain char and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its inverse, let you make such a program work with the opposite default. The type char is always a distinct type from each of signed char or unsigned char, even though its behavior is always just like one of those two. -fsigned-char Let the type char be signed, like signed char. Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Likewise, the option -fno-signed-char is equivalent to -funsigned-char. -fsigned-bitfields -funsigned-bitfields -fno-signed-bitfields -fno-unsigned-bitfields These options control whether a bit-field is signed or unsigned, when the declaration does not use either signed or unsigned. By default, such a bit-field is signed, because this is consistent: the basic integer types such as int are signed types. -fsso-struct=endianness Set the default scalar storage order of structures and unions to the specified endianness. The accepted values are 'big-endian', 'little-endian' and 'native' for the native endianness of the target (the default). This option is not supported for C++. Warning: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated without it if the specified endianness is not the native endianness of the target. ────────────────────────────────────────────────────────────────────────────── Next: C++ Dialect Options, Previous: Invoking G++, Up: Invoking GCC Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Invoking GCC Link: next: Objective-C and Objective-C++ Dialect Options Link: prev: C Dialect Options Next: Objective-C and Objective-C++ Dialect Options, Previous: C Dialect ────────────────────────────────────────────────────────────────────────────── 3.5 Options Controlling C++ Dialect This section describes the command-line options that are only meaningful for C++ programs. You can also use most of the GNU compiler options regardless of what language your program is in. For example, you might compile a file firstClass.C like this: g++ -g -fstrict-enums -O -c firstClass.C In this example, only -fstrict-enums is an option meant only for C++ programs; you can use the other options with any language supported by GCC. Some options for compiling C programs, such as -std, are also relevant for C++ programs. See Options Controlling C Dialect. Here is a list of options that are only for compiling C++ programs: -fabi-version=n Use version n of the C++ ABI. The default is version 0. Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed. Version 1 is the version of the C++ ABI that first appeared in G++ 3.2. Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through G++ 4.9. Version 3 corrects an error in mangling a constant address as a template argument. Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types. Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter. Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and –, and a class scope function used as a template argument. Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope. Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers. Version 9, which first appeared in G++ 5.2, corrects the alignment of nullptr_t. Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity, such as ia32 calling convention attributes (e.g. 'stdcall'). Version 11, which first appeared in G++ 7, corrects the mangling of sizeof... expressions and operator names. For multiple entities with the same name within a function, that are declared in different scopes, the mangling now changes starting with the twelfth occurrence. It also implies -fnew-inheriting-ctors. Version 12, which first appeared in G++ 8, corrects the calling conventions for empty classes on the x86_64 target and for classes with only deleted copy/move constructors. It accidentally changes the calling convention for classes with a deleted copy constructor and a trivial move constructor. Version 13, which first appeared in G++ 8.2, fixes the accidental change in version 12. See also -Wabi. -fabi-compat-version=n On targets that support strong aliases, G++ works around mangling changes by creating an alias with the correct mangled name when defining a symbol with an incorrect mangled name. This switch specifies which ABI version to use for the alias. With -fabi-version=0 (the default), this defaults to 11 (GCC 7 compatibility). If another ABI version is explicitly selected, this defaults to 0. For compatibility with GCC versions 3.2 through 4.9, use -fabi-compat-version=2. If this option is not provided but -Wabi=n is, that version is used for compatibility aliases. If this option is provided along with -Wabi (without the version), the version from this option is used for the warning. -fno-access-control Turn off all access checking. This switch is mainly useful for working around bugs in the access control code. -faligned-new Enable support for C++17 new of types that require more alignment than void* ::operator new(std::size_t) provides. A numeric argument such as -faligned-new=32 can be used to specify how much alignment (in bytes) is provided by that function, but few users will need to override the default of alignof(std::max_align_t). This flag is enabled by default for -std=c++17. -fchar8_t -fno-char8_t Enable support for char8_t as adopted for C++2a. This includes the addition of a new char8_t fundamental type, changes to the types of UTF-8 string and character literals, new signatures for user-defined literals, associated standard library updates, and new __cpp_char8_t and __cpp_lib_char8_t feature test macros. This option enables functions to be overloaded for ordinary and UTF-8 strings: int f(const char *); // #1 int f(const char8_t *); // #2 int v1 = f("text"); // Calls #1 int v2 = f(u8"text"); // Calls #2 and introduces new signatures for user-defined literals: int operator""_udl1(char8_t); int v3 = u8'x'_udl1; int operator""_udl2(const char8_t*, std::size_t); int v4 = u8"text"_udl2; template int operator""_udl3(); int v5 = u8"text"_udl3; The change to the types of UTF-8 string and character literals introduces incompatibilities with ISO C++11 and later standards. For example, the following code is well-formed under ISO C++11, but is ill-formed when -fchar8_t is specified. char ca[] = u8"xx"; // error: char-array initialized from wide // string const char *cp = u8"xx";// error: invalid conversion from // `const char8_t*' to `const char*' int f(const char*); auto v = f(u8"xx"); // error: invalid conversion from // `const char8_t*' to `const char*' std::string s{u8"xx"}; // error: no matching function for call to // `std::basic_string::basic_string()' using namespace std::literals; s = u8"xx"s; // error: conversion from // `basic_string' to non-scalar // type `basic_string' requested -fcheck-new Check that the pointer returned by operator new is non-null before attempting to modify the storage allocated. This check is normally unnecessary because the C++ standard specifies that operator new only returns 0 if it is declared throw(), in which case the compiler always checks the return value even without this option. In all other cases, when operator new has a non-empty exception specification, memory exhaustion is signalled by throwing std::bad_alloc. See also 'new (nothrow)'. -fconcepts Enable support for the C++ Extensions for Concepts Technical Specification, ISO 19217 (2015), which allows code like template concept bool Addable = requires (T t) { t + t; }; template T add (T a, T b) { return a + b; } -fconstexpr-depth=n Set the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed to detect endless recursion during constant expression evaluation. The minimum specified by the standard is 512. -fconstexpr-loop-limit=n Set the maximum number of iterations for a loop in C++14 constexpr functions to n. A limit is needed to detect infinite loops during constant expression evaluation. The default is 262144 (1<<18). -fconstexpr-ops-limit=n Set the maximum number of operations during a single constexpr evaluation. Even when number of iterations of a single loop is limited with the above limit, if there are several nested loops and each of them has many iterations but still smaller than the above limit, or if in a body of some loop or even outside of a loop too many expressions need to be evaluated, the resulting constexpr evaluation might take too long. The default is 33554432 (1<<25). -fdeduce-init-list Enable deduction of a template type parameter as std::initializer_list from a brace-enclosed initializer list, i.e. template auto forward(T t) -> decltype (realfn (t)) { return realfn (t); } void f() { forward({1,2}); // call forward> } This deduction was implemented as a possible extension to the originally proposed semantics for the C++11 standard, but was not part of the final standard, so it is disabled by default. This option is deprecated, and may be removed in a future version of G++. -fno-elide-constructors The C++ standard allows an implementation to omit creating a temporary that is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases. This option also causes G++ to call trivial member functions which otherwise would be expanded inline. In C++17, the compiler is required to omit these temporaries, but this option still affects trivial member functions. -fno-enforce-eh-specs Don't generate code to check for violation of exception specifications at run time. This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining NDEBUG. This does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time. -fextern-tls-init -fno-extern-tls-init The C++11 and OpenMP standards allow thread_local and threadprivate variables to have dynamic (runtime) initialization. To support this, any use of such a variable goes through a wrapper function that performs any necessary initialization. When the use and definition of the variable are in the same translation unit, this overhead can be optimized away, but when the use is in a different translation unit there is significant overhead even if the variable doesn't actually need dynamic initialization. If the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or a use of the variable in the defining TU will be executed before any uses in another TU), they can avoid this overhead with the -fno-extern-tls-init option. On targets that support symbol aliases, the default is -fextern-tls-init. On targets that do not support symbol aliases, the default is -fno-extern-tls-init. -fno-gnu-keywords Do not recognize typeof as a keyword, so that code can use this word as an identifier. You can use the keyword __typeof__ instead. This option is implied by the strict ISO C++ dialects: -ansi, -std=c++98, -std=c++11, etc. -fno-implicit-templates Never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. If you use this option, you must take care to structure your code to include all the necessary explicit instantiations to avoid getting undefined symbols at link time. See Template Instantiation, for more information. -fno-implicit-inline-templates Don't emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles with and without optimization need the same set of explicit instantiations. -fno-implement-inlines To save space, do not emit out-of-line copies of inline functions controlled by #pragma implementation. This causes linker errors if these functions are not inlined everywhere they are called. -fms-extensions Disable Wpedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax. -fnew-inheriting-ctors Enable the P0136 adjustment to the semantics of C++11 constructor inheritance. This is part of C++17 but also considered to be a Defect Report against C++11 and C++14. This flag is enabled by default unless -fabi-version=10 or lower is specified. -fnew-ttp-matching Enable the P0522 resolution to Core issue 150, template template parameters and default arguments: this allows a template with default template arguments as an argument for a template template parameter with fewer template parameters. This flag is enabled by default for -std=c++17. -fno-nonansi-builtins Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include ffs, alloca, _exit, index, bzero, conjf, and other related functions. -fnothrow-opt Treat a throw() exception specification as if it were a noexcept specification to reduce or eliminate the text size overhead relative to a function with no exception specification. If the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller because the EH cleanups for those variables can be optimized away. The semantic effect is that an exception thrown out of a function with such an exception specification results in a call to terminate rather than unexpected. -fno-operator-names Do not treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms as keywords. -fno-optional-diags Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class. -fpermissive Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using -fpermissive allows some nonconforming code to compile. -fno-pretty-templates When an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g. void f(T) [with T = int] rather than void f(int)) so that it's clear which template is involved. When an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. If either of these behaviors make it harder to understand the error message rather than easier, you can use -fno-pretty-templates to disable them. -frepo Enable automatic template instantiation at link time. This option also implies -fno-implicit-templates. See Template Instantiation, for more information. -fno-rtti Disable generation of information about every class with virtual functions for use by the C++ run-time type identification features (dynamic_cast and typeid). If you don't use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but G++ generates it as needed. The dynamic_cast operator can still be used for casts that do not require run-time type information, i.e. casts to void * or to unambiguous base classes. Mixing code compiled with -frtti with that compiled with -fno-rtti may not work. For example, programs may fail to link if a class compiled with -fno-rtti is used as a base for a class compiled with -frtti. -fsized-deallocation Enable the built-in global declarations void operator delete (void *, std::size_t) noexcept; void operator delete[] (void *, std::size_t) noexcept; as introduced in C++14. This is useful for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster. Enabled by default under -std=c++14 and above. The flag -Wsized-deallocation warns about places that might want to add a definition. -fstrict-enums Allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type. -fstrong-eval-order Evaluate member access, array subscripting, and shift expressions in left-to-right order, and evaluate assignment in right-to-left order, as adopted for C++17. Enabled by default with -std=c++17. -fstrong-eval-order=some enables just the ordering of member access and shift expressions, and is the default without -std=c++17. -ftemplate-backtrace-limit=n Set the maximum number of template instantiation notes for a single warning or error to n. The default value is 10. -ftemplate-depth=n Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11). The default value is 900, as the compiler can run out of stack space before hitting 1024 in some situations. -fno-threadsafe-statics Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn't need to be thread-safe. -fuse-cxa-atexit Register destructors for objects with static storage duration with the __cxa_atexit function rather than the atexit function. This option is required for fully standards-compliant handling of static destructors, but only works if your C library supports __cxa_atexit. -fno-use-cxa-get-exception-ptr Don't use the __cxa_get_exception_ptr runtime routine. This causes std::uncaught_exception to be incorrect, but is necessary if the runtime routine is not available. -fvisibility-inlines-hidden This switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects. The effect of this is that GCC may, effectively, mark inline methods with __attribute__ ((visibility ("hidden"))) so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates. The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object. You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility has no effect. Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary. See Template Instantiation. -fvisibility-ms-compat This flag attempts to use visibility settings to make GCC's C++ linkage model compatible with that of Microsoft Visual Studio. The flag makes these changes to GCC's linkage model: 1. It sets the default visibility to hidden, like -fvisibility=hidden. 2. Types, but not their members, are not hidden by default. 3. The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used. In new code it is better to use -fvisibility=hidden and export those classes that are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior. Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently. -fno-weak Do not use weak symbol support, even if it is provided by the linker. By default, G++ uses weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++. -nostdinc++ Do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library.) In addition, these optimization, warning, and code generation options have meanings only for C++ programs: -Wabi (C, Objective-C, C++ and Objective-C++ only) Warn when G++ it generates code that is probably not compatible with the vendor-neutral C++ ABI. Since G++ now defaults to updating the ABI with each major release, normally -Wabi will warn only if there is a check added later in a release series for an ABI issue discovered since the initial release. -Wabi will warn about more things if an older ABI version is selected (with -fabi-version=n). -Wabi can also be used with an explicit version number to warn about compatibility with a particular -fabi-version level, e.g. -Wabi=2 to warn about changes relative to -fabi-version=2. If an explicit version number is provided and -fabi-compat-version is not specified, the version number from this option is used for compatibility aliases. If no explicit version number is provided with this option, but -fabi-compat-version is specified, that version number is used for ABI warnings. Although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is generating incompatible code. There may also be cases where warnings are emitted even though the code that is generated is compatible. You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers. Known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4 to 4.9) include: * A template with a non-type template parameter of reference type was mangled incorrectly: extern int N; template struct S {}; void n (S) {2} This was fixed in -fabi-version=3. * SIMD vector types declared using __attribute ((vector_size)) were mangled in a non-standard way that does not allow for overloading of functions taking vectors of different sizes. The mangling was changed in -fabi-version=4. * __attribute ((const)) and noreturn were mangled as type qualifiers, and decltype of a plain declaration was folded away. These mangling issues were fixed in -fabi-version=5. * Scoped enumerators passed as arguments to a variadic function are promoted like unscoped enumerators, causing va_arg to complain. On most targets this does not actually affect the parameter passing ABI, as there is no way to pass an argument smaller than int. Also, the ABI changed the mangling of template argument packs, const_cast, static_cast, prefix increment/decrement, and a class scope function used as a template argument. These issues were corrected in -fabi-version=6. * Lambdas in default argument scope were mangled incorrectly, and the ABI changed the mangling of nullptr_t. These issues were corrected in -fabi-version=7. * When mangling a function type with function-cv-qualifiers, the un-qualified function type was incorrectly treated as a substitution candidate. This was fixed in -fabi-version=8, the default for GCC 5.1. * decltype(nullptr) incorrectly had an alignment of 1, leading to unaligned accesses. Note that this did not affect the ABI of a function with a nullptr_t parameter, as parameters have a minimum alignment. This was fixed in -fabi-version=9, the default for GCC 5.2. * Target-specific attributes that affect the identity of a type, such as ia32 calling conventions on a function type (stdcall, regparm, etc.), did not affect the mangled name, leading to name collisions when function pointers were used as template arguments. This was fixed in -fabi-version=10, the default for GCC 6.1. It also warns about psABI-related changes. The known psABI changes at this point include: * For SysV/x86-64, unions with long double members are passed in memory as specified in psABI. For example: union U { long double ld; int i; }; union U is always passed in memory. -Wabi-tag (C++ and Objective-C++ only) Warn when a type with an ABI tag is used in a context that does not have that ABI tag. See C++ Attributes for more information about ABI tags. -Wctor-dtor-privacy (C++ and Objective-C++ only) Warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions. Also warn if there are no non-private methods, and there's at least one private member function that isn't a constructor or destructor. -Wdelete-non-virtual-dtor (C++ and Objective-C++ only) Warn when delete is used to destroy an instance of a class that has virtual functions and non-virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor. This warning is enabled by -Wall. -Wdeprecated-copy (C++ and Objective-C++ only) Warn that the implicit declaration of a copy constructor or copy assignment operator is deprecated if the class has a user-provided copy constructor or copy assignment operator, in C++11 and up. This warning is enabled by -Wextra. With -Wdeprecated-copy-dtor, also deprecate if the class has a user-provided destructor. -Wno-init-list-lifetime (C++ and Objective-C++ only) Do not warn about uses of std::initializer_list that are likely to result in dangling pointers. Since the underlying array for an initializer_list is handled like a normal C++ temporary object, it is easy to inadvertently keep a pointer to the array past the end of the array's lifetime. For example: * If a function returns a temporary initializer_list, or a local initializer_list variable, the array's lifetime ends at the end of the return statement, so the value returned has a dangling pointer. * If a new-expression creates an initializer_list, the array only lives until the end of the enclosing full-expression, so the initializer_list in the heap has a dangling pointer. * When an initializer_list variable is assigned from a brace-enclosed initializer list, the temporary array created for the right side of the assignment only lives until the end of the full-expression, so at the next statement the initializer_list variable has a dangling pointer. // li's initial underlying array lives as long as li std::initializer_list li = { 1,2,3 }; // assignment changes li to point to a temporary array li = { 4, 5 }; // now the temporary is gone and li has a dangling pointer int i = li.begin()[0] // undefined behavior * When a list constructor stores the begin pointer from the initializer_list argument, this doesn't extend the lifetime of the array, so if a class variable is constructed from a temporary initializer_list, the pointer is left dangling by the end of the variable declaration statement. -Wliteral-suffix (C++ and Objective-C++ only) Warn when a string or character literal is followed by a ud-suffix which does not begin with an underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens in order to maintain backwards compatibility with code that uses formatting macros from . For example: #define __STDC_FORMAT_MACROS #include #include int main() { int64_t i64 = 123; printf("My int64: %" PRId64"\n", i64); } In this case, PRId64 is treated as a separate preprocessing token. Additionally, warn when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore. Literal suffix identifiers that don't begin with an underscore are reserved for future standardization. This warning is enabled by default. -Wlto-type-mismatch During the link-time optimization warn about type mismatches in global declarations from different compilation units. Requires -flto to be enabled. Enabled by default. -Wno-narrowing (C++ and Objective-C++ only) For C++11 and later standards, narrowing conversions are diagnosed by default, as required by the standard. A narrowing conversion from a constant produces an error, and a narrowing conversion from a non-constant produces a warning, but -Wno-narrowing suppresses the diagnostic. Note that this does not affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE contexts. With -Wnarrowing in C++98, warn when a narrowing conversion prohibited by C++11 occurs within '{ }', e.g. int i = { 2.2 }; // error: narrowing from double to int This flag is included in -Wall and -Wc++11-compat. -Wnoexcept (C++ and Objective-C++ only) Warn when a noexcept-expression evaluates to false because of a call to a function that does not have a non-throwing exception specification (i.e. throw() or noexcept) but is known by the compiler to never throw an exception. -Wnoexcept-type (C++ and Objective-C++ only) Warn if the C++17 feature making noexcept part of a function type changes the mangled name of a symbol relative to C++14. Enabled by -Wabi and -Wc++17-compat. As an example: template void f(T t) { t(); }; void g() noexcept; void h() { f(g); } In C++14, f calls f, but in C++17 it calls f. -Wclass-memaccess (C++ and Objective-C++ only) Warn when the destination of a call to a raw memory function such as memset or memcpy is an object of class type, and when writing into such an object might bypass the class non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers. Modifying the representation of such objects may violate invariants maintained by member functions of the class. For example, the call to memset below is undefined because it modifies a non-trivial class object and is, therefore, diagnosed. The safe way to either initialize or clear the storage of objects of such types is by using the appropriate constructor or assignment operator, if one is available. std::string str = "abc"; memset (&str, 0, sizeof str); The -Wclass-memaccess option is enabled by -Wall. Explicitly casting the pointer to the class object to void * or to a type that can be safely accessed by the raw memory function suppresses the warning. -Wnon-virtual-dtor (C++ and Objective-C++ only) Warn when a class has virtual functions and an accessible non-virtual destructor itself or in an accessible polymorphic base class, in which case it is possible but unsafe to delete an instance of a derived class through a pointer to the class itself or base class. This warning is automatically enabled if -Weffc++ is specified. -Wregister (C++ and Objective-C++ only) Warn on uses of the register storage class specifier, except when it is part of the GNU Explicit Register Variables extension. The use of the register keyword as storage class specifier has been deprecated in C++11 and removed in C++17. Enabled by default with -std=c++17. -Wreorder (C++ and Objective-C++ only) Warn when the order of member initializers given in the code does not match the order in which they must be executed. For instance: struct A { int i; int j; A(): j (0), i (1) { } }; The compiler rearranges the member initializers for i and j to match the declaration order of the members, emitting a warning to that effect. This warning is enabled by -Wall. -Wno-pessimizing-move (C++ and Objective-C++ only) This warning warns when a call to std::move prevents copy elision. A typical scenario when copy elision can occur is when returning in a function with a class return type, when the expression being returned is the name of a non-volatile automatic object, and is not a function parameter, and has the same type as the function return type. struct T { … }; T fn() { T t; … return std::move (t); } But in this example, the std::move call prevents copy elision. This warning is enabled by -Wall. -Wno-redundant-move (C++ and Objective-C++ only) This warning warns about redundant calls to std::move; that is, when a move operation would have been performed even without the std::move call. This happens because the compiler is forced to treat the object as if it were an rvalue in certain situations such as returning a local variable, where copy elision isn't applicable. Consider: struct T { … }; T fn(T t) { … return std::move (t); } Here, the std::move call is redundant. Because G++ implements Core Issue 1579, another example is: struct T { // convertible to U … }; struct U { … }; U fn() { T t; … return std::move (t); } In this example, copy elision isn't applicable because the type of the expression being returned and the function return type differ, yet G++ treats the return value as if it were designated by an rvalue. This warning is enabled by -Wextra. -fext-numeric-literals (C++ and Objective-C++ only) Accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off these suffixes are treated as C++11 user-defined literal numeric suffixes. This is on by default for all pre-C++11 dialects and all GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14. This option is off by default for ISO C++11 onwards (-std=c++11, ...). The following -W… options are not affected by -Wall. -Weffc++ (C++ and Objective-C++ only) Warn about violations of the following style guidelines from Scott Meyers' Effective C++ series of books: * Define a copy constructor and an assignment operator for classes with dynamically-allocated memory. * Prefer initialization to assignment in constructors. * Have operator= return a reference to *this. * Don't try to return a reference when you must return an object. * Distinguish between prefix and postfix forms of increment and decrement operators. * Never overload &&, ||, or ,. This option also enables -Wnon-virtual-dtor, which is also one of the effective C++ recommendations. However, the check is extended to warn about the lack of virtual destructor in accessible non-polymorphic bases classes too. When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use 'grep -v' to filter out those warnings. -Wstrict-null-sentinel (C++ and Objective-C++ only) Warn about the use of an uncasted NULL as sentinel. When compiling only with GCC this is a valid sentinel, as NULL is defined to __null. Although it is a null pointer constant rather than a null pointer, it is guaranteed to be of the same size as a pointer. But this use is not portable across different compilers. -Wno-non-template-friend (C++ and Objective-C++ only) Disable warnings when non-template friend functions are declared within a template. In very old versions of GCC that predate implementation of the ISO standard, declarations such as 'friend int foo(int)', where the name of the friend is an unqualified-id, could be interpreted as a particular specialization of a template function; the warning exists to diagnose compatibility problems, and is enabled by default. -Wold-style-cast (C++ and Objective-C++ only) Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts (dynamic_cast, static_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended effects and much easier to search for. -Woverloaded-virtual (C++ and Objective-C++ only) Warn when a function declaration hides virtual functions from a base class. For example, in: struct A { virtual void f(); }; struct B: public A { void f(int); }; the A class version of f is hidden in B, and code like: B* b; b->f(); fails to compile. -Wno-pmf-conversions (C++ and Objective-C++ only) Disable the diagnostic for converting a bound pointer to member function to a plain pointer. -Wsign-promo (C++ and Objective-C++ only) Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. Previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior. -Wtemplates (C++ and Objective-C++ only) Warn when a primary template declaration is encountered. Some coding rules disallow templates, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also instantiate or specialize templates. -Wmultiple-inheritance (C++ and Objective-C++ only) Warn when a class is defined with multiple direct base classes. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use multiple inheritance. -Wvirtual-inheritance Warn when a class is defined with a virtual direct base class. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use virtual inheritance. -Wnamespaces Warn when a namespace definition is opened. Some coding rules disallow namespaces, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also use using directives and qualified names. -Wno-terminate (C++ and Objective-C++ only) Disable the warning about a throw-expression that will immediately result in a call to terminate. -Wno-class-conversion (C++ and Objective-C++ only) Disable the warning about the case when a conversion function converts an object to the same type, to a base class of that type, or to void; such a conversion function will never be called. ────────────────────────────────────────────────────────────────────────────── Next: Objective-C and Objective-C++ Dialect Options, Previous: C Dialect Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Invoking GCC Link: next: Warning Options Link: prev: Objective-C and Objective-C++ Dialect Options Next: Warning Options, Previous: Objective-C and Objective-C++ Dialect ────────────────────────────────────────────────────────────────────────────── 3.7 Options to Control Diagnostic Messages Formatting Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, …). You can use the options described below to control the formatting algorithm for diagnostic messages, e.g. how many characters per line, how often source location information should be reported. Note that some language front ends may not honor these options. -fmessage-length=n Try to format error messages so that they fit on lines of about n characters. If n is zero, then no line-wrapping is done; each error message appears on a single line. This is the default for all front ends. Note - this option also affects the display of the '#error' and '#warning' pre-processor directives, and the 'deprecated' function/type/variable attribute. It does not however affect the 'pragma GCC warning' and 'pragma GCC error' pragmas. -fdiagnostics-show-location=once Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit source location information once; that is, in case the message is too long to fit on a single physical line and has to be wrapped, the source location won't be emitted (as prefix) again, over and over, in subsequent continuation lines. This is the default behavior. -fdiagnostics-show-location=every-line Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the same source location information (as prefix) for physical lines that result from the process of breaking a message which is too long to fit on a single line. -fdiagnostics-color[=WHEN] -fno-diagnostics-color Use color in diagnostics. WHEN is 'never', 'always', or 'auto'. The default depends on how the compiler has been configured, it can be any of the above WHEN options or also 'never' if GCC_COLORS environment variable isn't present in the environment, and 'auto' otherwise. 'auto' means to use color only when the standard error is a terminal. The forms -fdiagnostics-color and -fno-diagnostics-color are aliases for -fdiagnostics-color=always and -fdiagnostics-color=never, respectively. The colors are defined by the environment variable GCC_COLORS. Its value is a colon-separated list of capabilities and Select Graphic Rendition (SGR) substrings. SGR commands are interpreted by the terminal or terminal emulator. (See the section in the documentation of your text terminal for permitted values and their meanings as character attributes.) These substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate include '1' for bold, '4' for underline, '5' for blink, '7' for inverse, '39' for default foreground color, '30' to '37' for foreground colors, '90' to '97' for 16-color mode foreground colors, '38;5;0' to '38;5;255' for 88-color and 256-color modes foreground colors, '49' for default background color, '40' to '47' for background colors, '100' to '107' for 16-color mode background colors, and '48;5;0' to '48;5;255' for 88-color and 256-color modes background colors. The default GCC_COLORS is error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\ quote=01:fixit-insert=32:fixit-delete=31:\ diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\ type-diff=01;32 where '01;31' is bold red, '01;35' is bold magenta, '01;36' is bold cyan, '32' is green, '34' is blue, '01' is bold, and '31' is red. Setting GCC_COLORS to the empty string disables colors. Supported capabilities are as follows. error= SGR substring for error: markers. warning= SGR substring for warning: markers. note= SGR substring for note: markers. range1= SGR substring for first additional range. range2= SGR substring for second additional range. locus= SGR substring for location information, 'file:line' or 'file:line:column' etc. quote= SGR substring for information printed within quotes. fixit-insert= SGR substring for fix-it hints suggesting text to be inserted or replaced. fixit-delete= SGR substring for fix-it hints suggesting text to be deleted. diff-filename= SGR substring for filename headers within generated patches. diff-hunk= SGR substring for the starts of hunks within generated patches. diff-delete= SGR substring for deleted lines within generated patches. diff-insert= SGR substring for inserted lines within generated patches. type-diff= SGR substring for highlighting mismatching types within template arguments in the C++ frontend. -fno-diagnostics-show-option By default, each diagnostic emitted includes text indicating the command-line option that directly controls the diagnostic (if such an option is known to the diagnostic machinery). Specifying the -fno-diagnostics-show-option flag suppresses that behavior. -fno-diagnostics-show-caret By default, each diagnostic emitted includes the original source line and a caret '^' indicating the column. This option suppresses this information. The source line is truncated to n characters, if the -fmessage-length=n option is given. When the output is done to the terminal, the width is limited to the width given by the COLUMNS environment variable or, if not set, to the terminal width. -fno-diagnostics-show-labels By default, when printing source code (via -fdiagnostics-show-caret), diagnostics can label ranges of source code with pertinent information, such as the types of expressions: printf ("foo %s bar", long_i + long_j); ~^ ~~~~~~~~~~~~~~~ | | char * long int This option suppresses the printing of these labels (in the example above, the vertical bars and the “char *” and “long int” text). -fno-diagnostics-show-line-numbers By default, when printing source code (via -fdiagnostics-show-caret), a left margin is printed, showing line numbers. This option suppresses this left margin. -fdiagnostics-minimum-margin-width=width This option controls the minimum width of the left margin printed by -fdiagnostics-show-line-numbers. It defaults to 6. -fdiagnostics-parseable-fixits Emit fix-it hints in a machine-parseable format, suitable for consumption by IDEs. For each fix-it, a line will be printed after the relevant diagnostic, starting with the string “fix-it:”. For example: fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all" The location is expressed as a half-open range, expressed as a count of bytes, starting at byte 1 for the initial column. In the above example, bytes 3 through 20 of line 45 of “test.c” are to be replaced with the given string: 00000000011111111112222222222 12345678901234567890123456789 gtk_widget_showall (dlg); ^^^^^^^^^^^^^^^^^^ gtk_widget_show_all The filename and replacement string escape backslash as “\\", tab as “\t”, newline as “\n”, double quotes as “\"”, non-printable characters as octal (e.g. vertical tab as “\013”). An empty replacement string indicates that the given range is to be removed. An empty range (e.g. “45:3-45:3”) indicates that the string is to be inserted at the given position. -fdiagnostics-generate-patch Print fix-it hints to stderr in unified diff format, after any diagnostics are printed. For example: --- test.c +++ test.c @ -42,5 +42,5 @ void show_cb(GtkDialog *dlg) { - gtk_widget_showall(dlg); + gtk_widget_show_all(dlg); } The diff may or may not be colorized, following the same rules as for diagnostics (see -fdiagnostics-color). -fdiagnostics-show-template-tree In the C++ frontend, when printing diagnostics showing mismatching template types, such as: could not convert 'std::map >()' from 'map<[...],vector>' to 'map<[...],vector> the -fdiagnostics-show-template-tree flag enables printing a tree-like structure showing the common and differing parts of the types, such as: map< [...], vector< [double != float]>> The parts that differ are highlighted with color (“double” and “float” in this case). -fno-elide-type By default when the C++ frontend prints diagnostics showing mismatching template types, common parts of the types are printed as “[...]” to simplify the error message. For example: could not convert 'std::map >()' from 'map<[...],vector>' to 'map<[...],vector> Specifying the -fno-elide-type flag suppresses that behavior. This flag also affects the output of the -fdiagnostics-show-template-tree flag. -fno-show-column Do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that does not understand the column numbers, such as dejagnu. -fdiagnostics-format=FORMAT Select a different format for printing diagnostics. FORMAT is 'text' or 'json'. The default is 'text'. The 'json' format consists of a top-level JSON array containing JSON objects representing the diagnostics. The JSON is emitted as one line, without formatting; the examples below have been formatted for clarity. Diagnostics can have child diagnostics. For example, this error and note: misleading-indentation.c:15:3: warning: this 'if' clause does not guard... [-Wmisleading-indentation] 15 | if (flag) | ^~ misleading-indentation.c:17:5: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'if' 17 | y = 2; | ^ might be printed in JSON form (after formatting) like this: [ { "kind": "warning", "locations": [ { "caret": { "column": 3, "file": "misleading-indentation.c", "line": 15 }, "finish": { "column": 4, "file": "misleading-indentation.c", "line": 15 } } ], "message": "this \u2018if\u2019 clause does not guard...", "option": "-Wmisleading-indentation", "children": [ { "kind": "note", "locations": [ { "caret": { "column": 5, "file": "misleading-indentation.c", "line": 17 } } ], "message": "...this statement, but the latter is …" } ] }, … ] where the note is a child of the warning. A diagnostic has a kind. If this is warning, then there is an option key describing the command-line option controlling the warning. A diagnostic can contain zero or more locations. Each location has up to three positions within it: a caret position and optional start and finish positions. A location can also have an optional label string. For example, this error: bad-binary-ops.c:64:23: error: invalid operands to binary + (have 'S' {aka 'struct s'} and 'T' {aka 'struct t'}) 64 | return callee_4a () + callee_4b (); | ~~~~~~~~~~~~ ^ ~~~~~~~~~~~~ | | | | | T {aka struct t} | S {aka struct s} has three locations. Its primary location is at the “+” token at column 23. It has two secondary locations, describing the left and right-hand sides of the expression, which have labels. It might be printed in JSON form as: { "children": [], "kind": "error", "locations": [ { "caret": { "column": 23, "file": "bad-binary-ops.c", "line": 64 } }, { "caret": { "column": 10, "file": "bad-binary-ops.c", "line": 64 }, "finish": { "column": 21, "file": "bad-binary-ops.c", "line": 64 }, "label": "S {aka struct s}" }, { "caret": { "column": 25, "file": "bad-binary-ops.c", "line": 64 }, "finish": { "column": 36, "file": "bad-binary-ops.c", "line": 64 }, "label": "T {aka struct t}" } ], "message": "invalid operands to binary + …" } If a diagnostic contains fix-it hints, it has a fixits array, consisting of half-open intervals, similar to the output of -fdiagnostics-parseable-fixits. For example, this diagnostic with a replacement fix-it hint: demo.c:8:15: error: 'struct s' has no member named 'colour'; did you mean 'color'? 8 | return ptr->colour; | ^~~~~~ | color might be printed in JSON form as: { "children": [], "fixits": [ { "next": { "column": 21, "file": "demo.c", "line": 8 }, "start": { "column": 15, "file": "demo.c", "line": 8 }, "string": "color" } ], "kind": "error", "locations": [ { "caret": { "column": 15, "file": "demo.c", "line": 8 }, "finish": { "column": 20, "file": "demo.c", "line": 8 } } ], "message": "\u2018struct s\u2019 has no member named …" } where the fix-it hint suggests replacing the text from start up to but not including next with string's value. Deletions are expressed via an empty value for string, insertions by having start equal next. ────────────────────────────────────────────────────────────────────────────── Next: Warning Options, Previous: Objective-C and Objective-C++ Dialect Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Invoking GCC Link: next: Debugging Options Link: prev: Diagnostic Message Formatting Options Next: Debugging Options, Previous: Diagnostic Message Formatting Options, ────────────────────────────────────────────────────────────────────────────── 3.8 Options to Request or Suppress Warnings Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error. The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by GCC. -fsyntax-only Check the code for syntax errors, but don't do anything beyond that. -fmax-errors=n Limits the maximum number of error messages to n, at which point GCC bails out rather than attempting to continue processing the source code. If n is 0 (the default), there is no limit on the number of error messages produced. If -Wfatal-errors is also specified, then -Wfatal-errors takes precedence over this option. -w Inhibit all warning messages. -Werror Make all warnings into errors. -Werror= Make the specified warning into an error. The specifier for a warning is appended; for example -Werror=switch turns the warnings controlled by -Wswitch into errors. This switch takes a negative form, to be used to negate -Werror for specific warnings; for example -Wno-error=switch makes -Wswitch warnings not be errors, even when -Werror is in effect. The warning message for each controllable warning includes the option that controls the warning. That option can then be used with -Werror= and -Wno-error= as described above. (Printing of the option in the warning message can be disabled using the -fno-diagnostics-show-option flag.) Note that specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply anything. -Wfatal-errors This option causes the compiler to abort compilation on the first error occurred rather than trying to keep going and printing further error messages. You can request many specific warnings with options beginning with '-W', for example -Wimplicit to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning '-Wno-' to turn off warnings; for example, -Wno-implicit. This manual lists only one of the two forms, whichever is not the default. For further language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. Some options, such as -Wall and -Wextra, turn on other options, such as -Wunused, which may turn on further options, such as -Wunused-value. The combined effect of positive and negative forms is that more specific options have priority over less specific ones, independently of their position in the command-line. For options of the same specificity, the last one takes effect. Options enabled or disabled via pragmas (see Diagnostic Pragmas) take effect as if they appeared at the end of the command-line. When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC emits a diagnostic stating that the option is not recognized. However, if the -Wno- form is used, the behavior is slightly different: no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being produced. This allows the use of new -Wno- options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present. -Wpedantic -pedantic Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++. For ISO C, follows the version of the ISO C standard specified by any -std option used. Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few require -ansi or a -std option specifying the required version of ISO C). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are rejected. -Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with '__'. Pedantic warnings are also disabled in the expression that follows __extension__. However, only system header files should use these escape routes; application programs should avoid them. See Alternate Keywords. Some users try to use -Wpedantic to check programs for strict ISO C conformance. They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all—only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added. A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable additional work and would be quite different from -Wpedantic. We don't have plans to support such a feature in the near future. Where the standard specified with -std represents a GNU extended dialect of C, such as 'gnu90' or 'gnu99', there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from -Wpedantic are given where they are required by the base standard. (It does not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.) -pedantic-errors Give an error whenever the base standard (see -Wpedantic) requires a diagnostic, in some cases where there is undefined behavior at compile-time and in some other cases that do not prevent compilation of programs that are valid according to the standard. This is not equivalent to -Werror=pedantic, since there are errors enabled by this option and not enabled by the latter and vice versa. -Wall This enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with macros. This also enables some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. -Wall turns on the following warning flags: -Waddress -Warray-bounds=1 (only with -O2) -Wbool-compare -Wbool-operation -Wc++11-compat -Wc++14-compat -Wcatch-value (C++ and Objective-C++ only) -Wchar-subscripts -Wcomment -Wduplicate-decl-specifier (C and Objective-C only) -Wenum-compare (in C/ObjC; this is on by default in C++) -Wformat -Wint-in-bool-context -Wimplicit (C and Objective-C only) -Wimplicit-int (C and Objective-C only) -Wimplicit-function-declaration (C and Objective-C only) -Winit-self (only for C++) -Wlogical-not-parentheses -Wmain (only for C/ObjC and unless -ffreestanding) -Wmaybe-uninitialized -Wmemset-elt-size -Wmemset-transposed-args -Wmisleading-indentation (only for C/C++) -Wmissing-attributes -Wmissing-braces (only for C/ObjC) -Wmultistatement-macros -Wnarrowing (only for C++) -Wnonnull -Wnonnull-compare -Wopenmp-simd -Wparentheses -Wpessimizing-move (only for C++) -Wpointer-sign -Wreorder -Wrestrict -Wreturn-type -Wsequence-point -Wsign-compare (only in C++) -Wsizeof-pointer-div -Wsizeof-pointer-memaccess -Wstrict-aliasing -Wstrict-overflow=1 -Wswitch -Wtautological-compare -Wtrigraphs -Wuninitialized -Wunknown-pragmas -Wunused-function -Wunused-label -Wunused-value -Wunused-variable -Wvolatile-register-var Note that some warning flags are not implied by -Wall. Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning. Some of them are enabled by -Wextra but many of them must be enabled individually. -Wextra This enables some extra warning flags that are not enabled by -Wall. (This option used to be called -W. The older name is still supported, but the newer name is more descriptive.) -Wclobbered -Wcast-function-type -Wdeprecated-copy (C++ only) -Wempty-body -Wignored-qualifiers -Wimplicit-fallthrough=3 -Wmissing-field-initializers -Wmissing-parameter-type (C only) -Wold-style-declaration (C only) -Woverride-init -Wsign-compare (C only) -Wredundant-move (only for C++) -Wtype-limits -Wuninitialized -Wshift-negative-value (in C++03 and in C99 and newer) -Wunused-parameter (only with -Wunused or -Wall) -Wunused-but-set-parameter (only with -Wunused or -Wall) The option -Wextra also prints warning messages for the following cases: * A pointer is compared against integer zero with <, <=, >, or >=. * (C++ only) An enumerator and a non-enumerator both appear in a conditional expression. * (C++ only) Ambiguous virtual bases. * (C++ only) Subscripting an array that has been declared register. * (C++ only) Taking the address of a variable that has been declared register. * (C++ only) A base class is not initialized in the copy constructor of a derived class. -Wchar-subscripts Warn if an array subscript has type char. This is a common cause of error, as programmers often forget that this type is signed on some machines. This warning is enabled by -Wall. -Wno-coverage-mismatch Warn if feedback profiles do not match when using the -fprofile-use option. If a source file is changed between compiling with -fprofile-generate and with -fprofile-use, the files with the profile feedback can fail to match the source file and GCC cannot use the profile feedback information. By default, this warning is enabled and is treated as an error. -Wno-coverage-mismatch can be used to disable the warning or -Wno-error=coverage-mismatch can be used to disable the error. Disabling the error for this warning can result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to an existing code-base. Completely disabling the warning is not recommended. -Wno-cpp (C, Objective-C, C++, Objective-C++ and Fortran only) Suppress warning messages emitted by #warning directives. -Wdouble-promotion (C, C++, Objective-C and Objective-C++ only) Give a warning when a value of type float is implicitly promoted to double. CPUs with a 32-bit “single-precision” floating-point unit implement float in hardware, but emulate double in software. On such a machine, doing computations using double values is much more expensive because of the overhead required for software emulation. It is easy to accidentally do computations with double because floating-point literals are implicitly of type double. For example, in: float area(float radius) { return 3.14159 * radius * radius; } the compiler performs the entire computation with double because the floating-point literal is a double. -Wduplicate-decl-specifier (C and Objective-C only) Warn if a declaration has duplicate const, volatile, restrict or _Atomic specifier. This warning is enabled by -Wall. -Wformat -Wformat=n Check calls to printf and scanf, etc., to make sure that the arguments supplied have types appropriate to the format string specified, and that the conversions specified in the format string make sense. This includes standard functions, and others specified by format attributes (see Function Attributes), in the printf, scanf, strftime and strfmon (an X/Open extension, not in the C standard) families (or other target-specific families). Which functions are checked without format attributes having been specified depends on the standard version selected, and such checks of functions without the attribute specified are disabled by -ffreestanding or -fno-builtin. The formats are checked against the format features supported by GNU libc version 2.2. These include all ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU extensions. Other library implementations may not support all these features; GCC does not support warning about features that go beyond a particular library's limitations. However, if -Wpedantic is used with -Wformat, warnings are given about format features not in the selected standard version (but not for strfmon formats, since those are not in any version of the C standard). See Options Controlling C Dialect. -Wformat=1 -Wformat Option -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0. Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull. Some aspects of this level of format checking can be disabled by the options: -Wno-format-contains-nul, -Wno-format-extra-args, and -Wno-format-zero-length. -Wformat is enabled by -Wall. -Wno-format-contains-nul If -Wformat is specified, do not warn about format strings that contain NUL bytes. -Wno-format-extra-args If -Wformat is specified, do not warn about excess arguments to a printf or scanf format function. The C standard specifies that such arguments are ignored. Where the unused arguments lie between used arguments that are specified with '$' operand number specifications, normally warnings are still given, since the implementation could not know what type to pass to va_arg to skip the unused arguments. However, in the case of scanf formats, this option suppresses the warning if the unused arguments are all pointers, since the Single Unix Specification says that such unused arguments are allowed. -Wformat-overflow -Wformat-overflow=level Warn about calls to formatted input/output functions such as sprintf and vsprintf that might overflow the destination buffer. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. -Wformat-overflow -Wformat-overflow=1 Level 1 of -Wformat-overflow enabled by -Wformat employs a conservative approach that warns only about calls that most likely overflow the buffer. At this level, numeric arguments to format directives with unknown values are assumed to have the value of one, and strings of unknown length to be empty. Numeric arguments that are known to be bounded to a subrange of their type, or string arguments whose output is bounded either by their directive's precision or by a finite set of string literals, are assumed to take on the value within the range that results in the most bytes on output. For example, the call to sprintf below is diagnosed because even with both a and b equal to zero, the terminating NUL character ('\0') appended by the function to the destination buffer will be written past its end. Increasing the size of the buffer by a single byte is sufficient to avoid the warning, though it may not be sufficient to avoid the overflow. void f (int a, int b) { char buf [13]; sprintf (buf, "a = %i, b = %i\n", a, b); } -Wformat-overflow=2 Level 2 warns also about calls that might overflow the destination buffer given an argument of sufficient length or magnitude. At level 2, unknown numeric arguments are assumed to have the minimum representable value for signed types with a precision greater than 1, and the maximum representable value otherwise. Unknown string arguments whose length cannot be assumed to be bounded either by the directive's precision, or by a finite set of string literals they may evaluate to, or the character array they may point to, are assumed to be 1 character long. At level 2, the call in the example above is again diagnosed, but this time because with a equal to a 32-bit INT_MIN the first %i directive will write some of its digits beyond the end of the destination buffer. To make the call safe regardless of the values of the two variables, the size of the destination buffer must be increased to at least 34 bytes. GCC includes the minimum size of the buffer in an informational note following the warning. An alternative to increasing the size of the destination buffer is to constrain the range of formatted values. The maximum length of string arguments can be bounded by specifying the precision in the format directive. When numeric arguments of format directives can be assumed to be bounded by less than the precision of their type, choosing an appropriate length modifier to the format specifier will reduce the required buffer size. For example, if a and b in the example above can be assumed to be within the precision of the short int type then using either the %hi format directive or casting the argument to short reduces the maximum required size of the buffer to 24 bytes. void f (int a, int b) { char buf [23]; sprintf (buf, "a = %hi, b = %i\n", a, (short)b); } -Wno-format-zero-length If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed. -Wformat=2 Enable -Wformat plus additional format checks. Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k. -Wformat-nonliteral If -Wformat is specified, also warn if the format string is not a string literal and so cannot be checked, unless the format function takes its format arguments as a va_list. -Wformat-security If -Wformat is specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, as in printf (foo);. This may be a security hole if the format string came from untrusted input and contains '%n'. (This is currently a subset of what -Wformat-nonliteral warns about, but in future warnings may be added to -Wformat-security that are not included in -Wformat-nonliteral.) -Wformat-signedness If -Wformat is specified, also warn if the format string requires an unsigned argument and the argument is signed and vice versa. -Wformat-truncation -Wformat-truncation=level Warn about calls to formatted input/output functions such as snprintf and vsnprintf that might result in output truncation. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. Except as noted otherwise, the option uses the same logic -Wformat-overflow. -Wformat-truncation -Wformat-truncation=1 Level 1 of -Wformat-truncation enabled by -Wformat employs a conservative approach that warns only about calls to bounded functions whose return value is unused and that will most likely result in output truncation. -Wformat-truncation=2 Level 2 warns also about calls to bounded functions whose return value is used and that might result in truncation given an argument of sufficient length or magnitude. -Wformat-y2k If -Wformat is specified, also warn about strftime formats that may yield only a two-digit year. -Wnonnull Warn about passing a null pointer for arguments marked as requiring a non-null value by the nonnull function attribute. -Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option. -Wnonnull-compare Warn when comparing an argument marked with the nonnull function attribute against null inside the function. -Wnonnull-compare is included in -Wall. It can be disabled with the -Wno-nonnull-compare option. -Wnull-dereference Warn if the compiler detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. This option is only active when -fdelete-null-pointer-checks is active, which is enabled by optimizations in most targets. The precision of the warnings depends on the optimization options used. -Winit-self (C, C++, Objective-C and Objective-C++ only) Warn about uninitialized variables that are initialized with themselves. Note this option can only be used with the -Wuninitialized option. For example, GCC warns about i being uninitialized in the following snippet only when -Winit-self has been specified: int f() { int i = i; return i; } This warning is enabled by -Wall in C++. -Wimplicit-int (C and Objective-C only) Warn when a declaration does not specify a type. This warning is enabled by -Wall. -Wimplicit-function-declaration (C and Objective-C only) Give a warning whenever a function is used before being declared. In C99 mode (-std=c99 or -std=gnu99), this warning is enabled by default and it is made into an error by -pedantic-errors. This warning is also enabled by -Wall. -Wimplicit (C and Objective-C only) Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall. -Wimplicit-fallthrough -Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3 and -Wno-implicit-fallthrough is the same as -Wimplicit-fallthrough=0. -Wimplicit-fallthrough=n Warn when a switch case falls through. For example: switch (cond) { case 1: a = 1; break; case 2: a = 2; case 3: a = 3; break; } This warning does not warn when the last statement of a case cannot fall through, e.g. when there is a return statement or a call to function declared with the noreturn attribute. -Wimplicit-fallthrough= also takes into account control flow statements, such as ifs, and only warns when appropriate. E.g. switch (cond) { case 1: if (i > 3) { bar (5); break; } else if (i < 1) { bar (0); } else return; default: … } Since there are occasions where a switch case fall through is desirable, GCC provides an attribute, __attribute__ ((fallthrough)), that is to be used along with a null statement to suppress this warning that would normally occur: switch (cond) { case 1: bar (0); __attribute__ ((fallthrough)); default: … } C++17 provides a standard way to suppress the -Wimplicit-fallthrough warning using [[fallthrough]]; instead of the GNU attribute. In C++11 or C++14 users can use [[gnu::fallthrough]];, which is a GNU extension. Instead of these attributes, it is also possible to add a fallthrough comment to silence the warning. The whole body of the C or C++ style comment should match the given regular expressions listed below. The option argument n specifies what kind of comments are accepted: * -Wimplicit-fallthrough=0 disables the warning altogether. * -Wimplicit-fallthrough=1 matches .* regular expression, any comment is used as fallthrough comment. * -Wimplicit-fallthrough=2 case insensitively matches .*falls?[ \t-]*thr(ough|u).* regular expression. * -Wimplicit-fallthrough=3 case sensitively matches one of the following regular expressions: * -fallthrough * @fallthrough@ * lint -fallthrough[ \t]* * [ \t.!]*(ELSE,? |INTENTIONAL(LY)? )? FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)? * [ \t.!]*(Else,? |Intentional(ly)? )? Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)? * [ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )? fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)? * -Wimplicit-fallthrough=4 case sensitively matches one of the following regular expressions: * -fallthrough * @fallthrough@ * lint -fallthrough[ \t]* * [ \t]*FALLTHR(OUGH|U)[ \t]* * -Wimplicit-fallthrough=5 doesn't recognize any comments as fallthrough comments, only attributes disable the warning. The comment needs to be followed after optional whitespace and other comments by case or default keywords or by a user label that precedes some case or default label. switch (cond) { case 1: bar (0); /* FALLTHRU */ default: … } The -Wimplicit-fallthrough=3 warning is enabled by -Wextra. -Wif-not-aligned (C, C++, Objective-C and Objective-C++ only) Control if warning triggered by the warn_if_not_aligned attribute should be issued. This is enabled by default. Use -Wno-if-not-aligned to disable it. -Wignored-qualifiers (C and C++ only) Warn if the return type of a function has a type qualifier such as const. For ISO C such a type qualifier has no effect, since the value returned by a function is not an lvalue. For C++, the warning is only emitted for scalar types or void. ISO C prohibits qualified void return types on function definitions, so such return types always receive a warning even without this option. This warning is also enabled by -Wextra. -Wignored-attributes (C and C++ only) Warn when an attribute is ignored. This is different from the -Wattributes option in that it warns whenever the compiler decides to drop an attribute, not that the attribute is either unknown, used in a wrong place, etc. This warning is enabled by default. -Wmain Warn if the type of main is suspicious. main should be a function with external linkage, returning int, taking either zero arguments, two, or three arguments of appropriate types. This warning is enabled by default in C++ and is enabled by either -Wall or -Wpedantic. -Wmisleading-indentation (C and C++ only) Warn when the indentation of the code does not reflect the block structure. Specifically, a warning is issued for if, else, while, and for clauses with a guarded statement that does not use braces, followed by an unguarded statement with the same indentation. In the following example, the call to “bar” is misleadingly indented as if it were guarded by the “if” conditional. if (some_condition ()) foo (); bar (); /* Gotcha: this is not guarded by the "if". */ In the case of mixed tabs and spaces, the warning uses the -ftabstop= option to determine if the statements line up (defaulting to 8). The warning is not issued for code involving multiline preprocessor logic such as the following example. if (flagA) foo (0); #if SOME_CONDITION_THAT_DOES_NOT_HOLD if (flagB) #endif foo (1); The warning is not issued after a #line directive, since this typically indicates autogenerated code, and no assumptions can be made about the layout of the file that the directive references. This warning is enabled by -Wall in C and C++. -Wmissing-attributes Warn when a declaration of a function is missing one or more attributes that a related function is declared with and whose absence may adversely affect the correctness or efficiency of generated code. For example, the warning is issued for declarations of aliases that use attributes to specify less restrictive requirements than those of their targets. This typically represents a potential optimization opportunity. By contrast, the -Wattribute-alias=2 option controls warnings issued when the alias is more restrictive than the target, which could lead to incorrect code generation. Attributes considered include alloc_align, alloc_size, cold, const, hot, leaf, malloc, nonnull, noreturn, nothrow, pure, returns_nonnull, and returns_twice. In C++, the warning is issued when an explicit specialization of a primary template declared with attribute alloc_align, alloc_size, assume_aligned, format, format_arg, malloc, or nonnull is declared without it. Attributes deprecated, error, and warning suppress the warning. (see Function Attributes). You can use the copy attribute to apply the same set of attributes to a declaration as that on another declaration without explicitly enumerating the attributes. This attribute can be applied to declarations of functions (see Common Function Attributes), variables (see Common Variable Attributes), or types (see Common Type Attributes). -Wmissing-attributes is enabled by -Wall. For example, since the declaration of the primary function template below makes use of both attribute malloc and alloc_size the declaration of the explicit specialization of the template is diagnosed because it is missing one of the attributes. template T* __attribute__ ((malloc, alloc_size (1))) allocate (size_t); template <> void* __attribute__ ((malloc)) // missing alloc_size allocate (size_t); -Wmissing-braces Warn if an aggregate or union initializer is not fully bracketed. In the following example, the initializer for a is not fully bracketed, but that for b is fully bracketed. This warning is enabled by -Wall in C. int a[2][2] = { 0, 1, 2, 3 }; int b[2][2] = { { 0, 1 }, { 2, 3 } }; This warning is enabled by -Wall. -Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only) Warn if a user-supplied include directory does not exist. -Wmissing-profile Warn if feedback profiles are missing when using the -fprofile-use option. This option diagnoses those cases where a new function or a new file is added to the user code between compiling with -fprofile-generate and with -fprofile-use, without regenerating the profiles. In these cases, the profile feedback data files do not contain any profile feedback information for the newly added function or file respectively. Also, in the case when profile count data (.gcda) files are removed, GCC cannot use any profile feedback information. In all these cases, warnings are issued to inform the user that a profile generation step is due. -Wno-missing-profile can be used to disable the warning. Ignoring the warning can result in poorly optimized code. Completely disabling the warning is not recommended and should be done only when non-existent profile data is justified. -Wmultistatement-macros Warn about unsafe multiple statement macros that appear to be guarded by a clause such as if, else, for, switch, or while, in which only the first statement is actually guarded after the macro is expanded. For example: #define DOIT x++; y++ if (c) DOIT; will increment y unconditionally, not just when c holds. The can usually be fixed by wrapping the macro in a do-while loop: #define DOIT do { x++; y++; } while (0) if (c) DOIT; This warning is enabled by -Wall in C and C++. -Wparentheses Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about. Also warn if a comparison like x<=y<=z appears; this is equivalent to (x<=y ? 1 : 0) <= z, which is a different interpretation from that of ordinary mathematical notation. Also warn for dangerous uses of the GNU extension to ?: with omitted middle operand. When the condition in the ?: operator is a boolean expression, the omitted value is always 1. Often programmers expect it to be a value computed inside the conditional expression instead. For C++ this also warns for some cases of unnecessary parentheses in declarations, which can indicate an attempt at a function call instead of a declaration: { // Declares a local variable called mymutex. std::unique_lock (mymutex); // User meant std::unique_lock lock (mymutex); } This warning is enabled by -Wall. -Wsequence-point Warn about code that may have undefined semantics because of violations of sequence point rules in the C and C++ standards. The C and C++ standards define the order in which expressions in a C/C++ program are evaluated in terms of sequence points, which represent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those executed after it. These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a &&, ||, ? : or , (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is not specified. All these rules describe only a partial order rather than a total order, since, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified. However, the standards committee have ruled that function calls do not overlap. It is not specified when between sequence points modifications to the values of objects take effect. Programs whose behavior depends on this have undefined behavior; the C and C++ standards specify that “Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.”. If a program breaks these rules, the results on any particular implementation are entirely unpredictable. Examples of code with undefined behavior are a = a++;, a[n] = b[n++] and a[i++] = i;. Some more complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs. The C++17 standard will define the order of evaluation of operands in more cases: in particular it requires that the right-hand side of an assignment be evaluated before the left-hand side, so the above examples are no longer undefined. But this warning will still warn about them, to help people avoid writing code that is undefined in C and earlier revisions of C++. The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at http://gcc.gnu.org/readings.html. This warning is enabled by -Wall for C and C++. -Wno-return-local-addr Do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the function returns. -Wreturn-type Warn whenever a function is defined with a return type that defaults to int. Also warn about any return statement with no return value in a function whose return type is not void (falling off the end of the function body is considered returning without a value). For C only, warn about a return statement with an expression in a function whose return type is void, unless the expression type is also void. As a GNU extension, the latter case is accepted without a warning unless -Wpedantic is used. Attempting to use the return value of a non-void function other than main that flows off the end by reaching the closing curly brace that terminates the function is undefined. Unlike in C, in C++, flowing off the end of a non-void function other than main results in undefined behavior even when the value of the function is not used. This warning is enabled by default in C++ and by -Wall otherwise. -Wshift-count-negative Warn if shift count is negative. This warning is enabled by default. -Wshift-count-overflow Warn if shift count >= width of type. This warning is enabled by default. -Wshift-negative-value Warn if left shifting a negative value. This warning is enabled by -Wextra in C99 and C++11 modes (and newer). -Wshift-overflow -Wshift-overflow=n Warn about left shift overflows. This warning is enabled by default in C99 and C++11 modes (and newer). -Wshift-overflow=1 This is the warning level of -Wshift-overflow and is enabled by default in C99 and C++11 modes (and newer). This warning level does not warn about left-shifting 1 into the sign bit. (However, in C, such an overflow is still rejected in contexts where an integer constant expression is required.) No warning is emitted in C++2A mode (and newer), as signed left shifts always wrap. -Wshift-overflow=2 This warning level also warns about left-shifting 1 into the sign bit, unless C++14 mode (or newer) is active. -Wswitch Warn whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration. (The presence of a default label prevents this warning.) case labels outside the enumeration range also provoke warnings when this option is used (even if there is a default label). This warning is enabled by -Wall. -Wswitch-default Warn whenever a switch statement does not have a default case. -Wswitch-enum Warn whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration. case labels outside the enumeration range also provoke warnings when this option is used. The only difference between -Wswitch and this option is that this option gives a warning about an omitted enumeration code even if there is a default label. -Wswitch-bool Warn whenever a switch statement has an index of boolean type and the case values are outside the range of a boolean type. It is possible to suppress this warning by casting the controlling expression to a type other than bool. For example: switch ((int) (a == 4)) { … } This warning is enabled by default for C and C++ programs. -Wswitch-unreachable Warn whenever a switch statement contains statements between the controlling expression and the first case label, which will never be executed. For example: switch (cond) { i = 15; … case 5: … } -Wswitch-unreachable does not warn if the statement between the controlling expression and the first case label is just a declaration: switch (cond) { int i; … case 5: i = 5; … } This warning is enabled by default for C and C++ programs. -Wsync-nand (C and C++ only) Warn when __sync_fetch_and_nand and __sync_nand_and_fetch built-in functions are used. These functions changed semantics in GCC 4.4. -Wunused-but-set-parameter Warn whenever a function parameter is assigned to, but otherwise unused (aside from its declaration). To suppress this warning use the unused attribute (see Variable Attributes). This warning is also enabled by -Wunused together with -Wextra. -Wunused-but-set-variable Warn whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This warning is enabled by -Wall. To suppress this warning use the unused attribute (see Variable Attributes). This warning is also enabled by -Wunused, which is enabled by -Wall. -Wunused-function Warn whenever a static function is declared but not defined or a non-inline static function is unused. This warning is enabled by -Wall. -Wunused-label Warn whenever a label is declared but not used. This warning is enabled by -Wall. To suppress this warning use the unused attribute (see Variable Attributes). -Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only) Warn when a typedef locally defined in a function is not used. This warning is enabled by -Wall. -Wunused-parameter Warn whenever a function parameter is unused aside from its declaration. To suppress this warning use the unused attribute (see Variable Attributes). -Wno-unused-result Do not warn if a caller of a function marked with attribute warn_unused_result (see Function Attributes) does not use its return value. The default is -Wunused-result. -Wunused-variable Warn whenever a local or static variable is unused aside from its declaration. This option implies -Wunused-const-variable=1 for C, but not for C++. This warning is enabled by -Wall. To suppress this warning use the unused attribute (see Variable Attributes). -Wunused-const-variable -Wunused-const-variable=n Warn whenever a constant static variable is unused aside from its declaration. -Wunused-const-variable=1 is enabled by -Wunused-variable for C, but not for C++. In C this declares variable storage, but in C++ this is not an error since const variables take the place of #defines. To suppress this warning use the unused attribute (see Variable Attributes). -Wunused-const-variable=1 This is the warning level that is enabled by -Wunused-variable for C. It warns only about unused static const variables defined in the main compilation unit, but not about static const variables declared in any header included. -Wunused-const-variable=2 This warning level also warns for unused constant static variables in headers (excluding system headers). This is the warning level of -Wunused-const-variable and must be explicitly requested since in C++ this isn't an error and in C it might be harder to clean up all headers included. -Wunused-value Warn whenever a statement computes a result that is explicitly not used. To suppress this warning cast the unused expression to void. This includes an expression-statement or the left-hand side of a comma expression that contains no side effects. For example, an expression such as x[i,j] causes a warning, while x[(void)i,j] does not. This warning is enabled by -Wall. -Wunused All the above -Wunused options combined. In order to get a warning about an unused function parameter, you must either specify -Wextra -Wunused (note that -Wall implies -Wunused), or separately specify -Wunused-parameter. -Wuninitialized Warn if an automatic variable is used without first being initialized or if a variable may be clobbered by a setjmp call. In C++, warn if a non-static reference or non-static const member appears in a class without constructors. If you want to warn about code that uses the uninitialized value of the variable in its own initializer, use the -Winit-self option. These warnings occur for individual uninitialized or clobbered elements of structure, union or array variables as well as for variables that are uninitialized or clobbered as a whole. They do not occur for variables or elements declared volatile. Because these warnings depend on optimization, the exact variables or elements for which there are warnings depends on the precise optimization options and version of GCC used. Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed. -Winvalid-memory-model Warn for invocations of __atomic Builtins, __sync Builtins, and the C11 atomic generic functions with a memory consistency argument that is either invalid for the operation or outside the range of values of the memory_order enumeration. For example, since the __atomic_store and __atomic_store_n built-ins are only defined for the relaxed, release, and sequentially consistent memory orders the following code is diagnosed: void store (int *i) { __atomic_store_n (i, 0, memory_order_consume); } -Winvalid-memory-model is enabled by default. -Wmaybe-uninitialized For an automatic (i.e. local) variable, if there exists a path from the function entry to a use of the variable that is initialized, but there exist some other paths for which the variable is not initialized, the compiler emits a warning if it cannot prove the uninitialized paths are not executed at run time. These warnings are only possible in optimizing compilation, because otherwise GCC does not keep track of the state of variables. These warnings are made optional because GCC may not be able to determine when the code is correct in spite of appearing to have an error. Here is one example of how this can happen: { int x; switch (y) { case 1: x = 1; break; case 2: x = 4; break; case 3: x = 5; } foo (x); } If the value of y is always 1, 2 or 3, then x is always initialized, but GCC doesn't know this. To suppress the warning, you need to provide a default case with assert(0) or similar code. This option also warns when a non-volatile automatic variable might be changed by a call to longjmp. The compiler sees only the calls to setjmp. It cannot know where longjmp will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there is in fact no problem because longjmp cannot in fact be called at the place that would cause a problem. Some spurious warnings can be avoided if you declare all the functions you use that never return as noreturn. See Function Attributes. This warning is enabled by -Wall or -Wextra. -Wunknown-pragmas Warn when a #pragma directive is encountered that is not understood by GCC. If this command-line option is used, warnings are even issued for unknown pragmas in system header files. This is not the case if the warnings are only enabled by the -Wall command-line option. -Wno-pragmas Do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas. See also -Wunknown-pragmas. -Wno-prio-ctor-dtor Do not warn if a priority from 0 to 100 is used for constructor or destructor. The use of constructor and destructor attributes allow you to assign a priority to the constructor/destructor to control its order of execution before main is called or after it returns. The priority values must be greater than 100 as the compiler reserves priority values between 0–100 for the implementation. -Wstrict-aliasing This option is only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. The warning does not catch all cases, but does attempt to catch the more common pitfalls. It is included in -Wall. It is equivalent to -Wstrict-aliasing=3 -Wstrict-aliasing=n This option is only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. Higher levels correspond to higher accuracy (fewer false positives). Higher levels also correspond to more effort, similar to the way -O works. -Wstrict-aliasing is equivalent to -Wstrict-aliasing=3. Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but -fstrict-aliasing still breaks the code, as it has very few false negatives. However, it has many false positives. Warns for all pointer conversions between possibly incompatible types, even if never dereferenced. Runs in the front end only. Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. Warns about incomplete types. Runs in the front end only. Level 3 (default for -Wstrict-aliasing): Should have very few false positives and few false negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the front end: *(int*)&some_float. If optimization is enabled, it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types. -Wstrict-overflow -Wstrict-overflow=n This option is only active when signed overflow is undefined. It warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur. Note that it does not warn about all cases where the code might overflow: it only warns about cases where the compiler implements some optimization. Thus this warning depends on the optimization level. An optimization that assumes that signed overflow does not occur is perfectly safe if the values of the variables involved are such that overflow never does, in fact, occur. Therefore this warning can easily give a false positive: a warning about code that is not actually a problem. To help focus on important issues, several warning levels are defined. No warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop requires, in particular when determining whether a loop will be executed at all. -Wstrict-overflow=1 Warn about cases that are both questionable and easy to avoid. For example the compiler simplifies x + 1 > x to 1. This level of -Wstrict-overflow is enabled by -Wall; higher levels are not, and must be explicitly requested. -Wstrict-overflow=2 Also warn about other cases where a comparison is simplified to a constant. For example: abs (x) >= 0. This can only be simplified when signed integer overflow is undefined, because abs (INT_MIN) overflows to INT_MIN, which is less than zero. -Wstrict-overflow (with no level) is the same as -Wstrict-overflow=2. -Wstrict-overflow=3 Also warn about other cases where a comparison is simplified. For example: x + 1 > 1 is simplified to x > 0. -Wstrict-overflow=4 Also warn about other simplifications not covered by the above cases. For example: (x * 10) / 5 is simplified to x * 2. -Wstrict-overflow=5 Also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example: x + 2 > y is simplified to x + 1 >= y. This is reported only at the highest warning level because this simplification applies to many comparisons, so this warning level gives a very large number of false positives. -Wstringop-overflow -Wstringop-overflow=type Warn for calls to string manipulation functions such as memcpy and strcpy that are determined to overflow the destination buffer. The optional argument is one greater than the type of Object Size Checking to perform to determine the size of the destination. See Object Size Checking. The argument is meaningful only for functions that operate on character arrays but not for raw memory functions like memcpy which always make use of Object Size type-0. The option also warns for calls that specify a size in excess of the largest possible object or at most SIZE_MAX / 2 bytes. The option produces the best results with optimization enabled but can detect a small subset of simple buffer overflows even without optimization in calls to the GCC built-in functions like __builtin_memcpy that correspond to the standard functions. In any case, the option warns about just a subset of buffer overflows detected by the corresponding overflow checking built-ins. For example, the option will issue a warning for the strcpy call below because it copies at least 5 characters (the string "blue" including the terminating NUL) into the buffer of size 4. enum Color { blue, purple, yellow }; const char* f (enum Color clr) { static char buf [4]; const char *str; switch (clr) { case blue: str = "blue"; break; case purple: str = "purple"; break; case yellow: str = "yellow"; break; } return strcpy (buf, str); // warning here } Option -Wstringop-overflow=2 is enabled by default. -Wstringop-overflow -Wstringop-overflow=1 The -Wstringop-overflow=1 option uses type-zero Object Size Checking to determine the sizes of destination objects. This is the default setting of the option. At this setting the option will not warn for writes past the end of subobjects of larger objects accessed by pointers unless the size of the largest surrounding object is known. When the destination may be one of several objects it is assumed to be the largest one of them. On Linux systems, when optimization is enabled at this setting the option warns for the same code as when the _FORTIFY_SOURCE macro is defined to a non-zero value. -Wstringop-overflow=2 The -Wstringop-overflow=2 option uses type-one Object Size Checking to determine the sizes of destination objects. At this setting the option will warn about overflows when writing to members of the largest complete objects whose exact size is known. It will, however, not warn for excessive writes to the same members of unknown objects referenced by pointers since they may point to arrays containing unknown numbers of elements. -Wstringop-overflow=3 The -Wstringop-overflow=3 option uses type-two Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflowing the smallest object or data member. This is the most restrictive setting of the option that may result in warnings for safe code. -Wstringop-overflow=4 The -Wstringop-overflow=4 option uses type-three Object Size Checking to determine the sizes of destination objects. At this setting the option will warn about overflowing any data members, and when the destination is one of several objects it uses the size of the largest of them to decide whether to issue a warning. Similarly to -Wstringop-overflow=3 this setting of the option may result in warnings for benign code. -Wstringop-truncation Warn for calls to bounded string manipulation functions such as strncat, strncpy, and stpncpy that may either truncate the copied string or leave the destination unchanged. In the following example, the call to strncat specifies a bound that is less than the length of the source string. As a result, the copy of the source will be truncated and so the call is diagnosed. To avoid the warning use bufsize - strlen (buf) - 1) as the bound. void append (char *buf, size_t bufsize) { strncat (buf, ".txt", 3); } As another example, the following call to strncpy results in copying to d just the characters preceding the terminating NUL, without appending the NUL to the end. Assuming the result of strncpy is necessarily a NUL-terminated string is a common mistake, and so the call is diagnosed. To avoid the warning when the result is not expected to be NUL-terminated, call memcpy instead. void copy (char *d, const char *s) { strncpy (d, s, strlen (s)); } In the following example, the call to strncpy specifies the size of the destination buffer as the bound. If the length of the source string is equal to or greater than this size the result of the copy will not be NUL-terminated. Therefore, the call is also diagnosed. To avoid the warning, specify sizeof buf - 1 as the bound and set the last element of the buffer to NUL. void copy (const char *s) { char buf[80]; strncpy (buf, s, sizeof buf); … } In situations where a character array is intended to store a sequence of bytes with no terminating NUL such an array may be annotated with attribute nonstring to avoid this warning. Such arrays, however, are not suitable arguments to functions that expect NUL-terminated strings. To help detect accidental misuses of such arrays GCC issues warnings unless it can prove that the use is safe. See Common Variable Attributes. -Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc] Warn for cases where adding an attribute may be beneficial. The attributes currently supported are listed below. -Wsuggest-attribute=pure -Wsuggest-attribute=const -Wsuggest-attribute=noreturn -Wmissing-noreturn -Wsuggest-attribute=malloc Warn about functions that might be candidates for attributes pure, const or noreturn or malloc. The compiler only warns for functions visible in other compilation units or (in the case of pure and const) if it cannot prove that the function returns normally. A function returns normally if it doesn't contain an infinite loop or return abnormally by throwing, calling abort or trapping. This analysis requires option -fipa-pure-const, which is enabled by default at -O and higher. Higher optimization levels improve the accuracy of the analysis. -Wsuggest-attribute=format -Wmissing-format-attribute Warn about function pointers that might be candidates for format attributes. Note these are only possible candidates, not absolute ones. GCC guesses that function pointers with format attributes that are used in assignment, initialization, parameter passing or return statements should have a corresponding format attribute in the resulting type. I.e. the left-hand side of the assignment or initialization, the type of the parameter variable, or the return type of the containing function respectively should also have a format attribute to avoid the warning. GCC also warns about function definitions that might be candidates for format attributes. Again, these are only possible candidates. GCC guesses that format attributes might be appropriate for any function that calls a function like vprintf or vscanf, but this might not always be the case, and some functions for which format attributes are appropriate may not be detected. -Wsuggest-attribute=cold Warn about functions that might be candidates for cold attribute. This is based on static detection and generally will only warn about functions which always leads to a call to another cold function such as wrappers of C++ throw or fatal error reporting functions leading to abort. -Wsuggest-final-types Warn about types with virtual methods where code quality would be improved if the type were declared with the C++11 final specifier, or, if possible, declared in an anonymous namespace. This allows GCC to more aggressively devirtualize the polymorphic calls. This warning is more effective with link time optimization, where the information about the class hierarchy graph is more complete. -Wsuggest-final-methods Warn about virtual methods where code quality would be improved if the method were declared with the C++11 final specifier, or, if possible, its type were declared in an anonymous namespace or with the final specifier. This warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete. It is recommended to first consider suggestions of -Wsuggest-final-types and then rebuild with new annotations. -Wsuggest-override Warn about overriding virtual functions that are not marked with the override keyword. -Walloc-zero Warn about calls to allocation functions decorated with attribute alloc_size that specify zero bytes, including those to the built-in forms of the functions aligned_alloc, alloca, calloc, malloc, and realloc. Because the behavior of these functions when called with a zero size differs among implementations (and in the case of realloc has been deprecated) relying on it may result in subtle portability bugs and should be avoided. -Walloc-size-larger-than=byte-size Warn about calls to functions decorated with attribute alloc_size that attempt to allocate objects larger than the specified number of bytes, or where the result of the size computation in an integer type with infinite precision would exceed the value of 'PTRDIFF_MAX' on the target. -Walloc-size-larger-than='PTRDIFF_MAX' is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of 'SIZE_MAX' or more or by -Wno-alloc-size-larger-than. See Function Attributes. -Wno-alloc-size-larger-than Disable -Walloc-size-larger-than= warnings. The option is equivalent to -Walloc-size-larger-than='SIZE_MAX' or larger. -Walloca This option warns on all uses of alloca in the source. -Walloca-larger-than=byte-size This option warns on calls to alloca with an integer argument whose value is either zero, or that is not bounded by a controlling predicate that limits its value to at most byte-size. It also warns for calls to alloca where the bound value is unknown. Arguments of non-integer types are considered unbounded even if they appear to be constrained to the expected range. For example, a bounded case of alloca could be: void func (size_t n) { void *p; if (n <= 1000) p = alloca (n); else p = malloc (n); f (p); } In the above example, passing -Walloca-larger-than=1000 would not issue a warning because the call to alloca is known to be at most 1000 bytes. However, if -Walloca-larger-than=500 were passed, the compiler would emit a warning. Unbounded uses, on the other hand, are uses of alloca with no controlling predicate constraining its integer argument. For example: void func () { void *p = alloca (n); f (p); } If -Walloca-larger-than=500 were passed, the above would trigger a warning, but this time because of the lack of bounds checking. Note, that even seemingly correct code involving signed integers could cause a warning: void func (signed int n) { if (n < 500) { p = alloca (n); f (p); } } In the above example, n could be negative, causing a larger than expected argument to be implicitly cast into the alloca call. This option also warns when alloca is used in a loop. -Walloca-larger-than='PTRDIFF_MAX' is enabled by default but is usually only effective when -ftree-vrp is active (default for -O2 and above). See also -Wvla-larger-than='byte-size'. -Wno-alloca-larger-than Disable -Walloca-larger-than= warnings. The option is equivalent to -Walloca-larger-than='SIZE_MAX' or larger. -Warray-bounds -Warray-bounds=n This option is only active when -ftree-vrp is active (default for -O2 and above). It warns about subscripts to arrays that are always out of bounds. This warning is enabled by -Wall. -Warray-bounds=1 This is the warning level of -Warray-bounds and is enabled by -Wall; higher levels are not, and must be explicitly requested. -Warray-bounds=2 This warning level also warns about out of bounds access for arrays at the end of a struct and for arrays accessed through pointers. This warning level may give a larger number of false positives and is deactivated by default. -Wattribute-alias=n -Wno-attribute-alias Warn about declarations using the alias and similar attributes whose target is incompatible with the type of the alias. See Declaring Attributes of Functions. -Wattribute-alias=1 The default warning level of the -Wattribute-alias option diagnoses incompatibilities between the type of the alias declaration and that of its target. Such incompatibilities are typically indicative of bugs. -Wattribute-alias=2 At this level -Wattribute-alias also diagnoses cases where the attributes of the alias declaration are more restrictive than the attributes applied to its target. These mismatches can potentially result in incorrect code generation. In other cases they may be benign and could be resolved simply by adding the missing attribute to the target. For comparison, see the -Wmissing-attributes option, which controls diagnostics when the alias declaration is less restrictive than the target, rather than more restrictive. Attributes considered include alloc_align, alloc_size, cold, const, hot, leaf, malloc, nonnull, noreturn, nothrow, pure, returns_nonnull, and returns_twice. -Wattribute-alias is equivalent to -Wattribute-alias=1. This is the default. You can disable these warnings with either -Wno-attribute-alias or -Wattribute-alias=0. -Wbool-compare Warn about boolean expression compared with an integer value different from true/false. For instance, the following comparison is always false: int n = 5; … if ((n > 1) == 2) { … } This warning is enabled by -Wall. -Wbool-operation Warn about suspicious operations on expressions of a boolean type. For instance, bitwise negation of a boolean is very likely a bug in the program. For C, this warning also warns about incrementing or decrementing a boolean, which rarely makes sense. (In C++, decrementing a boolean is always invalid. Incrementing a boolean is invalid in C++17, and deprecated otherwise.) This warning is enabled by -Wall. -Wduplicated-branches Warn when an if-else has identical branches. This warning detects cases like if (p != NULL) return 0; else return 0; It doesn't warn when both branches contain just a null statement. This warning also warn for conditional operators: int i = x ? *p : *p; -Wduplicated-cond Warn about duplicated conditions in an if-else-if chain. For instance, warn for the following code: if (p->q != NULL) { … } else if (p->q != NULL) { … } -Wframe-address Warn when the '__builtin_frame_address' or '__builtin_return_address' is called with an argument greater than 0. Such calls may return indeterminate values or crash the program. The warning is included in -Wall. -Wno-discarded-qualifiers (C and Objective-C only) Do not warn if type qualifiers on pointers are being discarded. Typically, the compiler warns if a const char * variable is passed to a function that takes a char * parameter. This option can be used to suppress such a warning. -Wno-discarded-array-qualifiers (C and Objective-C only) Do not warn if type qualifiers on arrays which are pointer targets are being discarded. Typically, the compiler warns if a const int (*)[] variable is passed to a function that takes a int (*)[] parameter. This option can be used to suppress such a warning. -Wno-incompatible-pointer-types (C and Objective-C only) Do not warn when there is a conversion between pointers that have incompatible types. This warning is for cases not covered by -Wno-pointer-sign, which warns for pointer argument passing or assignment with different signedness. -Wno-int-conversion (C and Objective-C only) Do not warn about incompatible integer to pointer and pointer to integer conversions. This warning is about implicit conversions; for explicit conversions the warnings -Wno-int-to-pointer-cast and -Wno-pointer-to-int-cast may be used. -Wno-div-by-zero Do not warn about compile-time integer division by zero. Floating-point division by zero is not warned about, as it can be a legitimate way of obtaining infinities and NaNs. -Wsystem-headers Print warning messages for constructs found in system header files. Warnings from system headers are normally suppressed, on the assumption that they usually do not indicate real problems and would only make the compiler output harder to read. Using this command-line option tells GCC to emit warnings from system headers as if they occurred in user code. However, note that using -Wall in conjunction with this option does not warn about unknown pragmas in system headers—for that, -Wunknown-pragmas must also be used. -Wtautological-compare Warn if a self-comparison always evaluates to true or false. This warning detects various mistakes such as: int i = 1; … if (i > i) { … } This warning also warns about bitwise comparisons that always evaluate to true or false, for instance: if ((a & 16) == 10) { … } will always be false. This warning is enabled by -Wall. -Wtrampolines Warn about trampolines generated for pointers to nested functions. A trampoline is a small piece of data or code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. For some targets, it is made up of data only and thus requires no special treatment. But, for most targets, it is made up of code and thus requires the stack to be made executable in order for the program to work properly. -Wfloat-equal Warn if floating-point values are used in equality comparisons. The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you are doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem). In particular, instead of testing for equality, you should check to see whether the two values have ranges that overlap; and this is done with the relational operators, so equality comparisons are probably mistaken. -Wtraditional (C and Objective-C only) Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided. * Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes place within string literals, but in ISO C it does not. * In traditional C, some preprocessor directives did not exist. Traditional preprocessors only considered a line to be a directive if the '#' appeared in column 1 on the line. Therefore -Wtraditional warns about directives that traditional C understands but ignores because the '#' does not appear as the first character on the line. It also suggests you hide directives like #pragma not understood by traditional C by indenting them. Some traditional implementations do not recognize #elif, so this option suggests avoiding it altogether. * A function-like macro that appears without arguments. * The unary plus operator. * The 'U' integer constant suffix, or the 'F' or 'L' floating-point constant suffixes. (Traditional C does support the 'L' suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the '_MIN'/'_MAX' macros in . Use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases. * A function declared external in one block and then used after the end of the block. * A switch statement has an operand of type long. * A non-static function declaration follows a static one. This construct is not accepted by some traditional C compilers. * The ISO type of an integer constant has a different width or signedness from its traditional type. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about. * Usage of ISO string concatenation is detected. * Initialization of automatic aggregates. * Identifier conflicts with labels. Traditional C lacks a separate namespace for labels. * Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the assumption that the zero initializer in user code appears conditioned on e.g. __STDC__ to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case. * Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these prototypes when compiling with traditional C causes serious problems. This is a subset of the possible conversion warnings; for the full set use -Wtraditional-conversion. * Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features appear in your code when using libiberty's traditional C compatibility macros, PARAMS and VPARAMS. This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility. -Wtraditional-conversion (C and Objective-C only) Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed-point argument except when the same as the default promotion. -Wdeclaration-after-statement (C and Objective-C only) Warn when a declaration is found after a statement in a block. This construct, known from C++, was introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90. See Mixed Declarations. -Wshadow Warn whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C) or whenever a built-in function is shadowed. Note that in C++, the compiler warns if a local variable shadows an explicit typedef, but not if it shadows a struct/class/enum. Same as -Wshadow=global. -Wno-shadow-ivar (Objective-C only) Do not warn whenever a local variable shadows an instance variable in an Objective-C method. -Wshadow=global The default for -Wshadow. Warns for any (global) shadowing. -Wshadow=local Warn when a local variable shadows another local variable or parameter. This warning is enabled by -Wshadow=global. -Wshadow=compatible-local Warn when a local variable shadows another local variable or parameter whose type is compatible with that of the shadowing variable. In C++, type compatibility here means the type of the shadowing variable can be converted to that of the shadowed variable. The creation of this flag (in addition to -Wshadow=local) is based on the idea that when a local variable shadows another one of incompatible type, it is most likely intentional, not a bug or typo, as shown in the following example: for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i) { for (int i = 0; i < N; ++i) { ... } ... } Since the two variable i in the example above have incompatible types, enabling only -Wshadow=compatible-local will not emit a warning. Because their types are incompatible, if a programmer accidentally uses one in place of the other, type checking will catch that and emit an error or warning. So not warning (about shadowing) in this case will not lead to undetected bugs. Use of this flag instead of -Wshadow=local can possibly reduce the number of warnings triggered by intentional shadowing. This warning is enabled by -Wshadow=local. -Wlarger-than=byte-size Warn whenever an object is defined whose size exceeds byte-size. -Wlarger-than='PTRDIFF_MAX' is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of 'SIZE_MAX' or more or by -Wno-larger-than. -Wno-larger-than Disable -Wlarger-than= warnings. The option is equivalent to -Wlarger-than='SIZE_MAX' or larger. -Wframe-larger-than=byte-size Warn if the size of a function frame exceeds byte-size. The computation done to determine the stack frame size is approximate and not conservative. The actual requirements may be somewhat greater than byte-size even if you do not get a warning. In addition, any space allocated via alloca, variable-length arrays, or related constructs is not included by the compiler when determining whether or not to issue a warning. -Wframe-larger-than='PTRDIFF_MAX' is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of 'SIZE_MAX' or more or by -Wno-frame-larger-than. -Wno-frame-larger-than Disable -Wframe-larger-than= warnings. The option is equivalent to -Wframe-larger-than='SIZE_MAX' or larger. -Wno-free-nonheap-object Do not warn when attempting to free an object that was not allocated on the heap. -Wstack-usage=byte-size Warn if the stack usage of a function might exceed byte-size. The computation done to determine the stack usage is conservative. Any space allocated via alloca, variable-length arrays, or related constructs is included by the compiler when determining whether or not to issue a warning. The message is in keeping with the output of -fstack-usage. * If the stack usage is fully static but exceeds the specified amount, it's: warning: stack usage is 1120 bytes * If the stack usage is (partly) dynamic but bounded, it's: warning: stack usage might be 1648 bytes * If the stack usage is (partly) dynamic and not bounded, it's: warning: stack usage might be unbounded -Wstack-usage='PTRDIFF_MAX' is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of 'SIZE_MAX' or more or by -Wno-stack-usage. -Wno-stack-usage Disable -Wstack-usage= warnings. The option is equivalent to -Wstack-usage='SIZE_MAX' or larger. -Wunsafe-loop-optimizations Warn if the loop cannot be optimized because the compiler cannot assume anything on the bounds of the loop indices. With -funsafe-loop-optimizations warn if the compiler makes such assumptions. -Wno-pedantic-ms-format (MinGW targets only) When used in combination with -Wformat and -pedantic without GNU extensions, this option disables the warnings about non-ISO printf / scanf format width specifiers I32, I64, and I used on Windows targets, which depend on the MS runtime. -Waligned-new Warn about a new-expression of a type that requires greater alignment than the alignof(std::max_align_t) but uses an allocation function without an explicit alignment parameter. This option is enabled by -Wall. Normally this only warns about global allocation functions, but -Waligned-new=all also warns about class member allocation functions. -Wplacement-new -Wplacement-new=n Warn about placement new expressions with undefined behavior, such as constructing an object in a buffer that is smaller than the type of the object. For example, the placement new expression below is diagnosed because it attempts to construct an array of 64 integers in a buffer only 64 bytes large. char buf [64]; new (buf) int[64]; This warning is enabled by default. -Wplacement-new=1 This is the default warning level of -Wplacement-new. At this level the warning is not issued for some strictly undefined constructs that GCC allows as extensions for compatibility with legacy code. For example, the following new expression is not diagnosed at this level even though it has undefined behavior according to the C++ standard because it writes past the end of the one-element array. struct S { int n, a[1]; }; S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]); new (s->a)int [32](); -Wplacement-new=2 At this level, in addition to diagnosing all the same constructs as at level 1, a diagnostic is also issued for placement new expressions that construct an object in the last member of structure whose type is an array of a single element and whose size is less than the size of the object being constructed. While the previous example would be diagnosed, the following construct makes use of the flexible member array extension to avoid the warning at level 2. struct S { int n, a[]; }; S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]); new (s->a)int [32](); -Wpointer-arith Warn about anything that depends on the “size of” a function type or of void. GNU C assigns these types a size of 1, for convenience in calculations with void * pointers and pointers to functions. In C++, warn also when an arithmetic operation involves NULL. This warning is also enabled by -Wpedantic. -Wpointer-compare Warn if a pointer is compared with a zero character constant. This usually means that the pointer was meant to be dereferenced. For example: const char *p = foo (); if (p == '\0') return 42; Note that the code above is invalid in C++11. This warning is enabled by default. -Wtype-limits Warn if a comparison is always true or always false due to the limited range of the data type, but do not warn for constant expressions. For example, warn if an unsigned variable is compared against zero with < or >=. This warning is also enabled by -Wextra. -Wabsolute-value (C and Objective-C only) Warn for calls to standard functions that compute the absolute value of an argument when a more appropriate standard function is available. For example, calling abs(3.14) triggers the warning because the appropriate function to call to compute the absolute value of a double argument is fabs. The option also triggers warnings when the argument in a call to such a function has an unsigned type. This warning can be suppressed with an explicit type cast and it is also enabled by -Wextra. -Wcomment -Wcomments Warn whenever a comment-start sequence '/*' appears in a '/*' comment, or whenever a backslash-newline appears in a '//' comment. This warning is enabled by -Wall. -Wtrigraphs Warn if any trigraphs are encountered that might change the meaning of the program. Trigraphs within comments are not warned about, except those that would form escaped newlines. This option is implied by -Wall. If -Wall is not given, this option is still enabled unless trigraphs are enabled. To get trigraph conversion without warnings, but get the other -Wall warnings, use '-trigraphs -Wall -Wno-trigraphs'. -Wundef Warn if an undefined identifier is evaluated in an #if directive. Such identifiers are replaced with zero. -Wexpansion-to-defined Warn whenever 'defined' is encountered in the expansion of a macro (including the case where the macro is expanded by an '#if' directive). Such usage is not portable. This warning is also enabled by -Wpedantic and -Wextra. -Wunused-macros Warn about macros defined in the main file that are unused. A macro is used if it is expanded or tested for existence at least once. The preprocessor also warns if the macro has not been used at the time it is redefined or undefined. Built-in macros, macros defined on the command line, and macros defined in include files are not warned about. Note: If a macro is actually used, but only used in skipped conditional blocks, then the preprocessor reports it as unused. To avoid the warning in such a case, you might improve the scope of the macro's definition by, for example, moving it into the first skipped block. Alternatively, you could provide a dummy use with something like: #if defined the_macro_causing_the_warning #endif -Wno-endif-labels Do not warn whenever an #else or an #endif are followed by text. This sometimes happens in older programs with code of the form #if FOO … #else FOO … #endif FOO The second and third FOO should be in comments. This warning is on by default. -Wbad-function-cast (C and Objective-C only) Warn when a function call is cast to a non-matching type. For example, warn if a call to a function returning an integer type is cast to a pointer type. -Wc90-c99-compat (C and Objective-C only) Warn about features not present in ISO C90, but present in ISO C99. For instance, warn about use of variable length arrays, long long type, bool type, compound literals, designated initializers, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__. -Wc99-c11-compat (C and Objective-C only) Warn about features not present in ISO C99, but present in ISO C11. For instance, warn about use of anonymous structures and unions, _Atomic type qualifier, _Thread_local storage-class specifier, _Alignas specifier, Alignof operator, _Generic keyword, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__. -Wc11-c2x-compat (C and Objective-C only) Warn about features not present in ISO C11, but present in ISO C2X. For instance, warn about omitting the string in _Static_assert. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__. -Wc++-compat (C and Objective-C only) Warn about ISO C constructs that are outside of the common subset of ISO C and ISO C++, e.g. request for implicit conversion from void * to a pointer to non-void type. -Wc++11-compat (C++ and Objective-C++ only) Warn about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is enabled by -Wall. -Wc++14-compat (C++ and Objective-C++ only) Warn about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014. This warning is enabled by -Wall. -Wc++17-compat (C++ and Objective-C++ only) Warn about C++ constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017. This warning is enabled by -Wall. -Wcast-qual Warn whenever a pointer is cast so as to remove a type qualifier from the target type. For example, warn if a const char * is cast to an ordinary char *. Also warn when making a cast that introduces a type qualifier in an unsafe way. For example, casting char ** to const char ** is unsafe, as in this example: /* p is char ** value. */ const char **q = (const char **) p; /* Assignment of readonly string to const char * is OK. */ *q = "string"; /* Now char** pointer points to read-only memory. */ **p = 'b'; -Wcast-align Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a char * is cast to an int * on machines where integers can only be accessed at two- or four-byte boundaries. -Wcast-align=strict Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a char * is cast to an int * regardless of the target machine. -Wcast-function-type Warn when a function pointer is cast to an incompatible function pointer. In a cast involving function types with a variable argument list only the types of initial arguments that are provided are considered. Any parameter of pointer-type matches any other pointer-type. Any benign differences in integral types are ignored, like int vs. long on ILP32 targets. Likewise type qualifiers are ignored. The function type void (*) (void) is special and matches everything, which can be used to suppress this warning. In a cast involving pointer to member types this warning warns whenever the type cast is changing the pointer to member type. This warning is enabled by -Wextra. -Wwrite-strings When compiling C, give string constants the type const char[length] so that copying the address of one into a non-const char * pointer produces a warning. These warnings help you find at compile time code that can try to write into a string constant, but only if you have been very careful about using const in declarations and prototypes. Otherwise, it is just a nuisance. This is why we did not make -Wall request these warnings. When compiling C++, warn about the deprecated conversion from string literals to char *. This warning is enabled by default for C++ programs. -Wcatch-value -Wcatch-value=n (C++ and Objective-C++ only) Warn about catch handlers that do not catch via reference. With -Wcatch-value=1 (or -Wcatch-value for short) warn about polymorphic class types that are caught by value. With -Wcatch-value=2 warn about all class types that are caught by value. With -Wcatch-value=3 warn about all types that are not caught by reference. -Wcatch-value is enabled by -Wall. -Wclobbered Warn for variables that might be changed by longjmp or vfork. This warning is also enabled by -Wextra. -Wconditionally-supported (C++ and Objective-C++ only) Warn for conditionally-supported (C++11 [intro.defs]) constructs. -Wconversion Warn for implicit conversions that may alter a value. This includes conversions between real and integer, like abs (x) when x is double; conversions between signed and unsigned, like unsigned ui = -1; and conversions to smaller types, like sqrtf (M_PI). Do not warn for explicit casts like abs ((int) x) and ui = (unsigned) -1, or if the value is not changed by the conversion like in abs (2.0). Warnings about conversions between signed and unsigned integers can be disabled by using -Wno-sign-conversion. For C++, also warn for confusing overload resolution for user-defined conversions; and conversions that never use a type conversion operator: conversions to void, the same type, a base class or a reference to them. Warnings about conversions between signed and unsigned integers are disabled by default in C++ unless -Wsign-conversion is explicitly enabled. -Wno-conversion-null (C++ and Objective-C++ only) Do not warn for conversions between NULL and non-pointer types. -Wconversion-null is enabled by default. -Wzero-as-null-pointer-constant (C++ and Objective-C++ only) Warn when a literal '0' is used as null pointer constant. This can be useful to facilitate the conversion to nullptr in C++11. -Wsubobject-linkage (C++ and Objective-C++ only) Warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage. If a type A depends on a type B with no or internal linkage, defining it in multiple translation units would be an ODR violation because the meaning of B is different in each translation unit. If A only appears in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace as well. The compiler doesn't give this warning for types defined in the main .C file, as those are unlikely to have multiple definitions. -Wsubobject-linkage is enabled by default. -Wdangling-else Warn about constructions where there may be confusion to which if statement an else branch belongs. Here is an example of such a case: { if (a) if (b) foo (); else bar (); } In C/C++, every else branch belongs to the innermost possible if statement, which in this example is if (b). This is often not what the programmer expected, as illustrated in the above example by indentation the programmer chose. When there is the potential for this confusion, GCC issues a warning when this flag is specified. To eliminate the warning, add explicit braces around the innermost if statement so there is no way the else can belong to the enclosing if. The resulting code looks like this: { if (a) { if (b) foo (); else bar (); } } This warning is enabled by -Wparentheses. -Wdate-time Warn when macros __TIME__, __DATE__ or __TIMESTAMP__ are encountered as they might prevent bit-wise-identical reproducible compilations. -Wdelete-incomplete (C++ and Objective-C++ only) Warn when deleting a pointer to incomplete type, which may cause undefined behavior at runtime. This warning is enabled by default. -Wuseless-cast (C++ and Objective-C++ only) Warn when an expression is casted to its own type. -Wempty-body Warn if an empty body occurs in an if, else or do while statement. This warning is also enabled by -Wextra. -Wenum-compare Warn about a comparison between values of different enumerated types. In C++ enumerated type mismatches in conditional expressions are also diagnosed and the warning is enabled by default. In C this warning is enabled by -Wall. -Wextra-semi (C++, Objective-C++ only) Warn about redundant semicolon after in-class function definition. -Wjump-misses-init (C, Objective-C only) Warn if a goto statement or a switch statement jumps forward across the initialization of a variable, or jumps backward to a label after the variable has been initialized. This only warns about variables that are initialized when they are declared. This warning is only supported for C and Objective-C; in C++ this sort of branch is an error in any case. -Wjump-misses-init is included in -Wc++-compat. It can be disabled with the -Wno-jump-misses-init option. -Wsign-compare Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. In C++, this warning is also enabled by -Wall. In C, it is also enabled by -Wextra. -Wsign-conversion Warn for implicit conversions that may change the sign of an integer value, like assigning a signed integer expression to an unsigned integer variable. An explicit cast silences the warning. In C, this option is enabled also by -Wconversion. -Wfloat-conversion Warn for implicit conversions that reduce the precision of a real value. This includes conversions from real to integer, and from higher precision real to lower precision real values. This option is also enabled by -Wconversion. -Wno-scalar-storage-order Do not warn on suspicious constructs involving reverse scalar storage order. -Wsized-deallocation (C++ and Objective-C++ only) Warn about a definition of an unsized deallocation function void operator delete (void *) noexcept; void operator delete[] (void *) noexcept; without a definition of the corresponding sized deallocation function void operator delete (void *, std::size_t) noexcept; void operator delete[] (void *, std::size_t) noexcept; or vice versa. Enabled by -Wextra along with -fsized-deallocation. -Wsizeof-pointer-div Warn for suspicious divisions of two sizeof expressions that divide the pointer size by the element size, which is the usual way to compute the array size but won't work out correctly with pointers. This warning warns e.g. about sizeof (ptr) / sizeof (ptr[0]) if ptr is not an array, but a pointer. This warning is enabled by -Wall. -Wsizeof-pointer-memaccess Warn for suspicious length parameters to certain string and memory built-in functions if the argument uses sizeof. This warning triggers for example for memset (ptr, 0, sizeof (ptr)); if ptr is not an array, but a pointer, and suggests a possible fix, or about memcpy (&foo, ptr, sizeof (&foo));. -Wsizeof-pointer-memaccess also warns about calls to bounded string copy functions like strncat or strncpy that specify as the bound a sizeof expression of the source array. For example, in the following function the call to strncat specifies the size of the source string as the bound. That is almost certainly a mistake and so the call is diagnosed. void make_file (const char *name) { char path[PATH_MAX]; strncpy (path, name, sizeof path - 1); strncat (path, ".text", sizeof ".text"); … } The -Wsizeof-pointer-memaccess option is enabled by -Wall. -Wsizeof-array-argument Warn when the sizeof operator is applied to a parameter that is declared as an array in a function definition. This warning is enabled by default for C and C++ programs. -Wmemset-elt-size Warn for suspicious calls to the memset built-in function, if the first argument references an array, and the third argument is a number equal to the number of elements, but not equal to the size of the array in memory. This indicates that the user has omitted a multiplication by the element size. This warning is enabled by -Wall. -Wmemset-transposed-args Warn for suspicious calls to the memset built-in function where the second argument is not zero and the third argument is zero. For example, the call memset (buf, sizeof buf, 0) is diagnosed because memset (buf, 0, sizeof buf) was meant instead. The diagnostic is only emitted if the third argument is a literal zero. Otherwise, if it is an expression that is folded to zero, or a cast of zero to some type, it is far less likely that the arguments have been mistakenly transposed and no warning is emitted. This warning is enabled by -Wall. -Waddress Warn about suspicious uses of memory addresses. These include using the address of a function in a conditional expression, such as void func(void); if (func), and comparisons against the memory address of a string literal, such as if (x == "abc"). Such uses typically indicate a programmer error: the address of a function always evaluates to true, so their use in a conditional usually indicate that the programmer forgot the parentheses in a function call; and comparisons against string literals result in unspecified behavior and are not portable in C, so they usually indicate that the programmer intended to use strcmp. This warning is enabled by -Wall. -Waddress-of-packed-member Warn when the address of packed member of struct or union is taken, which usually results in an unaligned pointer value. This is enabled by default. -Wlogical-op Warn about suspicious uses of logical operators in expressions. This includes using logical operators in contexts where a bit-wise operator is likely to be expected. Also warns when the operands of a logical operator are the same: extern int a; if (a < 0 && a < 0) { … } -Wlogical-not-parentheses Warn about logical not used on the left hand side operand of a comparison. This option does not warn if the right operand is considered to be a boolean expression. Its purpose is to detect suspicious code like the following: int a; … if (!a > 1) { … } It is possible to suppress the warning by wrapping the LHS into parentheses: if ((!a) > 1) { … } This warning is enabled by -Wall. -Waggregate-return Warn if any functions that return structures or unions are defined or called. (In languages where you can return an array, this also elicits a warning.) -Wno-aggressive-loop-optimizations Warn if in a loop with constant number of iterations the compiler detects undefined behavior in some statement during one or more of the iterations. -Wno-attributes Do not warn if an unexpected __attribute__ is used, such as unrecognized attributes, function attributes applied to variables, etc. This does not stop errors for incorrect use of supported attributes. -Wno-builtin-declaration-mismatch Warn if a built-in function is declared with an incompatible signature or as a non-function, or when a built-in function declared with a type that does not include a prototype is called with arguments whose promoted types do not match those expected by the function. When -Wextra is specified, also warn when a built-in function that takes arguments is declared without a prototype. The -Wno-builtin-declaration-mismatch warning is enabled by default. To avoid the warning include the appropriate header to bring the prototypes of built-in functions into scope. For example, the call to memset below is diagnosed by the warning because the function expects a value of type size_t as its argument but the type of 32 is int. With -Wextra, the declaration of the function is diagnosed as well. extern void* memset (); void f (void *d) { memset (d, '\0', 32); } -Wno-builtin-macro-redefined Do not warn if certain built-in macros are redefined. This suppresses warnings for redefinition of __TIMESTAMP__, __TIME__, __DATE__, __FILE__, and __BASE_FILE__. -Wstrict-prototypes (C and Objective-C only) Warn if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if preceded by a declaration that specifies the argument types.) -Wold-style-declaration (C and Objective-C only) Warn for obsolescent usages, according to the C Standard, in a declaration. For example, warn if storage-class specifiers like static are not the first things in a declaration. This warning is also enabled by -Wextra. -Wold-style-definition (C and Objective-C only) Warn if an old-style function definition is used. A warning is given even if there is a previous prototype. -Wmissing-parameter-type (C and Objective-C only) A function parameter is declared without a type specifier in K&R-style functions: void foo(bar) { } This warning is also enabled by -Wextra. -Wmissing-prototypes (C and Objective-C only) Warn if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. Use this option to detect global functions that do not have a matching prototype declaration in a header file. This option is not valid for C++ because all function declarations provide prototypes and a non-matching declaration declares an overload rather than conflict with an earlier declaration. Use -Wmissing-declarations to detect missing declarations in C++. -Wmissing-declarations Warn if a global function is defined without a previous declaration. Do so even if the definition itself provides a prototype. Use this option to detect global functions that are not declared in header files. In C, no warnings are issued for functions with previous non-prototype declarations; use -Wmissing-prototypes to detect missing prototypes. In C++, no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces. -Wmissing-field-initializers Warn if a structure's initializer has some fields missing. For example, the following code causes such a warning, because x.h is implicitly zero: struct s { int f, g, h; }; struct s x = { 3, 4 }; This option does not warn about designated initializers, so the following modification does not trigger a warning: struct s { int f, g, h; }; struct s x = { .f = 3, .g = 4 }; In C this option does not warn about the universal zero initializer '{ 0 }': struct s { int f, g, h; }; struct s x = { 0 }; Likewise, in C++ this option does not warn about the empty { } initializer, for example: struct s { int f, g, h; }; s x = { }; This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-missing-field-initializers. -Wno-multichar Do not warn if a multicharacter constant (''FOOF'') is used. Usually they indicate a typo in the user's code, as they have implementation-defined values, and should not be used in portable code. -Wnormalized=[none|id|nfc|nfkc] In ISO C and ISO C++, two identifiers are different if they are different sequences of characters. However, sometimes when characters outside the basic ASCII character set are used, you can have two different character sequences that look the same. To avoid confusion, the ISO 10646 standard sets out some normalization rules which when applied ensure that two sequences that look the same are turned into the same sequence. GCC can warn you if you are using identifiers that have not been normalized; this option controls that warning. There are four levels of warning supported by GCC. The default is -Wnormalized=nfc, which warns about any identifier that is not in the ISO 10646 “C” normalized form, NFC. NFC is the recommended form for most uses. It is equivalent to -Wnormalized. Unfortunately, there are some characters allowed in identifiers by ISO C and ISO C++ that, when turned into NFC, are not allowed in identifiers. That is, there's no way to use these symbols in portable ISO C or C++ and have all your identifiers in NFC. -Wnormalized=id suppresses the warning for these characters. It is hoped that future versions of the standards involved will correct this, which is why this option is not the default. You can switch the warning off for all characters by writing -Wnormalized=none or -Wno-normalized. You should only do this if you are using some other normalization scheme (like “D”), because otherwise you can easily create bugs that are literally impossible to see. Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies, especially once formatting has been applied. For instance \u207F, “SUPERSCRIPT LATIN SMALL LETTER N”, displays just like a regular n that has been placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert all these into a standard form as well, and GCC warns if your code is not in NFKC if you use -Wnormalized=nfkc. This warning is comparable to warning about every identifier that contains the letter O because it might be confused with the digit 0, and so is not the default, but may be useful as a local coding convention if the programming environment cannot be fixed to display these characters distinctly. -Wno-attribute-warning Do not warn about usage of functions (see Function Attributes) declared with warning attribute. By default, this warning is enabled. -Wno-attribute-warning can be used to disable the warning or -Wno-error=attribute-warning can be used to disable the error when compiled with -Werror flag. -Wno-deprecated Do not warn about usage of deprecated features. See Deprecated Features. -Wno-deprecated-declarations Do not warn about uses of functions (see Function Attributes), variables (see Variable Attributes), and types (see Type Attributes) marked as deprecated by using the deprecated attribute. -Wno-overflow Do not warn about compile-time overflow in constant expressions. -Wno-odr Warn about One Definition Rule violations during link-time optimization. Requires -flto-odr-type-merging to be enabled. Enabled by default. -Wopenmp-simd Warn if the vectorizer cost model overrides the OpenMP simd directive set by user. The -fsimd-cost-model=unlimited option can be used to relax the cost model. -Woverride-init (C and Objective-C only) Warn if an initialized field without side effects is overridden when using designated initializers (see Designated Initializers). This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-override-init. -Woverride-init-side-effects (C and Objective-C only) Warn if an initialized field with side effects is overridden when using designated initializers (see Designated Initializers). This warning is enabled by default. -Wpacked Warn if a structure is given the packed attribute, but the packed attribute has no effect on the layout or size of the structure. Such structures may be mis-aligned for little benefit. For instance, in this code, the variable f.x in struct bar is misaligned even though struct bar does not itself have the packed attribute: struct foo { int x; char a, b, c, d; } __attribute__((packed)); struct bar { char z; struct foo f; }; -Wpacked-bitfield-compat The 4.1, 4.2 and 4.3 series of GCC ignore the packed attribute on bit-fields of type char. This has been fixed in GCC 4.4 but the change can lead to differences in the structure layout. GCC informs you when the offset of such a field has changed in GCC 4.4. For example there is no longer a 4-bit padding between field a and b in this structure: struct foo { char a:4; char b:8; } __attribute__ ((packed)); This warning is enabled by default. Use -Wno-packed-bitfield-compat to disable this warning. -Wpacked-not-aligned (C, C++, Objective-C and Objective-C++ only) Warn if a structure field with explicitly specified alignment in a packed struct or union is misaligned. For example, a warning will be issued on struct S, like, warning: alignment 1 of 'struct S' is less than 8, in this code: struct __attribute__ ((aligned (8))) S8 { char a[8]; }; struct __attribute__ ((packed)) S { struct S8 s8; }; This warning is enabled by -Wall. -Wpadded Warn if padding is included in a structure, either to align an element of the structure or to align the whole structure. Sometimes when this happens it is possible to rearrange the fields of the structure to reduce the padding and so make the structure smaller. -Wredundant-decls Warn if anything is declared more than once in the same scope, even in cases where multiple declaration is valid and changes nothing. -Wno-restrict Warn when an object referenced by a restrict-qualified parameter (or, in C++, a __restrict-qualified parameter) is aliased by another argument, or when copies between such objects overlap. For example, the call to the strcpy function below attempts to truncate the string by replacing its initial characters with the last four. However, because the call writes the terminating NUL into a[4], the copies overlap and the call is diagnosed. void foo (void) { char a[] = "abcd1234"; strcpy (a, a + 4); … } The -Wrestrict option detects some instances of simple overlap even without optimization but works best at -O2 and above. It is included in -Wall. -Wnested-externs (C and Objective-C only) Warn if an extern declaration is encountered within a function. -Wno-inherited-variadic-ctor Suppress warnings about use of C++11 inheriting constructors when the base class inherited from has a C variadic constructor; the warning is on by default because the ellipsis is not inherited. -Winline Warn if a function that is declared as inline cannot be inlined. Even with this option, the compiler does not warn about failures to inline functions declared in system headers. The compiler uses a variety of heuristics to determine whether or not to inline a function. For example, the compiler takes into account the size of the function being inlined and the amount of inlining that has already been done in the current function. Therefore, seemingly insignificant changes in the source program can cause the warnings produced by -Winline to appear or disappear. -Wno-invalid-offsetof (C++ and Objective-C++ only) Suppress warnings from applying the offsetof macro to a non-POD type. According to the 2014 ISO C++ standard, applying offsetof to a non-standard-layout type is undefined. In existing C++ implementations, however, offsetof typically gives meaningful results. This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it. The restrictions on offsetof may be relaxed in a future version of the C++ standard. -Wint-in-bool-context Warn for suspicious use of integer values where boolean values are expected, such as conditional expressions (?:) using non-boolean integer constants in boolean context, like if (a <= b ? 2 : 3). Or left shifting of signed integers in boolean context, like for (a = 0; 1 << a; a++);. Likewise for all kinds of multiplications regardless of the data type. This warning is enabled by -Wall. -Wno-int-to-pointer-cast Suppress warnings from casts to pointer type of an integer of a different size. In C++, casting to a pointer type of smaller size is an error. Wint-to-pointer-cast is enabled by default. -Wno-pointer-to-int-cast (C and Objective-C only) Suppress warnings from casts from a pointer to an integer type of a different size. -Winvalid-pch Warn if a precompiled header (see Precompiled Headers) is found in the search path but cannot be used. -Wlong-long Warn if long long type is used. This is enabled by either -Wpedantic or -Wtraditional in ISO C90 and C++98 modes. To inhibit the warning messages, use -Wno-long-long. -Wvariadic-macros Warn if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode. This is enabled by either -Wpedantic or -Wtraditional. To inhibit the warning messages, use -Wno-variadic-macros. -Wvarargs Warn upon questionable usage of the macros used to handle variable arguments like va_start. This is default. To inhibit the warning messages, use -Wno-varargs. -Wvector-operation-performance Warn if vector operation is not implemented via SIMD capabilities of the architecture. Mainly useful for the performance tuning. Vector operation can be implemented piecewise, which means that the scalar operation is performed on every vector element; in parallel, which means that the vector operation is implemented using scalars of wider type, which normally is more performance efficient; and as a single scalar, which means that vector fits into a scalar type. -Wno-virtual-move-assign Suppress warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. This is dangerous because if the virtual base is reachable along more than one path, it is moved multiple times, which can mean both objects end up in the moved-from state. If the move assignment operator is written to avoid moving from a moved-from object, this warning can be disabled. -Wvla Warn if a variable-length array is used in the code. -Wno-vla prevents the -Wpedantic warning of the variable-length array. -Wvla-larger-than=byte-size If this option is used, the compiler will warn for declarations of variable-length arrays whose size is either unbounded, or bounded by an argument that allows the array size to exceed byte-size bytes. This is similar to how -Walloca-larger-than=byte-size works, but with variable-length arrays. Note that GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning may not get triggered for such arrays. -Wvla-larger-than='PTRDIFF_MAX' is enabled by default but is typically only effective when -ftree-vrp is active (default for -O2 and above). See also -Walloca-larger-than=byte-size. -Wno-vla-larger-than Disable -Wvla-larger-than= warnings. The option is equivalent to -Wvla-larger-than='SIZE_MAX' or larger. -Wvolatile-register-var Warn if a register variable is declared volatile. The volatile modifier does not inhibit all optimizations that may eliminate reads and/or writes to register variables. This warning is enabled by -Wall. -Wdisabled-optimization Warn if a requested optimization pass is disabled. This warning does not generally indicate that there is anything wrong with your code; it merely indicates that GCC's optimizers are unable to handle the code effectively. Often, the problem is that your code is too big or too complex; GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time. -Wpointer-sign (C and Objective-C only) Warn for pointer argument passing or assignment with different signedness. This option is only supported for C and Objective-C. It is implied by -Wall and by -Wpedantic, which can be disabled with -Wno-pointer-sign. -Wstack-protector This option is only active when -fstack-protector is active. It warns about functions that are not protected against stack smashing. -Woverlength-strings Warn about string constants that are longer than the “minimum maximum” length specified in the C standard. Modern compilers generally allow string constants that are much longer than the standard's minimum limit, but very portable programs should avoid using longer strings. The limit applies after string constant concatenation, and does not count the trailing NUL. In C90, the limit was 509 characters; in C99, it was raised to 4095. C++98 does not specify a normative minimum maximum, so we do not diagnose overlength strings in C++. This option is implied by -Wpedantic, and can be disabled with -Wno-overlength-strings. -Wunsuffixed-float-constants (C and Objective-C only) Issue a warning for any floating constant that does not have a suffix. When used together with -Wsystem-headers it warns about such constants in system header files. This can be useful when preparing code to use with the FLOAT_CONST_DECIMAL64 pragma from the decimal floating-point extension to C99. -Wno-designated-init (C and Objective-C only) Suppress warnings when a positional initializer is used to initialize a structure that has been marked with the designated_init attribute. -Whsa Issue a warning when HSAIL cannot be emitted for the compiled function or OpenMP construct. ────────────────────────────────────────────────────────────────────────────── Next: Debugging Options, Previous: Diagnostic Message Formatting Options, Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Invoking GCC Link: next: Optimize Options Link: prev: Warning Options Next: Optimize Options, Previous: Warning Options, Up: Invoking GCC ────────────────────────────────────────────────────────────────────────────── 3.9 Options for Debugging Your Program To tell GCC to emit extra information for use by a debugger, in almost all cases you need only to add -g to your other options. GCC allows you to use -g with -O. The shortcuts taken by optimized code may occasionally be surprising: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values are already at hand; some statements may execute in different places because they have been moved out of loops. Nevertheless it is possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs. If you are not using some other optimization option, consider using -Og (see Optimize Options) with -g. With no -O option at all, some compiler passes that collect information useful for debugging do not run at all, so that -Og may result in a better debugging experience. -g Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information. On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but probably makes other debuggers crash or refuse to read the program. If you want to control for certain whether to generate the extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below). -ggdb Produce debugging information for use by GDB. This means to use the most expressive format available (DWARF, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible. -gdwarf -gdwarf-version Produce debugging information in DWARF format (if that is supported). The value of version may be either 2, 3, 4 or 5; the default version for most targets is 4. DWARF Version 5 is only experimental. Note that with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables. Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit. GCC no longer supports DWARF Version 1, which is substantially different than Version 2 and later. For historical reasons, some other DWARF-related options such as -fno-dwarf2-cfi-asm) retain a reference to DWARF Version 2 in their names, but apply to all currently-supported versions of DWARF. -gstabs Produce debugging information in stabs format (if that is supported), without GDB extensions. This is the format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output that is not understood by DBX. On System V Release 4 systems this option requires the GNU assembler. -gstabs+ Produce debugging information in stabs format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program. -gxcoff Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems. -gxcoff+ Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail with an error. -gvms Produce debugging information in Alpha/VMS debug format (if that is supported). This is the format used by DEBUG on Alpha/VMS systems. -glevel -ggdblevel -gstabslevel -gxcofflevel -gvmslevel Request debugging information and also use level to specify how much information. The default level is 2. Level 0 produces no debug information at all. Thus, -g0 negates -g. Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, and line number tables, but no information about local variables. Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3. If you use multiple -g options, with or without level numbers, the last such option is the one that is effective. -gdwarf does not accept a concatenated debug level, to avoid confusion with -gdwarf-level. Instead use an additional -glevel option to change the debug level for DWARF. -feliminate-unused-debug-symbols Produce debugging information in stabs format (if that is supported), for only symbols that are actually used. -femit-class-debug-always Instead of emitting debugging information for a C++ class in only one object file, emit it in all object files using the class. This option should be used only with debuggers that are unable to handle the way GCC normally emits debugging information for classes because using this option increases the size of debugging information by as much as a factor of two. -fno-merge-debug-strings Direct the linker to not merge together strings in the debugging information that are identical in different object files. Merging is not supported by all assemblers or linkers. Merging decreases the size of the debug information in the output file at the cost of increasing link processing time. Merging is enabled by default. -fdebug-prefix-map=old=new When compiling files residing in directory old, record debugging information describing them as if the files resided in directory new instead. This can be used to replace a build-time path with an install-time path in the debug info. It can also be used to change an absolute path to a relative path by using . for new. This can give more reproducible builds, which are location independent, but may require an extra command to tell GDB where to find the source files. See also -ffile-prefix-map. -fvar-tracking Run variable tracking pass. It computes where variables are stored at each position in code. Better debugging information is then generated (if the debugging information format supports this information). It is enabled by default when compiling with optimization (-Os, -O, -O2, …), debugging information (-g) and the debug info format supports it. -fvar-tracking-assignments Annotate assignments to user variables early in the compilation and attempt to carry the annotations over throughout the compilation all the way to the end, in an attempt to improve debug information while optimizing. Use of -gdwarf-4 is recommended along with it. It can be enabled even if var-tracking is disabled, in which case annotations are created and maintained, but discarded at the end. By default, this flag is enabled together with -fvar-tracking, except when selective scheduling is enabled. -gsplit-dwarf Separate as much DWARF debugging information as possible into a separate output file with the extension .dwo. This option allows the build system to avoid linking files with debug information. To be useful, this option requires a debugger capable of reading .dwo files. -gdescribe-dies Add description attributes to some DWARF DIEs that have no name attribute, such as artificial variables, external references and call site parameter DIEs. -gpubnames Generate DWARF .debug_pubnames and .debug_pubtypes sections. -ggnu-pubnames Generate .debug_pubnames and .debug_pubtypes sections in a format suitable for conversion into a GDB index. This option is only useful with a linker that can produce GDB index version 7. -fdebug-types-section When using DWARF Version 4 or higher, type DIEs can be put into their own .debug_types section instead of making them part of the .debug_info section. It is more efficient to put them in a separate comdat section since the linker can then remove duplicates. But not all DWARF consumers support .debug_types sections yet and on some objects .debug_types produces larger instead of smaller debugging information. -grecord-gcc-switches -gno-record-gcc-switches This switch causes the command-line options used to invoke the compiler that may affect code generation to be appended to the DW_AT_producer attribute in DWARF debugging information. The options are concatenated with spaces separating them from each other and from the compiler version. It is enabled by default. See also -frecord-gcc-switches for another way of storing compiler options into the object file. -gstrict-dwarf Disallow using extensions of later DWARF standard version than selected with -gdwarf-version. On most targets using non-conflicting DWARF extensions from later standard versions is allowed. -gno-strict-dwarf Allow using extensions of later DWARF standard version than selected with -gdwarf-version. -gas-loc-support Inform the compiler that the assembler supports .loc directives. It may then use them for the assembler to generate DWARF2+ line number tables. This is generally desirable, because assembler-generated line-number tables are a lot more compact than those the compiler can generate itself. This option will be enabled by default if, at GCC configure time, the assembler was found to support such directives. -gno-as-loc-support Force GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are to be generated. gas-locview-support Inform the compiler that the assembler supports view assignment and reset assertion checking in .loc directives. This option will be enabled by default if, at GCC configure time, the assembler was found to support them. gno-as-locview-support Force GCC to assign view numbers internally, if -gvariable-location-views are explicitly requested. -gcolumn-info -gno-column-info Emit location column information into DWARF debugging information, rather than just file and line. This option is enabled by default. -gstatement-frontiers -gno-statement-frontiers This option causes GCC to create markers in the internal representation at the beginning of statements, and to keep them roughly in place throughout compilation, using them to guide the output of is_stmt markers in the line number table. This is enabled by default when compiling with optimization (-Os, -O, -O2, …), and outputting DWARF 2 debug information at the normal level. -gvariable-location-views -gvariable-location-views=incompat5 -gno-variable-location-views Augment variable location lists with progressive view numbers implied from the line number table. This enables debug information consumers to inspect state at certain points of the program, even if no instructions associated with the corresponding source locations are present at that point. If the assembler lacks support for view numbers in line number tables, this will cause the compiler to emit the line number table, which generally makes them somewhat less compact. The augmented line number tables and location lists are fully backward-compatible, so they can be consumed by debug information consumers that are not aware of these augmentations, but they won't derive any benefit from them either. This is enabled by default when outputting DWARF 2 debug information at the normal level, as long as there is assembler support, -fvar-tracking-assignments is enabled and -gstrict-dwarf is not. When assembler support is not available, this may still be enabled, but it will force GCC to output internal line number tables, and if -ginternal-reset-location-views is not enabled, that will most certainly lead to silently mismatching location views. There is a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, that can be enabled with -gvariable-location-views=incompat5. This option may be removed in the future, is only provided as a reference implementation of the proposed representation. Debug information consumers are not expected to support this extended format, and they would be rendered unable to decode location lists using it. -ginternal-reset-location-views -gno-internal-reset-location-views Attempt to determine location views that can be omitted from location view lists. This requires the compiler to have very accurate insn length estimates, which isn't always the case, and it may cause incorrect view lists to be generated silently when using an assembler that does not support location view lists. The GNU assembler will flag any such error as a view number mismatch. This is only enabled on ports that define a reliable estimation function. -ginline-points -gno-inline-points Generate extended debug information for inlined functions. Location view tracking markers are inserted at inlined entry points, so that address and view numbers can be computed and output in debug information. This can be enabled independently of location views, in which case the view numbers won't be output, but it can only be enabled along with statement frontiers, and it is only enabled by default if location views are enabled. -gz[=type] Produce compressed debug sections in DWARF format, if that is supported. If type is not given, the default type depends on the capabilities of the assembler and linker used. type may be one of 'none' (don't compress debug sections), 'zlib' (use zlib compression in ELF gABI format), or 'zlib-gnu' (use zlib compression in traditional GNU format). If the linker doesn't support writing compressed debug sections, the option is rejected. Otherwise, if the assembler does not support them, -gz is silently ignored when producing object files. -femit-struct-debug-baseonly Emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the struct is defined. This option substantially reduces the size of debugging information, but at significant potential loss in type information to the debugger. See -femit-struct-debug-reduced for a less aggressive option. See -femit-struct-debug-detailed for more detailed control. This option works only with DWARF debug output. -femit-struct-debug-reduced Emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the type is defined, unless the struct is a template or defined in a system header. This option significantly reduces the size of debugging information, with some potential loss in type information to the debugger. See -femit-struct-debug-baseonly for a more aggressive option. See -femit-struct-debug-detailed for more detailed control. This option works only with DWARF debug output. -femit-struct-debug-detailed[=spec-list] Specify the struct-like types for which the compiler generates debug information. The intent is to reduce duplicate struct debug information between different object files within the same program. This option is a detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, which serves for most needs. A specification has the syntax ['dir:'|'ind:']['ord:'|'gen:']('any'|'sys'|'base'|'none') The optional first word limits the specification to structs that are used directly ('dir:') or used indirectly ('ind:'). A struct type is used directly when it is the type of a variable, member. Indirect uses arise through pointers to structs. That is, when use of an incomplete struct is valid, the use is indirect. An example is 'struct one direct; struct two * indirect;'. The optional second word limits the specification to ordinary structs ('ord:') or generic structs ('gen:'). Generic structs are a bit complicated to explain. For C++, these are non-explicit specializations of template classes, or non-template classes within the above. Other programming languages have generics, but -femit-struct-debug-detailed does not yet implement them. The third word specifies the source files for those structs for which the compiler should emit debug information. The values 'none' and 'any' have the normal meaning. The value 'base' means that the base of name of the file in which the type declaration appears must match the base of the name of the main compilation file. In practice, this means that when compiling foo.c, debug information is generated for types declared in that file and foo.h, but not other header files. The value 'sys' means those types satisfying 'base' or declared in system or compiler headers. You may need to experiment to determine the best settings for your application. The default is -femit-struct-debug-detailed=all. This option works only with DWARF debug output. -fno-dwarf2-cfi-asm Emit DWARF unwind info as compiler generated .eh_frame section instead of using GAS .cfi_* directives. -fno-eliminate-unused-debug-types Normally, when producing DWARF output, GCC avoids producing debug symbol output for types that are nowhere used in the source file being compiled. Sometimes it is useful to have GCC emit debugging information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit, for example if, in the debugger, you want to cast a value to a type that is not actually used in your program (but is declared). More often, however, this results in a significant amount of wasted space. ────────────────────────────────────────────────────────────────────────────── Next: Optimize Options, Previous: Warning Options, Up: Invoking GCC Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Invoking GCC Link: next: Instrumentation Options Link: prev: Debugging Options Next: Instrumentation Options, Previous: Debugging Options, Up: Invoking ────────────────────────────────────────────────────────────────────────────── 3.10 Options That Control Optimization These options control various sorts of optimizations. Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you expect from the source code. Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program. The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them. Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section. Most optimizations are completely disabled at -O0 or if an -O level is not set on the command line, even if individual optimization flags are specified. Similarly, -Og suppresses many optimization passes. Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each -O level than those listed here. You can invoke GCC with -Q --help=optimizers to find out the exact set of optimizations that are enabled at each level. See Overall Options, for examples. -O -O1 Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function. With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time. -O turns on the following optimization flags: -fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments -fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch -fdse -fforward-propagate -fguess-branch-probability -fif-conversion -fif-conversion2 -finline-functions-called-once -fipa-profile -fipa-pure-const -fipa-reference -fipa-reference-addressable -fmerge-constants -fmove-loop-invariants -fomit-frame-pointer -freorder-blocks -fshrink-wrap -fshrink-wrap-separate -fsplit-wide-types -fssa-backprop -fssa-phiopt -ftree-bit-ccp -ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -ftree-phiprop -ftree-pta -ftree-scev-cprop -ftree-sink -ftree-slsr -ftree-sra -ftree-ter -funit-at-a-time -O2 Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. As compared to -O, this option increases both compilation time and the performance of the generated code. -O2 turns on all optimization flags specified by -O. It also turns on the following optimization flags: -falign-functions -falign-jumps -falign-labels -falign-loops -fcaller-saves -fcode-hoisting -fcrossjumping -fcse-follow-jumps -fcse-skip-blocks -fdelete-null-pointer-checks -fdevirtualize -fdevirtualize-speculatively -fexpensive-optimizations -fgcse -fgcse-lm -fhoist-adjacent-loads -finline-small-functions -findirect-inlining -fipa-bit-cp -fipa-cp -fipa-icf -fipa-ra -fipa-sra -fipa-vrp -fisolate-erroneous-paths-dereference -flra-remat -foptimize-sibling-calls -foptimize-strlen -fpartial-inlining -fpeephole2 -freorder-blocks-algorithm=stc -freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop -fschedule-insns -fschedule-insns2 -fsched-interblock -fsched-spec -fstore-merging -fstrict-aliasing -fthread-jumps -ftree-builtin-call-dce -ftree-pre -ftree-switch-conversion -ftree-tail-merge -ftree-vrp Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos. -O3 Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the following optimization flags: -fgcse-after-reload -finline-functions -fipa-cp-clone -floop-interchange -floop-unroll-and-jam -fpeel-loops -fpredictive-commoning -fsplit-paths -ftree-loop-distribute-patterns -ftree-loop-distribution -ftree-loop-vectorize -ftree-partial-pre -ftree-slp-vectorize -funswitch-loops -fvect-cost-model -fversion-loops-for-strides -O0 Reduce compilation time and make debugging produce the expected results. This is the default. -Os Optimize for size. -Os enables all -O2 optimizations except those that often increase code size: -falign-functions -falign-jumps -falign-labels -falign-loops -fprefetch-loop-arrays -freorder-blocks-algorithm=stc It also enables -finline-functions, causes the compiler to tune for code size rather than execution speed, and performs further optimizations designed to reduce code size. -Ofast Disregard strict standards compliance. -Ofast enables all -O3 optimizations. It also enables optimizations that are not valid for all standard-compliant programs. It turns on -ffast-math and the Fortran-specific -fstack-arrays, unless -fmax-stack-var-size is specified, and -fno-protect-parens. -Og Optimize debugging experience. -Og should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience. It is a better choice than -O0 for producing debuggable code because some compiler passes that collect debug information are disabled at -O0. Like -O0, -Og completely disables a number of optimization passes so that individual options controlling them have no effect. Otherwise -Og enables all -O1 optimization flags except for those that may interfere with debugging: -fbranch-count-reg -fdelayed-branch -fif-conversion -fif-conversion2 -finline-functions-called-once -fmove-loop-invariants -fssa-phiopt -ftree-bit-ccp -ftree-pta -ftree-sra If you use multiple -O options, with or without level numbers, the last such option is the one that is effective. Options of the form -fflag specify machine-independent flags. Most flags have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed—the one you typically use. You can figure out the other form by either removing 'no-' or adding it. The following options control specific optimizations. They are either activated by -O options or are related to ones that are. You can use the following flags in the rare cases when “fine-tuning” of optimizations to be performed is desired. -fno-defer-pop For machines that must pop arguments after a function call, always pop the arguments as soon as each function returns. At levels -O1 and higher, -fdefer-pop is the default; this allows the compiler to let arguments accumulate on the stack for several function calls and pop them all at once. -fforward-propagate Perform a forward propagation pass on RTL. The pass tries to combine two instructions and checks if the result can be simplified. If loop unrolling is active, two passes are performed and the second is scheduled after loop unrolling. This option is enabled by default at optimization levels -O, -O2, -O3, -Os. -ffp-contract=style -ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables floating-point expression contraction such as forming of fused multiply-add operations if the target has native support for them. -ffp-contract=on enables floating-point expression contraction if allowed by the language standard. This is currently not implemented and treated equal to -ffp-contract=off. The default is -ffp-contract=fast. -fomit-frame-pointer Omit the frame pointer in functions that don't need one. This avoids the instructions to save, set up and restore the frame pointer; on many targets it also makes an extra register available. On some targets this flag has no effect because the standard calling sequence always uses a frame pointer, so it cannot be omitted. Note that -fno-omit-frame-pointer doesn't guarantee the frame pointer is used in all functions. Several targets always omit the frame pointer in leaf functions. Enabled by default at -O and higher. -foptimize-sibling-calls Optimize sibling and tail recursive calls. Enabled at levels -O2, -O3, -Os. -foptimize-strlen Optimize various standard C string functions (e.g. strlen, strchr or strcpy) and their _FORTIFY_SOURCE counterparts into faster alternatives. Enabled at levels -O2, -O3. -fno-inline Do not expand any functions inline apart from those marked with the always_inline attribute. This is the default when not optimizing. Single functions can be exempted from inlining by marking them with the noinline attribute. -finline-small-functions Integrate functions into their callers when their body is smaller than expected function call code (so overall size of program gets smaller). The compiler heuristically decides which functions are simple enough to be worth integrating in this way. This inlining applies to all functions, even those not declared inline. Enabled at levels -O2, -O3, -Os. -findirect-inlining Inline also indirect calls that are discovered to be known at compile time thanks to previous inlining. This option has any effect only when inlining itself is turned on by the -finline-functions or -finline-small-functions options. Enabled at levels -O2, -O3, -Os. -finline-functions Consider all functions for inlining, even if they are not declared inline. The compiler heuristically decides which functions are worth integrating in this way. If all calls to a given function are integrated, and the function is declared static, then the function is normally not output as assembler code in its own right. Enabled at levels -O3, -Os. Also enabled by -fprofile-use and -fauto-profile. -finline-functions-called-once Consider all static functions called once for inlining into their caller even if they are not marked inline. If a call to a given function is integrated, then the function is not output as assembler code in its own right. Enabled at levels -O1, -O2, -O3 and -Os, but not -Og. -fearly-inlining Inline functions marked by always_inline and functions whose body seems smaller than the function call overhead early before doing -fprofile-generate instrumentation and real inlining pass. Doing so makes profiling significantly cheaper and usually inlining faster on programs having large chains of nested wrapper functions. Enabled by default. -fipa-sra Perform interprocedural scalar replacement of aggregates, removal of unused parameters and replacement of parameters passed by reference by parameters passed by value. Enabled at levels -O2, -O3 and -Os. -finline-limit=n By default, GCC limits the size of functions that can be inlined. This flag allows coarse control of this limit. n is the size of functions that can be inlined in number of pseudo instructions. Inlining is actually controlled by a number of parameters, which may be specified individually by using --param name=value. The -finline-limit=n option sets some of these parameters as follows: max-inline-insns-single is set to n/2. max-inline-insns-auto is set to n/2. See below for a documentation of the individual parameters controlling inlining and for the defaults of these parameters. Note: there may be no value to -finline-limit that results in default behavior. Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size. In no way does it represent a count of assembly instructions and as such its exact meaning might change from one release to an another. -fno-keep-inline-dllexport This is a more fine-grained version of -fkeep-inline-functions, which applies only to functions that are declared using the dllexport attribute or declspec. See Declaring Attributes of Functions. -fkeep-inline-functions In C, emit static functions that are declared inline into the object file, even if the function has been inlined into all of its callers. This switch does not affect functions using the extern inline extension in GNU C90. In C++, emit any and all inline functions into the object file. -fkeep-static-functions Emit static functions into the object file, even if the function is never used. -fkeep-static-consts Emit variables declared static const when optimization isn't turned on, even if the variables aren't referenced. GCC enables this option by default. If you want to force the compiler to check if a variable is referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option. -fmerge-constants Attempt to merge identical constants (string constants and floating-point constants) across compilation units. This option is the default for optimized compilation if the assembler and linker support it. Use -fno-merge-constants to inhibit this behavior. Enabled at levels -O, -O2, -O3, -Os. -fmerge-all-constants Attempt to merge identical constants and identical variables. This option implies -fmerge-constants. In addition to -fmerge-constants this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating-point types. Languages like C or C++ require each variable, including multiple instances of the same variable in recursive calls, to have distinct locations, so using this option results in non-conforming behavior. -fmodulo-sched Perform swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost loops and reorders their instructions by overlapping different iterations. -fmodulo-sched-allow-regmoves Perform more aggressive SMS-based modulo scheduling with register moves allowed. By setting this flag certain anti-dependences edges are deleted, which triggers the generation of reg-moves based on the life-range analysis. This option is effective only with -fmodulo-sched enabled. -fno-branch-count-reg Disable the optimization pass that scans for opportunities to use “decrement and branch” instructions on a count register instead of instruction sequences that decrement a register, compare it against zero, and then branch based upon the result. This option is only meaningful on architectures that support such instructions, which include x86, PowerPC, IA-64 and S/390. Note that the -fno-branch-count-reg option doesn't remove the decrement and branch instructions from the generated instruction stream introduced by other optimization passes. The default is -fbranch-count-reg at -O1 and higher, except for -Og. -fno-function-cse Do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly. This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used. The default is -ffunction-cse -fno-zero-initialized-in-bss If the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS. This can save space in the resulting code. This option turns off this behavior because some programs explicitly rely on variables going to the data section—e.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that. The default is -fzero-initialized-in-bss. -fthread-jumps Perform optimizations that check to see if a jump branches to a location where another comparison subsumed by the first is found. If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false. Enabled at levels -O2, -O3, -Os. -fsplit-wide-types When using a type that occupies multiple registers, such as long long on a 32-bit system, split the registers apart and allocate them independently. This normally generates better code for those types, but may make debugging more difficult. Enabled at levels -O, -O2, -O3, -Os. -fcse-follow-jumps In common subexpression elimination (CSE), scan through jump instructions when the target of the jump is not reached by any other path. For example, when CSE encounters an if statement with an else clause, CSE follows the jump when the condition tested is false. Enabled at levels -O2, -O3, -Os. -fcse-skip-blocks This is similar to -fcse-follow-jumps, but causes CSE to follow jumps that conditionally skip over blocks. When CSE encounters a simple if statement with no else clause, -fcse-skip-blocks causes CSE to follow the jump around the body of the if. Enabled at levels -O2, -O3, -Os. -frerun-cse-after-loop Re-run common subexpression elimination after loop optimizations are performed. Enabled at levels -O2, -O3, -Os. -fgcse Perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation. Note: When compiling a program using computed gotos, a GCC extension, you may get better run-time performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line. Enabled at levels -O2, -O3, -Os. -fgcse-lm When -fgcse-lm is enabled, global common subexpression elimination attempts to move loads that are only killed by stores into themselves. This allows a loop containing a load/store sequence to be changed to a load outside the loop, and a copy/store within the loop. Enabled by default when -fgcse is enabled. -fgcse-sm When -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination. This pass attempts to move stores out of loops. When used in conjunction with -fgcse-lm, loops containing a load/store sequence can be changed to a load before the loop and a store after the loop. Not enabled at any optimization level. -fgcse-las When -fgcse-las is enabled, the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location (both partial and full redundancies). Not enabled at any optimization level. -fgcse-after-reload When -fgcse-after-reload is enabled, a redundant load elimination pass is performed after reload. The purpose of this pass is to clean up redundant spilling. Enabled by -fprofile-use and -fauto-profile. -faggressive-loop-optimizations This option tells the loop optimizer to use language constraints to derive bounds for the number of iterations of a loop. This assumes that loop code does not invoke undefined behavior by for example causing signed integer overflows or out-of-bound array accesses. The bounds for the number of iterations of a loop are used to guide loop unrolling and peeling and loop exit test optimizations. This option is enabled by default. -funconstrained-commons This option tells the compiler that variables declared in common blocks (e.g. Fortran) may later be overridden with longer trailing arrays. This prevents certain optimizations that depend on knowing the array bounds. -fcrossjumping Perform cross-jumping transformation. This transformation unifies equivalent code and saves code size. The resulting code may or may not perform better than without cross-jumping. Enabled at levels -O2, -O3, -Os. -fauto-inc-dec Combine increments or decrements of addresses with memory accesses. This pass is always skipped on architectures that do not have instructions to support this. Enabled by default at -O and higher on architectures that support this. -fdce Perform dead code elimination (DCE) on RTL. Enabled by default at -O and higher. -fdse Perform dead store elimination (DSE) on RTL. Enabled by default at -O and higher. -fif-conversion Attempt to transform conditional jumps into branch-less equivalents. This includes use of conditional moves, min, max, set flags and abs instructions, and some tricks doable by standard arithmetics. The use of conditional execution on chips where it is available is controlled by -fif-conversion2. Enabled at levels -O, -O2, -O3, -Os, but not with -Og. -fif-conversion2 Use conditional execution (where available) to transform conditional jumps into branch-less equivalents. Enabled at levels -O, -O2, -O3, -Os, but not with -Og. -fdeclone-ctor-dtor The C++ ABI requires multiple entry points for constructors and destructors: one for a base subobject, one for a complete object, and one for a virtual destructor that calls operator delete afterwards. For a hierarchy with virtual bases, the base and complete variants are clones, which means two copies of the function. With this option, the base and complete variants are changed to be thunks that call a common implementation. Enabled by -Os. -fdelete-null-pointer-checks Assume that programs cannot safely dereference null pointers, and that no code or data element resides at address zero. This option enables simple constant folding optimizations at all optimization levels. In addition, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers; these assume that a memory access to address zero always results in a trap, so that if a pointer is checked after it has already been dereferenced, it cannot be null. Note however that in some environments this assumption is not true. Use -fno-delete-null-pointer-checks to disable this optimization for programs that depend on that behavior. This option is enabled by default on most targets. On Nios II ELF, it defaults to off. On AVR, CR16, and MSP430, this option is completely disabled. Passes that use the dataflow information are enabled independently at different optimization levels. -fdevirtualize Attempt to convert calls to virtual functions to direct calls. This is done both within a procedure and interprocedurally as part of indirect inlining (-findirect-inlining) and interprocedural constant propagation (-fipa-cp). Enabled at levels -O2, -O3, -Os. -fdevirtualize-speculatively Attempt to convert calls to virtual functions to speculative direct calls. Based on the analysis of the type inheritance graph, determine for a given call the set of likely targets. If the set is small, preferably of size 1, change the call into a conditional deciding between direct and indirect calls. The speculative calls enable more optimizations, such as inlining. When they seem useless after further optimization, they are converted back into original form. -fdevirtualize-at-ltrans Stream extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode. This option enables more devirtualization but significantly increases the size of streamed data. For this reason it is disabled by default. -fexpensive-optimizations Perform a number of minor optimizations that are relatively expensive. Enabled at levels -O2, -O3, -Os. -free Attempt to remove redundant extension instructions. This is especially helpful for the x86-64 architecture, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half. Enabled for Alpha, AArch64 and x86 at levels -O2, -O3, -Os. -fno-lifetime-dse In C++ the value of an object is only affected by changes within its lifetime: when the constructor begins, the object has an indeterminate value, and any changes during the lifetime of the object are dead when the object is destroyed. Normally dead store elimination will take advantage of this; if your code relies on the value of the object storage persisting beyond the lifetime of the object, you can use this flag to disable this optimization. To preserve stores before the constructor starts (e.g. because your operator new clears the object storage) but still treat the object as dead after the destructor you, can use -flifetime-dse=1. The default behavior can be explicitly selected with -flifetime-dse=2. -flifetime-dse=0 is equivalent to -fno-lifetime-dse. -flive-range-shrinkage Attempt to decrease register pressure through register live range shrinkage. This is helpful for fast processors with small or moderate size register sets. -fira-algorithm=algorithm Use the specified coloring algorithm for the integrated register allocator. The algorithm argument can be 'priority', which specifies Chow's priority coloring, or 'CB', which specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring is not implemented for all architectures, but for those targets that do support it, it is the default because it generates better code. -fira-region=region Use specified regions for the integrated register allocator. The region argument should be one of the following: 'all' Use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set. 'mixed' Use all loops except for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed (-O, -O2, …). 'one' Use all functions as a single region. This typically results in the smallest code size, and is enabled by default for -Os or -O0. -fira-hoist-pressure Use IRA to evaluate register pressure in the code hoisting pass for decisions to hoist expressions. This option usually results in smaller code, but it can slow the compiler down. This option is enabled at level -Os for all targets. -fira-loop-pressure Use IRA to evaluate register pressure in loops for decisions to move loop invariants. This option usually results in generation of faster and smaller code on machines with large register files (>= 32 registers), but it can slow the compiler down. This option is enabled at level -O3 for some targets. -fno-ira-share-save-slots Disable sharing of stack slots used for saving call-used hard registers living through a call. Each hard register gets a separate stack slot, and as a result function stack frames are larger. -fno-ira-share-spill-slots Disable sharing of stack slots allocated for pseudo-registers. Each pseudo-register that does not get a hard register gets a separate stack slot, and as a result function stack frames are larger. -flra-remat Enable CFG-sensitive rematerialization in LRA. Instead of loading values of spilled pseudos, LRA tries to rematerialize (recalculate) values if it is profitable. Enabled at levels -O2, -O3, -Os. -fdelayed-branch If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions. Enabled at levels -O, -O2, -O3, -Os, but not at -Og. -fschedule-insns If supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to required data being unavailable. This helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating-point instruction is required. Enabled at levels -O2, -O3. -fschedule-insns2 Similar to -fschedule-insns, but requests an additional pass of instruction scheduling after register allocation has been done. This is especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle. Enabled at levels -O2, -O3, -Os. -fno-sched-interblock Disable instruction scheduling across basic blocks, which is normally enabled when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher. -fno-sched-spec Disable speculative motion of non-load instructions, which is normally enabled when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher. -fsched-pressure Enable register pressure sensitive insn scheduling before register allocation. This only makes sense when scheduling before register allocation is enabled, i.e. with -fschedule-insns or at -O2 or higher. Usage of this option can improve the generated code and decrease its size by preventing register pressure increase above the number of available hard registers and subsequent spills in register allocation. -fsched-spec-load Allow speculative motion of some load instructions. This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher. -fsched-spec-load-dangerous Allow speculative motion of more load instructions. This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher. -fsched-stalled-insns -fsched-stalled-insns=n Define how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready list during the second scheduling pass. -fno-sched-stalled-insns means that no insns are moved prematurely, -fsched-stalled-insns=0 means there is no limit on how many queued insns can be moved prematurely. -fsched-stalled-insns without a value is equivalent to -fsched-stalled-insns=1. -fsched-stalled-insns-dep -fsched-stalled-insns-dep=n Define how many insn groups (cycles) are examined for a dependency on a stalled insn that is a candidate for premature removal from the queue of stalled insns. This has an effect only during the second scheduling pass, and only if -fsched-stalled-insns is used. -fno-sched-stalled-insns-dep is equivalent to -fsched-stalled-insns-dep=0. -fsched-stalled-insns-dep without a value is equivalent to -fsched-stalled-insns-dep=1. -fsched2-use-superblocks When scheduling after register allocation, use superblock scheduling. This allows motion across basic block boundaries, resulting in faster schedules. This option is experimental, as not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm. This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher. -fsched-group-heuristic Enable the group heuristic in the scheduler. This heuristic favors the instruction that belongs to a schedule group. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. -fsched-critical-path-heuristic Enable the critical-path heuristic in the scheduler. This heuristic favors instructions on the critical path. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. -fsched-spec-insn-heuristic Enable the speculative instruction heuristic in the scheduler. This heuristic favors speculative instructions with greater dependency weakness. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. -fsched-rank-heuristic Enable the rank heuristic in the scheduler. This heuristic favors the instruction belonging to a basic block with greater size or frequency. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. -fsched-last-insn-heuristic Enable the last-instruction heuristic in the scheduler. This heuristic favors the instruction that is less dependent on the last instruction scheduled. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. -fsched-dep-count-heuristic Enable the dependent-count heuristic in the scheduler. This heuristic favors the instruction that has more instructions depending on it. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. -freschedule-modulo-scheduled-loops Modulo scheduling is performed before traditional scheduling. If a loop is modulo scheduled, later scheduling passes may change its schedule. Use this option to control that behavior. -fselective-scheduling Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass. -fselective-scheduling2 Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the second scheduler pass. -fsel-sched-pipelining Enable software pipelining of innermost loops during selective scheduling. This option has no effect unless one of -fselective-scheduling or -fselective-scheduling2 is turned on. -fsel-sched-pipelining-outer-loops When pipelining loops during selective scheduling, also pipeline outer loops. This option has no effect unless -fsel-sched-pipelining is turned on. -fsemantic-interposition Some object formats, like ELF, allow interposing of symbols by the dynamic linker. This means that for symbols exported from the DSO, the compiler cannot perform interprocedural propagation, inlining and other optimizations in anticipation that the function or variable in question may change. While this feature is useful, for example, to rewrite memory allocation functions by a debugging implementation, it is expensive in the terms of code quality. With -fno-semantic-interposition the compiler assumes that if interposition happens for functions the overwriting function will have precisely the same semantics (and side effects). Similarly if interposition happens for variables, the constructor of the variable will be the same. The flag has no effect for functions explicitly declared inline (where it is never allowed for interposition to change semantics) and for symbols explicitly declared weak. -fshrink-wrap Emit function prologues only before parts of the function that need it, rather than at the top of the function. This flag is enabled by default at -O and higher. -fshrink-wrap-separate Shrink-wrap separate parts of the prologue and epilogue separately, so that those parts are only executed when needed. This option is on by default, but has no effect unless -fshrink-wrap is also turned on and the target supports this. -fcaller-saves Enable allocation of values to registers that are clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls. Such allocation is done only when it seems to result in better code. This option is always enabled by default on certain machines, usually those which have no call-preserved registers to use instead. Enabled at levels -O2, -O3, -Os. -fcombine-stack-adjustments Tracks stack adjustments (pushes and pops) and stack memory references and then tries to find ways to combine them. Enabled by default at -O1 and higher. -fipa-ra Use caller save registers for allocation if those registers are not used by any called function. In that case it is not necessary to save and restore them around calls. This is only possible if called functions are part of same compilation unit as current function and they are compiled before it. Enabled at levels -O2, -O3, -Os, however the option is disabled if generated code will be instrumented for profiling (-p, or -pg) or if callee's register usage cannot be known exactly (this happens on targets that do not expose prologues and epilogues in RTL). -fconserve-stack Attempt to minimize stack usage. The compiler attempts to use less stack space, even if that makes the program slower. This option implies setting the large-stack-frame parameter to 100 and the large-stack-frame-growth parameter to 400. -ftree-reassoc Perform reassociation on trees. This flag is enabled by default at -O and higher. -fcode-hoisting Perform code hoisting. Code hoisting tries to move the evaluation of expressions executed on all paths to the function exit as early as possible. This is especially useful as a code size optimization, but it often helps for code speed as well. This flag is enabled by default at -O2 and higher. -ftree-pre Perform partial redundancy elimination (PRE) on trees. This flag is enabled by default at -O2 and -O3. -ftree-partial-pre Make partial redundancy elimination (PRE) more aggressive. This flag is enabled by default at -O3. -ftree-forwprop Perform forward propagation on trees. This flag is enabled by default at -O and higher. -ftree-fre Perform full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE only considers expressions that are computed on all paths leading to the redundant computation. This analysis is faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at -O and higher. -ftree-phiprop Perform hoisting of loads from conditional pointers on trees. This pass is enabled by default at -O and higher. -fhoist-adjacent-loads Speculatively hoist loads from both branches of an if-then-else if the loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction. This flag is enabled by default at -O2 and higher. -ftree-copy-prop Perform copy propagation on trees. This pass eliminates unnecessary copy operations. This flag is enabled by default at -O and higher. -fipa-pure-const Discover which functions are pure or constant. Enabled by default at -O and higher. -fipa-reference Discover which static variables do not escape the compilation unit. Enabled by default at -O and higher. -fipa-reference-addressable Discover read-only, write-only and non-addressable static variables. Enabled by default at -O and higher. -fipa-stack-alignment Reduce stack alignment on call sites if possible. Enabled by default. -fipa-pta Perform interprocedural pointer analysis and interprocedural modification and reference analysis. This option can cause excessive memory and compile-time usage on large compilation units. It is not enabled by default at any optimization level. -fipa-profile Perform interprocedural profile propagation. The functions called only from cold functions are marked as cold. Also functions executed once (such as cold, noreturn, static constructors or destructors) are identified. Cold functions and loop less parts of functions executed once are then optimized for size. Enabled by default at -O and higher. -fipa-cp Perform interprocedural constant propagation. This optimization analyzes the program to determine when values passed to functions are constants and then optimizes accordingly. This optimization can substantially increase performance if the application has constants passed to functions. This flag is enabled by default at -O2, -Os and -O3. It is also enabled by -fprofile-use and -fauto-profile. -fipa-cp-clone Perform function cloning to make interprocedural constant propagation stronger. When enabled, interprocedural constant propagation performs function cloning when externally visible function can be called with constant arguments. Because this optimization can create multiple copies of functions, it may significantly increase code size (see --param ipcp-unit-growth=value). This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile. -fipa-bit-cp When enabled, perform interprocedural bitwise constant propagation. This flag is enabled by default at -O2 and by -fprofile-use and -fauto-profile. It requires that -fipa-cp is enabled. -fipa-vrp When enabled, perform interprocedural propagation of value ranges. This flag is enabled by default at -O2. It requires that -fipa-cp is enabled. -fipa-icf Perform Identical Code Folding for functions and read-only variables. The optimization reduces code size and may disturb unwind stacks by replacing a function by equivalent one with a different name. The optimization works more effectively with link-time optimization enabled. Although the behavior is similar to the Gold Linker's ICF optimization, GCC ICF works on different levels and thus the optimizations are not same - there are equivalences that are found only by GCC and equivalences found only by Gold. This flag is enabled by default at -O2 and -Os. -flive-patching=level Control GCC's optimizations to produce output suitable for live-patching. If the compiler's optimization uses a function's body or information extracted from its body to optimize/change another function, the latter is called an impacted function of the former. If a function is patched, its impacted functions should be patched too. The impacted functions are determined by the compiler's interprocedural optimizations. For example, a caller is impacted when inlining a function into its caller, cloning a function and changing its caller to call this new clone, or extracting a function's pureness/constness information to optimize its direct or indirect callers, etc. Usually, the more IPA optimizations enabled, the larger the number of impacted functions for each function. In order to control the number of impacted functions and more easily compute the list of impacted function, IPA optimizations can be partially enabled at two different levels. The level argument should be one of the following: 'inline-clone' Only enable inlining and cloning optimizations, which includes inlining, cloning, interprocedural scalar replacement of aggregates and partial inlining. As a result, when patching a function, all its callers and its clones' callers are impacted, therefore need to be patched as well. -flive-patching=inline-clone disables the following optimization flags: -fwhole-program -fipa-pta -fipa-reference -fipa-ra -fipa-icf -fipa-icf-functions -fipa-icf-variables -fipa-bit-cp -fipa-vrp -fipa-pure-const -fipa-reference-addressable -fipa-stack-alignment 'inline-only-static' Only enable inlining of static functions. As a result, when patching a static function, all its callers are impacted and so need to be patched as well. In addition to all the flags that -flive-patching=inline-clone disables, -flive-patching=inline-only-static disables the following additional optimization flags: -fipa-cp-clone -fipa-sra -fpartial-inlining -fipa-cp When -flive-patching is specified without any value, the default value is inline-clone. This flag is disabled by default. Note that -flive-patching is not supported with link-time optimization (-flto). -fisolate-erroneous-paths-dereference Detect paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. Isolate those paths from the main control flow and turn the statement with erroneous or undefined behavior into a trap. This flag is enabled by default at -O2 and higher and depends on -fdelete-null-pointer-checks also being enabled. -fisolate-erroneous-paths-attribute Detect paths that trigger erroneous or undefined behavior due to a null value being used in a way forbidden by a returns_nonnull or nonnull attribute. Isolate those paths from the main control flow and turn the statement with erroneous or undefined behavior into a trap. This is not currently enabled, but may be enabled by -O2 in the future. -ftree-sink Perform forward store motion on trees. This flag is enabled by default at -O and higher. -ftree-bit-ccp Perform sparse conditional bit constant propagation on trees and propagate pointer alignment information. This pass only operates on local scalar variables and is enabled by default at -O1 and higher, except for -Og. It requires that -ftree-ccp is enabled. -ftree-ccp Perform sparse conditional constant propagation (CCP) on trees. This pass only operates on local scalar variables and is enabled by default at -O and higher. -fssa-backprop Propagate information about uses of a value up the definition chain in order to simplify the definitions. For example, this pass strips sign operations if the sign of a value never matters. The flag is enabled by default at -O and higher. -fssa-phiopt Perform pattern matching on SSA PHI nodes to optimize conditional code. This pass is enabled by default at -O1 and higher, except for -Og. -ftree-switch-conversion Perform conversion of simple initializations in a switch to initializations from a scalar array. This flag is enabled by default at -O2 and higher. -ftree-tail-merge Look for identical code sequences. When found, replace one with a jump to the other. This optimization is known as tail merging or cross jumping. This flag is enabled by default at -O2 and higher. The compilation time in this pass can be limited using max-tail-merge-comparisons parameter and max-tail-merge-iterations parameter. -ftree-dce Perform dead code elimination (DCE) on trees. This flag is enabled by default at -O and higher. -ftree-builtin-call-dce Perform conditional dead code elimination (DCE) for calls to built-in functions that may set errno but are otherwise free of side effects. This flag is enabled by default at -O2 and higher if -Os is not also specified. -ftree-dominator-opts Perform a variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range propagation and expression simplification) based on a dominator tree traversal. This also performs jump threading (to reduce jumps to jumps). This flag is enabled by default at -O and higher. -ftree-dse Perform dead store elimination (DSE) on trees. A dead store is a store into a memory location that is later overwritten by another store without any intervening loads. In this case the earlier store can be deleted. This flag is enabled by default at -O and higher. -ftree-ch Perform loop header copying on trees. This is beneficial since it increases effectiveness of code motion optimizations. It also saves one jump. This flag is enabled by default at -O and higher. It is not enabled for -Os, since it usually increases code size. -ftree-loop-optimize Perform loop optimizations on trees. This flag is enabled by default at -O and higher. -ftree-loop-linear -floop-strip-mine -floop-block Perform loop nest optimizations. Same as -floop-nest-optimize. To use this code transformation, GCC has to be configured with --with-isl to enable the Graphite loop transformation infrastructure. -fgraphite-identity Enable the identity transformation for graphite. For every SCoP we generate the polyhedral representation and transform it back to gimple. Using -fgraphite-identity we can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations are also performed by the code generator isl, like index splitting and dead code elimination in loops. -floop-nest-optimize Enable the isl based loop nest optimizer. This is a generic loop nest optimizer based on the Pluto optimization algorithms. It calculates a loop structure optimized for data-locality and parallelism. This option is experimental. -floop-parallelize-all Use the Graphite data dependence analysis to identify loops that can be parallelized. Parallelize all the loops that can be analyzed to not contain loop carried dependences without checking that it is profitable to parallelize the loops. -ftree-coalesce-vars While transforming the program out of the SSA representation, attempt to reduce copying by coalescing versions of different user-defined variables, instead of just compiler temporaries. This may severely limit the ability to debug an optimized program compiled with -fno-var-tracking-assignments. In the negated form, this flag prevents SSA coalescing of user variables. This option is enabled by default if optimization is enabled, and it does very little otherwise. -ftree-loop-if-convert Attempt to transform conditional jumps in the innermost loops to branch-less equivalents. The intent is to remove control-flow from the innermost loops in order to improve the ability of the vectorization pass to handle these loops. This is enabled by default if vectorization is enabled. -ftree-loop-distribution Perform loop distribution. This flag can improve cache performance on big loop bodies and allow further loop optimizations, like parallelization or vectorization, to take place. For example, the loop DO I = 1, N A(I) = B(I) + C D(I) = E(I) * F ENDDO is transformed to DO I = 1, N A(I) = B(I) + C ENDDO DO I = 1, N D(I) = E(I) * F ENDDO This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile. -ftree-loop-distribute-patterns Perform loop distribution of patterns that can be code generated with calls to a library. This flag is enabled by default at -O3, and by -fprofile-use and -fauto-profile. This pass distributes the initialization loops and generates a call to memset zero. For example, the loop DO I = 1, N A(I) = 0 B(I) = A(I) + I ENDDO is transformed to DO I = 1, N A(I) = 0 ENDDO DO I = 1, N B(I) = A(I) + I ENDDO and the initialization loop is transformed into a call to memset zero. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile. -floop-interchange Perform loop interchange outside of graphite. This flag can improve cache performance on loop nest and allow further loop optimizations, like vectorization, to take place. For example, the loop for (int i = 0; i < N; i++) for (int j = 0; j < N; j++) for (int k = 0; k < N; k++) c[i][j] = c[i][j] + a[i][k]*b[k][j]; is transformed to for (int i = 0; i < N; i++) for (int k = 0; k < N; k++) for (int j = 0; j < N; j++) c[i][j] = c[i][j] + a[i][k]*b[k][j]; This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile. -floop-unroll-and-jam Apply unroll and jam transformations on feasible loops. In a loop nest this unrolls the outer loop by some factor and fuses the resulting multiple inner loops. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile. -ftree-loop-im Perform loop invariant motion on trees. This pass moves only invariants that are hard to handle at RTL level (function calls, operations that expand to nontrivial sequences of insns). With -funswitch-loops it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion. -ftree-loop-ivcanon Create a canonical counter for number of iterations in loops for which determining number of iterations requires complicated analysis. Later optimizations then may determine the number easily. Useful especially in connection with unrolling. -ftree-scev-cprop Perform final value replacement. If a variable is modified in a loop in such a way that its value when exiting the loop can be determined using only its initial value and the number of loop iterations, replace uses of the final value by such a computation, provided it is sufficiently cheap. This reduces data dependencies and may allow further simplifications. Enabled by default at -O and higher. -fivopts Perform induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees. -ftree-parallelize-loops=n Parallelize loops, i.e., split their iteration space to run in n threads. This is only possible for loops whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g. by memory bandwidth. This option implies -pthread, and thus is only supported on targets that have support for -pthread. -ftree-pta Perform function-local points-to analysis on trees. This flag is enabled by default at -O1 and higher, except for -Og. -ftree-sra Perform scalar replacement of aggregates. This pass replaces structure references with scalars to prevent committing structures to memory too early. This flag is enabled by default at -O1 and higher, except for -Og. -fstore-merging Perform merging of narrow stores to consecutive memory addresses. This pass merges contiguous stores of immediate values narrower than a word into fewer wider stores to reduce the number of instructions. This is enabled by default at -O2 and higher as well as -Os. -ftree-ter Perform temporary expression replacement during the SSA->normal phase. Single use/single def temporaries are replaced at their use location with their defining expression. This results in non-GIMPLE code, but gives the expanders much more complex trees to work on resulting in better RTL generation. This is enabled by default at -O and higher. -ftree-slsr Perform straight-line strength reduction on trees. This recognizes related expressions involving multiplications and replaces them by less expensive calculations when possible. This is enabled by default at -O and higher. -ftree-vectorize Perform vectorization on trees. This flag enables -ftree-loop-vectorize and -ftree-slp-vectorize if not explicitly specified. -ftree-loop-vectorize Perform loop vectorization on trees. This flag is enabled by default at -O3 and by -ftree-vectorize, -fprofile-use, and -fauto-profile. -ftree-slp-vectorize Perform basic block vectorization on trees. This flag is enabled by default at -O3 and by -ftree-vectorize, -fprofile-use, and -fauto-profile. -fvect-cost-model=model Alter the cost model used for vectorization. The model argument should be one of 'unlimited', 'dynamic' or 'cheap'. With the 'unlimited' model the vectorized code-path is assumed to be profitable while with the 'dynamic' model a runtime check guards the vectorized code-path to enable it only for iteration counts that will likely execute faster than when executing the original scalar loop. The 'cheap' model disables vectorization of loops where doing so would be cost prohibitive for example due to required runtime checks for data dependence or alignment but otherwise is equal to the 'dynamic' model. The default cost model depends on other optimization flags and is either 'dynamic' or 'cheap'. -fsimd-cost-model=model Alter the cost model used for vectorization of loops marked with the OpenMP simd directive. The model argument should be one of 'unlimited', 'dynamic', 'cheap'. All values of model have the same meaning as described in -fvect-cost-model and by default a cost model defined with -fvect-cost-model is used. -ftree-vrp Perform Value Range Propagation on trees. This is similar to the constant propagation pass, but instead of values, ranges of values are propagated. This allows the optimizers to remove unnecessary range checks like array bound checks and null pointer checks. This is enabled by default at -O2 and higher. Null pointer check elimination is only done if -fdelete-null-pointer-checks is enabled. -fsplit-paths Split paths leading to loop backedges. This can improve dead code elimination and common subexpression elimination. This is enabled by default at -O3 and above. -fsplit-ivs-in-unroller Enables expression of values of induction variables in later iterations of the unrolled loop using the value in the first iteration. This breaks long dependency chains, thus improving efficiency of the scheduling passes. A combination of -fweb and CSE is often sufficient to obtain the same effect. However, that is not reliable in cases where the loop body is more complicated than a single basic block. It also does not work at all on some architectures due to restrictions in the CSE pass. This optimization is enabled by default. -fvariable-expansion-in-unroller With this option, the compiler creates multiple copies of some local variables when unrolling a loop, which can result in superior code. -fpartial-inlining Inline parts of functions. This option has any effect only when inlining itself is turned on by the -finline-functions or -finline-small-functions options. Enabled at levels -O2, -O3, -Os. -fpredictive-commoning Perform predictive commoning optimization, i.e., reusing computations (especially memory loads and stores) performed in previous iterations of loops. This option is enabled at level -O3. It is also enabled by -fprofile-use and -fauto-profile. -fprefetch-loop-arrays If supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays. This option may generate better or worse code; results are highly dependent on the structure of loops within the source code. Disabled at level -Os. -fno-printf-return-value Do not substitute constants for known return value of formatted output functions such as sprintf, snprintf, vsprintf, and vsnprintf (but not printf of fprintf). This transformation allows GCC to optimize or even eliminate branches based on the known return value of these functions called with arguments that are either constant, or whose values are known to be in a range that makes determining the exact return value possible. For example, when -fprintf-return-value is in effect, both the branch and the body of the if statement (but not the call to snprint) can be optimized away when i is a 32-bit or smaller integer because the return value is guaranteed to be at most 8. char buf[9]; if (snprintf (buf, "%08x", i) >= sizeof buf) … The -fprintf-return-value option relies on other optimizations and yields best results with -O2 and above. It works in tandem with the -Wformat-overflow and -Wformat-truncation options. The -fprintf-return-value option is enabled by default. -fno-peephole -fno-peephole2 Disable any machine-specific peephole optimizations. The difference between -fno-peephole and -fno-peephole2 is in how they are implemented in the compiler; some targets use one, some use the other, a few use both. -fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os. -fno-guess-branch-probability Do not guess branch probabilities using heuristics. GCC uses heuristics to guess branch probabilities if they are not provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are specified by __builtin_expect, then the heuristics are used to guess branch probabilities for the rest of the control flow graph, taking the __builtin_expect info into account. The interactions between the heuristics and __builtin_expect can be complex, and in some cases, it may be useful to disable the heuristics so that the effects of __builtin_expect are easier to understand. It is also possible to specify expected probability of the expression with __builtin_expect_with_probability built-in function. The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os. -freorder-blocks Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code locality. Enabled at levels -O, -O2, -O3, -Os. -freorder-blocks-algorithm=algorithm Use the specified algorithm for basic block reordering. The algorithm argument can be 'simple', which does not increase code size (except sometimes due to secondary effects like alignment), or 'stc', the “software trace cache” algorithm, which tries to put all often executed code together, minimizing the number of branches executed by making extra copies of code. The default is 'simple' at levels -O, -Os, and 'stc' at levels -O2, -O3. -freorder-blocks-and-partition In addition to reordering basic blocks in the compiled function, in order to reduce number of taken branches, partitions hot and cold basic blocks into separate sections of the assembly and .o files, to improve paging and cache locality performance. This optimization is automatically turned off in the presence of exception handling or unwind tables (on targets using setjump/longjump or target specific scheme), for linkonce sections, for functions with a user-defined section attribute and on any architecture that does not support named sections. When -fsplit-stack is used this option is not enabled by default (to avoid linker errors), but may be enabled explicitly (if using a working linker). Enabled for x86 at levels -O2, -O3, -Os. -freorder-functions Reorder functions in the object file in order to improve code locality. This is implemented by using special subsections .text.hot for most frequently executed functions and .text.unlikely for unlikely executed functions. Reordering is done by the linker so object file format must support named sections and linker must place them in a reasonable way. This option isn't effective unless you either provide profile feedback (see -fprofile-arcs for details) or manually annotate functions with hot or cold attributes (see Common Function Attributes). Enabled at levels -O2, -O3, -Os. -fstrict-aliasing Allow the compiler to assume the strictest aliasing rules applicable to the language being compiled. For C (and C++), this activates optimizations based on the type of expressions. In particular, an object of one type is assumed never to reside at the same address as an object of a different type, unless the types are almost the same. For example, an unsigned int can alias an int, but not a void* or a double. A character type may alias any other type. Pay special attention to code like this: union a_union { int i; double d; }; int f() { union a_union t; t.d = 3.0; return t.i; } The practice of reading from a different union member than the one most recently written to (called “type-punning”) is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type. So, the code above works as expected. See Structures unions enumerations and bit-fields implementation. However, this code might not: int f() { union a_union t; int* ip; t.d = 3.0; ip = &t.i; return *ip; } Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior, even if the cast uses a union type, e.g.: int f() { double d = 3.0; return ((union a_union *) &d)->i; } The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os. -falign-functions -falign-functions=n -falign-functions=n:m -falign-functions=n:m:n2 -falign-functions=n:m:n2:m2 Align the start of functions to the next power-of-two greater than n, skipping up to m-1 bytes. This ensures that at least the first m bytes of the function can be fetched by the CPU without crossing an n-byte alignment boundary. If m is not specified, it defaults to n. Examples: -falign-functions=32 aligns functions to the next 32-byte boundary, -falign-functions=24 aligns to the next 32-byte boundary only if this can be done by skipping 23 bytes or less, -falign-functions=32:7 aligns to the next 32-byte boundary only if this can be done by skipping 6 bytes or less. The second pair of n2:m2 values allows you to specify a secondary alignment: -falign-functions=64:7:32:3 aligns to the next 64-byte boundary if this can be done by skipping 6 bytes or less, otherwise aligns to the next 32-byte boundary if this can be done by skipping 2 bytes or less. If m2 is not specified, it defaults to n2. Some assemblers only support this flag when n is a power of two; in that case, it is rounded up. -fno-align-functions and -falign-functions=1 are equivalent and mean that functions are not aligned. If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536. Enabled at levels -O2, -O3. -flimit-function-alignment If this option is enabled, the compiler tries to avoid unnecessarily overaligning functions. It attempts to instruct the assembler to align by the amount specified by -falign-functions, but not to skip more bytes than the size of the function. -falign-labels -falign-labels=n -falign-labels=n:m -falign-labels=n:m:n2 -falign-labels=n:m:n2:m2 Align all branch targets to a power-of-two boundary. Parameters of this option are analogous to the -falign-functions option. -fno-align-labels and -falign-labels=1 are equivalent and mean that labels are not aligned. If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead. If n is not specified or is zero, use a machine-dependent default which is very likely to be '1', meaning no alignment. The maximum allowed n option value is 65536. Enabled at levels -O2, -O3. -falign-loops -falign-loops=n -falign-loops=n:m -falign-loops=n:m:n2 -falign-loops=n:m:n2:m2 Align loops to a power-of-two boundary. If the loops are executed many times, this makes up for any execution of the dummy padding instructions. Parameters of this option are analogous to the -falign-functions option. -fno-align-loops and -falign-loops=1 are equivalent and mean that loops are not aligned. The maximum allowed n option value is 65536. If n is not specified or is zero, use a machine-dependent default. Enabled at levels -O2, -O3. -falign-jumps -falign-jumps=n -falign-jumps=n:m -falign-jumps=n:m:n2 -falign-jumps=n:m:n2:m2 Align branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by jumping. In this case, no dummy operations need be executed. Parameters of this option are analogous to the -falign-functions option. -fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops are not aligned. If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536. Enabled at levels -O2, -O3. -funit-at-a-time This option is left for compatibility reasons. -funit-at-a-time has no effect, while -fno-unit-at-a-time implies -fno-toplevel-reorder and -fno-section-anchors. Enabled by default. -fno-toplevel-reorder Do not reorder top-level functions, variables, and asm statements. Output them in the same order that they appear in the input file. When this option is used, unreferenced static variables are not removed. This option is intended to support existing code that relies on a particular ordering. For new code, it is better to use attributes when possible. -ftoplevel-reorder is the default at -O1 and higher, and also at -O0 if -fsection-anchors is explicitly requested. Additionally -fno-toplevel-reorder implies -fno-section-anchors. -fweb Constructs webs as commonly used for register allocation purposes and assign each web individual pseudo register. This allows the register allocation pass to operate on pseudos directly, but also strengthens several other optimization passes, such as CSE, loop optimizer and trivial dead code remover. It can, however, make debugging impossible, since variables no longer stay in a “home register”. Enabled by default with -funroll-loops. -fwhole-program Assume that the current compilation unit represents the whole program being compiled. All public functions and variables with the exception of main and those merged by attribute externally_visible become static functions and in effect are optimized more aggressively by interprocedural optimizers. This option should not be used in combination with -flto. Instead relying on a linker plugin should provide safer and more precise information. -flto[=n] This option runs the standard link-time optimizer. When invoked with source code, it generates GIMPLE (one of GCC's internal representations) and writes it to special ELF sections in the object file. When the object files are linked together, all the function bodies are read from these ELF sections and instantiated as if they had been part of the same translation unit. To use the link-time optimizer, -flto and optimization options should be specified at compile time and during the final link. It is recommended that you compile all the files participating in the same link with the same options and also specify those options at link time. For example: gcc -c -O2 -flto foo.c gcc -c -O2 -flto bar.c gcc -o myprog -flto -O2 foo.o bar.o The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o. The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two files into a single internal image, and compiles the result as usual. Since both foo.o and bar.o are merged into a single image, this causes all the interprocedural analyses and optimizations in GCC to work across the two files as if they were a single one. This means, for example, that the inliner is able to inline functions in bar.o into functions in foo.o and vice-versa. Another (simpler) way to enable link-time optimization is: gcc -o myprog -flto -O2 foo.c bar.c The above generates bytecode for foo.c and bar.c, merges them together into a single GIMPLE representation and optimizes them as usual to produce myprog. The important thing to keep in mind is that to enable link-time optimizations you need to use the GCC driver to perform the link step. GCC automatically performs link-time optimization if any of the objects involved were compiled with the -flto command-line option. You can always override the automatic decision to do link-time optimization by passing -fno-lto to the link command. To make whole program optimization effective, it is necessary to make certain whole program assumptions. The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit. When supported by the linker, the linker plugin (see -fuse-linker-plugin) passes information to the compiler about used and externally visible symbols. When the linker plugin is not available, -fwhole-program should be used to allow the compiler to make these assumptions, which leads to more aggressive optimization decisions. When a file is compiled with -flto without -fuse-linker-plugin, the generated object file is larger than a regular object file because it contains GIMPLE bytecodes and the usual final code (see -ffat-lto-objects. This means that object files with LTO information can be linked as normal object files; if -fno-lto is passed to the linker, no interprocedural optimizations are applied. Note that when -fno-fat-lto-objects is enabled the compile stage is faster but you cannot perform a regular, non-LTO link on them. When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. Therefore, you can mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC automatically selects which files to optimize in LTO mode and which files to link without further processing. Generally, options specified at link time override those specified at compile time, although in some cases GCC attempts to infer link-time options from the settings used to compile the input files. If you do not specify an optimization level option -O at link time, then GCC uses the highest optimization level used when compiling the object files. Note that it is generally ineffective to specify an optimization level option only at link time and not at compile time, for two reasons. First, compiling without optimization suppresses compiler passes that gather information needed for effective optimization at link time. Second, some early optimization passes can be performed only at compile time and not at link time. There are some code generation flags preserved by GCC when generating bytecodes, as they need to be used during the final link. Currently, the following options and their settings are taken from the first object file that explicitly specifies them: -fPIC, -fpic, -fpie, -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags. Certain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value is ignored. This includes options such as -freg-struct-return and -fpcc-struct-return. Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting translation units. Specifically -fno-strict-overflow, -fwrapv and -fno-trapv take precedence; and for example -ffp-contract=off takes precedence over -ffp-contract=fast. You can override them at link time. If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The behavior is still undefined at run time. Similar diagnostics may be raised for other languages. Another feature of LTO is that it is possible to apply interprocedural optimizations on files written in different languages: gcc -c -flto foo.c g++ -c -flto bar.cc gfortran -c -flto baz.f90 g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran Notice that the final link is done with g++ to get the C++ runtime libraries and -lgfortran is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation. If object files containing GIMPLE bytecode are stored in a library archive, say libfoo.a, it is possible to extract and use them in an LTO link if you are using a linker with plugin support. To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; to show the symbols of object files with GIMPLE bytecode, use gcc-nm. Those commands require that ar, ranlib and nm have been compiled with plugin support. At link time, use the flag -fuse-linker-plugin to ensure that the library participates in the LTO optimization process: gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized. If you are not using a linker with plugin support and/or do not enable the linker plugin, then the objects inside libfoo.a are extracted and linked as usual, but they do not participate in the LTO optimization process. In order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects. Link-time optimizations do not require the presence of the whole program to operate. If the program does not require any symbols to be exported, it is possible to combine -flto and -fwhole-program to allow the interprocedural optimizers to use more aggressive assumptions which may lead to improved optimization opportunities. Use of -fwhole-program is not needed when linker plugin is active (see -fuse-linker-plugin). The current implementation of LTO makes no attempt to generate bytecode that is portable between different types of hosts. The bytecode files are versioned and there is a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC. Link-time optimization does not work well with generation of debugging information on systems other than those using a combination of ELF and DWARF. If you specify the optional n, the optimization and code generation done at link time is executed in parallel using n parallel jobs by utilizing an installed make program. The environment variable MAKE may be used to override the program used. The default value for n is 1. You can also specify -flto=jobserver to use GNU make's job server mode to determine the number of parallel jobs. This is useful when the Makefile calling GCC is already executing in parallel. You must prepend a '+' to the command recipe in the parent Makefile for this to work. This option likely only works if MAKE is GNU make. -flto-partition=alg Specify the partitioning algorithm used by the link-time optimizer. The value is either '1to1' to specify a partitioning mirroring the original source files or 'balanced' to specify partitioning into equally sized chunks (whenever possible) or 'max' to create new partition for every symbol where possible. Specifying 'none' as an algorithm disables partitioning and streaming completely. The default value is 'balanced'. While '1to1' can be used as an workaround for various code ordering issues, the 'max' partitioning is intended for internal testing only. The value 'one' specifies that exactly one partition should be used while the value 'none' bypasses partitioning and executes the link-time optimization step directly from the WPA phase. -flto-odr-type-merging Enable streaming of mangled types names of C++ types and their unification at link time. This increases size of LTO object files, but enables diagnostics about One Definition Rule violations. -flto-compression-level=n This option specifies the level of compression used for intermediate language written to LTO object files, and is only meaningful in conjunction with LTO mode (-flto). Valid values are 0 (no compression) to 9 (maximum compression). Values outside this range are clamped to either 0 or 9. If the option is not given, a default balanced compression setting is used. -fuse-linker-plugin Enables the use of a linker plugin during link-time optimization. This option relies on plugin support in the linker, which is available in gold or in GNU ld 2.21 or newer. This option enables the extraction of object files with GIMPLE bytecode out of library archives. This improves the quality of optimization by exposing more code to the link-time optimizer. This information specifies what symbols can be accessed externally (by non-LTO object or during dynamic linking). Resulting code quality improvements on binaries (and shared libraries that use hidden visibility) are similar to -fwhole-program. See -flto for a description of the effect of this flag and how to use it. This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker supporting plugins (GNU ld 2.21 or newer or gold). -ffat-lto-objects Fat LTO objects are object files that contain both the intermediate language and the object code. This makes them usable for both LTO linking and normal linking. This option is effective only when compiling with -flto and is ignored at link time. -fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete toolchain to be aware of LTO. It requires a linker with linker plugin support for basic functionality. Additionally, nm, ar and ranlib need to support linker plugins to allow a full-featured build environment (capable of building static libraries etc). GCC provides the gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the right options to these tools. With non fat LTO makefiles need to be modified to use them. Note that modern binutils provide plugin auto-load mechanism. Installing the linker plugin into $libdir/bfd-plugins has the same effect as usage of the command wrappers (gcc-ar, gcc-nm and gcc-ranlib). The default is -fno-fat-lto-objects on targets with linker plugin support. -fcompare-elim After register allocation and post-register allocation instruction splitting, identify arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic. If possible, eliminate the explicit comparison operation. This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete. Enabled at levels -O, -O2, -O3, -Os. -fcprop-registers After register allocation and post-register allocation instruction splitting, perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy. Enabled at levels -O, -O2, -O3, -Os. -fprofile-correction Profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates. When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected. This option is enabled by -fauto-profile. -fprofile-use -fprofile-use=path Enable profile feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available: -fbranch-probabilities -fprofile-values -funroll-loops -fpeel-loops -ftracer -fvpt -finline-functions -fipa-cp -fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops -funswitch-loops -fgcse-after-reload -ftree-loop-vectorize -ftree-slp-vectorize -fvect-cost-model=dynamic -ftree-loop-distribute-patterns -fprofile-reorder-functions Before you can use this option, you must first generate profiling information. See Instrumentation Options, for information about the -fprofile-generate option. By default, GCC emits an error message if the feedback profiles do not match the source code. This error can be turned into a warning by using -Wno-error=coverage-mismatch. Note this may result in poorly optimized code. Additionally, by default, GCC also emits a warning message if the feedback profiles do not exist (see -Wmissing-profile). If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir. -fauto-profile -fauto-profile=path Enable sampling-based feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available: -fbranch-probabilities -fprofile-values -funroll-loops -fpeel-loops -ftracer -fvpt -finline-functions -fipa-cp -fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops -funswitch-loops -fgcse-after-reload -ftree-loop-vectorize -ftree-slp-vectorize -fvect-cost-model=dynamic -ftree-loop-distribute-patterns -fprofile-correction path is the name of a file containing AutoFDO profile information. If omitted, it defaults to fbdata.afdo in the current directory. Producing an AutoFDO profile data file requires running your program with the perf utility on a supported GNU/Linux target system. For more information, see https://perf.wiki.kernel.org/. E.g. perf record -e br_inst_retired:near_taken -b -o perf.data \ -- your_program Then use the create_gcov tool to convert the raw profile data to a format that can be used by GCC. You must also supply the unstripped binary for your program to this tool. See https://github.com/google/autofdo. E.g. create_gcov --binary=your_program.unstripped --profile=perf.data \ --gcov=profile.afdo The following options control compiler behavior regarding floating-point arithmetic. These options trade off between speed and correctness. All must be specifically enabled. -ffloat-store Do not store floating-point variables in registers, and inhibit other options that might change whether a floating-point value is taken from a register or memory. This option prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a double is supposed to have. Similarly for the x86 architecture. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use -ffloat-store for such programs, after modifying them to store all pertinent intermediate computations into variables. -fexcess-precision=style This option allows further control over excess precision on machines where floating-point operations occur in a format with more precision or range than the IEEE standard and interchange floating-point types. By default, -fexcess-precision=fast is in effect; this means that operations may be carried out in a wider precision than the types specified in the source if that would result in faster code, and it is unpredictable when rounding to the types specified in the source code takes place. When compiling C, if -fexcess-precision=standard is specified then excess precision follows the rules specified in ISO C99; in particular, both casts and assignments cause values to be rounded to their semantic types (whereas -ffloat-store only affects assignments). This option is enabled by default for C if a strict conformance option such as -std=c99 is used. -ffast-math enables -fexcess-precision=fast by default regardless of whether a strict conformance option is used. -fexcess-precision=standard is not implemented for languages other than C. On the x86, it has no effect if -mfpmath=sse or -mfpmath=sse+387 is specified; in the former case, IEEE semantics apply without excess precision, and in the latter, rounding is unpredictable. -ffast-math Sets the options -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans, -fcx-limited-range and -fexcess-precision=fast. This option causes the preprocessor macro __FAST_MATH__ to be defined. This option is not turned on by any -O option besides -Ofast since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications. -fno-math-errno Do not set errno after calling math functions that are executed with a single instruction, e.g., sqrt. A program that relies on IEEE exceptions for math error handling may want to use this flag for speed while maintaining IEEE arithmetic compatibility. This option is not turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications. The default is -fmath-errno. On Darwin systems, the math library never sets errno. There is therefore no reason for the compiler to consider the possibility that it might, and -fno-math-errno is the default. -funsafe-math-optimizations Allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and (b) may violate IEEE or ANSI standards. When used at link time, it may include libraries or startup files that change the default FPU control word or other similar optimizations. This option is not turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications. Enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math. The default is -fno-unsafe-math-optimizations. -fassociative-math Allow re-association of operands in series of floating-point operations. This violates the ISO C and C++ language standard by possibly changing computation result. NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow (and thus cannot be used on code that relies on rounding behavior like (x + 2**52) - 2**52. May also reorder floating-point comparisons and thus may not be used when ordered comparisons are required. This option requires that both -fno-signed-zeros and -fno-trapping-math be in effect. Moreover, it doesn't make much sense with -frounding-math. For Fortran the option is automatically enabled when both -fno-signed-zeros and -fno-trapping-math are in effect. The default is -fno-associative-math. -freciprocal-math Allow the reciprocal of a value to be used instead of dividing by the value if this enables optimizations. For example x / y can be replaced with x * (1/y), which is useful if (1/y) is subject to common subexpression elimination. Note that this loses precision and increases the number of flops operating on the value. The default is -fno-reciprocal-math. -ffinite-math-only Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs. This option is not turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications. The default is -fno-finite-math-only. -fno-signed-zeros Allow optimizations for floating-point arithmetic that ignore the signedness of zero. IEEE arithmetic specifies the behavior of distinct +0.0 and -0.0 values, which then prohibits simplification of expressions such as x+0.0 or 0.0*x (even with -ffinite-math-only). This option implies that the sign of a zero result isn't significant. The default is -fsigned-zeros. -fno-trapping-math Compile code assuming that floating-point operations cannot generate user-visible traps. These traps include division by zero, overflow, underflow, inexact result and invalid operation. This option requires that -fno-signaling-nans be in effect. Setting this option may allow faster code if one relies on “non-stop” IEEE arithmetic, for example. This option should never be turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. The default is -ftrapping-math. -frounding-math Disable transformations and optimizations that assume default floating-point rounding behavior. This is round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic truncations. This option should be specified for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode. This option disables constant folding of floating-point expressions at compile time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes. The default is -fno-rounding-math. This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode. Future versions of GCC may provide finer control of this setting using C99's FENV_ACCESS pragma. This command-line option will be used to specify the default state for FENV_ACCESS. -fsignaling-nans Compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations. Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs. This option implies -ftrapping-math. This option causes the preprocessor macro __SUPPORT_SNAN__ to be defined. The default is -fno-signaling-nans. This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior. -fno-fp-int-builtin-inexact Do not allow the built-in functions ceil, floor, round and trunc, and their float and long double variants, to generate code that raises the “inexact” floating-point exception for noninteger arguments. ISO C99 and C11 allow these functions to raise the “inexact” exception, but ISO/IEC TS 18661-1:2014, the C bindings to IEEE 754-2008, does not allow these functions to do so. The default is -ffp-int-builtin-inexact, allowing the exception to be raised. This option does nothing unless -ftrapping-math is in effect. Even if -fno-fp-int-builtin-inexact is used, if the functions generate a call to a library function then the “inexact” exception may be raised if the library implementation does not follow TS 18661. -fsingle-precision-constant Treat floating-point constants as single precision instead of implicitly converting them to double-precision constants. -fcx-limited-range When enabled, this option states that a range reduction step is not needed when performing complex division. Also, there is no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case. The default is -fno-cx-limited-range, but is enabled by -ffast-math. This option controls the default setting of the ISO C99 CX_LIMITED_RANGE pragma. Nevertheless, the option applies to all languages. -fcx-fortran-rules Complex multiplication and division follow Fortran rules. Range reduction is done as part of complex division, but there is no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case. The default is -fno-cx-fortran-rules. The following options control optimizations that may improve performance, but are not enabled by any -O options. This section includes experimental options that may produce broken code. -fbranch-probabilities After running a program compiled with -fprofile-arcs (see Instrumentation Options), you can compile it a second time using -fbranch-probabilities, to improve optimizations based on the number of times each branch was taken. When a program compiled with -fprofile-arcs exits, it saves arc execution counts to a file called sourcename.gcda for each source file. The information in this data file is very dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations. With -fbranch-probabilities, GCC puts a 'REG_BR_PROB' note on each 'JUMP_INSN' and 'CALL_INSN'. These can be used to improve optimization. Currently, they are only used in one place: in reorg.c, instead of guessing which path a branch is most likely to take, the 'REG_BR_PROB' values are used to exactly determine which path is taken more often. Enabled by -fprofile-use and -fauto-profile. -fprofile-values If combined with -fprofile-arcs, it adds code so that some data about values of expressions in the program is gathered. With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions for usage in optimizations. Enabled by -fprofile-generate, -fprofile-use, and -fauto-profile. -fprofile-reorder-functions Function reordering based on profile instrumentation collects first time of execution of a function and orders these functions in ascending order. Enabled with -fprofile-use. -fvpt If combined with -fprofile-arcs, this option instructs the compiler to add code to gather information about values of expressions. With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based on them. Currently the optimizations include specialization of division operations using the knowledge about the value of the denominator. Enabled with -fprofile-use and -fauto-profile. -frename-registers Attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation. This optimization most benefits processors with lots of registers. Depending on the debug information format adopted by the target, however, it can make debugging impossible, since variables no longer stay in a “home register”. Enabled by default with -funroll-loops. -fschedule-fusion Performs a target dependent pass over the instruction stream to schedule instructions of same type together because target machine can execute them more efficiently if they are adjacent to each other in the instruction flow. Enabled at levels -O2, -O3, -Os. -ftracer Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing other optimizations to do a better job. Enabled by -fprofile-use and -fauto-profile. -funroll-loops Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. -funroll-loops implies -frerun-cse-after-loop, -fweb and -frename-registers. It also turns on complete loop peeling (i.e. complete removal of loops with a small constant number of iterations). This option makes code larger, and may or may not make it run faster. Enabled by -fprofile-use and -fauto-profile. -funroll-all-loops Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs run more slowly. -funroll-all-loops implies the same options as -funroll-loops. -fpeel-loops Peels loops for which there is enough information that they do not roll much (from profile feedback or static analysis). It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations). Enabled by -O3, -fprofile-use, and -fauto-profile. -fmove-loop-invariants Enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1 and higher, except for -Og. -fsplit-loops Split a loop into two if it contains a condition that's always true for one side of the iteration space and false for the other. Enabled by -fprofile-use and -fauto-profile. -funswitch-loops Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to result of the condition). Enabled by -fprofile-use and -fauto-profile. -fversion-loops-for-strides If a loop iterates over an array with a variable stride, create another version of the loop that assumes the stride is always one. For example: for (int i = 0; i < n; ++i) x[i * stride] = …; becomes: if (stride == 1) for (int i = 0; i < n; ++i) x[i] = …; else for (int i = 0; i < n; ++i) x[i * stride] = …; This is particularly useful for assumed-shape arrays in Fortran where (for example) it allows better vectorization assuming contiguous accesses. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile. -ffunction-sections -fdata-sections Place each function or data item into its own section in the output file if the target supports arbitrary sections. The name of the function or the name of the data item determines the section's name in the output file. Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format have linkers with such optimizations. On AIX, the linker rearranges sections (CSECTs) based on the call graph. The performance impact varies. Together with a linker garbage collection (linker --gc-sections option) these options may lead to smaller statically-linked executables (after stripping). On ELF/DWARF systems these options do not degenerate the quality of the debug information. There could be issues with other object files/debug info formats. Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker create larger object and executable files and are also slower. These options affect code generation. They prevent optimizations by the compiler and assembler using relative locations inside a translation unit since the locations are unknown until link time. An example of such an optimization is relaxing calls to short call instructions. -fbranch-target-load-optimize Perform branch target register load optimization before prologue / epilogue threading. The use of target registers can typically be exposed only during reload, thus hoisting loads out of loops and doing inter-block scheduling needs a separate optimization pass. -fbranch-target-load-optimize2 Perform branch target register load optimization after prologue / epilogue threading. -fbtr-bb-exclusive When performing branch target register load optimization, don't reuse branch target registers within any basic block. -fstdarg-opt Optimize the prologue of variadic argument functions with respect to usage of those arguments. -fsection-anchors Try to reduce the number of symbolic address calculations by using shared “anchor” symbols to address nearby objects. This transformation can help to reduce the number of GOT entries and GOT accesses on some targets. For example, the implementation of the following function foo: static int a, b, c; int foo (void) { return a + b + c; } usually calculates the addresses of all three variables, but if you compile it with -fsection-anchors, it accesses the variables from a common anchor point instead. The effect is similar to the following pseudocode (which isn't valid C): int foo (void) { register int *xr = &x; return xr[&a - &x] + xr[&b - &x] + xr[&c - &x]; } Not all targets support this option. --param name=value In some places, GCC uses various constants to control the amount of optimization that is done. For example, GCC does not inline functions that contain more than a certain number of instructions. You can control some of these constants on the command line using the --param option. The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases. In order to get minimal, maximal and default value of a parameter, one can use --help=param -Q options. In each case, the value is an integer. The allowable choices for name are: predictable-branch-outcome When branch is predicted to be taken with probability lower than this threshold (in percent), then it is considered well predictable. max-rtl-if-conversion-insns RTL if-conversion tries to remove conditional branches around a block and replace them with conditionally executed instructions. This parameter gives the maximum number of instructions in a block which should be considered for if-conversion. The compiler will also use other heuristics to decide whether if-conversion is likely to be profitable. max-rtl-if-conversion-predictable-cost max-rtl-if-conversion-unpredictable-cost RTL if-conversion will try to remove conditional branches around a block and replace them with conditionally executed instructions. These parameters give the maximum permissible cost for the sequence that would be generated by if-conversion depending on whether the branch is statically determined to be predictable or not. The units for this parameter are the same as those for the GCC internal seq_cost metric. The compiler will try to provide a reasonable default for this parameter using the BRANCH_COST target macro. max-crossjump-edges The maximum number of incoming edges to consider for cross-jumping. The algorithm used by -fcrossjumping is O(N^2) in the number of edges incoming to each block. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in executable size. min-crossjump-insns The minimum number of instructions that must be matched at the end of two blocks before cross-jumping is performed on them. This value is ignored in the case where all instructions in the block being cross-jumped from are matched. max-grow-copy-bb-insns The maximum code size expansion factor when copying basic blocks instead of jumping. The expansion is relative to a jump instruction. max-goto-duplication-insns The maximum number of instructions to duplicate to a block that jumps to a computed goto. To avoid O(N^2) behavior in a number of passes, GCC factors computed gotos early in the compilation process, and unfactors them as late as possible. Only computed jumps at the end of a basic blocks with no more than max-goto-duplication-insns are unfactored. max-delay-slot-insn-search The maximum number of instructions to consider when looking for an instruction to fill a delay slot. If more than this arbitrary number of instructions are searched, the time savings from filling the delay slot are minimal, so stop searching. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in execution time. max-delay-slot-live-search When trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compilation time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph. max-gcse-memory The approximate maximum amount of memory that can be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization is not done. max-gcse-insertion-ratio If the ratio of expression insertions to deletions is larger than this value for any expression, then RTL PRE inserts or removes the expression and thus leaves partially redundant computations in the instruction stream. max-pending-list-length The maximum number of pending dependencies scheduling allows before flushing the current state and starting over. Large functions with few branches or calls can create excessively large lists which needlessly consume memory and resources. max-modulo-backtrack-attempts The maximum number of backtrack attempts the scheduler should make when modulo scheduling a loop. Larger values can exponentially increase compilation time. max-inline-insns-single Several parameters control the tree inliner used in GCC. This number sets the maximum number of instructions (counted in GCC's internal representation) in a single function that the tree inliner considers for inlining. This only affects functions declared inline and methods implemented in a class declaration (C++). max-inline-insns-auto When you use -finline-functions (included in -O3), a lot of functions that would otherwise not be considered for inlining by the compiler are investigated. To those functions, a different (more restrictive) limit compared to functions declared inline can be applied. max-inline-insns-small This is bound applied to calls which are considered relevant with -finline-small-functions. max-inline-insns-size This is bound applied to calls which are optimized for size. Small growth may be desirable to anticipate optimization oppurtunities exposed by inlining. uninlined-function-insns Number of instructions accounted by inliner for function overhead such as function prologue and epilogue. uninlined-function-time Extra time accounted by inliner for function overhead such as time needed to execute function prologue and epilogue uninlined-thunk-insns uninlined-thunk-time Same as --param uninlined-function-insns and --param uninlined-function-time but applied to function thunks inline-min-speedup When estimated performance improvement of caller + callee runtime exceeds this threshold (in percent), the function can be inlined regardless of the limit on --param max-inline-insns-single and --param max-inline-insns-auto. large-function-insns The limit specifying really large functions. For functions larger than this limit after inlining, inlining is constrained by --param large-function-growth. This parameter is useful primarily to avoid extreme compilation time caused by non-linear algorithms used by the back end. large-function-growth Specifies maximal growth of large function caused by inlining in percents. For example, parameter value 100 limits large function growth to 2.0 times the original size. large-unit-insns The limit specifying large translation unit. Growth caused by inlining of units larger than this limit is limited by --param inline-unit-growth. For small units this might be too tight. For example, consider a unit consisting of function A that is inline and B that just calls A three times. If B is small relative to A, the growth of unit is 300\% and yet such inlining is very sane. For very large units consisting of small inlineable functions, however, the overall unit growth limit is needed to avoid exponential explosion of code size. Thus for smaller units, the size is increased to --param large-unit-insns before applying --param inline-unit-growth. inline-unit-growth Specifies maximal overall growth of the compilation unit caused by inlining. For example, parameter value 20 limits unit growth to 1.2 times the original size. Cold functions (either marked cold via an attribute or by profile feedback) are not accounted into the unit size. ipcp-unit-growth Specifies maximal overall growth of the compilation unit caused by interprocedural constant propagation. For example, parameter value 10 limits unit growth to 1.1 times the original size. large-stack-frame The limit specifying large stack frames. While inlining the algorithm is trying to not grow past this limit too much. large-stack-frame-growth Specifies maximal growth of large stack frames caused by inlining in percents. For example, parameter value 1000 limits large stack frame growth to 11 times the original size. max-inline-insns-recursive max-inline-insns-recursive-auto Specifies the maximum number of instructions an out-of-line copy of a self-recursive inline function can grow into by performing recursive inlining. --param max-inline-insns-recursive applies to functions declared inline. For functions not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled; --param max-inline-insns-recursive-auto applies instead. max-inline-recursive-depth max-inline-recursive-depth-auto Specifies the maximum recursion depth used for recursive inlining. --param max-inline-recursive-depth applies to functions declared inline. For functions not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled; --param max-inline-recursive-depth-auto applies instead. min-inline-recursive-probability Recursive inlining is profitable only for function having deep recursion in average and can hurt for function having little recursion depth by increasing the prologue size or complexity of function body to other optimizers. When profile feedback is available (see -fprofile-generate) the actual recursion depth can be guessed from the probability that function recurses via a given call expression. This parameter limits inlining only to call expressions whose probability exceeds the given threshold (in percents). early-inlining-insns Specify growth that the early inliner can make. In effect it increases the amount of inlining for code having a large abstraction penalty. max-early-inliner-iterations Limit of iterations of the early inliner. This basically bounds the number of nested indirect calls the early inliner can resolve. Deeper chains are still handled by late inlining. comdat-sharing-probability Probability (in percent) that C++ inline function with comdat visibility are shared across multiple compilation units. profile-func-internal-id A parameter to control whether to use function internal id in profile database lookup. If the value is 0, the compiler uses an id that is based on function assembler name and filename, which makes old profile data more tolerant to source changes such as function reordering etc. min-vect-loop-bound The minimum number of iterations under which loops are not vectorized when -ftree-vectorize is used. The number of iterations after vectorization needs to be greater than the value specified by this option to allow vectorization. gcse-cost-distance-ratio Scaling factor in calculation of maximum distance an expression can be moved by GCSE optimizations. This is currently supported only in the code hoisting pass. The bigger the ratio, the more aggressive code hoisting is with simple expressions, i.e., the expressions that have cost less than gcse-unrestricted-cost. Specifying 0 disables hoisting of simple expressions. gcse-unrestricted-cost Cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations do not constrain the distance an expression can travel. This is currently supported only in the code hoisting pass. The lesser the cost, the more aggressive code hoisting is. Specifying 0 allows all expressions to travel unrestricted distances. max-hoist-depth The depth of search in the dominator tree for expressions to hoist. This is used to avoid quadratic behavior in hoisting algorithm. The value of 0 does not limit on the search, but may slow down compilation of huge functions. max-tail-merge-comparisons The maximum amount of similar bbs to compare a bb with. This is used to avoid quadratic behavior in tree tail merging. max-tail-merge-iterations The maximum amount of iterations of the pass over the function. This is used to limit compilation time in tree tail merging. store-merging-allow-unaligned Allow the store merging pass to introduce unaligned stores if it is legal to do so. max-stores-to-merge The maximum number of stores to attempt to merge into wider stores in the store merging pass. max-unrolled-insns The maximum number of instructions that a loop may have to be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled. max-average-unrolled-insns The maximum number of instructions biased by probabilities of their execution that a loop may have to be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled. max-unroll-times The maximum number of unrollings of a single loop. max-peeled-insns The maximum number of instructions that a loop may have to be peeled. If a loop is peeled, this parameter also determines how many times the loop code is peeled. max-peel-times The maximum number of peelings of a single loop. max-peel-branches The maximum number of branches on the hot path through the peeled sequence. max-completely-peeled-insns The maximum number of insns of a completely peeled loop. max-completely-peel-times The maximum number of iterations of a loop to be suitable for complete peeling. max-completely-peel-loop-nest-depth The maximum depth of a loop nest suitable for complete peeling. max-unswitch-insns The maximum number of insns of an unswitched loop. max-unswitch-level The maximum number of branches unswitched in a single loop. lim-expensive The minimum cost of an expensive expression in the loop invariant motion. iv-consider-all-candidates-bound Bound on number of candidates for induction variables, below which all candidates are considered for each use in induction variable optimizations. If there are more candidates than this, only the most relevant ones are considered to avoid quadratic time complexity. iv-max-considered-uses The induction variable optimizations give up on loops that contain more induction variable uses. iv-always-prune-cand-set-bound If the number of candidates in the set is smaller than this value, always try to remove unnecessary ivs from the set when adding a new one. avg-loop-niter Average number of iterations of a loop. dse-max-object-size Maximum size (in bytes) of objects tracked bytewise by dead store elimination. Larger values may result in larger compilation times. dse-max-alias-queries-per-store Maximum number of queries into the alias oracle per store. Larger values result in larger compilation times and may result in more removed dead stores. scev-max-expr-size Bound on size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer. scev-max-expr-complexity Bound on the complexity of the expressions in the scalar evolutions analyzer. Complex expressions slow the analyzer. max-tree-if-conversion-phi-args Maximum number of arguments in a PHI supported by TREE if conversion unless the loop is marked with simd pragma. vect-max-version-for-alignment-checks The maximum number of run-time checks that can be performed when doing loop versioning for alignment in the vectorizer. vect-max-version-for-alias-checks The maximum number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer. vect-max-peeling-for-alignment The maximum number of loop peels to enhance access alignment for vectorizer. Value -1 means no limit. max-iterations-to-track The maximum number of iterations of a loop the brute-force algorithm for analysis of the number of iterations of the loop tries to evaluate. hot-bb-count-ws-permille A basic block profile count is considered hot if it contributes to the given permillage (i.e. 0...1000) of the entire profiled execution. hot-bb-frequency-fraction Select fraction of the entry block frequency of executions of basic block in function given basic block needs to have to be considered hot. max-predicted-iterations The maximum number of loop iterations we predict statically. This is useful in cases where a function contains a single loop with known bound and another loop with unknown bound. The known number of iterations is predicted correctly, while the unknown number of iterations average to roughly 10. This means that the loop without bounds appears artificially cold relative to the other one. builtin-expect-probability Control the probability of the expression having the specified value. This parameter takes a percentage (i.e. 0 ... 100) as input. builtin-string-cmp-inline-length The maximum length of a constant string for a builtin string cmp call eligible for inlining. align-threshold Select fraction of the maximal frequency of executions of a basic block in a function to align the basic block. align-loop-iterations A loop expected to iterate at least the selected number of iterations is aligned. tracer-dynamic-coverage tracer-dynamic-coverage-feedback This value is used to limit superblock formation once the given percentage of executed instructions is covered. This limits unnecessary code size expansion. The tracer-dynamic-coverage-feedback parameter is used only when profile feedback is available. The real profiles (as opposed to statically estimated ones) are much less balanced allowing the threshold to be larger value. tracer-max-code-growth Stop tail duplication once code growth has reached given percentage. This is a rather artificial limit, as most of the duplicates are eliminated later in cross jumping, so it may be set to much higher values than is the desired code growth. tracer-min-branch-ratio Stop reverse growth when the reverse probability of best edge is less than this threshold (in percent). tracer-min-branch-probability tracer-min-branch-probability-feedback Stop forward growth if the best edge has probability lower than this threshold. Similarly to tracer-dynamic-coverage two parameters are provided. tracer-min-branch-probability-feedback is used for compilation with profile feedback and tracer-min-branch-probability compilation without. The value for compilation with profile feedback needs to be more conservative (higher) in order to make tracer effective. stack-clash-protection-guard-size Specify the size of the operating system provided stack guard as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks. stack-clash-protection-probe-interval Stack clash protection involves probing stack space as it is allocated. This param controls the maximum distance between probes into the stack as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks. max-cse-path-length The maximum number of basic blocks on path that CSE considers. max-cse-insns The maximum number of instructions CSE processes before flushing. ggc-min-expand GCC uses a garbage collector to manage its own memory allocation. This parameter specifies the minimum percentage by which the garbage collector's heap should be allowed to expand between collections. Tuning this may improve compilation speed; it has no effect on code generation. The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If getrlimit is available, the notion of “RAM” is the smallest of actual RAM and RLIMIT_DATA or RLIMIT_AS. If GCC is not able to calculate RAM on a particular platform, the lower bound of 30% is used. Setting this parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity. This is extremely slow, but can be useful for debugging. ggc-min-heapsize Minimum size of the garbage collector's heap before it begins bothering to collect garbage. The first collection occurs after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. Again, tuning this may improve compilation speed, and has no effect on code generation. The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). If GCC is not able to calculate RAM on a particular platform, the lower bound is used. Setting this parameter very large effectively disables garbage collection. Setting this parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity. max-reload-search-insns The maximum number of instruction reload should look backward for equivalent register. Increasing values mean more aggressive optimization, making the compilation time increase with probably slightly better performance. max-cselib-memory-locations The maximum number of memory locations cselib should take into account. Increasing values mean more aggressive optimization, making the compilation time increase with probably slightly better performance. max-sched-ready-insns The maximum number of instructions ready to be issued the scheduler should consider at any given time during the first scheduling pass. Increasing values mean more thorough searches, making the compilation time increase with probably little benefit. max-sched-region-blocks The maximum number of blocks in a region to be considered for interblock scheduling. max-pipeline-region-blocks The maximum number of blocks in a region to be considered for pipelining in the selective scheduler. max-sched-region-insns The maximum number of insns in a region to be considered for interblock scheduling. max-pipeline-region-insns The maximum number of insns in a region to be considered for pipelining in the selective scheduler. min-spec-prob The minimum probability (in percents) of reaching a source block for interblock speculative scheduling. max-sched-extend-regions-iters The maximum number of iterations through CFG to extend regions. A value of 0 disables region extensions. max-sched-insn-conflict-delay The maximum conflict delay for an insn to be considered for speculative motion. sched-spec-prob-cutoff The minimal probability of speculation success (in percents), so that speculative insns are scheduled. sched-state-edge-prob-cutoff The minimum probability an edge must have for the scheduler to save its state across it. sched-mem-true-dep-cost Minimal distance (in CPU cycles) between store and load targeting same memory locations. selsched-max-lookahead The maximum size of the lookahead window of selective scheduling. It is a depth of search for available instructions. selsched-max-sched-times The maximum number of times that an instruction is scheduled during selective scheduling. This is the limit on the number of iterations through which the instruction may be pipelined. selsched-insns-to-rename The maximum number of best instructions in the ready list that are considered for renaming in the selective scheduler. sms-min-sc The minimum value of stage count that swing modulo scheduler generates. max-last-value-rtl The maximum size measured as number of RTLs that can be recorded in an expression in combiner for a pseudo register as last known value of that register. max-combine-insns The maximum number of instructions the RTL combiner tries to combine. integer-share-limit Small integer constants can use a shared data structure, reducing the compiler's memory usage and increasing its speed. This sets the maximum value of a shared integer constant. ssp-buffer-size The minimum size of buffers (i.e. arrays) that receive stack smashing protection when -fstack-protection is used. min-size-for-stack-sharing The minimum size of variables taking part in stack slot sharing when not optimizing. max-jump-thread-duplication-stmts Maximum number of statements allowed in a block that needs to be duplicated when threading jumps. max-fields-for-field-sensitive Maximum number of fields in a structure treated in a field sensitive manner during pointer analysis. prefetch-latency Estimate on average number of instructions that are executed before prefetch finishes. The distance prefetched ahead is proportional to this constant. Increasing this number may also lead to less streams being prefetched (see simultaneous-prefetches). simultaneous-prefetches Maximum number of prefetches that can run at the same time. l1-cache-line-size The size of cache line in L1 data cache, in bytes. l1-cache-size The size of L1 data cache, in kilobytes. l2-cache-size The size of L2 data cache, in kilobytes. prefetch-dynamic-strides Whether the loop array prefetch pass should issue software prefetch hints for strides that are non-constant. In some cases this may be beneficial, though the fact the stride is non-constant may make it hard to predict when there is clear benefit to issuing these hints. Set to 1 if the prefetch hints should be issued for non-constant strides. Set to 0 if prefetch hints should be issued only for strides that are known to be constant and below prefetch-minimum-stride. prefetch-minimum-stride Minimum constant stride, in bytes, to start using prefetch hints for. If the stride is less than this threshold, prefetch hints will not be issued. This setting is useful for processors that have hardware prefetchers, in which case there may be conflicts between the hardware prefetchers and the software prefetchers. If the hardware prefetchers have a maximum stride they can handle, it should be used here to improve the use of software prefetchers. A value of -1 means we don't have a threshold and therefore prefetch hints can be issued for any constant stride. This setting is only useful for strides that are known and constant. loop-interchange-max-num-stmts The maximum number of stmts in a loop to be interchanged. loop-interchange-stride-ratio The minimum ratio between stride of two loops for interchange to be profitable. min-insn-to-prefetch-ratio The minimum ratio between the number of instructions and the number of prefetches to enable prefetching in a loop. prefetch-min-insn-to-mem-ratio The minimum ratio between the number of instructions and the number of memory references to enable prefetching in a loop. use-canonical-types Whether the compiler should use the “canonical” type system. Should always be 1, which uses a more efficient internal mechanism for comparing types in C++ and Objective-C++. However, if bugs in the canonical type system are causing compilation failures, set this value to 0 to disable canonical types. switch-conversion-max-branch-ratio Switch initialization conversion refuses to create arrays that are bigger than switch-conversion-max-branch-ratio times the number of branches in the switch. max-partial-antic-length Maximum length of the partial antic set computed during the tree partial redundancy elimination optimization (-ftree-pre) when optimizing at -O3 and above. For some sorts of source code the enhanced partial redundancy elimination optimization can run away, consuming all of the memory available on the host machine. This parameter sets a limit on the length of the sets that are computed, which prevents the runaway behavior. Setting a value of 0 for this parameter allows an unlimited set length. rpo-vn-max-loop-depth Maximum loop depth that is value-numbered optimistically. When the limit hits the innermost rpo-vn-max-loop-depth loops and the outermost loop in the loop nest are value-numbered optimistically and the remaining ones not. sccvn-max-alias-queries-per-access Maximum number of alias-oracle queries we perform when looking for redundancies for loads and stores. If this limit is hit the search is aborted and the load or store is not considered redundant. The number of queries is algorithmically limited to the number of stores on all paths from the load to the function entry. ira-max-loops-num IRA uses regional register allocation by default. If a function contains more loops than the number given by this parameter, only at most the given number of the most frequently-executed loops form regions for regional register allocation. ira-max-conflict-table-size Although IRA uses a sophisticated algorithm to compress the conflict table, the table can still require excessive amounts of memory for huge functions. If the conflict table for a function could be more than the size in MB given by this parameter, the register allocator instead uses a faster, simpler, and lower-quality algorithm that does not require building a pseudo-register conflict table. ira-loop-reserved-regs IRA can be used to evaluate more accurate register pressure in loops for decisions to move loop invariants (see -O3). The number of available registers reserved for some other purposes is given by this parameter. Default of the parameter is the best found from numerous experiments. lra-inheritance-ebb-probability-cutoff LRA tries to reuse values reloaded in registers in subsequent insns. This optimization is called inheritance. EBB is used as a region to do this optimization. The parameter defines a minimal fall-through edge probability in percentage used to add BB to inheritance EBB in LRA. The default value was chosen from numerous runs of SPEC2000 on x86-64. loop-invariant-max-bbs-in-loop Loop invariant motion can be very expensive, both in compilation time and in amount of needed compile-time memory, with very large loops. Loops with more basic blocks than this parameter won't have loop invariant motion optimization performed on them. loop-max-datarefs-for-datadeps Building data dependencies is expensive for very large loops. This parameter limits the number of data references in loops that are considered for data dependence analysis. These large loops are no handled by the optimizations using loop data dependencies. max-vartrack-size Sets a maximum number of hash table slots to use during variable tracking dataflow analysis of any function. If this limit is exceeded with variable tracking at assignments enabled, analysis for that function is retried without it, after removing all debug insns from the function. If the limit is exceeded even without debug insns, var tracking analysis is completely disabled for the function. Setting the parameter to zero makes it unlimited. max-vartrack-expr-depth Sets a maximum number of recursion levels when attempting to map variable names or debug temporaries to value expressions. This trades compilation time for more complete debug information. If this is set too low, value expressions that are available and could be represented in debug information may end up not being used; setting this higher may enable the compiler to find more complex debug expressions, but compile time and memory use may grow. max-debug-marker-count Sets a threshold on the number of debug markers (e.g. begin stmt markers) to avoid complexity explosion at inlining or expanding to RTL. If a function has more such gimple stmts than the set limit, such stmts will be dropped from the inlined copy of a function, and from its RTL expansion. min-nondebug-insn-uid Use uids starting at this parameter for nondebug insns. The range below the parameter is reserved exclusively for debug insns created by -fvar-tracking-assignments, but debug insns may get (non-overlapping) uids above it if the reserved range is exhausted. ipa-sra-ptr-growth-factor IPA-SRA replaces a pointer to an aggregate with one or more new parameters only when their cumulative size is less or equal to ipa-sra-ptr-growth-factor times the size of the original pointer parameter. sra-max-scalarization-size-Ospeed sra-max-scalarization-size-Osize The two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) aim to replace scalar parts of aggregates with uses of independent scalar variables. These parameters control the maximum size, in storage units, of aggregate which is considered for replacement when compiling for speed (sra-max-scalarization-size-Ospeed) or size (sra-max-scalarization-size-Osize) respectively. tm-max-aggregate-size When making copies of thread-local variables in a transaction, this parameter specifies the size in bytes after which variables are saved with the logging functions as opposed to save/restore code sequence pairs. This option only applies when using -fgnu-tm. graphite-max-nb-scop-params To avoid exponential effects in the Graphite loop transforms, the number of parameters in a Static Control Part (SCoP) is bounded. A value of zero can be used to lift the bound. A variable whose value is unknown at compilation time and defined outside a SCoP is a parameter of the SCoP. loop-block-tile-size Loop blocking or strip mining transforms, enabled with -floop-block or -floop-strip-mine, strip mine each loop in the loop nest by a given number of iterations. The strip length can be changed using the loop-block-tile-size parameter. ipa-cp-value-list-size IPA-CP attempts to track all possible values and types passed to a function's parameter in order to propagate them and perform devirtualization. ipa-cp-value-list-size is the maximum number of values and types it stores per one formal parameter of a function. ipa-cp-eval-threshold IPA-CP calculates its own score of cloning profitability heuristics and performs those cloning opportunities with scores that exceed ipa-cp-eval-threshold. ipa-cp-recursion-penalty Percentage penalty the recursive functions will receive when they are evaluated for cloning. ipa-cp-single-call-penalty Percentage penalty functions containing a single call to another function will receive when they are evaluated for cloning. ipa-max-agg-items IPA-CP is also capable to propagate a number of scalar values passed in an aggregate. ipa-max-agg-items controls the maximum number of such values per one parameter. ipa-cp-loop-hint-bonus When IPA-CP determines that a cloning candidate would make the number of iterations of a loop known, it adds a bonus of ipa-cp-loop-hint-bonus to the profitability score of the candidate. ipa-cp-array-index-hint-bonus When IPA-CP determines that a cloning candidate would make the index of an array access known, it adds a bonus of ipa-cp-array-index-hint-bonus to the profitability score of the candidate. ipa-max-aa-steps During its analysis of function bodies, IPA-CP employs alias analysis in order to track values pointed to by function parameters. In order not spend too much time analyzing huge functions, it gives up and consider all memory clobbered after examining ipa-max-aa-steps statements modifying memory. lto-partitions Specify desired number of partitions produced during WHOPR compilation. The number of partitions should exceed the number of CPUs used for compilation. lto-min-partition Size of minimal partition for WHOPR (in estimated instructions). This prevents expenses of splitting very small programs into too many partitions. lto-max-partition Size of max partition for WHOPR (in estimated instructions). to provide an upper bound for individual size of partition. Meant to be used only with balanced partitioning. lto-max-streaming-parallelism Maximal number of parallel processes used for LTO streaming. cxx-max-namespaces-for-diagnostic-help The maximum number of namespaces to consult for suggestions when C++ name lookup fails for an identifier. sink-frequency-threshold The maximum relative execution frequency (in percents) of the target block relative to a statement's original block to allow statement sinking of a statement. Larger numbers result in more aggressive statement sinking. A small positive adjustment is applied for statements with memory operands as those are even more profitable so sink. max-stores-to-sink The maximum number of conditional store pairs that can be sunk. Set to 0 if either vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled. allow-store-data-races Allow optimizers to introduce new data races on stores. Set to 1 to allow, otherwise to 0. case-values-threshold The smallest number of different values for which it is best to use a jump-table instead of a tree of conditional branches. If the value is 0, use the default for the machine. tree-reassoc-width Set the maximum number of instructions executed in parallel in reassociated tree. This parameter overrides target dependent heuristics used by default if has non zero value. sched-pressure-algorithm Choose between the two available implementations of -fsched-pressure. Algorithm 1 is the original implementation and is the more likely to prevent instructions from being reordered. Algorithm 2 was designed to be a compromise between the relatively conservative approach taken by algorithm 1 and the rather aggressive approach taken by the default scheduler. It relies more heavily on having a regular register file and accurate register pressure classes. See haifa-sched.c in the GCC sources for more details. The default choice depends on the target. max-slsr-cand-scan Set the maximum number of existing candidates that are considered when seeking a basis for a new straight-line strength reduction candidate. asan-globals Enable buffer overflow detection for global objects. This kind of protection is enabled by default if you are using -fsanitize=address option. To disable global objects protection use --param asan-globals=0. asan-stack Enable buffer overflow detection for stack objects. This kind of protection is enabled by default when using -fsanitize=address. To disable stack protection use --param asan-stack=0 option. asan-instrument-reads Enable buffer overflow detection for memory reads. This kind of protection is enabled by default when using -fsanitize=address. To disable memory reads protection use --param asan-instrument-reads=0. asan-instrument-writes Enable buffer overflow detection for memory writes. This kind of protection is enabled by default when using -fsanitize=address. To disable memory writes protection use --param asan-instrument-writes=0 option. asan-memintrin Enable detection for built-in functions. This kind of protection is enabled by default when using -fsanitize=address. To disable built-in functions protection use --param asan-memintrin=0. asan-use-after-return Enable detection of use-after-return. This kind of protection is enabled by default when using the -fsanitize=address option. To disable it use --param asan-use-after-return=0. Note: By default the check is disabled at run time. To enable it, add detect_stack_use_after_return=1 to the environment variable ASAN_OPTIONS. asan-instrumentation-with-call-threshold If number of memory accesses in function being instrumented is greater or equal to this number, use callbacks instead of inline checks. E.g. to disable inline code use --param asan-instrumentation-with-call-threshold=0. use-after-scope-direct-emission-threshold If the size of a local variable in bytes is smaller or equal to this number, directly poison (or unpoison) shadow memory instead of using run-time callbacks. max-fsm-thread-path-insns Maximum number of instructions to copy when duplicating blocks on a finite state automaton jump thread path. max-fsm-thread-length Maximum number of basic blocks on a finite state automaton jump thread path. max-fsm-thread-paths Maximum number of new jump thread paths to create for a finite state automaton. parloops-chunk-size Chunk size of omp schedule for loops parallelized by parloops. parloops-schedule Schedule type of omp schedule for loops parallelized by parloops (static, dynamic, guided, auto, runtime). parloops-min-per-thread The minimum number of iterations per thread of an innermost parallelized loop for which the parallelized variant is preferred over the single threaded one. Note that for a parallelized loop nest the minimum number of iterations of the outermost loop per thread is two. max-ssa-name-query-depth Maximum depth of recursion when querying properties of SSA names in things like fold routines. One level of recursion corresponds to following a use-def chain. hsa-gen-debug-stores Enable emission of special debug stores within HSA kernels which are then read and reported by libgomp plugin. Generation of these stores is disabled by default, use --param hsa-gen-debug-stores=1 to enable it. max-speculative-devirt-maydefs The maximum number of may-defs we analyze when looking for a must-def specifying the dynamic type of an object that invokes a virtual call we may be able to devirtualize speculatively. max-vrp-switch-assertions The maximum number of assertions to add along the default edge of a switch statement during VRP. unroll-jam-min-percent The minimum percentage of memory references that must be optimized away for the unroll-and-jam transformation to be considered profitable. unroll-jam-max-unroll The maximum number of times the outer loop should be unrolled by the unroll-and-jam transformation. max-rtl-if-conversion-unpredictable-cost Maximum permissible cost for the sequence that would be generated by the RTL if-conversion pass for a branch that is considered unpredictable. max-variable-expansions-in-unroller If -fvariable-expansion-in-unroller is used, the maximum number of times that an individual variable will be expanded during loop unrolling. tracer-min-branch-probability-feedback Stop forward growth if the probability of best edge is less than this threshold (in percent). Used when profile feedback is available. partial-inlining-entry-probability Maximum probability of the entry BB of split region (in percent relative to entry BB of the function) to make partial inlining happen. max-tracked-strlens Maximum number of strings for which strlen optimization pass will track string lengths. gcse-after-reload-partial-fraction The threshold ratio for performing partial redundancy elimination after reload. gcse-after-reload-critical-fraction The threshold ratio of critical edges execution count that permit performing redundancy elimination after reload. max-loop-header-insns The maximum number of insns in loop header duplicated by the copy loop headers pass. vect-epilogues-nomask Enable loop epilogue vectorization using smaller vector size. slp-max-insns-in-bb Maximum number of instructions in basic block to be considered for SLP vectorization. avoid-fma-max-bits Maximum number of bits for which we avoid creating FMAs. sms-loop-average-count-threshold A threshold on the average loop count considered by the swing modulo scheduler. sms-dfa-history The number of cycles the swing modulo scheduler considers when checking conflicts using DFA. hot-bb-count-fraction Select fraction of the maximal count of repetitions of basic block in program given basic block needs to have to be considered hot (used in non-LTO mode) max-inline-insns-recursive-auto The maximum number of instructions non-inline function can grow to via recursive inlining. graphite-allow-codegen-errors Whether codegen errors should be ICEs when -fchecking. sms-max-ii-factor A factor for tuning the upper bound that swing modulo scheduler uses for scheduling a loop. lra-max-considered-reload-pseudos The max number of reload pseudos which are considered during spilling a non-reload pseudo. max-pow-sqrt-depth Maximum depth of sqrt chains to use when synthesizing exponentiation by a real constant. max-dse-active-local-stores Maximum number of active local stores in RTL dead store elimination. asan-instrument-allocas Enable asan allocas/VLAs protection. max-iterations-computation-cost Bound on the cost of an expression to compute the number of iterations. max-isl-operations Maximum number of isl operations, 0 means unlimited. graphite-max-arrays-per-scop Maximum number of arrays per scop. max-vartrack-reverse-op-size Max. size of loc list for which reverse ops should be added. unlikely-bb-count-fraction The minimum fraction of profile runs a given basic block execution count must be not to be considered unlikely. tracer-dynamic-coverage-feedback The percentage of function, weighted by execution frequency, that must be covered by trace formation. Used when profile feedback is available. max-inline-recursive-depth-auto The maximum depth of recursive inlining for non-inline functions. fsm-scale-path-stmts Scale factor to apply to the number of statements in a threading path when comparing to the number of (scaled) blocks. fsm-maximum-phi-arguments Maximum number of arguments a PHI may have before the FSM threader will not try to thread through its block. uninit-control-dep-attempts Maximum number of nested calls to search for control dependencies during uninitialized variable analysis. indir-call-topn-profile Track top N target addresses in indirect-call profile. max-once-peeled-insns The maximum number of insns of a peeled loop that rolls only once. sra-max-scalarization-size-Osize Maximum size, in storage units, of an aggregate which should be considered for scalarization when compiling for size. fsm-scale-path-blocks Scale factor to apply to the number of blocks in a threading path when comparing to the number of (scaled) statements. sched-autopref-queue-depth Hardware autoprefetcher scheduler model control flag. Number of lookahead cycles the model looks into; at ' ' only enable instruction sorting heuristic. loop-versioning-max-inner-insns The maximum number of instructions that an inner loop can have before the loop versioning pass considers it too big to copy. loop-versioning-max-outer-insns The maximum number of instructions that an outer loop can have before the loop versioning pass considers it too big to copy, discounting any instructions in inner loops that directly benefit from versioning. ssa-name-def-chain-limit The maximum number of SSA_NAME assignments to follow in determining a property of a variable such as its value. This limits the number of iterations or recursive calls GCC performs when optimizing certain statements or when determining their validity prior to issuing diagnostics. ────────────────────────────────────────────────────────────────────────────── Next: Instrumentation Options, Previous: Debugging Options, Up: Invoking Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Invoking GCC Link: next: Preprocessor Options Link: prev: Optimize Options Next: Preprocessor Options, Previous: Optimize Options, Up: Invoking GCC ────────────────────────────────────────────────────────────────────────────── 3.11 Program Instrumentation Options GCC supports a number of command-line options that control adding run-time instrumentation to the code it normally generates. For example, one purpose of instrumentation is collect profiling statistics for use in finding program hot spots, code coverage analysis, or profile-guided optimizations. Another class of program instrumentation is adding run-time checking to detect programming errors like invalid pointer dereferences or out-of-bounds array accesses, as well as deliberately hostile attacks such as stack smashing or C++ vtable hijacking. There is also a general hook which can be used to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes. -p -pg Generate extra code to write profile information suitable for the analysis program prof (for -p) or gprof (for -pg). You must use this option when compiling the source files you want data about, and you must also use it when linking. You can use the function attribute no_instrument_function to suppress profiling of individual functions when compiling with these options. See Common Function Attributes. -fprofile-arcs Add code so that program flow arcs are instrumented. During execution the program records how many times each branch and call is executed and how many times it is taken or returns. On targets that support constructors with priority support, profiling properly handles constructors, destructors and C++ constructors (and destructors) of classes which are used as a type of a global variable. When the compiled program exits it saves this data to a file called auxname.gcda for each source file. The data may be used for profile-directed optimizations (-fbranch-probabilities), or for test coverage analysis (-ftest-coverage). Each object file's auxname is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o). See Cross-profiling. --coverage This option is used to compile and link code instrumented for coverage analysis. The option is a synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the documentation for those options for more details. * Compile the source files with -fprofile-arcs plus optimization and code generation options. For test coverage analysis, use the additional -ftest-coverage option. You do not need to profile every source file in a program. * Compile the source files additionally with -fprofile-abs-path to create absolute path names in the .gcno files. This allows gcov to find the correct sources in projects where compilations occur with different working directories. * Link your object files with -lgcov or -fprofile-arcs (the latter implies the former). * Run the program on a representative workload to generate the arc profile information. This may be repeated any number of times. You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated. Unless a strict ISO C dialect option is in effect, fork calls are detected and correctly handled without double counting. * For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus -fbranch-probabilities (see Options that Control Optimization). * For test coverage analysis, use gcov to produce human readable information from the .gcno and .gcda files. Refer to the gcov documentation for further information. With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code. -ftest-coverage Produce a notes file that the gcov code-coverage utility (see gcov—a Test Coverage Program) can use to show program coverage. Each source file's note file is called auxname.gcno. Refer to the -fprofile-arcs option above for a description of auxname and instructions on how to generate test coverage data. Coverage data matches the source files more closely if you do not optimize. -fprofile-abs-path Automatically convert relative source file names to absolute path names in the .gcno files. This allows gcov to find the correct sources in projects where compilations occur with different working directories. -fprofile-dir=path Set the directory to search for the profile data files in to path. This option affects only the profile data generated by -fprofile-generate, -ftest-coverage, -fprofile-arcs and used by -fprofile-use and -fbranch-probabilities and its related options. Both absolute and relative paths can be used. By default, GCC uses the current directory as path, thus the profile data file appears in the same directory as the object file. In order to prevent the file name clashing, if the object file name is not an absolute path, we mangle the absolute path of the sourcename.gcda file and use it as the file name of a .gcda file. When an executable is run in a massive parallel environment, it is recommended to save profile to different folders. That can be done with variables in path that are exported during run-time: %p process ID. %q{VAR} value of environment variable VAR -fprofile-generate -fprofile-generate=path Enable options usually used for instrumenting application to produce profile useful for later recompilation with profile feedback based optimization. You must use -fprofile-generate both when compiling and when linking your program. The following options are enabled: -fprofile-arcs, -fprofile-values, -finline-functions, and -fipa-bit-cp. If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir. To optimize the program based on the collected profile information, use -fprofile-use. See Optimize Options, for more information. -fprofile-update=method Alter the update method for an application instrumented for profile feedback based optimization. The method argument should be one of 'single', 'atomic' or 'prefer-atomic'. The first one is useful for single-threaded applications, while the second one prevents profile corruption by emitting thread-safe code. Warning: When an application does not properly join all threads (or creates an detached thread), a profile file can be still corrupted. Using 'prefer-atomic' would be transformed either to 'atomic', when supported by a target, or to 'single' otherwise. The GCC driver automatically selects 'prefer-atomic' when -pthread is present in the command line. -fprofile-filter-files=regex Instrument only functions from files where names match any regular expression (separated by a semi-colon). For example, -fprofile-filter-files=main.c;module.*.c will instrument only main.c and all C files starting with 'module'. -fprofile-exclude-files=regex Instrument only functions from files where names do not match all the regular expressions (separated by a semi-colon). For example, -fprofile-exclude-files=/usr/* will prevent instrumentation of all files that are located in /usr/ folder. -fsanitize=address Enable AddressSanitizer, a fast memory error detector. Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs. The option enables -fsanitize-address-use-after-scope. See https://github.com/google/sanitizers/wiki/AddressSanitizer for more details. The run-time behavior can be influenced using the ASAN_OPTIONS environment variable. When set to help=1, the available options are shown at startup of the instrumented program. See https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags for a list of supported options. The option cannot be combined with -fsanitize=thread. -fsanitize=kernel-address Enable AddressSanitizer for Linux kernel. See https://github.com/google/kasan/wiki for more details. -fsanitize=pointer-compare Instrument comparison operation (<, <=, >, >=) with pointer operands. The option must be combined with either -fsanitize=kernel-address or -fsanitize=address The option cannot be combined with -fsanitize=thread. Note: By default the check is disabled at run time. To enable it, add detect_invalid_pointer_pairs=2 to the environment variable ASAN_OPTIONS. Using detect_invalid_pointer_pairs=1 detects invalid operation only when both pointers are non-null. -fsanitize=pointer-subtract Instrument subtraction with pointer operands. The option must be combined with either -fsanitize=kernel-address or -fsanitize=address The option cannot be combined with -fsanitize=thread. Note: By default the check is disabled at run time. To enable it, add detect_invalid_pointer_pairs=2 to the environment variable ASAN_OPTIONS. Using detect_invalid_pointer_pairs=1 detects invalid operation only when both pointers are non-null. -fsanitize=thread Enable ThreadSanitizer, a fast data race detector. Memory access instructions are instrumented to detect data race bugs. See https://github.com/google/sanitizers/wiki#threadsanitizer for more details. The run-time behavior can be influenced using the TSAN_OPTIONS environment variable; see https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags for a list of supported options. The option cannot be combined with -fsanitize=address, -fsanitize=leak. Note that sanitized atomic builtins cannot throw exceptions when operating on invalid memory addresses with non-call exceptions (-fnon-call-exceptions). -fsanitize=leak Enable LeakSanitizer, a memory leak detector. This option only matters for linking of executables and the executable is linked against a library that overrides malloc and other allocator functions. See https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer for more details. The run-time behavior can be influenced using the LSAN_OPTIONS environment variable. The option cannot be combined with -fsanitize=thread. -fsanitize=undefined Enable UndefinedBehaviorSanitizer, a fast undefined behavior detector. Various computations are instrumented to detect undefined behavior at runtime. Current suboptions are: -fsanitize=shift This option enables checking that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc. This option has two suboptions, -fsanitize=shift-base and -fsanitize=shift-exponent. -fsanitize=shift-exponent This option enables checking that the second argument of a shift operation is not negative and is smaller than the precision of the promoted first argument. -fsanitize=shift-base If the second argument of a shift operation is within range, check that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc. -fsanitize=integer-divide-by-zero Detect integer division by zero as well as INT_MIN / -1 division. -fsanitize=unreachable With this option, the compiler turns the __builtin_unreachable call into a diagnostics message call instead. When reaching the __builtin_unreachable call, the behavior is undefined. -fsanitize=vla-bound This option instructs the compiler to check that the size of a variable length array is positive. -fsanitize=null This option enables pointer checking. Particularly, the application built with this option turned on will issue an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue reference) is bound to a NULL pointer, or if a method is invoked on an object pointed by a NULL pointer. -fsanitize=return This option enables return statement checking. Programs built with this option turned on will issue an error message when the end of a non-void function is reached without actually returning a value. This option works in C++ only. -fsanitize=signed-integer-overflow This option enables signed integer overflow checking. We check that the result of +, *, and both unary and binary - does not overflow in the signed arithmetics. Note, integer promotion rules must be taken into account. That is, the following is not an overflow: signed char a = SCHAR_MAX; a++; -fsanitize=bounds This option enables instrumentation of array bounds. Various out of bounds accesses are detected. Flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented. -fsanitize=bounds-strict This option enables strict instrumentation of array bounds. Most out of bounds accesses are detected, including flexible array members and flexible array member-like arrays. Initializers of variables with static storage are not instrumented. -fsanitize=alignment This option enables checking of alignment of pointers when they are dereferenced, or when a reference is bound to insufficiently aligned target, or when a method or constructor is invoked on insufficiently aligned object. -fsanitize=object-size This option enables instrumentation of memory references using the __builtin_object_size function. Various out of bounds pointer accesses are detected. -fsanitize=float-divide-by-zero Detect floating-point division by zero. Unlike other similar options, -fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined, since floating-point division by zero can be a legitimate way of obtaining infinities and NaNs. -fsanitize=float-cast-overflow This option enables floating-point type to integer conversion checking. We check that the result of the conversion does not overflow. Unlike other similar options, -fsanitize=float-cast-overflow is not enabled by -fsanitize=undefined. This option does not work well with FE_INVALID exceptions enabled. -fsanitize=nonnull-attribute This option enables instrumentation of calls, checking whether null values are not passed to arguments marked as requiring a non-null value by the nonnull function attribute. -fsanitize=returns-nonnull-attribute This option enables instrumentation of return statements in functions marked with returns_nonnull function attribute, to detect returning of null values from such functions. -fsanitize=bool This option enables instrumentation of loads from bool. If a value other than 0/1 is loaded, a run-time error is issued. -fsanitize=enum This option enables instrumentation of loads from an enum type. If a value outside the range of values for the enum type is loaded, a run-time error is issued. -fsanitize=vptr This option enables instrumentation of C++ member function calls, member accesses and some conversions between pointers to base and derived classes, to verify the referenced object has the correct dynamic type. -fsanitize=pointer-overflow This option enables instrumentation of pointer arithmetics. If the pointer arithmetics overflows, a run-time error is issued. -fsanitize=builtin This option enables instrumentation of arguments to selected builtin functions. If an invalid value is passed to such arguments, a run-time error is issued. E.g. passing 0 as the argument to __builtin_ctz or __builtin_clz invokes undefined behavior and is diagnosed by this option. While -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives a diagnostic message. This currently works only for the C family of languages. -fno-sanitize=all This option disables all previously enabled sanitizers. -fsanitize=all is not allowed, as some sanitizers cannot be used together. -fasan-shadow-offset=number This option forces GCC to use custom shadow offset in AddressSanitizer checks. It is useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer. -fsanitize-sections=s1,s2,... Sanitize global variables in selected user-defined sections. si may contain wildcards. -fsanitize-recover[=opts] -fsanitize-recover= controls error recovery mode for sanitizers mentioned in comma-separated list of opts. Enabling this option for a sanitizer component causes it to attempt to continue running the program as if no error happened. This means multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported. The -fno-sanitize-recover= option can be used to alter this behavior: only the first detected error is reported and program then exits with a non-zero exit code. Currently this feature only works for -fsanitize=undefined (and its suboptions except for -fsanitize=unreachable and -fsanitize=return), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero, -fsanitize=bounds-strict, -fsanitize=kernel-address and -fsanitize=address. For these sanitizers error recovery is turned on by default, except -fsanitize=address, for which this feature is experimental. -fsanitize-recover=all and -fno-sanitize-recover=all is also accepted, the former enables recovery for all sanitizers that support it, the latter disables recovery for all sanitizers that support it. Even if a recovery mode is turned on the compiler side, it needs to be also enabled on the runtime library side, otherwise the failures are still fatal. The runtime library defaults to halt_on_error=0 for ThreadSanitizer and UndefinedBehaviorSanitizer, while default value for AddressSanitizer is halt_on_error=1. This can be overridden through setting the halt_on_error flag in the corresponding environment variable. Syntax without an explicit opts parameter is deprecated. It is equivalent to specifying an opts list of: undefined,float-cast-overflow,float-divide-by-zero,bounds-strict -fsanitize-address-use-after-scope Enable sanitization of local variables to detect use-after-scope bugs. The option sets -fstack-reuse to 'none'. -fsanitize-undefined-trap-on-error The -fsanitize-undefined-trap-on-error option instructs the compiler to report undefined behavior using __builtin_trap rather than a libubsan library routine. The advantage of this is that the libubsan library is not needed and is not linked in, so this is usable even in freestanding environments. -fsanitize-coverage=trace-pc Enable coverage-guided fuzzing code instrumentation. Inserts a call to __sanitizer_cov_trace_pc into every basic block. -fsanitize-coverage=trace-cmp Enable dataflow guided fuzzing code instrumentation. Inserts a call to __sanitizer_cov_trace_cmp1, __sanitizer_cov_trace_cmp2, __sanitizer_cov_trace_cmp4 or __sanitizer_cov_trace_cmp8 for integral comparison with both operands variable or __sanitizer_cov_trace_const_cmp1, __sanitizer_cov_trace_const_cmp2, __sanitizer_cov_trace_const_cmp4 or __sanitizer_cov_trace_const_cmp8 for integral comparison with one operand constant, __sanitizer_cov_trace_cmpf or __sanitizer_cov_trace_cmpd for float or double comparisons and __sanitizer_cov_trace_switch for switch statements. -fcf-protection=[full|branch|return|none] Enable code instrumentation of control-flow transfers to increase program security by checking that target addresses of control-flow transfer instructions (such as indirect function call, function return, indirect jump) are valid. This prevents diverting the flow of control to an unexpected target. This is intended to protect against such threats as Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP). The value branch tells the compiler to implement checking of validity of control-flow transfer at the point of indirect branch instructions, i.e. call/jmp instructions. The value return implements checking of validity at the point of returning from a function. The value full is an alias for specifying both branch and return. The value none turns off instrumentation. The macro __CET__ is defined when -fcf-protection is used. The first bit of __CET__ is set to 1 for the value branch and the second bit of __CET__ is set to 1 for the return. You can also use the nocf_check attribute to identify which functions and calls should be skipped from instrumentation (see Function Attributes). Currently the x86 GNU/Linux target provides an implementation based on Intel Control-flow Enforcement Technology (CET). -fstack-protector Emit extra code to check for buffer overflows, such as stack smashing attacks. This is done by adding a guard variable to functions with vulnerable objects. This includes functions that call alloca, and functions with buffers larger than 8 bytes. The guards are initialized when a function is entered and then checked when the function exits. If a guard check fails, an error message is printed and the program exits. -fstack-protector-all Like -fstack-protector except that all functions are protected. -fstack-protector-strong Like -fstack-protector but includes additional functions to be protected — those that have local array definitions, or have references to local frame addresses. -fstack-protector-explicit Like -fstack-protector but only protects those functions which have the stack_protect attribute. -fstack-check Generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag if you are running in an environment with multiple threads, but you only rarely need to specify it in a single-threaded environment since stack overflow is automatically detected on nearly all systems if there is only one stack. Note that this switch does not actually cause checking to be done; the operating system or the language runtime must do that. The switch causes generation of code to ensure that they see the stack being extended. You can additionally specify a string parameter: 'no' means no checking, 'generic' means force the use of old-style checking, 'specific' means use the best checking method and is equivalent to bare -fstack-check. Old-style checking is a generic mechanism that requires no specific target support in the compiler but comes with the following drawbacks: 1. Modified allocation strategy for large objects: they are always allocated dynamically if their size exceeds a fixed threshold. Note this may change the semantics of some code. 2. Fixed limit on the size of the static frame of functions: when it is topped by a particular function, stack checking is not reliable and a warning is issued by the compiler. 3. Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance is hampered. Note that old-style stack checking is also the fallback method for 'specific' if no target support has been added in the compiler. '-fstack-check=' is designed for Ada's needs to detect infinite recursion and stack overflows. 'specific' is an excellent choice when compiling Ada code. It is not generally sufficient to protect against stack-clash attacks. To protect against those you want '-fstack-clash-protection'. -fstack-clash-protection Generate code to prevent stack clash style attacks. When this option is enabled, the compiler will only allocate one page of stack space at a time and each page is accessed immediately after allocation. Thus, it prevents allocations from jumping over any stack guard page provided by the operating system. Most targets do not fully support stack clash protection. However, on those targets -fstack-clash-protection will protect dynamic stack allocations. -fstack-clash-protection may also provide limited protection for static stack allocations if the target supports -fstack-check=specific. -fstack-limit-register=reg -fstack-limit-symbol=sym -fno-stack-limit Generate code to ensure that the stack does not grow beyond a certain value, either the value of a register or the address of a symbol. If a larger stack is required, a signal is raised at run time. For most targets, the signal is raised before the stack overruns the boundary, so it is possible to catch the signal without taking special precautions. For instance, if the stack starts at absolute address '0x80000000' and grows downwards, you can use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Note that this may only work with the GNU linker. You can locally override stack limit checking by using the no_stack_limit function attribute (see Function Attributes). -fsplit-stack Generate code to automatically split the stack before it overflows. The resulting program has a discontiguous stack which can only overflow if the program is unable to allocate any more memory. This is most useful when running threaded programs, as it is no longer necessary to calculate a good stack size to use for each thread. This is currently only implemented for the x86 targets running GNU/Linux. When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack space available for the latter code to run. If compiling all code, including library code, with -fsplit-stack is not an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack. Support for this is implemented in the gold linker in GNU binutils release 2.21 and later. -fvtable-verify=[std|preinit|none] This option is only available when compiling C++ code. It turns on (or off, if using -fvtable-verify=none) the security feature that verifies at run time, for every virtual call, that the vtable pointer through which the call is made is valid for the type of the object, and has not been corrupted or overwritten. If an invalid vtable pointer is detected at run time, an error is reported and execution of the program is immediately halted. This option causes run-time data structures to be built at program startup, which are used for verifying the vtable pointers. The options 'std' and 'preinit' control the timing of when these data structures are built. In both cases the data structures are built before execution reaches main. Using -fvtable-verify=std causes the data structures to be built after shared libraries have been loaded and initialized. -fvtable-verify=preinit causes them to be built before shared libraries have been loaded and initialized. If this option appears multiple times in the command line with different values specified, 'none' takes highest priority over both 'std' and 'preinit'; 'preinit' takes priority over 'std'. -fvtv-debug When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, causes debug versions of the runtime functions for the vtable verification feature to be called. This flag also causes the compiler to log information about which vtable pointers it finds for each class. This information is written to a file named vtv_set_ptr_data.log in the directory named by the environment variable VTV_LOGS_DIR if that is defined or the current working directory otherwise. Note: This feature appends data to the log file. If you want a fresh log file, be sure to delete any existing one. -fvtv-counts This is a debugging flag. When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, this causes the compiler to keep track of the total number of virtual calls it encounters and the number of verifications it inserts. It also counts the number of calls to certain run-time library functions that it inserts and logs this information for each compilation unit. The compiler writes this information to a file named vtv_count_data.log in the directory named by the environment variable VTV_LOGS_DIR if that is defined or the current working directory otherwise. It also counts the size of the vtable pointer sets for each class, and writes this information to vtv_class_set_sizes.log in the same directory. Note: This feature appends data to the log files. To get fresh log files, be sure to delete any existing ones. -finstrument-functions Generate instrumentation calls for entry and exit to functions. Just after function entry and just before function exit, the following profiling functions are called with the address of the current function and its call site. (On some platforms, __builtin_return_address does not work beyond the current function, so the call site information may not be available to the profiling functions otherwise.) void __cyg_profile_func_enter (void *this_fn, void *call_site); void __cyg_profile_func_exit (void *this_fn, void *call_site); The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table. This instrumentation is also done for functions expanded inline in other functions. The profiling calls indicate where, conceptually, the inline function is entered and exited. This means that addressable versions of such functions must be available. If all your uses of a function are expanded inline, this may mean an additional expansion of code size. If you use extern inline in your C code, an addressable version of such functions must be provided. (This is normally the case anyway, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.) A function may be given the attribute no_instrument_function, in which case this instrumentation is not done. This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called (perhaps signal handlers, if the profiling routines generate output or allocate memory). See Common Function Attributes. -finstrument-functions-exclude-file-list=file,file,… Set the list of functions that are excluded from instrumentation (see the description of -finstrument-functions). If the file that contains a function definition matches with one of file, then that function is not instrumented. The match is done on substrings: if the file parameter is a substring of the file name, it is considered to be a match. For example: -finstrument-functions-exclude-file-list=/bits/stl,include/sys excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys. If, for some reason, you want to include letter ',' in one of sym, write '\,'. For example, -finstrument-functions-exclude-file-list='\,\,tmp' (note the single quote surrounding the option). -finstrument-functions-exclude-function-list=sym,sym,… This is similar to -finstrument-functions-exclude-file-list, but this option sets the list of function names to be excluded from instrumentation. The function name to be matched is its user-visible name, such as vector blah(const vector &), not the internal mangled name (e.g., _Z4blahRSt6vectorIiSaIiEE). The match is done on substrings: if the sym parameter is a substring of the function name, it is considered to be a match. For C99 and C++ extended identifiers, the function name must be given in UTF-8, not using universal character names. -fpatchable-function-entry=N[,M] Generate N NOPs right at the beginning of each function, with the function entry point before the Mth NOP. If M is omitted, it defaults to 0 so the function entry points to the address just at the first NOP. The NOP instructions reserve extra space which can be used to patch in any desired instrumentation at run time, provided that the code segment is writable. The amount of space is controllable indirectly via the number of NOPs; the NOP instruction used corresponds to the instruction emitted by the internal GCC back-end interface gen_nop. This behavior is target-specific and may also depend on the architecture variant and/or other compilation options. For run-time identification, the starting addresses of these areas, which correspond to their respective function entries minus M, are additionally collected in the __patchable_function_entries section of the resulting binary. Note that the value of __attribute__ ((patchable_function_entry (N,M))) takes precedence over command-line option -fpatchable-function-entry=N,M. This can be used to increase the area size or to remove it completely on a single function. If N=0, no pad location is recorded. The NOP instructions are inserted at—and maybe before, depending on M—the function entry address, even before the prologue. ────────────────────────────────────────────────────────────────────────────── Next: Preprocessor Options, Previous: Optimize Options, Up: Invoking GCC Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Invoking GCC Link: next: Assembler Options Link: prev: Instrumentation Options Next: Assembler Options, Previous: Instrumentation Options, Up: Invoking ────────────────────────────────────────────────────────────────────────────── 3.12 Options Controlling the Preprocessor These options control the C preprocessor, which is run on each C source file before actual compilation. If you use the -E option, nothing is done except preprocessing. Some of these options make sense only together with -E because they cause the preprocessor output to be unsuitable for actual compilation. In addition to the options listed here, there are a number of options to control search paths for include files documented in Directory Options. Options to control preprocessor diagnostics are listed in Warning Options. -D name Predefine name as a macro, with definition 1. -D name=definition The contents of definition are tokenized and processed as if they appeared during translation phase three in a '#define' directive. In particular, the definition is truncated by embedded newline characters. If you are invoking the preprocessor from a shell or shell-like program you may need to use the shell's quoting syntax to protect characters such as spaces that have a meaning in the shell syntax. If you wish to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you should quote the option. With sh and csh, -D'name(args…)=definition' works. -D and -U options are processed in the order they are given on the command line. All -imacros file and -include file options are processed after all -D and -U options. -U name Cancel any previous definition of name, either built in or provided with a -D option. -include file Process file as if #include "file" appeared as the first line of the primary source file. However, the first directory searched for file is the preprocessor's working directory instead of the directory containing the main source file. If not found there, it is searched for in the remainder of the #include "…" search chain as normal. If multiple -include options are given, the files are included in the order they appear on the command line. -imacros file Exactly like -include, except that any output produced by scanning file is thrown away. Macros it defines remain defined. This allows you to acquire all the macros from a header without also processing its declarations. All files specified by -imacros are processed before all files specified by -include. -undef Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined. -pthread Define additional macros required for using the POSIX threads library. You should use this option consistently for both compilation and linking. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. -M Instead of outputting the result of preprocessing, output a rule suitable for make describing the dependencies of the main source file. The preprocessor outputs one make rule containing the object file name for that source file, a colon, and the names of all the included files, including those coming from -include or -imacros command-line options. Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any suffix replaced with object file suffix and with any leading directory parts removed. If there are many included files then the rule is split into several lines using '\'-newline. The rule has no commands. This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such debug output with the dependency rules you should explicitly specify the dependency output file with -MF, or use an environment variable like DEPENDENCIES_OUTPUT (see Environment Variables). Debug output is still sent to the regular output stream as normal. Passing -M to the driver implies -E, and suppresses warnings with an implicit -w. -MM Like -M but do not mention header files that are found in system header directories, nor header files that are included, directly or indirectly, from such a header. This implies that the choice of angle brackets or double quotes in an '#include' directive does not in itself determine whether that header appears in -MM dependency output. -MF file When used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given the preprocessor sends the rules to the same place it would send preprocessed output. When used with the driver options -MD or -MMD, -MF overrides the default dependency output file. If file is -, then the dependencies are written to stdout. -MG In conjunction with an option such as -M requesting dependency generation, -MG assumes missing header files are generated files and adds them to the dependency list without raising an error. The dependency filename is taken directly from the #include directive without prepending any path. -MG also suppresses preprocessed output, as a missing header file renders this useless. This feature is used in automatic updating of makefiles. -MP This option instructs CPP to add a phony target for each dependency other than the main file, causing each to depend on nothing. These dummy rules work around errors make gives if you remove header files without updating the Makefile to match. This is typical output: test.o: test.c test.h test.h: -MT target Change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input file, deletes any directory components and any file suffix such as '.c', and appends the platform's usual object suffix. The result is the target. An -MT option sets the target to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to -MT, or use multiple -MT options. For example, -MT '$(objpfx)foo.o' might give $(objpfx)foo.o: foo.c -MQ target Same as -MT, but it quotes any characters which are special to Make. -MQ '$(objpfx)foo.o' gives $$(objpfx)foo.o: foo.c The default target is automatically quoted, as if it were given with -MQ. -MD -MD is equivalent to -M -MF file, except that -E is not implied. The driver determines file based on whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d, otherwise it takes the name of the input file, removes any directory components and suffix, and applies a .d suffix. If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file (see -MF), but if used without -E, each -o is understood to specify a target object file. Since -E is not implied, -MD can be used to generate a dependency output file as a side effect of the compilation process. -MMD Like -MD except mention only user header files, not system header files. -fpreprocessed Indicate to the preprocessor that the input file has already been preprocessed. This suppresses things like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes comments, so that you can pass a file preprocessed with -C to the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends. -fpreprocessed is implicit if the input file has one of the extensions '.i', '.ii' or '.mi'. These are the extensions that GCC uses for preprocessed files created by -save-temps. -fdirectives-only When preprocessing, handle directives, but do not expand macros. The option's behavior depends on the -E and -fpreprocessed options. With -E, preprocessing is limited to the handling of directives such as #define, #ifdef, and #error. Other preprocessor operations, such as macro expansion and trigraph conversion are not performed. In addition, the -dD option is implicitly enabled. With -fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros such as __LINE__, which are contextually dependent, are handled normally. This enables compilation of files previously preprocessed with -E -fdirectives-only. With both -E and -fpreprocessed, the rules for -fpreprocessed take precedence. This enables full preprocessing of files previously preprocessed with -E -fdirectives-only. -fdollars-in-identifiers Accept '$' in identifiers. -fextended-identifiers Accept universal character names in identifiers. This option is enabled by default for C99 (and later C standard versions) and C++. -fno-canonical-system-headers When preprocessing, do not shorten system header paths with canonicalization. -ftabstop=width Set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is ignored. The default is 8. -ftrack-macro-expansion[=level] Track locations of tokens across macro expansions. This allows the compiler to emit diagnostic about the current macro expansion stack when a compilation error occurs in a macro expansion. Using this option makes the preprocessor and the compiler consume more memory. The level parameter can be used to choose the level of precision of token location tracking thus decreasing the memory consumption if necessary. Value '0' of level de-activates this option. Value '1' tracks tokens locations in a degraded mode for the sake of minimal memory overhead. In this mode all tokens resulting from the expansion of an argument of a function-like macro have the same location. Value '2' tracks tokens locations completely. This value is the most memory hungry. When this option is given no argument, the default parameter value is '2'. Note that -ftrack-macro-expansion=2 is activated by default. -fmacro-prefix-map=old=new When preprocessing files residing in directory old, expand the __FILE__ and __BASE_FILE__ macros as if the files resided in directory new instead. This can be used to change an absolute path to a relative path by using . for new which can result in more reproducible builds that are location independent. This option also affects __builtin_FILE() during compilation. See also -ffile-prefix-map. -fexec-charset=charset Set the execution character set, used for string and character constants. The default is UTF-8. charset can be any encoding supported by the system's iconv library routine. -fwide-exec-charset=charset Set the wide execution character set, used for wide string and character constants. The default is UTF-32 or UTF-16, whichever corresponds to the width of wchar_t. As with -fexec-charset, charset can be any encoding supported by the system's iconv library routine; however, you will have problems with encodings that do not fit exactly in wchar_t. -finput-charset=charset Set the input character set, used for translation from the character set of the input file to the source character set used by GCC. If the locale does not specify, or GCC cannot get this information from the locale, the default is UTF-8. This can be overridden by either the locale or this command-line option. Currently the command-line option takes precedence if there's a conflict. charset can be any encoding supported by the system's iconv library routine. -fpch-deps When using precompiled headers (see Precompiled Headers), this flag causes the dependency-output flags to also list the files from the precompiled header's dependencies. If not specified, only the precompiled header are listed and not the files that were used to create it, because those files are not consulted when a precompiled header is used. -fpch-preprocess This option allows use of a precompiled header (see Precompiled Headers) together with -E. It inserts a special #pragma, #pragma GCC pch_preprocess "filename" in the output to mark the place where the precompiled header was found, and its filename. When -fpreprocessed is in use, GCC recognizes this #pragma and loads the PCH. This option is off by default, because the resulting preprocessed output is only really suitable as input to GCC. It is switched on by -save-temps. You should not write this #pragma in your own code, but it is safe to edit the filename if the PCH file is available in a different location. The filename may be absolute or it may be relative to GCC's current directory. -fworking-directory Enable generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preprocessor emits, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC uses this directory, when it's present in the preprocessed input, as the directory emitted as the current working directory in some debugging information formats. This option is implicitly enabled if debugging information is enabled, but this can be inhibited with the negated form -fno-working-directory. If the -P flag is present in the command line, this option has no effect, since no #line directives are emitted whatsoever. -A predicate=answer Make an assertion with the predicate predicate and answer answer. This form is preferred to the older form -A predicate(answer), which is still supported, because it does not use shell special characters. -A -predicate=answer Cancel an assertion with the predicate predicate and answer answer. -C Do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive. You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a '#'. -CC Do not discard comments, including during macro expansion. This is like -C, except that comments contained within macros are also passed through to the output file where the macro is expanded. In addition to the side effects of the -C option, the -CC option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line. The -CC option is generally used to support lint comments. -P Inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the preprocessor on something that is not C code, and will be sent to a program which might be confused by the linemarkers. -traditional -traditional-cpp Try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors. See the GNU CPP manual for details. Note that GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are only supported with the -E switch, or when invoking CPP explicitly. -trigraphs Support ISO C trigraphs. These are three-character sequences, all starting with '??', that are defined by ISO C to stand for single characters. For example, '??/' stands for '\', so ''??/n'' is a character constant for a newline. The nine trigraphs and their replacements are Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??- Replacement: [ ] { } # \ ^ | ~ By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the -std and -ansi options. -remap Enable special code to work around file systems which only permit very short file names, such as MS-DOS. -H Print the name of each header file used, in addition to other normal activities. Each name is indented to show how deep in the '#include' stack it is. Precompiled header files are also printed, even if they are found to be invalid; an invalid precompiled header file is printed with '...x' and a valid one with '...!' . -dletters Says to make debugging dumps during compilation as specified by letters. The flags documented here are those relevant to the preprocessor. Other letters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify letters whose behavior conflicts, the result is undefined. See Developer Options, for more information. -dM Instead of the normal output, generate a list of '#define' directives for all the macros defined during the execution of the preprocessor, including predefined macros. This gives you a way of finding out what is predefined in your version of the preprocessor. Assuming you have no file foo.h, the command touch foo.h; cpp -dM foo.h shows all the predefined macros. If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach. See (gcc)Developer Options. -dD Like -dM except in two respects: it does not include the predefined macros, and it outputs both the '#define' directives and the result of preprocessing. Both kinds of output go to the standard output file. -dN Like -dD, but emit only the macro names, not their expansions. -dI Output '#include' directives in addition to the result of preprocessing. -dU Like -dD except that only macros that are expanded, or whose definedness is tested in preprocessor directives, are output; the output is delayed until the use or test of the macro; and '#undef' directives are also output for macros tested but undefined at the time. -fdebug-cpp This option is only useful for debugging GCC. When used from CPP or with -E, it dumps debugging information about location maps. Every token in the output is preceded by the dump of the map its location belongs to. When used from GCC without -E, this option has no effect. -Wp,option You can use -Wp,option to bypass the compiler driver and pass option directly through to the preprocessor. If option contains commas, it is split into multiple options at the commas. However, many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses this phase. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and let the driver handle the options instead. -Xpreprocessor option Pass option as an option to the preprocessor. You can use this to supply system-specific preprocessor options that GCC does not recognize. If you want to pass an option that takes an argument, you must use -Xpreprocessor twice, once for the option and once for the argument. -no-integrated-cpp Perform preprocessing as a separate pass before compilation. By default, GCC performs preprocessing as an integrated part of input tokenization and parsing. If this option is provided, the appropriate language front end (cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively) is instead invoked twice, once for preprocessing only and once for actual compilation of the preprocessed input. This option may be useful in conjunction with the -B or -wrapper options to specify an alternate preprocessor or perform additional processing of the program source between normal preprocessing and compilation. ────────────────────────────────────────────────────────────────────────────── Next: Assembler Options, Previous: Instrumentation Options, Up: Invoking Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Invoking GCC Link: next: Link Options Link: prev: Preprocessor Options Next: Link Options, Previous: Preprocessor Options, Up: Invoking GCC ────────────────────────────────────────────────────────────────────────────── 3.13 Passing Options to the Assembler You can pass options to the assembler. -Wa,option Pass option as an option to the assembler. If option contains commas, it is split into multiple options at the commas. -Xassembler option Pass option as an option to the assembler. You can use this to supply system-specific assembler options that GCC does not recognize. If you want to pass an option that takes an argument, you must use -Xassembler twice, once for the option and once for the argument. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Invoking GCC Link: next: Directory Options Link: prev: Assembler Options Next: Directory Options, Previous: Assembler Options, Up: Invoking GCC ────────────────────────────────────────────────────────────────────────────── 3.14 Options for Linking These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step. object-file-name A file name that does not end in a special recognized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file contents.) If linking is done, these object files are used as input to the linker. -c -S -E If any of these options is used, then the linker is not run, and object file names should not be used as arguments. See Overall Options. -flinker-output=type This option controls the code generation of the link time optimizer. By default the linker output is determined by the linker plugin automatically. For debugging the compiler and in the case of incremental linking to non-lto object file is desired, it may be useful to control the type manually. If type is 'exec' the code generation is configured to produce static binary. In this case -fpic and -fpie are both disabled. If type is 'dyn' the code generation is configured to produce shared library. In this case -fpic or -fPIC is preserved, but not enabled automatically. This makes it possible to build shared libraries without position independent code on architectures this is possible, i.e. on x86. If type is 'pie' the code generation is configured to produce -fpie executable. This result in similar optimizations as 'exec' except that -fpie is not disabled if specified at compilation time. If type is 'rel' the compiler assumes that incremental linking is done. The sections containing intermediate code for link-time optimization are merged, pre-optimized, and output to the resulting object file. In addition, if -ffat-lto-objects is specified the binary code is produced for future non-lto linking. The object file produced by incremental linking will be smaller than a static library produced from the same object files. At link-time the result of incremental linking will also load faster to compiler than a static library assuming that majority of objects in the library are used. Finally 'nolto-rel' configure compiler to for incremental linking where code generation is forced, final binary is produced and the intermediate code for later link-time optimization is stripped. When multiple object files are linked together the resulting code will be optimized better than with link time optimizations disabled (for example, the cross-module inlining will happen), most of benefits of whole program optimizations are however lost. During the incremental link (by -r) the linker plugin will default to rel. With current interfaces to GNU Binutils it is however not possible to link incrementally LTO objects and non-LTO objects into a single mixed object file. In the case any of object files in incremental link cannot be used for link-time optimization the linker plugin will output warning and use 'nolto-rel'. To maintain the whole program optimization it is recommended to link such objects into static library instead. Alternatively it is possible to use H.J. Lu's binutils with support for mixed objects. -fuse-ld=bfd Use the bfd linker instead of the default linker. -fuse-ld=gold Use the gold linker instead of the default linker. -fuse-ld=lld Use the LLVM lld linker instead of the default linker. -llibrary -l library Search the library named library when linking. (The second alternative with the library as a separate argument is only for POSIX compliance and is not recommended.) The -l option is passed directly to the linker by GCC. Refer to your linker documentation for exact details. The general description below applies to the GNU linker. The linker searches a standard list of directories for the library. The directories searched include several standard system directories plus any that you specify with -L. Static libraries are archives of object files, and have file names like liblibrary.a. Some targets also support shared libraries, which typically have names like liblibrary.so. If both static and shared libraries are found, the linker gives preference to linking with the shared library unless the -static option is used. It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, 'foo.o -lz bar.o' searches library 'z' after file foo.o but before bar.o. If bar.o refers to functions in 'z', those functions may not be loaded. -lobjc You need this special case of the -l option in order to link an Objective-C or Objective-C++ program. -nostartfiles Do not use the standard system startup files when linking. The standard system libraries are used normally, unless -nostdlib, -nolibc, or -nodefaultlibs is used. -nodefaultlibs Do not use the standard system libraries when linking. Only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored. The standard startup files are used normally, unless -nostartfiles is used. The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified. -nolibc Do not use the C library or system libraries tightly coupled with it when linking. Still link with the startup files, libgcc or toolchain provided language support libraries such as libgnat, libgfortran or libstdc++ unless options preventing their inclusion are used as well. This typically removes -lc from the link command line, as well as system libraries that normally go with it and become meaningless when absence of a C library is assumed, for example -lpthread or -lm in some configurations. This is intended for bare-board targets when there is indeed no C library available. -nostdlib Do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored. The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified. One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages. (See Interfacing to GCC Output in GNU Compiler Collection (GCC) Internals, for more discussion of libgcc.a.) In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that you have no unresolved references to internal GCC library subroutines. (An example of such an internal subroutine is __main, used to ensure C++ constructors are called; see collect2 in GNU Compiler Collection (GCC) Internals.) -e entry --entry=entry Specify that the program entry point is entry. The argument is interpreted by the linker; the GNU linker accepts either a symbol name or an address. -pie Produce a dynamically linked position independent executable on targets that support it. For predictable results, you must also specify the same set of options used for compilation (-fpie, -fPIE, or model suboptions) when you specify this linker option. -no-pie Don't produce a dynamically linked position independent executable. -static-pie Produce a static position independent executable on targets that support it. A static position independent executable is similar to a static executable, but can be loaded at any address without a dynamic linker. For predictable results, you must also specify the same set of options used for compilation (-fpie, -fPIE, or model suboptions) when you specify this linker option. -pthread Link with the POSIX threads library. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. On some targets this option also sets flags for the preprocessor, so it should be used consistently for both compilation and linking. -r Produce a relocatable object as output. This is also known as partial linking. -rdynamic Pass the flag -export-dynamic to the ELF linker, on targets that support it. This instructs the linker to add all symbols, not only used ones, to the dynamic symbol table. This option is needed for some uses of dlopen or to allow obtaining backtraces from within a program. -s Remove all symbol table and relocation information from the executable. -static On systems that support dynamic linking, this overrides -pie and prevents linking with the shared libraries. On other systems, this option has no effect. -shared Produce a shared object which can then be linked with other objects to form an executable. Not all systems support this option. For predictable results, you must also specify the same set of options used for compilation (-fpic, -fPIC, or model suboptions) when you specify this linker option.^1 -shared-libgcc -static-libgcc On systems that provide libgcc as a shared library, these options force the use of either the shared or static version, respectively. If no shared version of libgcc was built when the compiler was configured, these options have no effect. There are several situations in which an application should use the shared libgcc instead of the static version. The most common of these is when the application wishes to throw and catch exceptions across different shared libraries. In that case, each of the libraries as well as the application itself should use the shared libgcc. Therefore, the G++ driver automatically adds -shared-libgcc whenever you build a shared library or a main executable, because C++ programs typically use exceptions, so this is the right thing to do. If, instead, you use the GCC driver to create shared libraries, you may find that they are not always linked with the shared libgcc. If GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that does not support option --eh-frame-hdr, it links the shared version of libgcc into shared libraries by default. Otherwise, it takes advantage of the linker and optimizes away the linking with the shared version of libgcc, linking with the static version of libgcc by default. This allows exceptions to propagate through such shared libraries, without incurring relocation costs at library load time. However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ driver, or using the option -shared-libgcc, such that it is linked with the shared libgcc. -static-libasan When the -fsanitize=address option is used to link a program, the GCC driver automatically links against libasan. If libasan is available as a shared library, and the -static option is not used, then this links against the shared version of libasan. The -static-libasan option directs the GCC driver to link libasan statically, without necessarily linking other libraries statically. -static-libtsan When the -fsanitize=thread option is used to link a program, the GCC driver automatically links against libtsan. If libtsan is available as a shared library, and the -static option is not used, then this links against the shared version of libtsan. The -static-libtsan option directs the GCC driver to link libtsan statically, without necessarily linking other libraries statically. -static-liblsan When the -fsanitize=leak option is used to link a program, the GCC driver automatically links against liblsan. If liblsan is available as a shared library, and the -static option is not used, then this links against the shared version of liblsan. The -static-liblsan option directs the GCC driver to link liblsan statically, without necessarily linking other libraries statically. -static-libubsan When the -fsanitize=undefined option is used to link a program, the GCC driver automatically links against libubsan. If libubsan is available as a shared library, and the -static option is not used, then this links against the shared version of libubsan. The -static-libubsan option directs the GCC driver to link libubsan statically, without necessarily linking other libraries statically. -static-libstdc++ When the g++ program is used to link a C++ program, it normally automatically links against libstdc++. If libstdc++ is available as a shared library, and the -static option is not used, then this links against the shared version of libstdc++. That is normally fine. However, it is sometimes useful to freeze the version of libstdc++ used by the program without going all the way to a fully static link. The -static-libstdc++ option directs the g++ driver to link libstdc++ statically, without necessarily linking other libraries statically. -symbolic Bind references to global symbols when building a shared object. Warn about any unresolved references (unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this option. -T script Use script as the linker script. This option is supported by most systems using the GNU linker. On some targets, such as bare-board targets without an operating system, the -T option may be required when linking to avoid references to undefined symbols. -Xlinker option Pass option as an option to the linker. You can use this to supply system-specific linker options that GCC does not recognize. If you want to pass an option that takes a separate argument, you must use -Xlinker twice, once for the option and once for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions. It does not work to write -Xlinker "-assert definitions", because this passes the entire string as a single argument, which is not what the linker expects. When using the GNU linker, it is usually more convenient to pass arguments to linker options using the option=value syntax than as separate arguments. For example, you can specify -Xlinker -Map=output.map rather than -Xlinker -Map -Xlinker output.map. Other linkers may not support this syntax for command-line options. -Wl,option Pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas. You can use this syntax to pass an argument to the option. For example, -Wl,-Map,output.map passes -Map output.map to the linker. When using the GNU linker, you can also get the same effect with -Wl,-Map=output.map. -u symbol Pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use -u multiple times with different symbols to force loading of additional library modules. -z keyword -z is passed directly on to the linker along with the keyword keyword. See the section in the documentation of your linker for permitted values and their meanings. Footnotes (1) On some systems, 'gcc -shared' needs to build supplementary stub code for constructors to work. On multi-libbed systems, 'gcc -shared' must select the correct support libraries to link against. Failing to supply the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is innocuous. ────────────────────────────────────────────────────────────────────────────── Next: Directory Options, Previous: Assembler Options, Up: Invoking GCC Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Invoking GCC Link: next: Code Gen Options Link: prev: Link Options Next: Code Gen Options, Previous: Link Options, Up: Invoking GCC ────────────────────────────────────────────────────────────────────────────── 3.15 Options for Directory Search These options specify directories to search for header files, for libraries and for parts of the compiler: -I dir -iquote dir -isystem dir -idirafter dir Add the directory dir to the list of directories to be searched for header files during preprocessing. If dir begins with '=' or $SYSROOT, then the '=' or $SYSROOT is replaced by the sysroot prefix; see --sysroot and -isysroot. Directories specified with -iquote apply only to the quote form of the directive, #include "file". Directories specified with -I, -isystem, or -idirafter apply to lookup for both the #include "file" and #include directives. You can specify any number or combination of these options on the command line to search for header files in several directories. The lookup order is as follows: 1. For the quote form of the include directive, the directory of the current file is searched first. 2. For the quote form of the include directive, the directories specified by -iquote options are searched in left-to-right order, as they appear on the command line. 3. Directories specified with -I options are scanned in left-to-right order. 4. Directories specified with -isystem options are scanned in left-to-right order. 5. Standard system directories are scanned. 6. Directories specified with -idirafter options are scanned in left-to-right order. You can use -I to override a system header file, substituting your own version, since these directories are searched before the standard system header file directories. However, you should not use this option to add directories that contain vendor-supplied system header files; use -isystem for that. The -isystem and -idirafter options also mark the directory as a system directory, so that it gets the same special treatment that is applied to the standard system directories. If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option is ignored. The directory is still searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the #include_next directive are not inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options. -I- Split the include path. This option has been deprecated. Please use -iquote instead for -I directories before the -I- and remove the -I- option. Any directories specified with -I options before -I- are searched only for headers requested with #include "file"; they are not searched for #include . If additional directories are specified with -I options after the -I-, those directories are searched for all '#include' directives. In addition, -I- inhibits the use of the directory of the current file directory as the first search directory for #include "file". There is no way to override this effect of -I-. -iprefix prefix Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you should include the final '/'. -iwithprefix dir -iwithprefixbefore dir Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. -iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it where -idirafter would. -isysroot dir This option is like the --sysroot option, but applies only to header files (except for Darwin targets, where it applies to both header files and libraries). See the --sysroot option for more information. -imultilib dir Use dir as a subdirectory of the directory containing target-specific C++ headers. -nostdinc Do not search the standard system directories for header files. Only the directories explicitly specified with -I, -iquote, -isystem, and/or -idirafter options (and the directory of the current file, if appropriate) are searched. -nostdinc++ Do not search for header files in the C++-specific standard directories, but do still search the other standard directories. (This option is used when building the C++ library.) -iplugindir=dir Set the directory to search for plugins that are passed by -fplugin=name instead of -fplugin=path/name.so. This option is not meant to be used by the user, but only passed by the driver. -Ldir Add directory dir to the list of directories to be searched for -l. -Bprefix This option specifies where to find the executables, libraries, include files, and data files of the compiler itself. The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a prefix for each program it tries to run, both with and without 'machine/version/' for the corresponding target machine and compiler version. For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name is not found, or if -B is not specified, the driver tries two standard prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your PATH environment variable. The compiler checks to see if the path provided by -B refers to a directory, and if necessary it adds a directory separator character at the end of the path. -B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker. They also apply to include files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor. In this case, the compiler appends 'include' to the prefix. The runtime support file libgcc.a can also be searched for using the -B prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means. Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX. See Environment Variables. As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it is replaced by [dir/]include. This is to help with boot-strapping the compiler. -no-canonical-prefixes Do not expand any symbolic links, resolve references to '/../' or '/./', or make the path absolute when generating a relative prefix. --sysroot=dir Use dir as the logical root directory for headers and libraries. For example, if the compiler normally searches for headers in /usr/include and libraries in /usr/lib, it instead searches dir/usr/include and dir/usr/lib. If you use both this option and the -isysroot option, then the --sysroot option applies to libraries, but the -isysroot option applies to header files. The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker does not support this option, the header file aspect of --sysroot still works, but the library aspect does not. --no-sysroot-suffix For some targets, a suffix is added to the root directory specified with --sysroot, depending on the other options used, so that headers may for example be found in dir/suffix/usr/include instead of dir/usr/include. This option disables the addition of such a suffix. ────────────────────────────────────────────────────────────────────────────── Next: Code Gen Options, Previous: Link Options, Up: Invoking GCC Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Invoking GCC Link: next: Developer Options Link: prev: Directory Options Next: Developer Options, Previous: Directory Options, Up: Invoking GCC ────────────────────────────────────────────────────────────────────────────── 3.16 Options for Code Generation Conventions These machine-independent options control the interface conventions used in code generation. Most of them have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed—the one that is not the default. You can figure out the other form by either removing 'no-' or adding it. -fstack-reuse=reuse-level This option controls stack space reuse for user declared local/auto variables and compiler generated temporaries. reuse_level can be 'all', 'named_vars', or 'none'. 'all' enables stack reuse for all local variables and temporaries, 'named_vars' enables the reuse only for user defined local variables with names, and 'none' disables stack reuse completely. The default value is 'all'. The option is needed when the program extends the lifetime of a scoped local variable or a compiler generated temporary beyond the end point defined by the language. When a lifetime of a variable ends, and if the variable lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. Legacy code extending local lifetime is likely to break with the stack reuse optimization. For example, int *p; { int local1; p = &local1; local1 = 10; .... } { int local2; local2 = 20; ... } if (*p == 10) // out of scope use of local1 { } Another example: struct A { A(int k) : i(k), j(k) { } int i; int j; }; A *ap; void foo(const A& ar) { ap = &ar; } void bar() { foo(A(10)); // temp object's lifetime ends when foo returns { A a(20); .... } ap->i+= 10; // ap references out of scope temp whose space // is reused with a. What is the value of ap->i? } The lifetime of a compiler generated temporary is well defined by the C++ standard. When a lifetime of a temporary ends, and if the temporary lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. However some of the legacy code relies on the behavior of older compilers in which temporaries' stack space is not reused, the aggressive stack reuse can lead to runtime errors. This option is used to control the temporary stack reuse optimization. -ftrapv This option generates traps for signed overflow on addition, subtraction, multiplication operations. The options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being effective. -fwrapv This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation. This flag enables some optimizations and disables others. The options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being effective. -fwrapv-pointer This option instructs the compiler to assume that pointer arithmetic overflow on addition and subtraction wraps around using twos-complement representation. This flag disables some optimizations which assume pointer overflow is invalid. -fstrict-overflow This option implies -fno-wrapv -fno-wrapv-pointer and when negated implies -fwrapv -fwrapv-pointer. -fexceptions Enable exception handling. Generates extra code needed to propagate exceptions. For some targets, this implies GCC generates frame unwind information for all functions, which can produce significant data size overhead, although it does not affect execution. If you do not specify this option, GCC enables it by default for languages like C++ that normally require exception handling, and disables it for languages like C that do not normally require it. However, you may need to enable this option when compiling C code that needs to interoperate properly with exception handlers written in C++. You may also wish to disable this option if you are compiling older C++ programs that don't use exception handling. -fnon-call-exceptions Generate code that allows trapping instructions to throw exceptions. Note that this requires platform-specific runtime support that does not exist everywhere. Moreover, it only allows trapping instructions to throw exceptions, i.e. memory references or floating-point instructions. It does not allow exceptions to be thrown from arbitrary signal handlers such as SIGALRM. -fdelete-dead-exceptions Consider that instructions that may throw exceptions but don't otherwise contribute to the execution of the program can be optimized away. This option is enabled by default for the Ada front end, as permitted by the Ada language specification. Optimization passes that cause dead exceptions to be removed are enabled independently at different optimization levels. -funwind-tables Similar to -fexceptions, except that it just generates any needed static data, but does not affect the generated code in any other way. You normally do not need to enable this option; instead, a language processor that needs this handling enables it on your behalf. -fasynchronous-unwind-tables Generate unwind table in DWARF format, if supported by target machine. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector). -fno-gnu-unique On systems with recent GNU assembler and C library, the C++ compiler uses the STB_GNU_UNIQUE binding to make sure that definitions of template static data members and static local variables in inline functions are unique even in the presence of RTLD_LOCAL; this is necessary to avoid problems with a library used by two different RTLD_LOCAL plugins depending on a definition in one of them and therefore disagreeing with the other one about the binding of the symbol. But this causes dlclose to be ignored for affected DSOs; if your program relies on reinitialization of a DSO via dlclose and dlopen, you can use -fno-gnu-unique. -fpcc-struct-return Return “short” struct and union values in memory like longer ones, rather than in registers. This convention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files compiled with other compilers, particularly the Portable C Compiler (pcc). The precise convention for returning structures in memory depends on the target configuration macros. Short structures and unions are those whose size and alignment match that of some integer type. Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled with the -freg-struct-return switch. Use it to conform to a non-default application binary interface. -freg-struct-return Return struct and union values in registers when possible. This is more efficient for small structures than -fpcc-struct-return. If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target. If there is no standard convention, GCC defaults to -fpcc-struct-return, except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative. Warning: code compiled with the -freg-struct-return switch is not binary compatible with code compiled with the -fpcc-struct-return switch. Use it to conform to a non-default application binary interface. -fshort-enums Allocate to an enum type only as many bytes as it needs for the declared range of possible values. Specifically, the enum type is equivalent to the smallest integer type that has enough room. Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. -fshort-wchar Override the underlying type for wchar_t to be short unsigned int instead of the default for the target. This option is useful for building programs to run under WINE. Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. -fno-common In C code, this option controls the placement of global variables defined without an initializer, known as tentative definitions in the C standard. Tentative definitions are distinct from declarations of a variable with the extern keyword, which do not allocate storage. Unix C compilers have traditionally allocated storage for uninitialized global variables in a common block. This allows the linker to resolve all tentative definitions of the same variable in different compilation units to the same object, or to a non-tentative definition. This is the behavior specified by -fcommon, and is the default for GCC on most targets. On the other hand, this behavior is not required by ISO C, and on some targets may carry a speed or code size penalty on variable references. The -fno-common option specifies that the compiler should instead place uninitialized global variables in the BSS section of the object file. This inhibits the merging of tentative definitions by the linker so you get a multiple-definition error if the same variable is defined in more than one compilation unit. Compiling with -fno-common is useful on targets for which it provides better performance, or if you wish to verify that the program will work on other systems that always treat uninitialized variable definitions this way. -fno-ident Ignore the #ident directive. -finhibit-size-directive Don't output a .size assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory. This option is used when compiling crtstuff.c; you should not need to use it for anything else. -fverbose-asm Put extra commentary information in the generated assembly code to make it more readable. This option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself). -fno-verbose-asm, the default, causes the extra information to be omitted and is useful when comparing two assembler files. The added comments include: * information on the compiler version and command-line options, * the source code lines associated with the assembly instructions, in the form FILENAME:LINENUMBER:CONTENT OF LINE, * hints on which high-level expressions correspond to the various assembly instruction operands. For example, given this C source file: int test (int n) { int i; int total = 0; for (i = 0; i < n; i++) total += i * i; return total; } compiling to (x86_64) assembly via -S and emitting the result direct to stdout via -o - gcc -S test.c -fverbose-asm -Os -o - gives output similar to this: .file "test.c" # GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu) [...snip...] # options passed: [...snip...] .text .globl test .type test, @function test: .LFB0: .cfi_startproc # test.c:4: int total = 0; xorl %eax, %eax # # test.c:6: for (i = 0; i < n; i++) xorl %edx, %edx # i .L2: # test.c:6: for (i = 0; i < n; i++) cmpl %edi, %edx # n, i jge .L5 #, # test.c:7: total += i * i; movl %edx, %ecx # i, tmp92 imull %edx, %ecx # i, tmp92 # test.c:6: for (i = 0; i < n; i++) incl %edx # i # test.c:7: total += i * i; addl %ecx, %eax # tmp92, jmp .L2 # .L5: # test.c:10: } ret .cfi_endproc .LFE0: .size test, .-test .ident "GCC: (GNU) 7.0.0 20160809 (experimental)" .section .note.GNU-stack,"",@progbits The comments are intended for humans rather than machines and hence the precise format of the comments is subject to change. -frecord-gcc-switches This switch causes the command line used to invoke the compiler to be recorded into the object file that is being created. This switch is only implemented on some targets and the exact format of the recording is target and binary file format dependent, but it usually takes the form of a section containing ASCII text. This switch is related to the -fverbose-asm switch, but that switch only records information in the assembler output file as comments, so it never reaches the object file. See also -grecord-gcc-switches for another way of storing compiler options into the object file. -fpic Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that -fpic does not work; in that case, recompile with -fPIC instead. (These maximums are 8k on the SPARC, 28k on AArch64 and 32k on the m68k and RS/6000. The x86 has no such limit.) Position-independent code requires special support, and therefore works only on certain machines. For the x86, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent. When this flag is set, the macros __pic__ and __PIC__ are defined to 1. -fPIC If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on AArch64, m68k, PowerPC and SPARC. Position-independent code requires special support, and therefore works only on certain machines. When this flag is set, the macros __pic__ and __PIC__ are defined to 2. -fpie -fPIE These options are similar to -fpic and -fPIC, but the generated position-independent code can be only linked into executables. Usually these options are used to compile code that will be linked using the -pie GCC option. -fpie and -fPIE both define the macros __pie__ and __PIE__. The macros have the value 1 for -fpie and 2 for -fPIE. -fno-plt Do not use the PLT for external function calls in position-independent code. Instead, load the callee address at call sites from the GOT and branch to it. This leads to more efficient code by eliminating PLT stubs and exposing GOT loads to optimizations. On architectures such as 32-bit x86 where PLT stubs expect the GOT pointer in a specific register, this gives more register allocation freedom to the compiler. Lazy binding requires use of the PLT; with -fno-plt all external symbols are resolved at load time. Alternatively, the function attribute noplt can be used to avoid calls through the PLT for specific external functions. In position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT to use the GOT instead. -fno-jump-tables Do not use jump tables for switch statements even where it would be more efficient than other code generation strategies. This option is of use in conjunction with -fpic or -fPIC for building code that forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables do not require a GOT and this option is not needed. -ffixed-reg Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role). reg must be the name of a register. The register names accepted are machine-specific and are defined in the REGISTER_NAMES macro in the machine description macro file. This flag does not have a negative form, because it specifies a three-way choice. -fcall-used-reg Treat the register named reg as an allocable register that is clobbered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way do not save and restore the register reg. It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results. This flag does not have a negative form, because it specifies a three-way choice. -fcall-saved-reg Treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way save and restore the register reg if they use it. It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results. A different sort of disaster results from the use of this flag for a register in which function values may be returned. This flag does not have a negative form, because it specifies a three-way choice. -fpack-struct[=n] Without a value specified, pack all structure members together without holes. When a value is specified (which must be a small power of two), pack structure members according to this value, representing the maximum alignment (that is, objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location. Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary interface. -fleading-underscore This option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented in the object file. One use is to help link with legacy assembly code. Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch. -ftls-model=model Alter the thread-local storage model to be used (see Thread-Local). The model argument should be one of 'global-dynamic', 'local-dynamic', 'initial-exec' or 'local-exec'. Note that the choice is subject to optimization: the compiler may use a more efficient model for symbols not visible outside of the translation unit, or if -fpic is not given on the command line. The default without -fpic is 'initial-exec'; with -fpic the default is 'global-dynamic'. -ftrampolines For targets that normally need trampolines for nested functions, always generate them instead of using descriptors. Otherwise, for targets that do not need them, like for example HP-PA or IA-64, do nothing. A trampoline is a small piece of code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. Therefore, it requires the stack to be made executable in order for the program to work properly. -fno-trampolines is enabled by default on a language by language basis to let the compiler avoid generating them, if it computes that this is safe, and replace them with descriptors. Descriptors are made up of data only, but the generated code must be prepared to deal with them. As of this writing, -fno-trampolines is enabled by default only for Ada. Moreover, code compiled with -ftrampolines and code compiled with -fno-trampolines are not binary compatible if nested functions are present. This option must therefore be used on a program-wide basis and be manipulated with extreme care. -fvisibility=[default|internal|hidden|protected] Set the default ELF image symbol visibility to the specified option—all symbols are marked with this unless overridden within the code. Using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent symbol clashes. It is strongly recommended that you use this in any shared objects you distribute. Despite the nomenclature, 'default' always means public; i.e., available to be linked against from outside the shared object. 'protected' and 'internal' are pretty useless in real-world usage so the only other commonly used option is 'hidden'. The default if -fvisibility isn't specified is 'default', i.e., make every symbol public. A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by “How To Write Shared Libraries” by Ulrich Drepper (which can be found at https://www.akkadia.org/drepper/)—however a superior solution made possible by this option to marking things hidden when the default is public is to make the default hidden and mark things public. This is the norm with DLLs on Windows and with -fvisibility=hidden and __attribute__ ((visibility("default"))) instead of __declspec(dllexport) you get almost identical semantics with identical syntax. This is a great boon to those working with cross-platform projects. For those adding visibility support to existing code, you may find #pragma GCC visibility of use. This works by you enclosing the declarations you wish to set visibility for with (for example) #pragma GCC visibility push(hidden) and #pragma GCC visibility pop. Bear in mind that symbol visibility should be viewed as part of the API interface contract and thus all new code should always specify visibility when it is not the default; i.e., declarations only for use within the local DSO should always be marked explicitly as hidden as so to avoid PLT indirection overheads—making this abundantly clear also aids readability and self-documentation of the code. Note that due to ISO C++ specification requirements, operator new and operator delete must always be of default visibility. Be aware that headers from outside your project, in particular system headers and headers from any other library you use, may not be expecting to be compiled with visibility other than the default. You may need to explicitly say #pragma GCC visibility push(default) before including any such headers. extern declarations are not affected by -fvisibility, so a lot of code can be recompiled with -fvisibility=hidden with no modifications. However, this means that calls to extern functions with no explicit visibility use the PLT, so it is more effective to use __attribute ((visibility)) and/or #pragma GCC visibility to tell the compiler which extern declarations should be treated as hidden. Note that -fvisibility does affect C++ vague linkage entities. This means that, for instance, an exception class that is be thrown between DSOs must be explicitly marked with default visibility so that the 'type_info' nodes are unified between the DSOs. An overview of these techniques, their benefits and how to use them is at http://gcc.gnu.org/wiki/Visibility. -fstrict-volatile-bitfields This option should be used if accesses to volatile bit-fields (or other structure fields, although the compiler usually honors those types anyway) should use a single access of the width of the field's type, aligned to a natural alignment if possible. For example, targets with memory-mapped peripheral registers might require all such accesses to be 16 bits wide; with this flag you can declare all peripheral bit-fields as unsigned short (assuming short is 16 bits on these targets) to force GCC to use 16-bit accesses instead of, perhaps, a more efficient 32-bit access. If this option is disabled, the compiler uses the most efficient instruction. In the previous example, that might be a 32-bit load instruction, even though that accesses bytes that do not contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated. In some cases, such as when the packed attribute is applied to a structure field, it may not be possible to access the field with a single read or write that is correctly aligned for the target machine. In this case GCC falls back to generating multiple accesses rather than code that will fault or truncate the result at run time. Note: Due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members. It is therefore recommended to define all bits of the field's type as bit-field members. The default value of this option is determined by the application binary interface for the target processor. -fsync-libcalls This option controls whether any out-of-line instance of the __sync family of functions may be used to implement the C++11 __atomic family of functions. The default value of this option is enabled, thus the only useful form of the option is -fno-sync-libcalls. This option is used in the implementation of the libatomic runtime library. ────────────────────────────────────────────────────────────────────────────── Next: Developer Options, Previous: Directory Options, Up: Invoking GCC Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Invoking GCC Link: next: Submodel Options Link: prev: Code Gen Options Next: Submodel Options, Previous: Code Gen Options, Up: Invoking GCC ────────────────────────────────────────────────────────────────────────────── 3.17 GCC Developer Options This section describes command-line options that are primarily of interest to GCC developers, including options to support compiler testing and investigation of compiler bugs and compile-time performance problems. This includes options that produce debug dumps at various points in the compilation; that print statistics such as memory use and execution time; and that print information about GCC's configuration, such as where it searches for libraries. You should rarely need to use any of these options for ordinary compilation and linking tasks. Many developer options that cause GCC to dump output to a file take an optional '=filename' suffix. You can specify 'stdout' or '-' to dump to standard output, and 'stderr' for standard error. If '=filename' is omitted, a default dump file name is constructed by concatenating the base dump file name, a pass number, phase letter, and pass name. The base dump file name is the name of output file produced by the compiler if explicitly specified and not an executable; otherwise it is the source file name. The pass number is determined by the order passes are registered with the compiler's pass manager. This is generally the same as the order of execution, but passes registered by plugins, target-specific passes, or passes that are otherwise registered late are numbered higher than the pass named 'final', even if they are executed earlier. The phase letter is one of 'i' (inter-procedural analysis), 'l' (language-specific), 'r' (RTL), or 't' (tree). The files are created in the directory of the output file. -dletters -fdump-rtl-pass -fdump-rtl-pass=filename Says to make debugging dumps during compilation at times specified by letters. This is used for debugging the RTL-based passes of the compiler. Some -dletters switches have different meaning when -E is used for preprocessing. See Preprocessor Options, for information about preprocessor-specific dump options. Debug dumps can be enabled with a -fdump-rtl switch or some -d option letters. Here are the possible letters for use in pass and letters, and their meanings: -fdump-rtl-alignments Dump after branch alignments have been computed. -fdump-rtl-asmcons Dump after fixing rtl statements that have unsatisfied in/out constraints. -fdump-rtl-auto_inc_dec Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions. -fdump-rtl-barriers Dump after cleaning up the barrier instructions. -fdump-rtl-bbpart Dump after partitioning hot and cold basic blocks. -fdump-rtl-bbro Dump after block reordering. -fdump-rtl-btl1 -fdump-rtl-btl2 -fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two branch target load optimization passes. -fdump-rtl-bypass Dump after jump bypassing and control flow optimizations. -fdump-rtl-combine Dump after the RTL instruction combination pass. -fdump-rtl-compgotos Dump after duplicating the computed gotos. -fdump-rtl-ce1 -fdump-rtl-ce2 -fdump-rtl-ce3 -fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if conversion passes. -fdump-rtl-cprop_hardreg Dump after hard register copy propagation. -fdump-rtl-csa Dump after combining stack adjustments. -fdump-rtl-cse1 -fdump-rtl-cse2 -fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common subexpression elimination passes. -fdump-rtl-dce Dump after the standalone dead code elimination passes. -fdump-rtl-dbr Dump after delayed branch scheduling. -fdump-rtl-dce1 -fdump-rtl-dce2 -fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store elimination passes. -fdump-rtl-eh Dump after finalization of EH handling code. -fdump-rtl-eh_ranges Dump after conversion of EH handling range regions. -fdump-rtl-expand Dump after RTL generation. -fdump-rtl-fwprop1 -fdump-rtl-fwprop2 -fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward propagation passes. -fdump-rtl-gcse1 -fdump-rtl-gcse2 -fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common subexpression elimination. -fdump-rtl-init-regs Dump after the initialization of the registers. -fdump-rtl-initvals Dump after the computation of the initial value sets. -fdump-rtl-into_cfglayout Dump after converting to cfglayout mode. -fdump-rtl-ira Dump after iterated register allocation. -fdump-rtl-jump Dump after the second jump optimization. -fdump-rtl-loop2 -fdump-rtl-loop2 enables dumping after the rtl loop optimization passes. -fdump-rtl-mach Dump after performing the machine dependent reorganization pass, if that pass exists. -fdump-rtl-mode_sw Dump after removing redundant mode switches. -fdump-rtl-rnreg Dump after register renumbering. -fdump-rtl-outof_cfglayout Dump after converting from cfglayout mode. -fdump-rtl-peephole2 Dump after the peephole pass. -fdump-rtl-postreload Dump after post-reload optimizations. -fdump-rtl-pro_and_epilogue Dump after generating the function prologues and epilogues. -fdump-rtl-sched1 -fdump-rtl-sched2 -fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic block scheduling passes. -fdump-rtl-ree Dump after sign/zero extension elimination. -fdump-rtl-seqabstr Dump after common sequence discovery. -fdump-rtl-shorten Dump after shortening branches. -fdump-rtl-sibling Dump after sibling call optimizations. -fdump-rtl-split1 -fdump-rtl-split2 -fdump-rtl-split3 -fdump-rtl-split4 -fdump-rtl-split5 These options enable dumping after five rounds of instruction splitting. -fdump-rtl-sms Dump after modulo scheduling. This pass is only run on some architectures. -fdump-rtl-stack Dump after conversion from GCC's “flat register file” registers to the x87's stack-like registers. This pass is only run on x86 variants. -fdump-rtl-subreg1 -fdump-rtl-subreg2 -fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after the two subreg expansion passes. -fdump-rtl-unshare Dump after all rtl has been unshared. -fdump-rtl-vartrack Dump after variable tracking. -fdump-rtl-vregs Dump after converting virtual registers to hard registers. -fdump-rtl-web Dump after live range splitting. -fdump-rtl-regclass -fdump-rtl-subregs_of_mode_init -fdump-rtl-subregs_of_mode_finish -fdump-rtl-dfinit -fdump-rtl-dfinish These dumps are defined but always produce empty files. -da -fdump-rtl-all Produce all the dumps listed above. -dA Annotate the assembler output with miscellaneous debugging information. -dD Dump all macro definitions, at the end of preprocessing, in addition to normal output. -dH Produce a core dump whenever an error occurs. -dp Annotate the assembler output with a comment indicating which pattern and alternative is used. The length and cost of each instruction are also printed. -dP Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp annotation. -dx Just generate RTL for a function instead of compiling it. Usually used with -fdump-rtl-expand. -fdump-debug Dump debugging information generated during the debug generation phase. -fdump-earlydebug Dump debugging information generated during the early debug generation phase. -fdump-noaddr When doing debugging dumps, suppress address output. This makes it more feasible to use diff on debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations. -freport-bug Collect and dump debug information into a temporary file if an internal compiler error (ICE) occurs. -fdump-unnumbered When doing debugging dumps, suppress instruction numbers and address output. This makes it more feasible to use diff on debugging dumps for compiler invocations with different options, in particular with and without -g. -fdump-unnumbered-links When doing debugging dumps (see -d option above), suppress instruction numbers for the links to the previous and next instructions in a sequence. -fdump-ipa-switch -fdump-ipa-switch-options Control the dumping at various stages of inter-procedural analysis language tree to a file. The file name is generated by appending a switch specific suffix to the source file name, and the file is created in the same directory as the output file. The following dumps are possible: 'all' Enables all inter-procedural analysis dumps. 'cgraph' Dumps information about call-graph optimization, unused function removal, and inlining decisions. 'inline' Dump after function inlining. Additionally, the options -optimized, -missed, -note, and -all can be provided, with the same meaning as for -fopt-info, defaulting to -optimized. For example, -fdump-ipa-inline-optimized-missed will emit information on callsites that were inlined, along with callsites that were not inlined. By default, the dump will contain messages about successful optimizations (equivalent to -optimized) together with low-level details about the analysis. -fdump-lang-all -fdump-lang-switch -fdump-lang-switch-options -fdump-lang-switch-options=filename Control the dumping of language-specific information. The options and filename portions behave as described in the -fdump-tree option. The following switch values are accepted: 'all' Enable all language-specific dumps. 'class' Dump class hierarchy information. Virtual table information is emitted unless 'slim' is specified. This option is applicable to C++ only. 'raw' Dump the raw internal tree data. This option is applicable to C++ only. -fdump-passes Print on stderr the list of optimization passes that are turned on and off by the current command-line options. -fdump-statistics-option Enable and control dumping of pass statistics in a separate file. The file name is generated by appending a suffix ending in '.statistics' to the source file name, and the file is created in the same directory as the output file. If the '-option' form is used, '-stats' causes counters to be summed over the whole compilation unit while '-details' dumps every event as the passes generate them. The default with no option is to sum counters for each function compiled. -fdump-tree-all -fdump-tree-switch -fdump-tree-switch-options -fdump-tree-switch-options=filename Control the dumping at various stages of processing the intermediate language tree to a file. If the '-options' form is used, options is a list of '-' separated options which control the details of the dump. Not all options are applicable to all dumps; those that are not meaningful are ignored. The following options are available 'address' Print the address of each node. Usually this is not meaningful as it changes according to the environment and source file. Its primary use is for tying up a dump file with a debug environment. 'asmname' If DECL_ASSEMBLER_NAME has been set for a given decl, use that in the dump instead of DECL_NAME. Its primary use is ease of use working backward from mangled names in the assembly file. 'slim' When dumping front-end intermediate representations, inhibit dumping of members of a scope or body of a function merely because that scope has been reached. Only dump such items when they are directly reachable by some other path. When dumping pretty-printed trees, this option inhibits dumping the bodies of control structures. When dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation. 'raw' Print a raw representation of the tree. By default, trees are pretty-printed into a C-like representation. 'details' Enable more detailed dumps (not honored by every dump option). Also include information from the optimization passes. 'stats' Enable dumping various statistics about the pass (not honored by every dump option). 'blocks' Enable showing basic block boundaries (disabled in raw dumps). 'graph' For each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with GraphViz to file.passid.pass.dot. Each function in the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot. This option currently only works for RTL dumps, and the RTL is always dumped in slim form. 'vops' Enable showing virtual operands for every statement. 'lineno' Enable showing line numbers for statements. 'uid' Enable showing the unique ID (DECL_UID) for each variable. 'verbose' Enable showing the tree dump for each statement. 'eh' Enable showing the EH region number holding each statement. 'scev' Enable showing scalar evolution analysis details. 'optimized' Enable showing optimization information (only available in certain passes). 'missed' Enable showing missed optimization information (only available in certain passes). 'note' Enable other detailed optimization information (only available in certain passes). 'all' Turn on all options, except raw, slim, verbose and lineno. 'optall' Turn on all optimization options, i.e., optimized, missed, and note. To determine what tree dumps are available or find the dump for a pass of interest follow the steps below. 1. Invoke GCC with -fdump-passes and in the stderr output look for a code that corresponds to the pass you are interested in. For example, the codes tree-evrp, tree-vrp1, and tree-vrp2 correspond to the three Value Range Propagation passes. The number at the end distinguishes distinct invocations of the same pass. 2. To enable the creation of the dump file, append the pass code to the -fdump- option prefix and invoke GCC with it. For example, to enable the dump from the Early Value Range Propagation pass, invoke GCC with the -fdump-tree-evrp option. Optionally, you may specify the name of the dump file. If you don't specify one, GCC creates as described below. 3. Find the pass dump in a file whose name is composed of three components separated by a period: the name of the source file GCC was invoked to compile, a numeric suffix indicating the pass number followed by the letter 't' for tree passes (and the letter 'r' for RTL passes), and finally the pass code. For example, the Early VRP pass dump might be in a file named myfile.c.038t.evrp in the current working directory. Note that the numeric codes are not stable and may change from one version of GCC to another. -fopt-info -fopt-info-options -fopt-info-options=filename Controls optimization dumps from various optimization passes. If the '-options' form is used, options is a list of '-' separated option keywords to select the dump details and optimizations. The options can be divided into three groups: 1. options describing what kinds of messages should be emitted, 2. options describing the verbosity of the dump, and 3. options describing which optimizations should be included. The options from each group can be freely mixed as they are non-overlapping. However, in case of any conflicts, the later options override the earlier options on the command line. The following options control which kinds of messages should be emitted: 'optimized' Print information when an optimization is successfully applied. It is up to a pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops which are successfully vectorized. 'missed' Print information about missed optimizations. Individual passes control which information to include in the output. 'note' Print verbose information about optimizations, such as certain transformations, more detailed messages about decisions etc. 'all' Print detailed optimization information. This includes 'optimized', 'missed', and 'note'. The following option controls the dump verbosity: 'internals' By default, only “high-level” messages are emitted. This option enables additional, more detailed, messages, which are likely to only be of interest to GCC developers. One or more of the following option keywords can be used to describe a group of optimizations: 'ipa' Enable dumps from all interprocedural optimizations. 'loop' Enable dumps from all loop optimizations. 'inline' Enable dumps from all inlining optimizations. 'omp' Enable dumps from all OMP (Offloading and Multi Processing) optimizations. 'vec' Enable dumps from all vectorization optimizations. 'optall' Enable dumps from all optimizations. This is a superset of the optimization groups listed above. If options is omitted, it defaults to 'optimized-optall', which means to dump messages about successful optimizations from all the passes, omitting messages that are treated as “internals”. If the filename is provided, then the dumps from all the applicable optimizations are concatenated into the filename. Otherwise the dump is output onto stderr. Though multiple -fopt-info options are accepted, only one of them can include a filename. If other filenames are provided then all but the first such option are ignored. Note that the output filename is overwritten in case of multiple translation units. If a combined output from multiple translation units is desired, stderr should be used instead. In the following example, the optimization info is output to stderr: gcc -O3 -fopt-info This example: gcc -O3 -fopt-info-missed=missed.all outputs missed optimization report from all the passes into missed.all, and this one: gcc -O2 -ftree-vectorize -fopt-info-vec-missed prints information about missed optimization opportunities from vectorization passes on stderr. Note that -fopt-info-vec-missed is equivalent to -fopt-info-missed-vec. The order of the optimization group names and message types listed after -fopt-info does not matter. As another example, gcc -O3 -fopt-info-inline-optimized-missed=inline.txt outputs information about missed optimizations as well as optimized locations from all the inlining passes into inline.txt. Finally, consider: gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt Here the two output filenames vec.miss and loop.opt are in conflict since only one output file is allowed. In this case, only the first option takes effect and the subsequent options are ignored. Thus only vec.miss is produced which contains dumps from the vectorizer about missed opportunities. -fsave-optimization-record Write a SRCFILE.opt-record.json.gz file detailing what optimizations were performed, for those optimizations that support -fopt-info. This option is experimental and the format of the data within the compressed JSON file is subject to change. It is roughly equivalent to a machine-readable version of -fopt-info-all, as a collection of messages with source file, line number and column number, with the following additional data for each message: * the execution count of the code being optimized, along with metadata about whether this was from actual profile data, or just an estimate, allowing consumers to prioritize messages by code hotness, * the function name of the code being optimized, where applicable, * the “inlining chain” for the code being optimized, so that when a function is inlined into several different places (which might themselves be inlined), the reader can distinguish between the copies, * objects identifying those parts of the message that refer to expressions, statements or symbol-table nodes, which of these categories they are, and, when available, their source code location, * the GCC pass that emitted the message, and * the location in GCC's own code from which the message was emitted Additionally, some messages are logically nested within other messages, reflecting implementation details of the optimization passes. -fsched-verbose=n On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints to the dump files. For n greater than zero, -fsched-verbose outputs the same information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one, it also output basic block probabilities, detailed ready list information and unit/insn info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n over four, -fsched-verbose also includes dependence info. -fenable-kind-pass -fdisable-kind-pass=range-list This is a set of options that are used to explicitly disable/enable optimization passes. These options are intended for use for debugging GCC. Compiler users should use regular options for enabling/disabling passes instead. -fdisable-ipa-pass Disable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1. -fdisable-rtl-pass -fdisable-rtl-pass=range-list Disable RTL pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1. range-list is a comma-separated list of function ranges or assembler names. Each range is a number pair separated by a colon. The range is inclusive in both ends. If the range is trivial, the number pair can be simplified as a single number. If the function's call graph node's uid falls within one of the specified ranges, the pass is disabled for that function. The uid is shown in the function header of a dump file, and the pass names can be dumped by using option -fdump-passes. -fdisable-tree-pass -fdisable-tree-pass=range-list Disable tree pass pass. See -fdisable-rtl for the description of option arguments. -fenable-ipa-pass Enable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1. -fenable-rtl-pass -fenable-rtl-pass=range-list Enable RTL pass pass. See -fdisable-rtl for option argument description and examples. -fenable-tree-pass -fenable-tree-pass=range-list Enable tree pass pass. See -fdisable-rtl for the description of option arguments. Here are some examples showing uses of these options. # disable ccp1 for all functions -fdisable-tree-ccp1 # disable complete unroll for function whose cgraph node uid is 1 -fenable-tree-cunroll=1 # disable gcse2 for functions at the following ranges [1,1], # [300,400], and [400,1000] # disable gcse2 for functions foo and foo2 -fdisable-rtl-gcse2=foo,foo2 # disable early inlining -fdisable-tree-einline # disable ipa inlining -fdisable-ipa-inline # enable tree full unroll -fenable-tree-unroll -fchecking -fchecking=n Enable internal consistency checking. The default depends on the compiler configuration. -fchecking=2 enables further internal consistency checking that might affect code generation. -frandom-seed=string This option provides a seed that GCC uses in place of random numbers in generating certain symbol names that have to be different in every compiled file. It is also used to place unique stamps in coverage data files and the object files that produce them. You can use the -frandom-seed option to produce reproducibly identical object files. The string can either be a number (decimal, octal or hex) or an arbitrary string (in which case it's converted to a number by computing CRC32). The string should be different for every file you compile. -save-temps -save-temps=cwd Store the usual “temporary” intermediate files permanently; place them in the current directory and name them based on the source file. Thus, compiling foo.c with -c -save-temps produces files foo.i and foo.s, as well as foo.o. This creates a preprocessed foo.i output file even though the compiler now normally uses an integrated preprocessor. When used in combination with the -x command-line option, -save-temps is sensible enough to avoid over writing an input source file with the same extension as an intermediate file. The corresponding intermediate file may be obtained by renaming the source file before using -save-temps. If you invoke GCC in parallel, compiling several different source files that share a common base name in different subdirectories or the same source file compiled for multiple output destinations, it is likely that the different parallel compilers will interfere with each other, and overwrite the temporary files. For instance: gcc -save-temps -o outdir1/foo.o indir1/foo.c& gcc -save-temps -o outdir2/foo.o indir2/foo.c& may result in foo.i and foo.o being written to simultaneously by both compilers. -save-temps=obj Store the usual “temporary” intermediate files permanently. If the -o option is used, the temporary files are based on the object file. If the -o option is not used, the -save-temps=obj switch behaves like -save-temps. For example: gcc -save-temps=obj -c foo.c gcc -save-temps=obj -c bar.c -o dir/xbar.o gcc -save-temps=obj foobar.c -o dir2/yfoobar creates foo.i, foo.s, dir/xbar.i, dir/xbar.s, dir2/yfoobar.i, dir2/yfoobar.s, and dir2/yfoobar.o. -time[=file] Report the CPU time taken by each subprocess in the compilation sequence. For C source files, this is the compiler proper and assembler (plus the linker if linking is done). Without the specification of an output file, the output looks like this: # cc1 0.12 0.01 # as 0.00 0.01 The first number on each line is the “user time”, that is time spent executing the program itself. The second number is “system time”, time spent executing operating system routines on behalf of the program. Both numbers are in seconds. With the specification of an output file, the output is appended to the named file, and it looks like this: 0.12 0.01 cc1 options 0.00 0.01 as options The “user time” and the “system time” are moved before the program name, and the options passed to the program are displayed, so that one can later tell what file was being compiled, and with which options. -fdump-final-insns[=file] Dump the final internal representation (RTL) to file. If the optional argument is omitted (or if file is .), the name of the dump file is determined by appending .gkd to the compilation output file name. -fcompare-debug[=opts] If no error occurs during compilation, run the compiler a second time, adding opts and -fcompare-debug-second to the arguments passed to the second compilation. Dump the final internal representation in both compilations, and print an error if they differ. If the equal sign is omitted, the default -gtoggle is used. The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and nonzero, implicitly enables -fcompare-debug. If GCC_COMPARE_DEBUG is defined to a string starting with a dash, then it is used for opts, otherwise the default -gtoggle is used. -fcompare-debug=, with the equal sign but without opts, is equivalent to -fno-compare-debug, which disables the dumping of the final representation and the second compilation, preventing even GCC_COMPARE_DEBUG from taking effect. To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking). To get just a warning, setting GCC_COMPARE_DEBUG to '-w%n-fcompare-debug not overridden' will do. -fcompare-debug-second This option is implicitly passed to the compiler for the second compilation requested by -fcompare-debug, along with options to silence warnings, and omitting other options that would cause the compiler to produce output to files or to standard output as a side effect. Dump files and preserved temporary files are renamed so as to contain the .gk additional extension during the second compilation, to avoid overwriting those generated by the first. When this option is passed to the compiler driver, it causes the first compilation to be skipped, which makes it useful for little other than debugging the compiler proper. -gtoggle Turn off generation of debug info, if leaving out this option generates it, or turn it on at level 2 otherwise. The position of this argument in the command line does not matter; it takes effect after all other options are processed, and it does so only once, no matter how many times it is given. This is mainly intended to be used with -fcompare-debug. -fvar-tracking-assignments-toggle Toggle -fvar-tracking-assignments, in the same way that -gtoggle toggles -g. -Q Makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes. -ftime-report Makes the compiler print some statistics about the time consumed by each pass when it finishes. -ftime-report-details Record the time consumed by infrastructure parts separately for each pass. -fira-verbose=n Control the verbosity of the dump file for the integrated register allocator. The default value is 5. If the value n is greater or equal to 10, the dump output is sent to stderr using the same format as n minus 10. -flto-report Prints a report with internal details on the workings of the link-time optimizer. The contents of this report vary from version to version. It is meant to be useful to GCC developers when processing object files in LTO mode (via -flto). Disabled by default. -flto-report-wpa Like -flto-report, but only print for the WPA phase of Link Time Optimization. -fmem-report Makes the compiler print some statistics about permanent memory allocation when it finishes. -fmem-report-wpa Makes the compiler print some statistics about permanent memory allocation for the WPA phase only. -fpre-ipa-mem-report -fpost-ipa-mem-report Makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization. -fprofile-report Makes the compiler print some statistics about consistency of the (estimated) profile and effect of individual passes. -fstack-usage Makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending .su to the auxname. auxname is generated from the name of the output file, if explicitly specified and it is not an executable, otherwise it is the basename of the source file. An entry is made up of three fields: * The name of the function. * A number of bytes. * One or more qualifiers: static, dynamic, bounded. The qualifier static means that the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes. The qualifier dynamic means that the function manipulates the stack dynamically: in addition to the static allocation described above, stack adjustments are made in the body of the function, for example to push/pop arguments around function calls. If the qualifier bounded is also present, the amount of these adjustments is bounded at compile time and the second field is an upper bound of the total amount of stack used by the function. If it is not present, the amount of these adjustments is not bounded at compile time and the second field only represents the bounded part. -fstats Emit statistics about front-end processing at the end of the compilation. This option is supported only by the C++ front end, and the information is generally only useful to the G++ development team. -fdbg-cnt-list Print the name and the counter upper bound for all debug counters. -fdbg-cnt=counter-value-list Set the internal debug counter lower and upper bound. counter-value-list is a comma-separated list of name:lower_bound:upper_bound tuples which sets the lower and the upper bound of each debug counter name. The lower_bound is optional and is zero initialized if not set. All debug counters have the initial upper bound of UINT_MAX; thus dbg_cnt returns true always unless the upper bound is set by this option. For example, with -fdbg-cnt=dce:2:4,tail_call:10, dbg_cnt(dce) returns true only for third and fourth invocation. For dbg_cnt(tail_call) true is returned for first 10 invocations. -print-file-name=library Print the full absolute name of the library file library that would be used when linking—and don't do anything else. With this option, GCC does not compile or link anything; it just prints the file name. -print-multi-directory Print the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist in GCC_EXEC_PREFIX. -print-multi-lib Print the mapping from multilib directory names to compiler switches that enable them. The directory name is separated from the switches by ';', and each switch starts with an '@' instead of the '-', without spaces between multiple switches. This is supposed to ease shell processing. -print-multi-os-directory Print the path to OS libraries for the selected multilib, relative to some lib subdirectory. If OS libraries are present in the lib subdirectory and no multilibs are used, this is usually just ., if OS libraries are present in libsuffix sibling directories this prints e.g. ../lib64, ../lib or ../lib32, or if OS libraries are present in lib/subdir subdirectories it prints e.g. amd64, sparcv9 or ev6. -print-multiarch Print the path to OS libraries for the selected multiarch, relative to some lib subdirectory. -print-prog-name=program Like -print-file-name, but searches for a program such as cpp. -print-libgcc-file-name Same as -print-file-name=libgcc.a. This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You can do: gcc -nostdlib files… `gcc -print-libgcc-file-name` -print-search-dirs Print the name of the configured installation directory and a list of program and library directories gcc searches—and don't do anything else. This is useful when gcc prints the error message 'installation problem, cannot exec cpp0: No such file or directory'. To resolve this you either need to put cpp0 and the other compiler components where gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed them. Don't forget the trailing '/'. See Environment Variables. -print-sysroot Print the target sysroot directory that is used during compilation. This is the target sysroot specified either at configure time or using the --sysroot option, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing. -print-sysroot-headers-suffix Print the suffix added to the target sysroot when searching for headers, or give an error if the compiler is not configured with such a suffix—and don't do anything else. -dumpmachine Print the compiler's target machine (for example, 'i686-pc-linux-gnu')—and don't do anything else. -dumpversion Print the compiler version (for example, 3.0, 6.3.0 or 7)—and don't do anything else. This is the compiler version used in filesystem paths and specs. Depending on how the compiler has been configured it can be just a single number (major version), two numbers separated by a dot (major and minor version) or three numbers separated by dots (major, minor and patchlevel version). -dumpfullversion Print the full compiler version—and don't do anything else. The output is always three numbers separated by dots, major, minor and patchlevel version. -dumpspecs Print the compiler's built-in specs—and don't do anything else. (This is used when GCC itself is being built.) See Spec Files. ────────────────────────────────────────────────────────────────────────────── Next: Submodel Options, Previous: Code Gen Options, Up: Invoking GCC Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Invoking GCC Link: next: AArch64 Options Link: prev: Developer Options Next: Spec Files, Previous: Developer Options, Up: Invoking GCC ────────────────────────────────────────────────────────────────────────────── 3.18 Machine-Dependent Options Each target machine supported by GCC can have its own options—for example, to allow you to compile for a particular processor variant or ABI, or to control optimizations specific to that machine. By convention, the names of machine-specific options start with '-m'. Some configurations of the compiler also support additional target-specific options, usually for compatibility with other compilers on the same platform. • AArch64 Options: • Adapteva Epiphany Options: • AMD GCN Options: • ARC Options: • ARM Options: • AVR Options: • Blackfin Options: • C6X Options: • CRIS Options: • CR16 Options: • C-SKY Options: • Darwin Options: • DEC Alpha Options: • FR30 Options: • FT32 Options: • FRV Options: • GNU/Linux Options: • H8/300 Options: • HPPA Options: • IA-64 Options: • LM32 Options: • M32C Options: • M32R/D Options: • M680x0 Options: • MCore Options: • MeP Options: • MicroBlaze Options: • MIPS Options: • MMIX Options: • MN10300 Options: • Moxie Options: • MSP430 Options: • NDS32 Options: • Nios II Options: • Nvidia PTX Options: • OpenRISC Options: • PDP-11 Options: • picoChip Options: • PowerPC Options: • RISC-V Options: • RL78 Options: • RS/6000 and PowerPC Options: • RX Options: • S/390 and zSeries Options: • Score Options: • SH Options: • Solaris 2 Options: • SPARC Options: • SPU Options: • System V Options: • TILE-Gx Options: • TILEPro Options: • V850 Options: • VAX Options: • Visium Options: • VMS Options: • VxWorks Options: • x86 Options: • x86 Windows Options: • Xstormy16 Options: • Xtensa Options: • zSeries Options: ────────────────────────────────────────────────────────────────────────────── Next: Spec Files, Previous: Developer Options, Up: Invoking GCC Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Submodel Options Link: next: ARC Options Link: prev: Adapteva Epiphany Options Next: ARC Options, Previous: Adapteva Epiphany Options, Up: Submodel ────────────────────────────────────────────────────────────────────────────── 3.18.3 AMD GCN Options These options are defined specifically for the AMD GCN port. -march=gpu -mtune=gpu Set architecture type or tuning for gpu. Supported values for gpu are 'fiji' Compile for GCN3 Fiji devices (gfx803). 'gfx900' Compile for GCN5 Vega 10 devices (gfx900). -mstack-size=bytes Specify how many bytes of stack space will be requested for each GPU thread (wave-front). Beware that there may be many threads and limited memory available. The size of the stack allocation may also have an impact on run-time performance. The default is 32KB when using OpenACC or OpenMP, and 1MB otherwise. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Submodel Options Link: next: Darwin Options Link: prev: CR16 Options Next: Darwin Options, Previous: CR16 Options, Up: Submodel Options ────────────────────────────────────────────────────────────────────────────── 3.18.11 C-SKY Options GCC supports these options when compiling for C-SKY V2 processors. -march=arch Specify the C-SKY target architecture. Valid values for arch are: 'ck801', 'ck802', 'ck803', 'ck807', and 'ck810'. The default is 'ck810'. -mcpu=cpu Specify the C-SKY target processor. Valid values for cpu are: 'ck801', 'ck801t', 'ck802', 'ck802t', 'ck802j', 'ck803', 'ck803h', 'ck803t', 'ck803ht', 'ck803f', 'ck803fh', 'ck803e', 'ck803eh', 'ck803et', 'ck803eht', 'ck803ef', 'ck803efh', 'ck803ft', 'ck803eft', 'ck803efht', 'ck803r1', 'ck803hr1', 'ck803tr1', 'ck803htr1', 'ck803fr1', 'ck803fhr1', 'ck803er1', 'ck803ehr1', 'ck803etr1', 'ck803ehtr1', 'ck803efr1', 'ck803efhr1', 'ck803ftr1', 'ck803eftr1', 'ck803efhtr1', 'ck803s', 'ck803st', 'ck803se', 'ck803sf', 'ck803sef', 'ck803seft', 'ck807e', 'ck807ef', 'ck807', 'ck807f', 'ck810e', 'ck810et', 'ck810ef', 'ck810eft', 'ck810', 'ck810v', 'ck810f', 'ck810t', 'ck810fv', 'ck810tv', 'ck810ft', and 'ck810ftv'. -mbig-endian -EB -mlittle-endian -EL Select big- or little-endian code. The default is little-endian. -mhard-float -msoft-float Select hardware or software floating-point implementations. The default is soft float. -mdouble-float -mno-double-float When -mhard-float is in effect, enable generation of double-precision float instructions. This is the default except when compiling for CK803. -mfdivdu -mno-fdivdu When -mhard-float is in effect, enable generation of frecipd, fsqrtd, and fdivd instructions. This is the default except when compiling for CK803. -mfpu=fpu Select the floating-point processor. This option can only be used with -mhard-float. Values for fpu are 'fpv2_sf' (equivalent to '-mno-double-float -mno-fdivdu'), 'fpv2' ('-mdouble-float -mno-divdu'), and 'fpv2_divd' ('-mdouble-float -mdivdu'). -melrw -mno-elrw Enable the extended lrw instruction. This option defaults to on for CK801 and off otherwise. -mistack -mno-istack Enable interrupt stack instructions; the default is off. The -mistack option is required to handle the interrupt and isr function attributes (see C-SKY Function Attributes). -mmp Enable multiprocessor instructions; the default is off. -mcp Enable coprocessor instructions; the default is off. -mcache Enable coprocessor instructions; the default is off. -msecurity Enable C-SKY security instructions; the default is off. -mtrust Enable C-SKY trust instructions; the default is off. -mdsp -medsp -mvdsp Enable C-SKY DSP, Enhanced DSP, or Vector DSP instructions, respectively. All of these options default to off. -mdiv -mno-div Generate divide instructions. Default is off. -msmart -mno-smart Generate code for Smart Mode, using only registers numbered 0-7 to allow use of 16-bit instructions. This option is ignored for CK801 where this is the required behavior, and it defaults to on for CK802. For other targets, the default is off. -mhigh-registers -mno-high-registers Generate code using the high registers numbered 16-31. This option is not supported on CK801, CK802, or CK803, and is enabled by default for other processors. -manchor -mno-anchor Generate code using global anchor symbol addresses. -mpushpop -mno-pushpop Generate code using push and pop instructions. This option defaults to on. -mmultiple-stld -mstm -mno-multiple-stld -mno-stm Generate code using stm and ldm instructions. This option isn't supported on CK801 but is enabled by default on other processors. -mconstpool -mno-constpool Create constant pools in the compiler instead of deferring it to the assembler. This option is the default and required for correct code generation on CK801 and CK802, and is optional on other processors. -mstack-size -mno-stack-size Emit .stack_size directives for each function in the assembly output. This option defaults to off. -mccrt -mno-ccrt Generate code for the C-SKY compiler runtime instead of libgcc. This option defaults to off. -mbranch-cost=n Set the branch costs to roughly n instructions. The default is 1. -msched-prolog -mno-sched-prolog Permit scheduling of function prologue and epilogue sequences. Using this option can result in code that is not compliant with the C-SKY V2 ABI prologue requirements and that cannot be debugged or backtraced. It is disabled by default. ────────────────────────────────────────────────────────────────────────────── Next: Darwin Options, Previous: CR16 Options, Up: Submodel Options Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Submodel Options Link: next: FRV Options Link: prev: FR30 Options Next: FRV Options, Previous: FR30 Options, Up: Submodel Options ────────────────────────────────────────────────────────────────────────────── 3.18.15 FT32 Options These options are defined specifically for the FT32 port. -msim Specifies that the program will be run on the simulator. This causes an alternate runtime startup and library to be linked. You must not use this option when generating programs that will run on real hardware; you must provide your own runtime library for whatever I/O functions are needed. -mlra Enable Local Register Allocation. This is still experimental for FT32, so by default the compiler uses standard reload. -mnodiv Do not use div and mod instructions. -mft32b Enable use of the extended instructions of the FT32B processor. -mcompress Compress all code using the Ft32B code compression scheme. -mnopm Do not generate code that reads program memory. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Submodel Options Link: next: H8/300 Options Link: prev: FRV Options Next: H8/300 Options, Previous: FRV Options, Up: Submodel Options ────────────────────────────────────────────────────────────────────────────── 3.18.17 GNU/Linux Options These '-m' options are defined for GNU/Linux targets: -mglibc Use the GNU C library. This is the default except on '*-*-linux-*uclibc*', '*-*-linux-*musl*' and '*-*-linux-*android*' targets. -muclibc Use uClibc C library. This is the default on '*-*-linux-*uclibc*' targets. -mmusl Use the musl C library. This is the default on '*-*-linux-*musl*' targets. -mbionic Use Bionic C library. This is the default on '*-*-linux-*android*' targets. -mandroid Compile code compatible with Android platform. This is the default on '*-*-linux-*android*' targets. When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. When linking, this option makes the GCC driver pass Android-specific options to the linker. Finally, this option causes the preprocessor macro __ANDROID__ to be defined. -tno-android-cc Disable compilation effects of -mandroid, i.e., do not enable -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. -tno-android-ld Disable linking effects of -mandroid, i.e., pass standard Linux linking options to the linker. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Submodel Options Link: next: SH Options Link: prev: S/390 and zSeries Options Next: SH Options, Previous: S/390 and zSeries Options, Up: Submodel ────────────────────────────────────────────────────────────────────────────── 3.18.45 Score Options These options are defined for Score implementations: -meb Compile code for big-endian mode. This is the default. -mel Compile code for little-endian mode. -mnhwloop Disable generation of bcnz instructions. -muls Enable generation of unaligned load and store instructions. -mmac Enable the use of multiply-accumulate instructions. Disabled by default. -mscore5 Specify the SCORE5 as the target architecture. -mscore5u Specify the SCORE5U of the target architecture. -mscore7 Specify the SCORE7 as the target architecture. This is the default. -mscore7d Specify the SCORE7D as the target architecture. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Submodel Options Link: next: SPARC Options Link: prev: SH Options Next: SPARC Options, Previous: SH Options, Up: Submodel Options ────────────────────────────────────────────────────────────────────────────── 3.18.47 Solaris 2 Options These '-m' options are supported on Solaris 2: -mclear-hwcap -mclear-hwcap tells the compiler to remove the hardware capabilities generated by the Solaris assembler. This is only necessary when object files use ISA extensions not supported by the current machine, but check at runtime whether or not to use them. -mimpure-text -mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when linking a shared object. Using this option, you can link position-dependent code into a shared object. -mimpure-text suppresses the “relocations remain against allocatable but non-writable sections” linker error message. However, the necessary relocations trigger copy-on-write, and the shared object is not actually shared across processes. Instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC. These switches are supported in addition to the above on Solaris 2: -pthreads This is a synonym for -pthread. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Submodel Options Link: next: TILE-Gx Options Link: prev: SPU Options Next: TILE-Gx Options, Previous: SPU Options, Up: Submodel Options ────────────────────────────────────────────────────────────────────────────── 3.18.50 Options for System V These additional options are available on System V Release 4 for compatibility with other compilers on those systems: -G Create a shared object. It is recommended that -symbolic or -shared be used instead. -Qy Identify the versions of each tool used by the compiler, in a .ident assembler directive in the output. -Qn Refrain from adding .ident directives to the output file (this is the default). -YP,dirs Search the directories dirs, and no others, for libraries specified with -l. -Ym,dir Look in the directory dir to find the M4 preprocessor. The assembler uses this option. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Submodel Options Link: next: x86 Windows Options Link: prev: VxWorks Options Next: x86 Windows Options, Previous: VxWorks Options, Up: Submodel Options ────────────────────────────────────────────────────────────────────────────── 3.18.58 x86 Options These '-m' options are defined for the x86 family of computers. -march=cpu-type Generate instructions for the machine type cpu-type. In contrast to -mtune=cpu-type, which merely tunes the generated code for the specified cpu-type, -march=cpu-type allows GCC to generate code that may not run at all on processors other than the one indicated. Specifying -march=cpu-type implies -mtune=cpu-type. The choices for cpu-type are: 'native' This selects the CPU to generate code for at compilation time by determining the processor type of the compiling machine. Using -march=native enables all instruction subsets supported by the local machine (hence the result might not run on different machines). Using -mtune=native produces code optimized for the local machine under the constraints of the selected instruction set. 'x86-64' A generic CPU with 64-bit extensions. 'i386' Original Intel i386 CPU. 'i486' Intel i486 CPU. (No scheduling is implemented for this chip.) 'i586' 'pentium' Intel Pentium CPU with no MMX support. 'lakemont' Intel Lakemont MCU, based on Intel Pentium CPU. 'pentium-mmx' Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support. 'pentiumpro' Intel Pentium Pro CPU. 'i686' When used with -march, the Pentium Pro instruction set is used, so the code runs on all i686 family chips. When used with -mtune, it has the same meaning as 'generic'. 'pentium2' Intel Pentium II CPU, based on Pentium Pro core with MMX instruction set support. 'pentium3' 'pentium3m' Intel Pentium III CPU, based on Pentium Pro core with MMX and SSE instruction set support. 'pentium-m' Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE and SSE2 instruction set support. Used by Centrino notebooks. 'pentium4' 'pentium4m' Intel Pentium 4 CPU with MMX, SSE and SSE2 instruction set support. 'prescott' Improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support. 'nocona' Improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2 and SSE3 instruction set support. 'core2' Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. 'nehalem' Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2 and POPCNT instruction set support. 'westmere' Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES and PCLMUL instruction set support. 'sandybridge' Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES and PCLMUL instruction set support. 'ivybridge' Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES, PCLMUL, FSGSBASE, RDRND and F16C instruction set support. 'haswell' Intel Haswell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2 and F16C instruction set support. 'broadwell' Intel Broadwell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX and PREFETCHW instruction set support. 'skylake' Intel Skylake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC and XSAVES instruction set support. 'bonnell' Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. 'silvermont' Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PCLMUL and RDRND instruction set support. 'goldmont' Intel Goldmont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT and FSGSBASE instruction set support. 'goldmont-plus' Intel Goldmont Plus CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX and UMIP instruction set support. 'tremont' Intel Tremont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, UMIP, GFNI-SSE, CLWB and ENCLV instruction set support. 'knl' Intel Knight's Landing CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, AVX512F, AVX512PF, AVX512ER and AVX512CD instruction set support. 'knm' Intel Knights Mill CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, AVX512F, AVX512PF, AVX512ER, AVX512CD, AVX5124VNNIW, AVX5124FMAPS and AVX512VPOPCNTDQ instruction set support. 'skylake-avx512' Intel Skylake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ and AVX512CD instruction set support. 'cannonlake' Intel Cannonlake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA and UMIP instruction set support. 'icelake-client' Intel Icelake Client CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI, VPCLMULQDQ, VAES instruction set support. 'icelake-server' Intel Icelake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI, VPCLMULQDQ, VAES, PCONFIG and WBNOINVD instruction set support. 'cascadelake' Intel Cascadelake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD and AVX512VNNI instruction set support. 'k6' AMD K6 CPU with MMX instruction set support. 'k6-2' 'k6-3' Improved versions of AMD K6 CPU with MMX and 3DNow! instruction set support. 'athlon' 'athlon-tbird' AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions support. 'athlon-4' 'athlon-xp' 'athlon-mp' Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and full SSE instruction set support. 'k8' 'opteron' 'athlon64' 'athlon-fx' Processors based on the AMD K8 core with x86-64 instruction set support, including the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (This supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and 64-bit instruction set extensions.) 'k8-sse3' 'opteron-sse3' 'athlon64-sse3' Improved versions of AMD K8 cores with SSE3 instruction set support. 'amdfam10' 'barcelona' CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions.) 'bdver1' CPUs based on AMD Family 15h cores with x86-64 instruction set support. (This supersets FMA4, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.) 'bdver2' AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.) 'bdver3' AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions. 'bdver4' AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, AVX2, XOP, LWP, AES, PCL_MUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions. 'znver1' AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCL_MUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit instruction set extensions. 'znver2' AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, ,CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCL_MUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit instruction set extensions.) 'btver1' CPUs based on AMD Family 14h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A, CX16, ABM and 64-bit instruction set extensions.) 'btver2' CPUs based on AMD Family 16h cores with x86-64 instruction set support. This includes MOVBE, F16C, BMI, AVX, PCL_MUL, AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and 64-bit instruction set extensions. 'winchip-c6' IDT WinChip C6 CPU, dealt in same way as i486 with additional MMX instruction set support. 'winchip2' IDT WinChip 2 CPU, dealt in same way as i486 with additional MMX and 3DNow! instruction set support. 'c3' VIA C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.) 'c3-2' VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.) 'c7' VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.) 'samuel-2' VIA Eden Samuel 2 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.) 'nehemiah' VIA Eden Nehemiah CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.) 'esther' VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.) 'eden-x2' VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.) 'eden-x4' VIA Eden X4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and AVX2 instruction set support. (No scheduling is implemented for this chip.) 'nano' Generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.) 'nano-1000' VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.) 'nano-2000' VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.) 'nano-3000' VIA Nano 3xxx CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.) 'nano-x2' VIA Nano Dual Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.) 'nano-x4' VIA Nano Quad Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.) 'geode' AMD Geode embedded processor with MMX and 3DNow! instruction set support. -mtune=cpu-type Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. While picking a specific cpu-type schedules things appropriately for that particular chip, the compiler does not generate any code that cannot run on the default machine type unless you use a -march=cpu-type option. For example, if GCC is configured for i686-pc-linux-gnu then -mtune=pentium4 generates code that is tuned for Pentium 4 but still runs on i686 machines. The choices for cpu-type are the same as for -march. In addition, -mtune supports 2 extra choices for cpu-type: 'generic' Produce code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=generic. But, if you do not know exactly what CPU users of your application will have, then you should use this option. As new processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the processors that are most common at the time that version of GCC is released. There is no -march=generic option because -march indicates the instruction set the compiler can use, and there is no generic instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized. 'intel' Produce code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=intel. But, if you want your application performs better on both Haswell and Silvermont, then you should use this option. As new Intel processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the most current Intel processors at the time that version of GCC is released. There is no -march=intel option because -march indicates the instruction set the compiler can use, and there is no common instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized. -mcpu=cpu-type A deprecated synonym for -mtune. -mfpmath=unit Generate floating-point arithmetic for selected unit unit. The choices for unit are: '387' Use the standard 387 floating-point coprocessor present on the majority of chips and emulated otherwise. Code compiled with this option runs almost everywhere. The temporary results are computed in 80-bit precision instead of the precision specified by the type, resulting in slightly different results compared to most of other chips. See -ffloat-store for more detailed description. This is the default choice for non-Darwin x86-32 targets. 'sse' Use scalar floating-point instructions present in the SSE instruction set. This instruction set is supported by Pentium III and newer chips, and in the AMD line by Athlon-4, Athlon XP and Athlon MP chips. The earlier version of the SSE instruction set supports only single-precision arithmetic, thus the double and extended-precision arithmetic are still done using 387. A later version, present only in Pentium 4 and AMD x86-64 chips, supports double-precision arithmetic too. For the x86-32 compiler, you must use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and make this option effective. For the x86-64 compiler, these extensions are enabled by default. The resulting code should be considerably faster in the majority of cases and avoid the numerical instability problems of 387 code, but may break some existing code that expects temporaries to be 80 bits. This is the default choice for the x86-64 compiler, Darwin x86-32 targets, and the default choice for x86-32 targets with the SSE2 instruction set when -ffast-math is enabled. 'sse,387' 'sse+387' 'both' Attempt to utilize both instruction sets at once. This effectively doubles the amount of available registers, and on chips with separate execution units for 387 and SSE the execution resources too. Use this option with care, as it is still experimental, because the GCC register allocator does not model separate functional units well, resulting in unstable performance. -masm=dialect Output assembly instructions using selected dialect. Also affects which dialect is used for basic asm (see Basic Asm) and extended asm (see Extended Asm). Supported choices (in dialect order) are 'att' or 'intel'. The default is 'att'. Darwin does not support 'intel'. -mieee-fp -mno-ieee-fp Control whether or not the compiler uses IEEE floating-point comparisons. These correctly handle the case where the result of a comparison is unordered. -m80387 -mhard-float Generate output containing 80387 instructions for floating point. -mno-80387 -msoft-float Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. On machines where a function returns floating-point results in the 80387 register stack, some floating-point opcodes may be emitted even if -msoft-float is used. -mno-fp-ret-in-387 Do not use the FPU registers for return values of functions. The usual calling convention has functions return values of types float and double in an FPU register, even if there is no FPU. The idea is that the operating system should emulate an FPU. The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead. -mno-fancy-math-387 Some 387 emulators do not support the sin, cos and sqrt instructions for the 387. Specify this option to avoid generating those instructions. This option is overridden when -march indicates that the target CPU always has an FPU and so the instruction does not need emulation. These instructions are not generated unless you also use the -funsafe-math-optimizations switch. -malign-double -mno-align-double Control whether GCC aligns double, long double, and long long variables on a two-word boundary or a one-word boundary. Aligning double variables on a two-word boundary produces code that runs somewhat faster on a Pentium at the expense of more memory. On x86-64, -malign-double is enabled by default. Warning: if you use the -malign-double switch, structures containing the above types are aligned differently than the published application binary interface specifications for the x86-32 and are not binary compatible with structures in code compiled without that switch. -m96bit-long-double -m128bit-long-double These switches control the size of long double type. The x86-32 application binary interface specifies the size to be 96 bits, so -m96bit-long-double is the default in 32-bit mode. Modern architectures (Pentium and newer) prefer long double to be aligned to an 8- or 16-byte boundary. In arrays or structures conforming to the ABI, this is not possible. So specifying -m128bit-long-double aligns long double to a 16-byte boundary by padding the long double with an additional 32-bit zero. In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that long double is aligned on 16-byte boundary. Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a long double. Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, as well as modifying the function calling convention for functions taking long double. Hence they are not binary-compatible with code compiled without that switch. -mlong-double-64 -mlong-double-80 -mlong-double-128 These switches control the size of long double type. A size of 64 bits makes the long double type equivalent to the double type. This is the default for 32-bit Bionic C library. A size of 128 bits makes the long double type equivalent to the __float128 type. This is the default for 64-bit Bionic C library. Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, as well as modifying the function calling convention for functions taking long double. Hence they are not binary-compatible with code compiled without that switch. -malign-data=type Control how GCC aligns variables. Supported values for type are 'compat' uses increased alignment value compatible uses GCC 4.8 and earlier, 'abi' uses alignment value as specified by the psABI, and 'cacheline' uses increased alignment value to match the cache line size. 'compat' is the default. -mlarge-data-threshold=threshold When -mcmodel=medium is specified, data objects larger than threshold are placed in the large data section. This value must be the same across all objects linked into the binary, and defaults to 65535. -mrtd Use a different function-calling convention, in which functions that take a fixed number of arguments return with the ret num instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there. You can specify that an individual function is called with this calling sequence with the function attribute stdcall. You can also override the -mrtd option by using the function attribute cdecl. See Function Attributes. Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code is generated for calls to those functions. In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.) -mregparm=num Control how many registers are used to pass integer arguments. By default, no registers are used to pass arguments, and at most 3 registers can be used. You can control this behavior for a specific function by using the function attribute regparm. See Function Attributes. Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules. -msseregparm Use SSE register passing conventions for float and double arguments and return values. You can control this behavior for a specific function by using the function attribute sseregparm. See Function Attributes. Warning: if you use this switch then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules. -mvect8-ret-in-mem Return 8-byte vectors in memory instead of MMX registers. This is the default on Solaris 8 and 9 and VxWorks to match the ABI of the Sun Studio compilers until version 12. Later compiler versions (starting with Studio 12 Update 1) follow the ABI used by other x86 targets, which is the default on Solaris 10 and later. Only use this option if you need to remain compatible with existing code produced by those previous compiler versions or older versions of GCC. -mpc32 -mpc64 -mpc80 Set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32 is specified, the significands of results of floating-point operations are rounded to 24 bits (single precision); -mpc64 rounds the significands of results of floating-point operations to 53 bits (double precision) and -mpc80 rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default. When this option is used, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly. Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more. Note that some mathematical libraries assume that extended-precision (80-bit) floating-point operations are enabled by default; routines in such libraries could suffer significant loss of accuracy, typically through so-called “catastrophic cancellation”, when this option is used to set the precision to less than extended precision. -mstackrealign Realign the stack at entry. On the x86, the -mstackrealign option generates an alternate prologue and epilogue that realigns the run-time stack if necessary. This supports mixing legacy codes that keep 4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE compatibility. See also the attribute force_align_arg_pointer, applicable to individual functions. -mpreferred-stack-boundary=num Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits). Warning: When generating code for the x86-64 architecture with SSE extensions disabled, -mpreferred-stack-boundary=3 can be used to keep the stack boundary aligned to 8 byte boundary. Since x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and intended to be used in controlled environment where stack space is important limitation. This option leads to wrong code when functions compiled with 16 byte stack alignment (such as functions from a standard library) are called with misaligned stack. In this case, SSE instructions may lead to misaligned memory access traps. In addition, variable arguments are handled incorrectly for 16 byte aligned objects (including x87 long double and __int128), leading to wrong results. You must build all modules with -mpreferred-stack-boundary=3, including any libraries. This includes the system libraries and startup modules. -mincoming-stack-boundary=num Assume the incoming stack is aligned to a 2 raised to num byte boundary. If -mincoming-stack-boundary is not specified, the one specified by -mpreferred-stack-boundary is used. On Pentium and Pentium Pro, double and long double values should be aligned to an 8-byte boundary (see -malign-double) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 may not work properly if it is not 16-byte aligned. To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. It is recommended that libraries that use callbacks always use the default setting. This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to -mpreferred-stack-boundary=2. -mmmx -msse -msse2 -msse3 -mssse3 -msse4 -msse4a -msse4.1 -msse4.2 -mavx -mavx2 -mavx512f -mavx512pf -mavx512er -mavx512cd -mavx512vl -mavx512bw -mavx512dq -mavx512ifma -mavx512vbmi -msha -maes -mpclmul -mclflushopt -mclwb -mfsgsbase -mptwrite -mrdrnd -mf16c -mfma -mpconfig -mwbnoinvd -mfma4 -mprfchw -mrdpid -mprefetchwt1 -mrdseed -msgx -mxop -mlwp -m3dnow -m3dnowa -mpopcnt -mabm -madx -mbmi -mbmi2 -mlzcnt -mfxsr -mxsave -mxsaveopt -mxsavec -mxsaves -mrtm -mhle -mtbm -mmwaitx -mclzero -mpku -mavx512vbmi2 -mgfni -mvaes -mwaitpkg -mvpclmulqdq -mavx512bitalg -mmovdiri -mmovdir64b -mavx512vpopcntdq -mavx5124fmaps -mavx512vnni -mavx5124vnniw -mcldemote These switches enable the use of instructions in the MMX, SSE, SSE2, SSE3, SSSE3, SSE4, SSE4A, SSE4.1, SSE4.2, AVX, AVX2, AVX512F, AVX512PF, AVX512ER, AVX512CD, AVX512VL, AVX512BW, AVX512DQ, AVX512IFMA, AVX512VBMI, SHA, AES, PCLMUL, CLFLUSHOPT, CLWB, FSGSBASE, PTWRITE, RDRND, F16C, FMA, PCONFIG, WBNOINVD, FMA4, PREFETCHW, RDPID, PREFETCHWT1, RDSEED, SGX, XOP, LWP, 3DNow!, enhanced 3DNow!, POPCNT, ABM, ADX, BMI, BMI2, LZCNT, FXSR, XSAVE, XSAVEOPT, XSAVEC, XSAVES, RTM, HLE, TBM, MWAITX, CLZERO, PKU, AVX512VBMI2, GFNI, VAES, WAITPKG, VPCLMULQDQ, AVX512BITALG, MOVDIRI, MOVDIR64B, AVX512VPOPCNTDQ, AVX5124FMAPS, AVX512VNNI, AVX5124VNNIW, or CLDEMOTE extended instruction sets. Each has a corresponding -mno- option to disable use of these instructions. These extensions are also available as built-in functions: see x86 Built-in Functions, for details of the functions enabled and disabled by these switches. To generate SSE/SSE2 instructions automatically from floating-point code (as opposed to 387 instructions), see -mfpmath=sse. GCC depresses SSEx instructions when -mavx is used. Instead, it generates new AVX instructions or AVX equivalence for all SSEx instructions when needed. These options enable GCC to use these extended instructions in generated code, even without -mfpmath=sse. Applications that perform run-time CPU detection must compile separate files for each supported architecture, using the appropriate flags. In particular, the file containing the CPU detection code should be compiled without these options. -mdump-tune-features This option instructs GCC to dump the names of the x86 performance tuning features and default settings. The names can be used in -mtune-ctrl=feature-list. -mtune-ctrl=feature-list This option is used to do fine grain control of x86 code generation features. feature-list is a comma separated list of feature names. See also -mdump-tune-features. When specified, the feature is turned on if it is not preceded with '^', otherwise, it is turned off. -mtune-ctrl=feature-list is intended to be used by GCC developers. Using it may lead to code paths not covered by testing and can potentially result in compiler ICEs or runtime errors. -mno-default This option instructs GCC to turn off all tunable features. See also -mtune-ctrl=feature-list and -mdump-tune-features. -mcld This option instructs GCC to emit a cld instruction in the prologue of functions that use string instructions. String instructions depend on the DF flag to select between autoincrement or autodecrement mode. While the ABI specifies the DF flag to be cleared on function entry, some operating systems violate this specification by not clearing the DF flag in their exception dispatchers. The exception handler can be invoked with the DF flag set, which leads to wrong direction mode when string instructions are used. This option can be enabled by default on 32-bit x86 targets by configuring GCC with the --enable-cld configure option. Generation of cld instructions can be suppressed with the -mno-cld compiler option in this case. -mvzeroupper This option instructs GCC to emit a vzeroupper instruction before a transfer of control flow out of the function to minimize the AVX to SSE transition penalty as well as remove unnecessary zeroupper intrinsics. -mprefer-avx128 This option instructs GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer. -mprefer-vector-width=opt This option instructs GCC to use opt-bit vector width in instructions instead of default on the selected platform. 'none' No extra limitations applied to GCC other than defined by the selected platform. '128' Prefer 128-bit vector width for instructions. '256' Prefer 256-bit vector width for instructions. '512' Prefer 512-bit vector width for instructions. -mcx16 This option enables GCC to generate CMPXCHG16B instructions in 64-bit code to implement compare-and-exchange operations on 16-byte aligned 128-bit objects. This is useful for atomic updates of data structures exceeding one machine word in size. The compiler uses this instruction to implement __sync Builtins. However, for __atomic Builtins operating on 128-bit integers, a library call is always used. -msahf This option enables generation of SAHF instructions in 64-bit code. Early Intel Pentium 4 CPUs with Intel 64 support, prior to the introduction of Pentium 4 G1 step in December 2005, lacked the LAHF and SAHF instructions which are supported by AMD64. These are load and store instructions, respectively, for certain status flags. In 64-bit mode, the SAHF instruction is used to optimize fmod, drem, and remainder built-in functions; see Other Builtins for details. -mmovbe This option enables use of the movbe instruction to implement __builtin_bswap32 and __builtin_bswap64. -mshstk The -mshstk option enables shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET). -mcrc32 This option enables built-in functions __builtin_ia32_crc32qi, __builtin_ia32_crc32hi, __builtin_ia32_crc32si and __builtin_ia32_crc32di to generate the crc32 machine instruction. -mrecip This option enables use of RCPSS and RSQRTSS instructions (and their vectorized variants RCPPS and RSQRTPS) with an additional Newton-Raphson step to increase precision instead of DIVSS and SQRTSS (and their vectorized variants) for single-precision floating-point arguments. These instructions are generated only when -funsafe-math-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math. Note that while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994). Note that GCC implements 1.0f/sqrtf(x) in terms of RSQRTSS (or RSQRTPS) already with -ffast-math (or the above option combination), and doesn't need -mrecip. Also note that GCC emits the above sequence with additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) already with -ffast-math (or the above option combination), and doesn't need -mrecip. -mrecip=opt This option controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a '!' to invert the option: 'all' Enable all estimate instructions. 'default' Enable the default instructions, equivalent to -mrecip. 'none' Disable all estimate instructions, equivalent to -mno-recip. 'div' Enable the approximation for scalar division. 'vec-div' Enable the approximation for vectorized division. 'sqrt' Enable the approximation for scalar square root. 'vec-sqrt' Enable the approximation for vectorized square root. So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for square root. -mveclibabi=type Specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values for type are 'svml' for the Intel short vector math library and 'acml' for the AMD math core library. To use this option, both -ftree-vectorize and -funsafe-math-optimizations have to be enabled, and an SVML or ACML ABI-compatible library must be specified at link time. GCC currently emits calls to vmldExp2, vmldLn2, vmldLog102, vmldPow2, vmldTanh2, vmldTan2, vmldAtan2, vmldAtanh2, vmldCbrt2, vmldSinh2, vmldSin2, vmldAsinh2, vmldAsin2, vmldCosh2, vmldCos2, vmldAcosh2, vmldAcos2, vmlsExp4, vmlsLn4, vmlsLog104, vmlsPow4, vmlsTanh4, vmlsTan4, vmlsAtan4, vmlsAtanh4, vmlsCbrt4, vmlsSinh4, vmlsSin4, vmlsAsinh4, vmlsAsin4, vmlsCosh4, vmlsCos4, vmlsAcosh4 and vmlsAcos4 for corresponding function type when -mveclibabi=svml is used, and __vrd2_sin, __vrd2_cos, __vrd2_exp, __vrd2_log, __vrd2_log2, __vrd2_log10, __vrs4_sinf, __vrs4_cosf, __vrs4_expf, __vrs4_logf, __vrs4_log2f, __vrs4_log10f and __vrs4_powf for the corresponding function type when -mveclibabi=acml is used. -mabi=name Generate code for the specified calling convention. Permissible values are 'sysv' for the ABI used on GNU/Linux and other systems, and 'ms' for the Microsoft ABI. The default is to use the Microsoft ABI when targeting Microsoft Windows and the SysV ABI on all other systems. You can control this behavior for specific functions by using the function attributes ms_abi and sysv_abi. See Function Attributes. -mforce-indirect-call Force all calls to functions to be indirect. This is useful when using Intel Processor Trace where it generates more precise timing information for function calls. -mmanual-endbr Insert ENDBR instruction at function entry only via the cf_check function attribute. This is useful when used with the option -fcf-protection=branch to control ENDBR insertion at the function entry. -mcall-ms2sysv-xlogues Due to differences in 64-bit ABIs, any Microsoft ABI function that calls a System V ABI function must consider RSI, RDI and XMM6-15 as clobbered. By default, the code for saving and restoring these registers is emitted inline, resulting in fairly lengthy prologues and epilogues. Using -mcall-ms2sysv-xlogues emits prologues and epilogues that use stubs in the static portion of libgcc to perform these saves and restores, thus reducing function size at the cost of a few extra instructions. -mtls-dialect=type Generate code to access thread-local storage using the 'gnu' or 'gnu2' conventions. 'gnu' is the conservative default; 'gnu2' is more efficient, but it may add compile- and run-time requirements that cannot be satisfied on all systems. -mpush-args -mno-push-args Use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as method using SUB/MOV operations and is enabled by default. In some cases disabling it may improve performance because of improved scheduling and reduced dependencies. -maccumulate-outgoing-args If enabled, the maximum amount of space required for outgoing arguments is computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when the preferred stack boundary is not equal to 2. The drawback is a notable increase in code size. This switch implies -mno-push-args. -mthreads Support thread-safe exception handling on MinGW. Programs that rely on thread-safe exception handling must compile and link all code with the -mthreads option. When compiling, -mthreads defines -D_MT; when linking, it links in a special thread helper library -lmingwthrd which cleans up per-thread exception-handling data. -mms-bitfields -mno-ms-bitfields Enable/disable bit-field layout compatible with the native Microsoft Windows compiler. If packed is used on a structure, or if bit-fields are used, it may be that the Microsoft ABI lays out the structure differently than the way GCC normally does. Particularly when moving packed data between functions compiled with GCC and the native Microsoft compiler (either via function call or as data in a file), it may be necessary to access either format. This option is enabled by default for Microsoft Windows targets. This behavior can also be controlled locally by use of variable or type attributes. For more information, see x86 Variable Attributes and x86 Type Attributes. The Microsoft structure layout algorithm is fairly simple with the exception of the bit-field packing. The padding and alignment of members of structures and whether a bit-field can straddle a storage-unit boundary are determine by these rules: 1. Structure members are stored sequentially in the order in which they are declared: the first member has the lowest memory address and the last member the highest. 2. Every data object has an alignment requirement. The alignment requirement for all data except structures, unions, and arrays is either the size of the object or the current packing size (specified with either the aligned attribute or the pack pragma), whichever is less. For structures, unions, and arrays, the alignment requirement is the largest alignment requirement of its members. Every object is allocated an offset so that: offset % alignment_requirement == 0 3. Adjacent bit-fields are packed into the same 1-, 2-, or 4-byte allocation unit if the integral types are the same size and if the next bit-field fits into the current allocation unit without crossing the boundary imposed by the common alignment requirements of the bit-fields. MSVC interprets zero-length bit-fields in the following ways: 1. If a zero-length bit-field is inserted between two bit-fields that are normally coalesced, the bit-fields are not coalesced. For example: struct { unsigned long bf_1 : 12; unsigned long : 0; unsigned long bf_2 : 12; } t1; The size of t1 is 8 bytes with the zero-length bit-field. If the zero-length bit-field were removed, t1's size would be 4 bytes. 2. If a zero-length bit-field is inserted after a bit-field, foo, and the alignment of the zero-length bit-field is greater than the member that follows it, bar, bar is aligned as the type of the zero-length bit-field. For example: struct { char foo : 4; short : 0; char bar; } t2; struct { char foo : 4; short : 0; double bar; } t3; For t2, bar is placed at offset 2, rather than offset 1. Accordingly, the size of t2 is 4. For t3, the zero-length bit-field does not affect the alignment of bar or, as a result, the size of the structure. Taking this into account, it is important to note the following: 1. If a zero-length bit-field follows a normal bit-field, the type of the zero-length bit-field may affect the alignment of the structure as whole. For example, t2 has a size of 4 bytes, since the zero-length bit-field follows a normal bit-field, and is of type short. 2. Even if a zero-length bit-field is not followed by a normal bit-field, it may still affect the alignment of the structure: struct { char foo : 6; long : 0; } t4; Here, t4 takes up 4 bytes. 3. Zero-length bit-fields following non-bit-field members are ignored: struct { char foo; long : 0; char bar; } t5; Here, t5 takes up 2 bytes. -mno-align-stringops Do not align the destination of inlined string operations. This switch reduces code size and improves performance in case the destination is already aligned, but GCC doesn't know about it. -minline-all-stringops By default GCC inlines string operations only when the destination is known to be aligned to least a 4-byte boundary. This enables more inlining and increases code size, but may improve performance of code that depends on fast memcpy, strlen, and memset for short lengths. -minline-stringops-dynamically For string operations of unknown size, use run-time checks with inline code for small blocks and a library call for large blocks. -mstringop-strategy=alg Override the internal decision heuristic for the particular algorithm to use for inlining string operations. The allowed values for alg are: 'rep_byte' 'rep_4byte' 'rep_8byte' Expand using i386 rep prefix of the specified size. 'byte_loop' 'loop' 'unrolled_loop' Expand into an inline loop. 'libcall' Always use a library call. -mmemcpy-strategy=strategy Override the internal decision heuristic to decide if __builtin_memcpy should be inlined and what inline algorithm to use when the expected size of the copy operation is known. strategy is a comma-separated list of alg:max_size:dest_align triplets. alg is specified in -mstringop-strategy, max_size specifies the max byte size with which inline algorithm alg is allowed. For the last triplet, the max_size must be -1. The max_size of the triplets in the list must be specified in increasing order. The minimal byte size for alg is 0 for the first triplet and max_size + 1 of the preceding range. -mmemset-strategy=strategy The option is similar to -mmemcpy-strategy= except that it is to control __builtin_memset expansion. -momit-leaf-frame-pointer Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up, and restore frame pointers and makes an extra register available in leaf functions. The option -fomit-leaf-frame-pointer removes the frame pointer for leaf functions, which might make debugging harder. -mtls-direct-seg-refs -mno-tls-direct-seg-refs Controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for 32-bit, %fs for 64-bit), or whether the thread base pointer must be added. Whether or not this is valid depends on the operating system, and whether it maps the segment to cover the entire TLS area. For systems that use the GNU C Library, the default is on. -msse2avx -mno-sse2avx Specify that the assembler should encode SSE instructions with VEX prefix. The option -mavx turns this on by default. -mfentry -mno-fentry If profiling is active (-pg), put the profiling counter call before the prologue. Note: On x86 architectures the attribute ms_hook_prologue isn't possible at the moment for -mfentry and -pg. -mrecord-mcount -mno-record-mcount If profiling is active (-pg), generate a __mcount_loc section that contains pointers to each profiling call. This is useful for automatically patching and out calls. -mnop-mcount -mno-nop-mcount If profiling is active (-pg), generate the calls to the profiling functions as NOPs. This is useful when they should be patched in later dynamically. This is likely only useful together with -mrecord-mcount. -minstrument-return=type Instrument function exit in -pg -mfentry instrumented functions with call to specified function. This only instruments true returns ending with ret, but not sibling calls ending with jump. Valid types are none to not instrument, call to generate a call to __return__, or nop5 to generate a 5 byte nop. -mrecord-return -mno-record-return Generate a __return_loc section pointing to all return instrumentation code. -mfentry-name=name Set name of __fentry__ symbol called at function entry for -pg -mfentry functions. -mfentry-section=name Set name of section to record -mrecord-mcount calls (default __mcount_loc). -mskip-rax-setup -mno-skip-rax-setup When generating code for the x86-64 architecture with SSE extensions disabled, -mskip-rax-setup can be used to skip setting up RAX register when there are no variable arguments passed in vector registers. Warning: Since RAX register is used to avoid unnecessarily saving vector registers on stack when passing variable arguments, the impacts of this option are callees may waste some stack space, misbehave or jump to a random location. GCC 4.4 or newer don't have those issues, regardless the RAX register value. -m8bit-idiv -mno-8bit-idiv On some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than 32-bit/64-bit integer divide. This option generates a run-time check. If both dividend and divisor are within range of 0 to 255, 8-bit unsigned integer divide is used instead of 32-bit/64-bit integer divide. -mavx256-split-unaligned-load -mavx256-split-unaligned-store Split 32-byte AVX unaligned load and store. -mstack-protector-guard=guard -mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset Generate stack protection code using canary at guard. Supported locations are 'global' for global canary or 'tls' for per-thread canary in the TLS block (the default). This option has effect only when -fstack-protector or -fstack-protector-all is specified. With the latter choice the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which segment register (%fs or %gs) to use as base register for reading the canary, and from what offset from that base register. The default for those is as specified in the relevant ABI. -mgeneral-regs-only Generate code that uses only the general-purpose registers. This prevents the compiler from using floating-point, vector, mask and bound registers. -mindirect-branch=choice Convert indirect call and jump with choice. The default is 'keep', which keeps indirect call and jump unmodified. 'thunk' converts indirect call and jump to call and return thunk. 'thunk-inline' converts indirect call and jump to inlined call and return thunk. 'thunk-extern' converts indirect call and jump to external call and return thunk provided in a separate object file. You can control this behavior for a specific function by using the function attribute indirect_branch. See Function Attributes. Note that -mcmodel=large is incompatible with -mindirect-branch=thunk and -mindirect-branch=thunk-extern since the thunk function may not be reachable in the large code model. Note that -mindirect-branch=thunk-extern is incompatible with -fcf-protection=branch since the external thunk cannot be modified to disable control-flow check. -mfunction-return=choice Convert function return with choice. The default is 'keep', which keeps function return unmodified. 'thunk' converts function return to call and return thunk. 'thunk-inline' converts function return to inlined call and return thunk. 'thunk-extern' converts function return to external call and return thunk provided in a separate object file. You can control this behavior for a specific function by using the function attribute function_return. See Function Attributes. Note that -mcmodel=large is incompatible with -mfunction-return=thunk and -mfunction-return=thunk-extern since the thunk function may not be reachable in the large code model. -mindirect-branch-register Force indirect call and jump via register. These '-m' switches are supported in addition to the above on x86-64 processors in 64-bit environments. -m32 -m64 -mx32 -m16 -miamcu Generate code for a 16-bit, 32-bit or 64-bit environment. The -m32 option sets int, long, and pointer types to 32 bits, and generates code that runs on any i386 system. The -m64 option sets int to 32 bits and long and pointer types to 64 bits, and generates code for the x86-64 architecture. For Darwin only the -m64 option also turns off the -fno-pic and -mdynamic-no-pic options. The -mx32 option sets int, long, and pointer types to 32 bits, and generates code for the x86-64 architecture. The -m16 option is the same as -m32, except for that it outputs the .code16gcc assembly directive at the beginning of the assembly output so that the binary can run in 16-bit mode. The -miamcu option generates code which conforms to Intel MCU psABI. It requires the -m32 option to be turned on. -mno-red-zone Do not use a so-called “red zone” for x86-64 code. The red zone is mandated by the x86-64 ABI; it is a 128-byte area beyond the location of the stack pointer that is not modified by signal or interrupt handlers and therefore can be used for temporary data without adjusting the stack pointer. The flag -mno-red-zone disables this red zone. -mcmodel=small Generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the default code model. -mcmodel=kernel Generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space. This model has to be used for Linux kernel code. -mcmodel=medium Generate code for the medium model: the program is linked in the lower 2 GB of the address space. Small symbols are also placed there. Symbols with sizes larger than -mlarge-data-threshold are put into large data or BSS sections and can be located above 2GB. Programs can be statically or dynamically linked. -mcmodel=large Generate code for the large model. This model makes no assumptions about addresses and sizes of sections. -maddress-mode=long Generate code for long address mode. This is only supported for 64-bit and x32 environments. It is the default address mode for 64-bit environments. -maddress-mode=short Generate code for short address mode. This is only supported for 32-bit and x32 environments. It is the default address mode for 32-bit and x32 environments. ────────────────────────────────────────────────────────────────────────────── Next: x86 Windows Options, Previous: VxWorks Options, Up: Submodel Options Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Submodel Options Link: next: Xstormy16 Options Link: prev: x86 Options Next: Xstormy16 Options, Previous: x86 Options, Up: Submodel Options ────────────────────────────────────────────────────────────────────────────── 3.18.59 x86 Windows Options These additional options are available for Microsoft Windows targets: -mconsole This option specifies that a console application is to be generated, by instructing the linker to set the PE header subsystem type required for console applications. This option is available for Cygwin and MinGW targets and is enabled by default on those targets. -mdll This option is available for Cygwin and MinGW targets. It specifies that a DLL—a dynamic link library—is to be generated, enabling the selection of the required runtime startup object and entry point. -mnop-fun-dllimport This option is available for Cygwin and MinGW targets. It specifies that the dllimport attribute should be ignored. -mthread This option is available for MinGW targets. It specifies that MinGW-specific thread support is to be used. -municode This option is available for MinGW-w64 targets. It causes the UNICODE preprocessor macro to be predefined, and chooses Unicode-capable runtime startup code. -mwin32 This option is available for Cygwin and MinGW targets. It specifies that the typical Microsoft Windows predefined macros are to be set in the pre-processor, but does not influence the choice of runtime library/startup code. -mwindows This option is available for Cygwin and MinGW targets. It specifies that a GUI application is to be generated by instructing the linker to set the PE header subsystem type appropriately. -fno-set-stack-executable This option is available for MinGW targets. It specifies that the executable flag for the stack used by nested functions isn't set. This is necessary for binaries running in kernel mode of Microsoft Windows, as there the User32 API, which is used to set executable privileges, isn't available. -fwritable-relocated-rdata This option is available for MinGW and Cygwin targets. It specifies that relocated-data in read-only section is put into the .data section. This is a necessary for older runtimes not supporting modification of .rdata sections for pseudo-relocation. -mpe-aligned-commons This option is available for Cygwin and MinGW targets. It specifies that the GNU extension to the PE file format that permits the correct alignment of COMMON variables should be used when generating code. It is enabled by default if GCC detects that the target assembler found during configuration supports the feature. See also under x86 Options for standard options. ────────────────────────────────────────────────────────────────────────────── Next: Xstormy16 Options, Previous: x86 Options, Up: Submodel Options Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Invoking GCC Link: next: Environment Variables Link: prev: zSeries Options Next: Environment Variables, Previous: Submodel Options, Up: Invoking GCC ────────────────────────────────────────────────────────────────────────────── 3.19 Specifying Subprocesses and the Switches to Pass to Them gcc is a driver program. It performs its job by invoking a sequence of other programs to do the work of compiling, assembling and linking. GCC interprets its command-line parameters and uses these to deduce which programs it should invoke, and which command-line options it ought to place on their command lines. This behavior is controlled by spec strings. In most cases there is one spec string for each program that GCC can invoke, but a few programs have multiple spec strings to control their behavior. The spec strings built into GCC can be overridden by using the -specs= command-line switch to specify a spec file. Spec files are plain-text files that are used to construct spec strings. They consist of a sequence of directives separated by blank lines. The type of directive is determined by the first non-whitespace character on the line, which can be one of the following: %command Issues a command to the spec file processor. The commands that can appear here are: %include Search for file and insert its text at the current point in the specs file. %include_noerr Just like '%include', but do not generate an error message if the include file cannot be found. %rename old_name new_name Rename the spec string old_name to new_name. *[spec_name]: This tells the compiler to create, override or delete the named spec string. All lines after this directive up to the next directive or blank line are considered to be the text for the spec string. If this results in an empty string then the spec is deleted. (Or, if the spec did not exist, then nothing happens.) Otherwise, if the spec does not currently exist a new spec is created. If the spec does exist then its contents are overridden by the text of this directive, unless the first character of that text is the '+' character, in which case the text is appended to the spec. [suffix]: Creates a new '[suffix] spec' pair. All lines after this directive and up to the next directive or blank line are considered to make up the spec string for the indicated suffix. When the compiler encounters an input file with the named suffix, it processes the spec string in order to work out how to compile that file. For example: .ZZ: z-compile -input %i This says that any input file whose name ends in '.ZZ' should be passed to the program 'z-compile', which should be invoked with the command-line switch -input and with the result of performing the '%i' substitution. (See below.) As an alternative to providing a spec string, the text following a suffix directive can be one of the following: @language This says that the suffix is an alias for a known language. This is similar to using the -x command-line switch to GCC to specify a language explicitly. For example: .ZZ: @c++ Says that .ZZ files are, in fact, C++ source files. #name This causes an error messages saying: name compiler not installed on this system. GCC already has an extensive list of suffixes built into it. This directive adds an entry to the end of the list of suffixes, but since the list is searched from the end backwards, it is effectively possible to override earlier entries using this technique. GCC has the following spec strings built into it. Spec files can override these strings or create their own. Note that individual targets can also add their own spec strings to this list. asm Options to pass to the assembler asm_final Options to pass to the assembler post-processor cpp Options to pass to the C preprocessor cc1 Options to pass to the C compiler cc1plus Options to pass to the C++ compiler endfile Object files to include at the end of the link link Options to pass to the linker lib Libraries to include on the command line to the linker libgcc Decides which GCC support library to pass to the linker linker Sets the name of the linker predefines Defines to be passed to the C preprocessor signed_char Defines to pass to CPP to say whether char is signed by default startfile Object files to include at the start of the link Here is a small example of a spec file: %rename lib old_lib *lib: --start-group -lgcc -lc -leval1 --end-group %(old_lib) This example renames the spec called 'lib' to 'old_lib' and then overrides the previous definition of 'lib' with a new one. The new definition adds in some extra command-line options before including the text of the old definition. Spec strings are a list of command-line options to be passed to their corresponding program. In addition, the spec strings can contain '%'-prefixed sequences to substitute variable text or to conditionally insert text into the command line. Using these constructs it is possible to generate quite complex command lines. Here is a table of all defined '%'-sequences for spec strings. Note that spaces are not generated automatically around the results of expanding these sequences. Therefore you can concatenate them together or combine them with constant text in a single argument. %% Substitute one '%' into the program name or argument. %i Substitute the name of the input file being processed. %b Substitute the basename of the input file being processed. This is the substring up to (and not including) the last period and not including the directory. %B This is the same as '%b', but include the file suffix (text after the last period). %d Marks the argument containing or following the '%d' as a temporary file name, so that that file is deleted if GCC exits successfully. Unlike '%g', this contributes no text to the argument. %gsuffix Substitute a file name that has suffix suffix and is chosen once per compilation, and mark the argument in the same way as '%d'. To reduce exposure to denial-of-service attacks, the file name is now chosen in a way that is hard to predict even when previously chosen file names are known. For example, '%g.s … %g.o … %g.s' might turn into 'ccUVUUAU.s ccXYAXZ12.o ccUVUUAU.s'. suffix matches the regexp '[.A-Za-z]*' or the special string '%O', which is treated exactly as if '%O' had been preprocessed. Previously, '%g' was simply substituted with a file name chosen once per compilation, without regard to any appended suffix (which was therefore treated just like ordinary text), making such attacks more likely to succeed. %usuffix Like '%g', but generates a new temporary file name each time it appears instead of once per compilation. %Usuffix Substitutes the last file name generated with '%usuffix', generating a new one if there is no such last file name. In the absence of any '%usuffix', this is just like '%gsuffix', except they don't share the same suffix space, so '%g.s … %U.s … %g.s … %U.s' involves the generation of two distinct file names, one for each '%g.s' and another for each '%U.s'. Previously, '%U' was simply substituted with a file name chosen for the previous '%u', without regard to any appended suffix. %jsuffix Substitutes the name of the HOST_BIT_BUCKET, if any, and if it is writable, and if -save-temps is not used; otherwise, substitute the name of a temporary file, just like '%u'. This temporary file is not meant for communication between processes, but rather as a junk disposal mechanism. %|suffix %msuffix Like '%g', except if -pipe is in effect. In that case '%|' substitutes a single dash and '%m' substitutes nothing at all. These are the two most common ways to instruct a program that it should read from standard input or write to standard output. If you need something more elaborate you can use an '%{pipe:X}' construct: see for example f/lang-specs.h. %.SUFFIX Substitutes .SUFFIX for the suffixes of a matched switch's args when it is subsequently output with '%*'. SUFFIX is terminated by the next space or %. %w Marks the argument containing or following the '%w' as the designated output file of this compilation. This puts the argument into the sequence of arguments that '%o' substitutes. %o Substitutes the names of all the output files, with spaces automatically placed around them. You should write spaces around the '%o' as well or the results are undefined. '%o' is for use in the specs for running the linker. Input files whose names have no recognized suffix are not compiled at all, but they are included among the output files, so they are linked. %O Substitutes the suffix for object files. Note that this is handled specially when it immediately follows '%g, %u, or %U', because of the need for those to form complete file names. The handling is such that '%O' is treated exactly as if it had already been substituted, except that '%g, %u, and %U' do not currently support additional suffix characters following '%O' as they do following, for example, '.o'. %p Substitutes the standard macro predefinitions for the current target machine. Use this when running cpp. %P Like '%p', but puts '__' before and after the name of each predefined macro, except for macros that start with '__' or with '_L', where L is an uppercase letter. This is for ISO C. %I Substitute any of -iprefix (made from GCC_EXEC_PREFIX), -isysroot (made from TARGET_SYSTEM_ROOT), -isystem (made from COMPILER_PATH and -B options) and -imultilib as necessary. %s Current argument is the name of a library or startup file of some sort. Search for that file in a standard list of directories and substitute the full name found. The current working directory is included in the list of directories scanned. %T Current argument is the name of a linker script. Search for that file in the current list of directories to scan for libraries. If the file is located insert a --script option into the command line followed by the full path name found. If the file is not found then generate an error message. Note: the current working directory is not searched. %estr Print str as an error message. str is terminated by a newline. Use this when inconsistent options are detected. %(name) Substitute the contents of spec string name at this point. %x{option} Accumulate an option for '%X'. %X Output the accumulated linker options specified by -Wl or a '%x' spec string. %Y Output the accumulated assembler options specified by -Wa. %Z Output the accumulated preprocessor options specified by -Wp. %a Process the asm spec. This is used to compute the switches to be passed to the assembler. %A Process the asm_final spec. This is a spec string for passing switches to an assembler post-processor, if such a program is needed. %l Process the link spec. This is the spec for computing the command line passed to the linker. Typically it makes use of the '%L %G %S %D and %E' sequences. %D Dump out a -L option for each directory that GCC believes might contain startup files. If the target supports multilibs then the current multilib directory is prepended to each of these paths. %L Process the lib spec. This is a spec string for deciding which libraries are included on the command line to the linker. %G Process the libgcc spec. This is a spec string for deciding which GCC support library is included on the command line to the linker. %S Process the startfile spec. This is a spec for deciding which object files are the first ones passed to the linker. Typically this might be a file named crt0.o. %E Process the endfile spec. This is a spec string that specifies the last object files that are passed to the linker. %C Process the cpp spec. This is used to construct the arguments to be passed to the C preprocessor. %1 Process the cc1 spec. This is used to construct the options to be passed to the actual C compiler (cc1). %2 Process the cc1plus spec. This is used to construct the options to be passed to the actual C++ compiler (cc1plus). %* Substitute the variable part of a matched option. See below. Note that each comma in the substituted string is replaced by a single space. %>' acts on negative numbers by sign extension. As an extension to the C language, GCC does not use the latitude given in C99 and C11 only to treat certain aspects of signed '<<' as undefined. However, -fsanitize=shift (and -fsanitize=undefined) will diagnose such cases. They are also diagnosed where constant expressions are required. * The sign of the remainder on integer division (C90 6.3.5). GCC always follows the C99 and C11 requirement that the result of division is truncated towards zero. ────────────────────────────────────────────────────────────────────────────── Next: Floating point implementation, Previous: Characters implementation, Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Implementation Link: next: Arrays and pointers implementation Link: prev: Integers implementation Next: Arrays and pointers implementation, Previous: Integers ────────────────────────────────────────────────────────────────────────────── 4.6 Floating Point * The accuracy of the floating-point operations and of the library functions in and that return floating-point results (C90, C99 and C11 5.2.4.2.2). The accuracy is unknown. * The rounding behaviors characterized by non-standard values of FLT_ROUNDS (C90, C99 and C11 5.2.4.2.2). GCC does not use such values. * The evaluation methods characterized by non-standard negative values of FLT_EVAL_METHOD (C99 and C11 5.2.4.2.2). GCC does not use such values. * The direction of rounding when an integer is converted to a floating-point number that cannot exactly represent the original value (C90 6.2.1.3, C99 and C11 6.3.1.4). C99 Annex F is followed. * The direction of rounding when a floating-point number is converted to a narrower floating-point number (C90 6.2.1.4, C99 and C11 6.3.1.5). C99 Annex F is followed. * How the nearest representable value or the larger or smaller representable value immediately adjacent to the nearest representable value is chosen for certain floating constants (C90 6.1.3.1, C99 and C11 6.4.4.2). C99 Annex F is followed. * Whether and how floating expressions are contracted when not disallowed by the FP_CONTRACT pragma (C99 and C11 6.5). Expressions are currently only contracted if -ffp-contract=fast, -funsafe-math-optimizations or -ffast-math are used. This is subject to change. * The default state for the FENV_ACCESS pragma (C99 and C11 7.6.1). This pragma is not implemented, but the default is to “off” unless -frounding-math is used in which case it is “on”. * Additional floating-point exceptions, rounding modes, environments, and classifications, and their macro names (C99 and C11 7.6, C99 and C11 7.12). This is dependent on the implementation of the C library, and is not defined by GCC itself. * The default state for the FP_CONTRACT pragma (C99 and C11 7.12.2). This pragma is not implemented. Expressions are currently only contracted if -ffp-contract=fast, -funsafe-math-optimizations or -ffast-math are used. This is subject to change. * Whether the “inexact” floating-point exception can be raised when the rounded result actually does equal the mathematical result in an IEC 60559 conformant implementation (C99 F.9). This is dependent on the implementation of the C library, and is not defined by GCC itself. * Whether the “underflow” (and “inexact”) floating-point exception can be raised when a result is tiny but not inexact in an IEC 60559 conformant implementation (C99 F.9). This is dependent on the implementation of the C library, and is not defined by GCC itself. ────────────────────────────────────────────────────────────────────────────── Next: Arrays and pointers implementation, Previous: Integers Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Implementation Link: next: Hints implementation Link: prev: Floating point implementation Next: Hints implementation, Previous: Floating point implementation, Up: C ────────────────────────────────────────────────────────────────────────────── 4.7 Arrays and Pointers * The result of converting a pointer to an integer or vice versa (C90 6.3.4, C99 and C11 6.3.2.3). A cast from pointer to integer discards most-significant bits if the pointer representation is larger than the integer type, sign-extends^2 if the pointer representation is smaller than the integer type, otherwise the bits are unchanged. A cast from integer to pointer discards most-significant bits if the pointer representation is smaller than the integer type, extends according to the signedness of the integer type if the pointer representation is larger than the integer type, otherwise the bits are unchanged. When casting from pointer to integer and back again, the resulting pointer must reference the same object as the original pointer, otherwise the behavior is undefined. That is, one may not use integer arithmetic to avoid the undefined behavior of pointer arithmetic as proscribed in C99 and C11 6.5.6/8. * The size of the result of subtracting two pointers to elements of the same array (C90 6.3.6, C99 and C11 6.5.6). The value is as specified in the standard and the type is determined by the ABI. Footnotes (2) Future versions of GCC may zero-extend, or use a target-defined ptr_extend pattern. Do not rely on sign extension. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Implementation Link: next: Structures unions enumerations and bit-fields implementation Link: prev: Arrays and pointers implementation Next: Structures unions enumerations and bit-fields implementation, Previous: Arrays and pointers implementation, Up: C Implementation ────────────────────────────────────────────────────────────────────────────── 4.8 Hints * The extent to which suggestions made by using the register storage-class specifier are effective (C90 6.5.1, C99 and C11 6.7.1). The register specifier affects code generation only in these ways: * When used as part of the register variable extension, see Explicit Register Variables. * When -O0 is in use, the compiler allocates distinct stack memory for all variables that do not have the register storage-class specifier; if register is specified, the variable may have a shorter lifespan than the code would indicate and may never be placed in memory. * On some rare x86 targets, setjmp doesn't save the registers in all circumstances. In those cases, GCC doesn't allocate any variables in registers unless they are marked register. * The extent to which suggestions made by using the inline function specifier are effective (C99 and C11 6.7.4). GCC will not inline any functions if the -fno-inline option is used or if -O0 is used. Otherwise, GCC may still be unable to inline a function for many reasons; the -Winline option may be used to determine if a function has not been inlined and why not. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Implementation Link: next: Qualifiers implementation Link: prev: Hints implementation Next: Qualifiers implementation, Previous: Hints implementation, Up: C ────────────────────────────────────────────────────────────────────────────── 4.9 Structures, Unions, Enumerations, and Bit-Fields * A member of a union object is accessed using a member of a different type (C90 6.3.2.3). The relevant bytes of the representation of the object are treated as an object of the type used for the access. See Type-punning. This may be a trap representation. * Whether a “plain” int bit-field is treated as a signed int bit-field or as an unsigned int bit-field (C90 6.5.2, C90 6.5.2.1, C99 and C11 6.7.2, C99 and C11 6.7.2.1). By default it is treated as signed int but this may be changed by the -funsigned-bitfields option. * Allowable bit-field types other than _Bool, signed int, and unsigned int (C99 and C11 6.7.2.1). Other integer types, such as long int, and enumerated types are permitted even in strictly conforming mode. * Whether atomic types are permitted for bit-fields (C11 6.7.2.1). Atomic types are not permitted for bit-fields. * Whether a bit-field can straddle a storage-unit boundary (C90 6.5.2.1, C99 and C11 6.7.2.1). Determined by ABI. * The order of allocation of bit-fields within a unit (C90 6.5.2.1, C99 and C11 6.7.2.1). Determined by ABI. * The alignment of non-bit-field members of structures (C90 6.5.2.1, C99 and C11 6.7.2.1). Determined by ABI. * The integer type compatible with each enumerated type (C90 6.5.2.2, C99 and C11 6.7.2.2). Normally, the type is unsigned int if there are no negative values in the enumeration, otherwise int. If -fshort-enums is specified, then if there are negative values it is the first of signed char, short and int that can represent all the values, otherwise it is the first of unsigned char, unsigned short and unsigned int that can represent all the values. On some targets, -fshort-enums is the default; this is determined by the ABI. ────────────────────────────────────────────────────────────────────────────── Next: Qualifiers implementation, Previous: Hints implementation, Up: C Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Implementation Link: next: Declarators implementation Link: prev: Structures unions enumerations and bit-fields implementation Next: Declarators implementation, Previous: Structures unions enumerations ────────────────────────────────────────────────────────────────────────────── 4.10 Qualifiers * What constitutes an access to an object that has volatile-qualified type (C90 6.5.3, C99 and C11 6.7.3). Such an object is normally accessed by pointers and used for accessing hardware. In most expressions, it is intuitively obvious what is a read and what is a write. For example volatile int *dst = somevalue; volatile int *src = someothervalue; *dst = *src; will cause a read of the volatile object pointed to by src and store the value into the volatile object pointed to by dst. There is no guarantee that these reads and writes are atomic, especially for objects larger than int. However, if the volatile storage is not being modified, and the value of the volatile storage is not used, then the situation is less obvious. For example volatile int *src = somevalue; *src; According to the C standard, such an expression is an rvalue whose type is the unqualified version of its original type, i.e. int. Whether GCC interprets this as a read of the volatile object being pointed to or only as a request to evaluate the expression for its side effects depends on this type. If it is a scalar type, or on most targets an aggregate type whose only member object is of a scalar type, or a union type whose member objects are of scalar types, the expression is interpreted by GCC as a read of the volatile object; in the other cases, the expression is only evaluated for its side effects. ────────────────────────────────────────────────────────────────────────────── Next: Declarators implementation, Previous: Structures unions enumerations Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Implementation Link: next: Statements implementation Link: prev: Qualifiers implementation Next: Statements implementation, Previous: Qualifiers implementation, Up: ────────────────────────────────────────────────────────────────────────────── 4.11 Declarators * The maximum number of declarators that may modify an arithmetic, structure or union type (C90 6.5.4). GCC is only limited by available memory. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Implementation Link: next: Preprocessing directives implementation Link: prev: Declarators implementation Next: Preprocessing directives implementation, Previous: Declarators ────────────────────────────────────────────────────────────────────────────── 4.12 Statements * The maximum number of case values in a switch statement (C90 6.6.4.2). GCC is only limited by available memory. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Implementation Link: next: Library functions implementation Link: prev: Statements implementation Next: Library functions implementation, Previous: Statements ────────────────────────────────────────────────────────────────────────────── 4.13 Preprocessing Directives See Implementation-defined behavior in The C Preprocessor, for details of these aspects of implementation-defined behavior. * The locations within #pragma directives where header name preprocessing tokens are recognized (C11 6.4, C11 6.4.7). * How sequences in both forms of header names are mapped to headers or external source file names (C90 6.1.7, C99 and C11 6.4.7). * Whether the value of a character constant in a constant expression that controls conditional inclusion matches the value of the same character constant in the execution character set (C90 6.8.1, C99 and C11 6.10.1). * Whether the value of a single-character character constant in a constant expression that controls conditional inclusion may have a negative value (C90 6.8.1, C99 and C11 6.10.1). * The places that are searched for an included '<>' delimited header, and how the places are specified or the header is identified (C90 6.8.2, C99 and C11 6.10.2). * How the named source file is searched for in an included '""' delimited header (C90 6.8.2, C99 and C11 6.10.2). * The method by which preprocessing tokens (possibly resulting from macro expansion) in a #include directive are combined into a header name (C90 6.8.2, C99 and C11 6.10.2). * The nesting limit for #include processing (C90 6.8.2, C99 and C11 6.10.2). * Whether the '#' operator inserts a '\' character before the '\' character that begins a universal character name in a character constant or string literal (C99 and C11 6.10.3.2). * The behavior on each recognized non-STDC #pragma directive (C90 6.8.6, C99 and C11 6.10.6). See Pragmas in The C Preprocessor, for details of pragmas accepted by GCC on all targets. See Pragmas Accepted by GCC, for details of target-specific pragmas. * The definitions for __DATE__ and __TIME__ when respectively, the date and time of translation are not available (C90 6.8.8, C99 6.10.8, C11 6.10.8.1). ────────────────────────────────────────────────────────────────────────────── Next: Library functions implementation, Previous: Statements Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Implementation Link: next: Architecture implementation Link: prev: Preprocessing directives implementation Next: Architecture implementation, Previous: Preprocessing directives ────────────────────────────────────────────────────────────────────────────── 4.14 Library Functions The behavior of most of these points are dependent on the implementation of the C library, and are not defined by GCC itself. * The null pointer constant to which the macro NULL expands (C90 7.1.6, C99 7.17, C11 7.19). In , NULL expands to ((void *)0). GCC does not provide the other headers which define NULL and some library implementations may use other definitions in those headers. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Implementation Link: next: Locale-specific behavior implementation Link: prev: Library functions implementation Next: Locale-specific behavior implementation, Previous: Library functions ────────────────────────────────────────────────────────────────────────────── 4.15 Architecture * The values or expressions assigned to the macros specified in the headers , , and (C90, C99 and C11 5.2.4.2, C99 7.18.2, C99 7.18.3, C11 7.20.2, C11 7.20.3). Determined by ABI. * The result of attempting to indirectly access an object with automatic or thread storage duration from a thread other than the one with which it is associated (C11 6.2.4). Such accesses are supported, subject to the same requirements for synchronization for concurrent accesses as for concurrent accesses to any object. * The number, order, and encoding of bytes in any object (when not explicitly specified in this International Standard) (C99 and C11 6.2.6.1). Determined by ABI. * Whether any extended alignments are supported and the contexts in which they are supported (C11 6.2.8). Extended alignments up to 2^{28} (bytes) are supported for objects of automatic storage duration. Alignments supported for objects of static and thread storage duration are determined by the ABI. * Valid alignment values other than those returned by an _Alignof expression for fundamental types, if any (C11 6.2.8). Valid alignments are powers of 2 up to and including 2^{28}. * The value of the result of the sizeof and _Alignof operators (C90 6.3.3.4, C99 and C11 6.5.3.4). Determined by ABI. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Implementation Link: next: C++ Implementation Link: prev: Architecture implementation Previous: Architecture implementation, Up: C Implementation ────────────────────────────────────────────────────────────────────────────── 4.16 Locale-Specific Behavior The behavior of these points are dependent on the implementation of the C library, and are not defined by GCC itself. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Top Link: next: Conditionally-supported behavior Link: prev: Locale-specific behavior implementation Next: C Extensions, Previous: C Implementation, Up: Top ────────────────────────────────────────────────────────────────────────────── 5 C++ Implementation-Defined Behavior A conforming implementation of ISO C++ is required to document its choice of behavior in each of the areas that are designated “implementation defined”. The following lists all such areas, along with the section numbers from the ISO/IEC 14882:1998 and ISO/IEC 14882:2003 standards. Some areas are only implementation-defined in one version of the standard. Some choices depend on the externally determined ABI for the platform (including standard character encodings) which GCC follows; these are listed as “determined by ABI” below. See Binary Compatibility, and http://gcc.gnu.org/readings.html. Some choices are documented in the preprocessor manual. See Implementation-defined behavior in The C Preprocessor. Some choices are documented in the corresponding document for the C language. See C Implementation. Some choices are made by the library and operating system (or other environment when compiling for a freestanding environment); refer to their documentation for details. • Conditionally-supported behavior: • Exception handling: Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C++ Implementation Link: next: Exception handling Link: prev: C++ Implementation ────────────────────────────────────────────────────────────────────────────── 5.1 Conditionally-Supported Behavior Each implementation shall include documentation that identifies all conditionally-supported constructs that it does not support (C++0x 1.4). * Whether an argument of class type with a non-trivial copy constructor or destructor can be passed to ... (C++0x 5.2.2). Such argument passing is supported, using the same pass-by-invisible-reference approach used for normal function arguments of such types. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C++ Implementation Link: next: C Extensions Link: prev: Conditionally-supported behavior Previous: Conditionally-supported behavior, Up: C++ Implementation ────────────────────────────────────────────────────────────────────────────── 5.2 Exception Handling * In the situation where no matching handler is found, it is implementation-defined whether or not the stack is unwound before std::terminate() is called (C++98 15.5.1). The stack is not unwound before std::terminate is called. c Copyright (C) 1988-2019 Free Software Foundation, Inc. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Top Link: next: Statement Exprs Link: prev: Exception handling Next: C++ Extensions, Previous: C++ Implementation, Up: Top ────────────────────────────────────────────────────────────────────────────── 6 Extensions to the C Language Family GNU C provides several language features not found in ISO standard C. (The -pedantic option directs GCC to print a warning message if any of these features is used.) To test for the availability of these features in conditional compilation, check for a predefined macro __GNUC__, which is always defined under GCC. These extensions are available in C and Objective-C. Most of them are also available in C++. See Extensions to the C++ Language, for extensions that apply only to C++. Some features that are in ISO C99 but not C90 or C++ are also, as extensions, accepted by GCC in C90 mode and in C++. ────────────────────────────────────────────────────────────────────────────── 6.1 Statements and Declarations in Expressions A compound statement enclosed in parentheses may appear as an expression in GNU C. This allows you to use loops, switches, and local variables within an expression. Recall that a compound statement is a sequence of statements surrounded by braces; in this construct, parentheses go around the braces. For example: ({ int y = foo (); int z; if (y > 0) z = y; else z = - y; z; }) is a valid (though slightly more complex than necessary) expression for the absolute value of foo (). The last thing in the compound statement should be an expression followed by a semicolon; the value of this subexpression serves as the value of the entire construct. (If you use some other kind of statement last within the braces, the construct has type void, and thus effectively no value.) This feature is especially useful in making macro definitions “safe” (so that they evaluate each operand exactly once). For example, the “maximum” function is commonly defined as a macro in standard C as follows: #define max(a,b) ((a) > (b) ? (a) : (b)) But this definition computes either a or b twice, with bad results if the operand has side effects. In GNU C, if you know the type of the operands (here taken as int), you can avoid this problem by defining the macro as follows: #define maxint(a,b) \ ({int _a = (a), _b = (b); _a > _b ? _a : _b; }) Note that introducing variable declarations (as we do in maxint) can cause variable shadowing, so while this example using the max macro produces correct results: int _a = 1, _b = 2, c; c = max (_a, _b); this example using maxint will not: int _a = 1, _b = 2, c; c = maxint (_a, _b); This problem may for instance occur when we use this pattern recursively, like so: #define maxint3(a, b, c) \ ({int _a = (a), _b = (b), _c = (c); maxint (maxint (_a, _b), _c); }) Embedded statements are not allowed in constant expressions, such as the value of an enumeration constant, the width of a bit-field, or the initial value of a static variable. If you don't know the type of the operand, you can still do this, but you must use typeof or __auto_type (see Typeof). In G++, the result value of a statement expression undergoes array and function pointer decay, and is returned by value to the enclosing expression. For instance, if A is a class, then A a; ({a;}).Foo () constructs a temporary A object to hold the result of the statement expression, and that is used to invoke Foo. Therefore the this pointer observed by Foo is not the address of a. In a statement expression, any temporaries created within a statement are destroyed at that statement's end. This makes statement expressions inside macros slightly different from function calls. In the latter case temporaries introduced during argument evaluation are destroyed at the end of the statement that includes the function call. In the statement expression case they are destroyed during the statement expression. For instance, #define macro(a) ({__typeof__(a) b = (a); b + 3; }) template T function(T a) { T b = a; return b + 3; } void foo () { macro (X ()); function (X ()); } has different places where temporaries are destroyed. For the macro case, the temporary X is destroyed just after the initialization of b. In the function case that temporary is destroyed when the function returns. These considerations mean that it is probably a bad idea to use statement expressions of this form in header files that are designed to work with C++. (Note that some versions of the GNU C Library contained header files using statement expressions that lead to precisely this bug.) Jumping into a statement expression with goto or using a switch statement outside the statement expression with a case or default label inside the statement expression is not permitted. Jumping into a statement expression with a computed goto (see Labels as Values) has undefined behavior. Jumping out of a statement expression is permitted, but if the statement expression is part of a larger expression then it is unspecified which other subexpressions of that expression have been evaluated except where the language definition requires certain subexpressions to be evaluated before or after the statement expression. A break or continue statement inside of a statement expression used in while, do or for loop or switch statement condition or for statement init or increment expressions jumps to an outer loop or switch statement if any (otherwise it is an error), rather than to the loop or switch statement in whose condition or init or increment expression it appears. In any case, as with a function call, the evaluation of a statement expression is not interleaved with the evaluation of other parts of the containing expression. For example, foo (), (({ bar1 (); goto a; 0; }) + bar2 ()), baz(); calls foo and bar1 and does not call baz but may or may not call bar2. If bar2 is called, it is called after foo and before bar1. ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Labels as Values Link: prev: Statement Exprs Next: Labels as Values, Previous: Statement Exprs, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.2 Locally Declared Labels GCC allows you to declare local labels in any nested block scope. A local label is just like an ordinary label, but you can only reference it (with a goto statement, or by taking its address) within the block in which it is declared. A local label declaration looks like this: __label__ label; or __label__ label1, label2, /* … */; Local label declarations must come at the beginning of the block, before any ordinary declarations or statements. The label declaration defines the label name, but does not define the label itself. You must do this in the usual way, with label:, within the statements of the statement expression. The local label feature is useful for complex macros. If a macro contains nested loops, a goto can be useful for breaking out of them. However, an ordinary label whose scope is the whole function cannot be used: if the macro can be expanded several times in one function, the label is multiply defined in that function. A local label avoids this problem. For example: #define SEARCH(value, array, target) \ do { \ __label__ found; \ typeof (target) _SEARCH_target = (target); \ typeof (*(array)) *_SEARCH_array = (array); \ int i, j; \ int value; \ for (i = 0; i < max; i++) \ for (j = 0; j < max; j++) \ if (_SEARCH_array[i][j] == _SEARCH_target) \ { (value) = i; goto found; } \ (value) = -1; \ found:; \ } while (0) This could also be written using a statement expression: #define SEARCH(array, target) \ ({ \ __label__ found; \ typeof (target) _SEARCH_target = (target); \ typeof (*(array)) *_SEARCH_array = (array); \ int i, j; \ int value; \ for (i = 0; i < max; i++) \ for (j = 0; j < max; j++) \ if (_SEARCH_array[i][j] == _SEARCH_target) \ { value = i; goto found; } \ value = -1; \ found: \ value; \ }) Local label declarations also make the labels they declare visible to nested functions, if there are any. See Nested Functions, for details. ────────────────────────────────────────────────────────────────────────────── Next: Labels as Values, Previous: Statement Exprs, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Nested Functions Link: prev: Local Labels Next: Nested Functions, Previous: Local Labels, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.3 Labels as Values You can get the address of a label defined in the current function (or a containing function) with the unary operator '&&'. The value has type void *. This value is a constant and can be used wherever a constant of that type is valid. For example: void *ptr; /* … */ ptr = &&foo; To use these values, you need to be able to jump to one. This is done with the computed goto statement^3, goto *exp;. For example, goto *ptr; Any expression of type void * is allowed. One way of using these constants is in initializing a static array that serves as a jump table: static void *array[] = { &&foo, &&bar, &&hack }; Then you can select a label with indexing, like this: goto *array[i]; Note that this does not check whether the subscript is in bounds—array indexing in C never does that. Such an array of label values serves a purpose much like that of the switch statement. The switch statement is cleaner, so use that rather than an array unless the problem does not fit a switch statement very well. Another use of label values is in an interpreter for threaded code. The labels within the interpreter function can be stored in the threaded code for super-fast dispatching. You may not use this mechanism to jump to code in a different function. If you do that, totally unpredictable things happen. The best way to avoid this is to store the label address only in automatic variables and never pass it as an argument. An alternate way to write the above example is static const int array[] = { &&foo - &&foo, &&bar - &&foo, &&hack - &&foo }; goto *(&&foo + array[i]); This is more friendly to code living in shared libraries, as it reduces the number of dynamic relocations that are needed, and by consequence, allows the data to be read-only. This alternative with label differences is not supported for the AVR target, please use the first approach for AVR programs. The &&foo expressions for the same label might have different values if the containing function is inlined or cloned. If a program relies on them being always the same, __attribute__((__noinline__,__noclone__)) should be used to prevent inlining and cloning. If &&foo is used in a static variable initializer, inlining and cloning is forbidden. Footnotes (3) The analogous feature in Fortran is called an assigned goto, but that name seems inappropriate in C, where one can do more than simply store label addresses in label variables. ────────────────────────────────────────────────────────────────────────────── Next: Nested Functions, Previous: Local Labels, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Nonlocal Gotos Link: prev: Labels as Values Next: Nonlocal Gotos, Previous: Labels as Values, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.4 Nested Functions A nested function is a function defined inside another function. Nested functions are supported as an extension in GNU C, but are not supported by GNU C++. The nested function's name is local to the block where it is defined. For example, here we define a nested function named square, and call it twice: foo (double a, double b) { double square (double z) { return z * z; } return square (a) + square (b); } The nested function can access all the variables of the containing function that are visible at the point of its definition. This is called lexical scoping. For example, here we show a nested function which uses an inherited variable named offset: bar (int *array, int offset, int size) { int access (int *array, int index) { return array[index + offset]; } int i; /* … */ for (i = 0; i < size; i++) /* … */ access (array, i) /* … */ } Nested function definitions are permitted within functions in the places where variable definitions are allowed; that is, in any block, mixed with the other declarations and statements in the block. It is possible to call the nested function from outside the scope of its name by storing its address or passing the address to another function: hack (int *array, int size) { void store (int index, int value) { array[index] = value; } intermediate (store, size); } Here, the function intermediate receives the address of store as an argument. If intermediate calls store, the arguments given to store are used to store into array. But this technique works only so long as the containing function (hack, in this example) does not exit. If you try to call the nested function through its address after the containing function exits, all hell breaks loose. If you try to call it after a containing scope level exits, and if it refers to some of the variables that are no longer in scope, you may be lucky, but it's not wise to take the risk. If, however, the nested function does not refer to anything that has gone out of scope, you should be safe. GCC implements taking the address of a nested function using a technique called trampolines. This technique was described in Lexical Closures for C++ (Thomas M. Breuel, USENIX C++ Conference Proceedings, October 17-21, 1988). A nested function can jump to a label inherited from a containing function, provided the label is explicitly declared in the containing function (see Local Labels). Such a jump returns instantly to the containing function, exiting the nested function that did the goto and any intermediate functions as well. Here is an example: bar (int *array, int offset, int size) { __label__ failure; int access (int *array, int index) { if (index > size) goto failure; return array[index + offset]; } int i; /* … */ for (i = 0; i < size; i++) /* … */ access (array, i) /* … */ /* … */ return 0; /* Control comes here from access if it detects an error. */ failure: return -1; } A nested function always has no linkage. Declaring one with extern or static is erroneous. If you need to declare the nested function before its definition, use auto (which is otherwise meaningless for function declarations). bar (int *array, int offset, int size) { __label__ failure; auto int access (int *, int); /* … */ int access (int *array, int index) { if (index > size) goto failure; return array[index + offset]; } /* … */ } ────────────────────────────────────────────────────────────────────────────── Next: Nonlocal Gotos, Previous: Labels as Values, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Constructing Calls Link: prev: Nested Functions Next: Constructing Calls, Previous: Nested Functions, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.5 Nonlocal Gotos GCC provides the built-in functions __builtin_setjmp and __builtin_longjmp which are similar to, but not interchangeable with, the C library functions setjmp and longjmp. The built-in versions are used internally by GCC's libraries to implement exception handling on some targets. You should use the standard C library functions declared in in user code instead of the builtins. The built-in versions of these functions use GCC's normal mechanisms to save and restore registers using the stack on function entry and exit. The jump buffer argument buf holds only the information needed to restore the stack frame, rather than the entire set of saved register values. An important caveat is that GCC arranges to save and restore only those registers known to the specific architecture variant being compiled for. This can make __builtin_setjmp and __builtin_longjmp more efficient than their library counterparts in some cases, but it can also cause incorrect and mysterious behavior when mixing with code that uses the full register set. You should declare the jump buffer argument buf to the built-in functions as: #include intptr_t buf[5]; Built-in Function: int __builtin_setjmp (intptr_t *buf) This function saves the current stack context in buf. __builtin_setjmp returns 0 when returning directly, and 1 when returning from __builtin_longjmp using the same buf. Built-in Function: void __builtin_longjmp (intptr_t *buf, int val) This function restores the stack context in buf, saved by a previous call to __builtin_setjmp. After __builtin_longjmp is finished, the program resumes execution as if the matching __builtin_setjmp returns the value val, which must be 1. Because __builtin_longjmp depends on the function return mechanism to restore the stack context, it cannot be called from the same function calling __builtin_setjmp to initialize buf. It can only be called from a function called (directly or indirectly) from the function calling __builtin_setjmp. ────────────────────────────────────────────────────────────────────────────── Next: Constructing Calls, Previous: Nested Functions, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Typeof Link: prev: Nonlocal Gotos Next: Typeof, Previous: Nonlocal Gotos, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.6 Constructing Function Calls Using the built-in functions described below, you can record the arguments a function received, and call another function with the same arguments, without knowing the number or types of the arguments. You can also record the return value of that function call, and later return that value, without knowing what data type the function tried to return (as long as your caller expects that data type). However, these built-in functions may interact badly with some sophisticated features or other extensions of the language. It is, therefore, not recommended to use them outside very simple functions acting as mere forwarders for their arguments. Built-in Function: void * __builtin_apply_args () This built-in function returns a pointer to data describing how to perform a call with the same arguments as are passed to the current function. The function saves the arg pointer register, structure value address, and all registers that might be used to pass arguments to a function into a block of memory allocated on the stack. Then it returns the address of that block. Built-in Function: void * __builtin_apply (void (*function)(), void *arguments, size_t size) This built-in function invokes function with a copy of the parameters described by arguments and size. The value of arguments should be the value returned by __builtin_apply_args. The argument size specifies the size of the stack argument data, in bytes. This function returns a pointer to data describing how to return whatever value is returned by function. The data is saved in a block of memory allocated on the stack. It is not always simple to compute the proper value for size. The value is used by __builtin_apply to compute the amount of data that should be pushed on the stack and copied from the incoming argument area. Built-in Function: void __builtin_return (void *result) This built-in function returns the value described by result from the containing function. You should specify, for result, a value returned by __builtin_apply. Built-in Function: __builtin_va_arg_pack () This built-in function represents all anonymous arguments of an inline function. It can be used only in inline functions that are always inlined, never compiled as a separate function, such as those using __attribute__ ((__always_inline__)) or __attribute__ ((__gnu_inline__)) extern inline functions. It must be only passed as last argument to some other function with variable arguments. This is useful for writing small wrapper inlines for variable argument functions, when using preprocessor macros is undesirable. For example: extern int myprintf (FILE *f, const char *format, ...); extern inline __attribute__ ((__gnu_inline__)) int myprintf (FILE *f, const char *format, ...) { int r = fprintf (f, "myprintf: "); if (r < 0) return r; int s = fprintf (f, format, __builtin_va_arg_pack ()); if (s < 0) return s; return r + s; } Built-in Function: size_t __builtin_va_arg_pack_len () This built-in function returns the number of anonymous arguments of an inline function. It can be used only in inline functions that are always inlined, never compiled as a separate function, such as those using __attribute__ ((__always_inline__)) or __attribute__ ((__gnu_inline__)) extern inline functions. For example following does link- or run-time checking of open arguments for optimized code: #ifdef __OPTIMIZE__ extern inline __attribute__((__gnu_inline__)) int myopen (const char *path, int oflag, ...) { if (__builtin_va_arg_pack_len () > 1) warn_open_too_many_arguments (); if (__builtin_constant_p (oflag)) { if ((oflag & O_CREAT) != 0 && __builtin_va_arg_pack_len () < 1) { warn_open_missing_mode (); return __open_2 (path, oflag); } return open (path, oflag, __builtin_va_arg_pack ()); } if (__builtin_va_arg_pack_len () < 1) return __open_2 (path, oflag); return open (path, oflag, __builtin_va_arg_pack ()); } #endif ────────────────────────────────────────────────────────────────────────────── Next: Typeof, Previous: Nonlocal Gotos, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Conditionals Link: prev: Constructing Calls Next: Conditionals, Previous: Constructing Calls, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.7 Referring to a Type with typeof Another way to refer to the type of an expression is with typeof. The syntax of using of this keyword looks like sizeof, but the construct acts semantically like a type name defined with typedef. There are two ways of writing the argument to typeof: with an expression or with a type. Here is an example with an expression: typeof (x[0](1)) This assumes that x is an array of pointers to functions; the type described is that of the values of the functions. Here is an example with a typename as the argument: typeof (int *) Here the type described is that of pointers to int. If you are writing a header file that must work when included in ISO C programs, write __typeof__ instead of typeof. See Alternate Keywords. A typeof construct can be used anywhere a typedef name can be used. For example, you can use it in a declaration, in a cast, or inside of sizeof or typeof. The operand of typeof is evaluated for its side effects if and only if it is an expression of variably modified type or the name of such a type. typeof is often useful in conjunction with statement expressions (see Statement Exprs). Here is how the two together can be used to define a safe “maximum” macro which operates on any arithmetic type and evaluates each of its arguments exactly once: #define max(a,b) \ ({ typeof (a) _a = (a); \ typeof (b) _b = (b); \ _a > _b ? _a : _b; }) The reason for using names that start with underscores for the local variables is to avoid conflicts with variable names that occur within the expressions that are substituted for a and b. Eventually we hope to design a new form of declaration syntax that allows you to declare variables whose scopes start only after their initializers; this will be a more reliable way to prevent such conflicts. Some more examples of the use of typeof: * This declares y with the type of what x points to. typeof (*x) y; * This declares y as an array of such values. typeof (*x) y[4]; * This declares y as an array of pointers to characters: typeof (typeof (char *)[4]) y; It is equivalent to the following traditional C declaration: char *y[4]; To see the meaning of the declaration using typeof, and why it might be a useful way to write, rewrite it with these macros: #define pointer(T) typeof(T *) #define array(T, N) typeof(T [N]) Now the declaration can be rewritten this way: array (pointer (char), 4) y; Thus, array (pointer (char), 4) is the type of arrays of 4 pointers to char. In GNU C, but not GNU C++, you may also declare the type of a variable as __auto_type. In that case, the declaration must declare only one variable, whose declarator must just be an identifier, the declaration must be initialized, and the type of the variable is determined by the initializer; the name of the variable is not in scope until after the initializer. (In C++, you should use C++11 auto for this purpose.) Using __auto_type, the “maximum” macro above could be written as: #define max(a,b) \ ({ __auto_type _a = (a); \ __auto_type _b = (b); \ _a > _b ? _a : _b; }) Using __auto_type instead of typeof has two advantages: * Each argument to the macro appears only once in the expansion of the macro. This prevents the size of the macro expansion growing exponentially when calls to such macros are nested inside arguments of such macros. * If the argument to the macro has variably modified type, it is evaluated only once when using __auto_type, but twice if typeof is used. ────────────────────────────────────────────────────────────────────────────── Next: Conditionals, Previous: Constructing Calls, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: __int128 Link: prev: Typeof ────────────────────────────────────────────────────────────────────────────── 6.8 Conditionals with Omitted Operands The middle operand in a conditional expression may be omitted. Then if the first operand is nonzero, its value is the value of the conditional expression. Therefore, the expression x ? : y has the value of x if that is nonzero; otherwise, the value of y. This example is perfectly equivalent to x ? x : y In this simple case, the ability to omit the middle operand is not especially useful. When it becomes useful is when the first operand does, or may (if it is a macro argument), contain a side effect. Then repeating the operand in the middle would perform the side effect twice. Omitting the middle operand uses the value already computed without the undesirable effects of recomputing it. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Long Long Link: prev: Conditionals Next: Long Long, Previous: Conditionals, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.9 128-bit Integers As an extension the integer scalar type __int128 is supported for targets which have an integer mode wide enough to hold 128 bits. Simply write __int128 for a signed 128-bit integer, or unsigned __int128 for an unsigned 128-bit integer. There is no support in GCC for expressing an integer constant of type __int128 for targets with long long integer less than 128 bits wide. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Complex Link: prev: __int128 ────────────────────────────────────────────────────────────────────────────── 6.10 Double-Word Integers ISO C99 and ISO C++11 support data types for integers that are at least 64 bits wide, and as an extension GCC supports them in C90 and C++98 modes. Simply write long long int for a signed integer, or unsigned long long int for an unsigned integer. To make an integer constant of type long long int, add the suffix 'LL' to the integer. To make an integer constant of type unsigned long long int, add the suffix 'ULL' to the integer. You can use these types in arithmetic like any other integer types. Addition, subtraction, and bitwise boolean operations on these types are open-coded on all types of machines. Multiplication is open-coded if the machine supports a fullword-to-doubleword widening multiply instruction. Division and shifts are open-coded only on machines that provide special support. The operations that are not open-coded use special library routines that come with GCC. There may be pitfalls when you use long long types for function arguments without function prototypes. If a function expects type int for its argument, and you pass a value of type long long int, confusion results because the caller and the subroutine disagree about the number of bytes for the argument. Likewise, if the function expects long long int and you pass int. The best way to avoid such problems is to use prototypes. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Floating Types Link: prev: Long Long Next: Floating Types, Previous: Long Long, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.11 Complex Numbers ISO C99 supports complex floating data types, and as an extension GCC supports them in C90 mode and in C++. GCC also supports complex integer data types which are not part of ISO C99. You can declare complex types using the keyword _Complex. As an extension, the older GNU keyword __complex__ is also supported. For example, '_Complex double x;' declares x as a variable whose real part and imaginary part are both of type double. '_Complex short int y;' declares y to have real and imaginary parts of type short int; this is not likely to be useful, but it shows that the set of complex types is complete. To write a constant with a complex data type, use the suffix 'i' or 'j' (either one; they are equivalent). For example, 2.5fi has type _Complex float and 3i has type _Complex int. Such a constant always has a pure imaginary value, but you can form any complex value you like by adding one to a real constant. This is a GNU extension; if you have an ISO C99 conforming C library (such as the GNU C Library), and want to construct complex constants of floating type, you should include and use the macros I or _Complex_I instead. The ISO C++14 library also defines the 'i' suffix, so C++14 code that includes the '' header cannot use 'i' for the GNU extension. The 'j' suffix still has the GNU meaning. To extract the real part of a complex-valued expression exp, write __real__ exp. Likewise, use __imag__ to extract the imaginary part. This is a GNU extension; for values of floating type, you should use the ISO C99 functions crealf, creal, creall, cimagf, cimag and cimagl, declared in and also provided as built-in functions by GCC. The operator '~' performs complex conjugation when used on a value with a complex type. This is a GNU extension; for values of floating type, you should use the ISO C99 functions conjf, conj and conjl, declared in and also provided as built-in functions by GCC. GCC can allocate complex automatic variables in a noncontiguous fashion; it's even possible for the real part to be in a register while the imaginary part is on the stack (or vice versa). Only the DWARF debug info format can represent this, so use of DWARF is recommended. If you are using the stabs debug info format, GCC describes a noncontiguous complex variable as if it were two separate variables of noncomplex type. If the variable's actual name is foo, the two fictitious variables are named foo$real and foo$imag. You can examine and set these two fictitious variables with your debugger. ────────────────────────────────────────────────────────────────────────────── Next: Floating Types, Previous: Long Long, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Half-Precision Link: prev: Complex Next: Half-Precision, Previous: Complex, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.12 Additional Floating Types ISO/IEC TS 18661-3:2015 defines C support for additional floating types _Floatn and _Floatnx, and GCC supports these type names; the set of types supported depends on the target architecture. These types are not supported when compiling C++. Constants with these types use suffixes fn or Fn and fnx or Fnx. These type names can be used together with _Complex to declare complex types. As an extension, GNU C and GNU C++ support additional floating types, which are not supported by all targets. * __float128 is available on i386, x86_64, IA-64, and hppa HP-UX, as well as on PowerPC GNU/Linux targets that enable the vector scalar (VSX) instruction set. __float128 supports the 128-bit floating type. On i386, x86_64, PowerPC, and IA-64 other than HP-UX, __float128 is an alias for _Float128. On hppa and IA-64 HP-UX, __float128 is an alias for long double. * __float80 is available on the i386, x86_64, and IA-64 targets, and supports the 80-bit (XFmode) floating type. It is an alias for the type name _Float64x on these targets. * __ibm128 is available on PowerPC targets, and provides access to the IBM extended double format which is the current format used for long double. When long double transitions to __float128 on PowerPC in the future, __ibm128 will remain for use in conversions between the two types. Support for these additional types includes the arithmetic operators: add, subtract, multiply, divide; unary arithmetic operators; relational operators; equality operators; and conversions to and from integer and other floating types. Use a suffix 'w' or 'W' in a literal constant of type __float80 or type __ibm128. Use a suffix 'q' or 'Q' for _float128. In order to use _Float128, __float128, and __ibm128 on PowerPC Linux systems, you must use the -mfloat128 option. It is expected in future versions of GCC that _Float128 and __float128 will be enabled automatically. The _Float128 type is supported on all systems where __float128 is supported or where long double has the IEEE binary128 format. The _Float64x type is supported on all systems where __float128 is supported. The _Float32 type is supported on all systems supporting IEEE binary32; the _Float64 and _Float32x types are supported on all systems supporting IEEE binary64. The _Float16 type is supported on AArch64 systems by default, and on ARM systems when the IEEE format for 16-bit floating-point types is selected with -mfp16-format=ieee. GCC does not currently support _Float128x on any systems. On the i386, x86_64, IA-64, and HP-UX targets, you can declare complex types using the corresponding internal complex type, XCmode for __float80 type and TCmode for __float128 type: typedef _Complex float __attribute__((mode(TC))) _Complex128; typedef _Complex float __attribute__((mode(XC))) _Complex80; On the PowerPC Linux VSX targets, you can declare complex types using the corresponding internal complex type, KCmode for __float128 type and ICmode for __ibm128 type: typedef _Complex float __attribute__((mode(KC))) _Complex_float128; typedef _Complex float __attribute__((mode(IC))) _Complex_ibm128; ────────────────────────────────────────────────────────────────────────────── Next: Half-Precision, Previous: Complex, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Decimal Float Link: prev: Floating Types Next: Decimal Float, Previous: Floating Types, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.13 Half-Precision Floating Point On ARM and AArch64 targets, GCC supports half-precision (16-bit) floating point via the __fp16 type defined in the ARM C Language Extensions. On ARM systems, you must enable this type explicitly with the -mfp16-format command-line option in order to use it. ARM targets support two incompatible representations for half-precision floating-point values. You must choose one of the representations and use it consistently in your program. Specifying -mfp16-format=ieee selects the IEEE 754-2008 format. This format can represent normalized values in the range of 2^{-14} to 65504. There are 11 bits of significand precision, approximately 3 decimal digits. Specifying -mfp16-format=alternative selects the ARM alternative format. This representation is similar to the IEEE format, but does not support infinities or NaNs. Instead, the range of exponents is extended, so that this format can represent normalized values in the range of 2^{-14} to 131008. The GCC port for AArch64 only supports the IEEE 754-2008 format, and does not require use of the -mfp16-format command-line option. The __fp16 type may only be used as an argument to intrinsics defined in , or as a storage format. For purposes of arithmetic and other operations, __fp16 values in C or C++ expressions are automatically promoted to float. The ARM target provides hardware support for conversions between __fp16 and float values as an extension to VFP and NEON (Advanced SIMD), and from ARMv8-A provides hardware support for conversions between __fp16 and double values. GCC generates code using these hardware instructions if you compile with options to select an FPU that provides them; for example, -mfpu=neon-fp16 -mfloat-abi=softfp, in addition to the -mfp16-format option to select a half-precision format. Language-level support for the __fp16 data type is independent of whether GCC generates code using hardware floating-point instructions. In cases where hardware support is not specified, GCC implements conversions between __fp16 and other types as library calls. It is recommended that portable code use the _Float16 type defined by ISO/IEC TS 18661-3:2015. See Floating Types. ────────────────────────────────────────────────────────────────────────────── Next: Decimal Float, Previous: Floating Types, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Hex Floats Link: prev: Half-Precision Next: Hex Floats, Previous: Half-Precision, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.14 Decimal Floating Types As an extension, GNU C supports decimal floating types as defined in the N1312 draft of ISO/IEC WDTR24732. Support for decimal floating types in GCC will evolve as the draft technical report changes. Calling conventions for any target might also change. Not all targets support decimal floating types. The decimal floating types are _Decimal32, _Decimal64, and _Decimal128. They use a radix of ten, unlike the floating types float, double, and long double whose radix is not specified by the C standard but is usually two. Support for decimal floating types includes the arithmetic operators add, subtract, multiply, divide; unary arithmetic operators; relational operators; equality operators; and conversions to and from integer and other floating types. Use a suffix 'df' or 'DF' in a literal constant of type _Decimal32, 'dd' or 'DD' for _Decimal64, and 'dl' or 'DL' for _Decimal128. GCC support of decimal float as specified by the draft technical report is incomplete: * When the value of a decimal floating type cannot be represented in the integer type to which it is being converted, the result is undefined rather than the result value specified by the draft technical report. * GCC does not provide the C library functionality associated with math.h, fenv.h, stdio.h, stdlib.h, and wchar.h, which must come from a separate C library implementation. Because of this the GNU C compiler does not define macro __STDC_DEC_FP__ to indicate that the implementation conforms to the technical report. Types _Decimal32, _Decimal64, and _Decimal128 are supported by the DWARF debug information format. ────────────────────────────────────────────────────────────────────────────── Next: Hex Floats, Previous: Half-Precision, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Fixed-Point Link: prev: Decimal Float Next: Fixed-Point, Previous: Decimal Float, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.15 Hex Floats ISO C99 and ISO C++17 support floating-point numbers written not only in the usual decimal notation, such as 1.55e1, but also numbers such as 0x1.fp3 written in hexadecimal format. As a GNU extension, GCC supports this in C90 mode (except in some cases when strictly conforming) and in C++98, C++11 and C++14 modes. In that format the '0x' hex introducer and the 'p' or 'P' exponent field are mandatory. The exponent is a decimal number that indicates the power of 2 by which the significant part is multiplied. Thus '0x1.f' is 1 15/16, 'p3' multiplies it by 8, and the value of 0x1.fp3 is the same as 1.55e1. Unlike for floating-point numbers in the decimal notation the exponent is always required in the hexadecimal notation. Otherwise the compiler would not be able to resolve the ambiguity of, e.g., 0x1.f. This could mean 1.0f or 1.9375 since 'f' is also the extension for floating-point constants of type float. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Named Address Spaces Link: prev: Hex Floats Next: Named Address Spaces, Previous: Hex Floats, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.16 Fixed-Point Types As an extension, GNU C supports fixed-point types as defined in the N1169 draft of ISO/IEC DTR 18037. Support for fixed-point types in GCC will evolve as the draft technical report changes. Calling conventions for any target might also change. Not all targets support fixed-point types. The fixed-point types are short _Fract, _Fract, long _Fract, long long _Fract, unsigned short _Fract, unsigned _Fract, unsigned long _Fract, unsigned long long _Fract, _Sat short _Fract, _Sat _Fract, _Sat long _Fract, _Sat long long _Fract, _Sat unsigned short _Fract, _Sat unsigned _Fract, _Sat unsigned long _Fract, _Sat unsigned long long _Fract, short _Accum, _Accum, long _Accum, long long _Accum, unsigned short _Accum, unsigned _Accum, unsigned long _Accum, unsigned long long _Accum, _Sat short _Accum, _Sat _Accum, _Sat long _Accum, _Sat long long _Accum, _Sat unsigned short _Accum, _Sat unsigned _Accum, _Sat unsigned long _Accum, _Sat unsigned long long _Accum. Fixed-point data values contain fractional and optional integral parts. The format of fixed-point data varies and depends on the target machine. Support for fixed-point types includes: * prefix and postfix increment and decrement operators (++, --) * unary arithmetic operators (+, -, !) * binary arithmetic operators (+, -, *, /) * binary shift operators (<<, >>) * relational operators (<, <=, >=, >) * equality operators (==, !=) * assignment operators (+=, -=, *=, /=, <<=, >>=) * conversions to and from integer, floating-point, or fixed-point types Use a suffix in a fixed-point literal constant: * 'hr' or 'HR' for short _Fract and _Sat short _Fract * 'r' or 'R' for _Fract and _Sat _Fract * 'lr' or 'LR' for long _Fract and _Sat long _Fract * 'llr' or 'LLR' for long long _Fract and _Sat long long _Fract * 'uhr' or 'UHR' for unsigned short _Fract and _Sat unsigned short _Fract * 'ur' or 'UR' for unsigned _Fract and _Sat unsigned _Fract * 'ulr' or 'ULR' for unsigned long _Fract and _Sat unsigned long _Fract * 'ullr' or 'ULLR' for unsigned long long _Fract and _Sat unsigned long long _Fract * 'hk' or 'HK' for short _Accum and _Sat short _Accum * 'k' or 'K' for _Accum and _Sat _Accum * 'lk' or 'LK' for long _Accum and _Sat long _Accum * 'llk' or 'LLK' for long long _Accum and _Sat long long _Accum * 'uhk' or 'UHK' for unsigned short _Accum and _Sat unsigned short _Accum * 'uk' or 'UK' for unsigned _Accum and _Sat unsigned _Accum * 'ulk' or 'ULK' for unsigned long _Accum and _Sat unsigned long _Accum * 'ullk' or 'ULLK' for unsigned long long _Accum and _Sat unsigned long long _Accum GCC support of fixed-point types as specified by the draft technical report is incomplete: * Pragmas to control overflow and rounding behaviors are not implemented. Fixed-point types are supported by the DWARF debug information format. ────────────────────────────────────────────────────────────────────────────── Next: Named Address Spaces, Previous: Hex Floats, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Zero Length Link: prev: Fixed-Point Next: Zero Length, Previous: Fixed-Point, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.17 Named Address Spaces As an extension, GNU C supports named address spaces as defined in the N1275 draft of ISO/IEC DTR 18037. Support for named address spaces in GCC will evolve as the draft technical report changes. Calling conventions for any target might also change. At present, only the AVR, SPU, M32C, RL78, and x86 targets support address spaces other than the generic address space. Address space identifiers may be used exactly like any other C type qualifier (e.g., const or volatile). See the N1275 document for more details. 6.17.1 AVR Named Address Spaces On the AVR target, there are several address spaces that can be used in order to put read-only data into the flash memory and access that data by means of the special instructions LPM or ELPM needed to read from flash. Devices belonging to avrtiny and avrxmega3 can access flash memory by means of LD* instructions because the flash memory is mapped into the RAM address space. There is no need for language extensions like __flash or attribute progmem. The default linker description files for these devices cater for that feature and .rodata stays in flash: The compiler just generates LD* instructions, and the linker script adds core specific offsets to all .rodata symbols: 0x4000 in the case of avrtiny and 0x8000 in the case of avrxmega3. See AVR Options for a list of respective devices. For devices not in avrtiny or avrxmega3, any data including read-only data is located in RAM (the generic address space) because flash memory is not visible in the RAM address space. In order to locate read-only data in flash memory and to generate the right instructions to access this data without using (inline) assembler code, special address spaces are needed. __flash The __flash qualifier locates data in the .progmem.data section. Data is read using the LPM instruction. Pointers to this address space are 16 bits wide. __flash1 __flash2 __flash3 __flash4 __flash5 These are 16-bit address spaces locating data in section .progmemN.data where N refers to address space __flashN. The compiler sets the RAMPZ segment register appropriately before reading data by means of the ELPM instruction. __memx This is a 24-bit address space that linearizes flash and RAM: If the high bit of the address is set, data is read from RAM using the lower two bytes as RAM address. If the high bit of the address is clear, data is read from flash with RAMPZ set according to the high byte of the address. See __builtin_avr_flash_segment. Objects in this address space are located in .progmemx.data. Example char my_read (const __flash char ** p) { /* p is a pointer to RAM that points to a pointer to flash. The first indirection of p reads that flash pointer from RAM and the second indirection reads a char from this flash address. */ return **p; } /* Locate array[] in flash memory */ const __flash int array[] = { 3, 5, 7, 11, 13, 17, 19 }; int i = 1; int main (void) { /* Return 17 by reading from flash memory */ return array[array[i]]; } For each named address space supported by avr-gcc there is an equally named but uppercase built-in macro defined. The purpose is to facilitate testing if respective address space support is available or not: #ifdef __FLASH const __flash int var = 1; int read_var (void) { return var; } #else #include /* From AVR-LibC */ const int var PROGMEM = 1; int read_var (void) { return (int) pgm_read_word (&var); } #endif /* __FLASH */ Notice that attribute progmem locates data in flash but accesses to these data read from generic address space, i.e. from RAM, so that you need special accessors like pgm_read_byte from AVR-LibC together with attribute progmem. Limitations and caveats * Reading across the 64 KiB section boundary of the __flash or __flashN address spaces shows undefined behavior. The only address space that supports reading across the 64 KiB flash segment boundaries is __memx. * If you use one of the __flashN address spaces you must arrange your linker script to locate the .progmemN.data sections according to your needs. * Any data or pointers to the non-generic address spaces must be qualified as const, i.e. as read-only data. This still applies if the data in one of these address spaces like software version number or calibration lookup table are intended to be changed after load time by, say, a boot loader. In this case the right qualification is const volatile so that the compiler must not optimize away known values or insert them as immediates into operands of instructions. * The following code initializes a variable pfoo located in static storage with a 24-bit address: extern const __memx char foo; const __memx void *pfoo = &foo; * On the reduced Tiny devices like ATtiny40, no address spaces are supported. Just use vanilla C / C++ code without overhead as outlined above. Attribute progmem is supported but works differently, see AVR Variable Attributes. 6.17.2 M32C Named Address Spaces On the M32C target, with the R8C and M16C CPU variants, variables qualified with __far are accessed using 32-bit addresses in order to access memory beyond the first 64 Ki bytes. If __far is used with the M32CM or M32C CPU variants, it has no effect. 6.17.3 RL78 Named Address Spaces On the RL78 target, variables qualified with __far are accessed with 32-bit pointers (20-bit addresses) rather than the default 16-bit addresses. Non-far variables are assumed to appear in the topmost 64 KiB of the address space. 6.17.4 SPU Named Address Spaces On the SPU target variables may be declared as belonging to another address space by qualifying the type with the __ea address space identifier: extern int __ea i; The compiler generates special code to access the variable i. It may use runtime library support, or generate special machine instructions to access that address space. 6.17.5 x86 Named Address Spaces On the x86 target, variables may be declared as being relative to the %fs or %gs segments. __seg_fs __seg_gs The object is accessed with the respective segment override prefix. The respective segment base must be set via some method specific to the operating system. Rather than require an expensive system call to retrieve the segment base, these address spaces are not considered to be subspaces of the generic (flat) address space. This means that explicit casts are required to convert pointers between these address spaces and the generic address space. In practice the application should cast to uintptr_t and apply the segment base offset that it installed previously. The preprocessor symbols __SEG_FS and __SEG_GS are defined when these address spaces are supported. ────────────────────────────────────────────────────────────────────────────── Next: Zero Length, Previous: Fixed-Point, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Empty Structures Link: prev: Named Address Spaces Next: Empty Structures, Previous: Named Address Spaces, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.18 Arrays of Length Zero Declaring zero-length arrays is allowed in GNU C as an extension. A zero-length array can be useful as the last element of a structure that is really a header for a variable-length object: struct line { int length; char contents[0]; }; struct line *thisline = (struct line *) malloc (sizeof (struct line) + this_length); thisline->length = this_length; Although the size of a zero-length array is zero, an array member of this kind may increase the size of the enclosing type as a result of tail padding. The offset of a zero-length array member from the beginning of the enclosing structure is the same as the offset of an array with one or more elements of the same type. The alignment of a zero-length array is the same as the alignment of its elements. Declaring zero-length arrays in other contexts, including as interior members of structure objects or as non-member objects, is discouraged. Accessing elements of zero-length arrays declared in such contexts is undefined and may be diagnosed. In the absence of the zero-length array extension, in ISO C90 the contents array in the example above would typically be declared to have a single element. Unlike a zero-length array which only contributes to the size of the enclosing structure for the purposes of alignment, a one-element array always occupies at least as much space as a single object of the type. Although using one-element arrays this way is discouraged, GCC handles accesses to trailing one-element array members analogously to zero-length arrays. The preferred mechanism to declare variable-length types like struct line above is the ISO C99 flexible array member, with slightly different syntax and semantics: * Flexible array members are written as contents[] without the 0. * Flexible array members have incomplete type, and so the sizeof operator may not be applied. As a quirk of the original implementation of zero-length arrays, sizeof evaluates to zero. * Flexible array members may only appear as the last member of a struct that is otherwise non-empty. * A structure containing a flexible array member, or a union containing such a structure (possibly recursively), may not be a member of a structure or an element of an array. (However, these uses are permitted by GCC as extensions.) Non-empty initialization of zero-length arrays is treated like any case where there are more initializer elements than the array holds, in that a suitable warning about “excess elements in array” is given, and the excess elements (all of them, in this case) are ignored. GCC allows static initialization of flexible array members. This is equivalent to defining a new structure containing the original structure followed by an array of sufficient size to contain the data. E.g. in the following, f1 is constructed as if it were declared like f2. struct f1 { int x; int y[]; } f1 = { 1, { 2, 3, 4 } }; struct f2 { struct f1 f1; int data[3]; } f2 = { { 1 }, { 2, 3, 4 } }; The convenience of this extension is that f1 has the desired type, eliminating the need to consistently refer to f2.f1. This has symmetry with normal static arrays, in that an array of unknown size is also written with []. Of course, this extension only makes sense if the extra data comes at the end of a top-level object, as otherwise we would be overwriting data at subsequent offsets. To avoid undue complication and confusion with initialization of deeply nested arrays, we simply disallow any non-empty initialization except when the structure is the top-level object. For example: struct foo { int x; int y[]; }; struct bar { struct foo z; }; struct foo a = { 1, { 2, 3, 4 } }; // Valid. struct bar b = { { 1, { 2, 3, 4 } } }; // Invalid. struct bar c = { { 1, { } } }; // Valid. struct foo d[1] = { { 1, { 2, 3, 4 } } }; // Invalid. ────────────────────────────────────────────────────────────────────────────── Next: Empty Structures, Previous: Named Address Spaces, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Variable Length Link: prev: Zero Length Next: Variable Length, Previous: Zero Length, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.19 Structures with No Members GCC permits a C structure to have no members: struct empty { }; The structure has size zero. In C++, empty structures are part of the language. G++ treats empty structures as if they had a single member of type char. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Variadic Macros Link: prev: Empty Structures Next: Variadic Macros, Previous: Empty Structures, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.20 Arrays of Variable Length Variable-length automatic arrays are allowed in ISO C99, and as an extension GCC accepts them in C90 mode and in C++. These arrays are declared like any other automatic arrays, but with a length that is not a constant expression. The storage is allocated at the point of declaration and deallocated when the block scope containing the declaration exits. For example: FILE * concat_fopen (char *s1, char *s2, char *mode) { char str[strlen (s1) + strlen (s2) + 1]; strcpy (str, s1); strcat (str, s2); return fopen (str, mode); } Jumping or breaking out of the scope of the array name deallocates the storage. Jumping into the scope is not allowed; you get an error message for it. As an extension, GCC accepts variable-length arrays as a member of a structure or a union. For example: void foo (int n) { struct S { int x[n]; }; } You can use the function alloca to get an effect much like variable-length arrays. The function alloca is available in many other C implementations (but not in all). On the other hand, variable-length arrays are more elegant. There are other differences between these two methods. Space allocated with alloca exists until the containing function returns. The space for a variable-length array is deallocated as soon as the array name's scope ends, unless you also use alloca in this scope. You can also use variable-length arrays as arguments to functions: struct entry tester (int len, char data[len][len]) { /* … */ } The length of an array is computed once when the storage is allocated and is remembered for the scope of the array in case you access it with sizeof. If you want to pass the array first and the length afterward, you can use a forward declaration in the parameter list—another GNU extension. struct entry tester (int len; char data[len][len], int len) { /* … */ } The 'int len' before the semicolon is a parameter forward declaration, and it serves the purpose of making the name len known when the declaration of data is parsed. You can write any number of such parameter forward declarations in the parameter list. They can be separated by commas or semicolons, but the last one must end with a semicolon, which is followed by the “real” parameter declarations. Each forward declaration must match a “real” declaration in parameter name and data type. ISO C99 does not support parameter forward declarations. ────────────────────────────────────────────────────────────────────────────── Next: Variadic Macros, Previous: Empty Structures, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Escaped Newlines Link: prev: Variable Length Next: Escaped Newlines, Previous: Variable Length, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.21 Macros with a Variable Number of Arguments. In the ISO C standard of 1999, a macro can be declared to accept a variable number of arguments much as a function can. The syntax for defining the macro is similar to that of a function. Here is an example: #define debug(format, ...) fprintf (stderr, format, __VA_ARGS__) Here '…' is a variable argument. In the invocation of such a macro, it represents the zero or more tokens until the closing parenthesis that ends the invocation, including any commas. This set of tokens replaces the identifier __VA_ARGS__ in the macro body wherever it appears. See the CPP manual for more information. GCC has long supported variadic macros, and used a different syntax that allowed you to give a name to the variable arguments just like any other argument. Here is an example: #define debug(format, args...) fprintf (stderr, format, args) This is in all ways equivalent to the ISO C example above, but arguably more readable and descriptive. GNU CPP has two further variadic macro extensions, and permits them to be used with either of the above forms of macro definition. In standard C, you are not allowed to leave the variable argument out entirely; but you are allowed to pass an empty argument. For example, this invocation is invalid in ISO C, because there is no comma after the string: debug ("A message") GNU CPP permits you to completely omit the variable arguments in this way. In the above examples, the compiler would complain, though since the expansion of the macro still has the extra comma after the format string. To help solve this problem, CPP behaves specially for variable arguments used with the token paste operator, '##'. If instead you write #define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__) and if the variable arguments are omitted or empty, the '##' operator causes the preprocessor to remove the comma before it. If you do provide some variable arguments in your macro invocation, GNU CPP does not complain about the paste operation and instead places the variable arguments after the comma. Just like any other pasted macro argument, these arguments are not macro expanded. ────────────────────────────────────────────────────────────────────────────── Next: Escaped Newlines, Previous: Variable Length, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Subscripting Link: prev: Variadic Macros Next: Subscripting, Previous: Variadic Macros, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.22 Slightly Looser Rules for Escaped Newlines The preprocessor treatment of escaped newlines is more relaxed than that specified by the C90 standard, which requires the newline to immediately follow a backslash. GCC's implementation allows whitespace in the form of spaces, horizontal and vertical tabs, and form feeds between the backslash and the subsequent newline. The preprocessor issues a warning, but treats it as a valid escaped newline and combines the two lines to form a single logical line. This works within comments and tokens, as well as between tokens. Comments are not treated as whitespace for the purposes of this relaxation, since they have not yet been replaced with spaces. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Pointer Arith Link: prev: Escaped Newlines Next: Pointer Arith, Previous: Escaped Newlines, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.23 Non-Lvalue Arrays May Have Subscripts In ISO C99, arrays that are not lvalues still decay to pointers, and may be subscripted, although they may not be modified or used after the next sequence point and the unary '&' operator may not be applied to them. As an extension, GNU C allows such arrays to be subscripted in C90 mode, though otherwise they do not decay to pointers outside C99 mode. For example, this is valid in GNU C though not valid in C90: struct foo {int a[4];}; struct foo f(); bar (int index) { return f().a[index]; } Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Variadic Pointer Args Link: prev: Subscripting Next: Variadic Pointer Args, Previous: Subscripting, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.24 Arithmetic on void- and Function-Pointers In GNU C, addition and subtraction operations are supported on pointers to void and on pointers to functions. This is done by treating the size of a void or of a function as 1. A consequence of this is that sizeof is also allowed on void and on function types, and returns 1. The option -Wpointer-arith requests a warning if these extensions are used. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Pointers to Arrays Link: prev: Pointer Arith Next: Pointers to Arrays, Previous: Pointer Arith, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.25 Pointer Arguments in Variadic Functions Standard C requires that pointer types used with va_arg in functions with variable argument lists either must be compatible with that of the actual argument, or that one type must be a pointer to void and the other a pointer to a character type. GNU C implements the POSIX XSI extension that additionally permits the use of va_arg with a pointer type to receive arguments of any other pointer type. In particular, in GNU C 'va_arg (ap, void *)' can safely be used to consume an argument of any pointer type. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Initializers Link: prev: Variadic Pointer Args Next: Initializers, Previous: Variadic Pointer Args, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.26 Pointers to Arrays with Qualifiers Work as Expected In GNU C, pointers to arrays with qualifiers work similar to pointers to other qualified types. For example, a value of type int (*)[5] can be used to initialize a variable of type const int (*)[5]. These types are incompatible in ISO C because the const qualifier is formally attached to the element type of the array and not the array itself. extern void transpose (int N, int M, double out[M][N], const double in[N][M]); double x[3][2]; double y[2][3]; … transpose(3, 2, y, x); Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Compound Literals Link: prev: Pointers to Arrays Next: Compound Literals, Previous: Pointers to Arrays, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.27 Non-Constant Initializers As in standard C++ and ISO C99, the elements of an aggregate initializer for an automatic variable are not required to be constant expressions in GNU C. Here is an example of an initializer with run-time varying elements: foo (float f, float g) { float beat_freqs[2] = { f-g, f+g }; /* … */ } Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Designated Inits Link: prev: Initializers Next: Designated Inits, Previous: Initializers, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.28 Compound Literals A compound literal looks like a cast of a brace-enclosed aggregate initializer list. Its value is an object of the type specified in the cast, containing the elements specified in the initializer. Unlike the result of a cast, a compound literal is an lvalue. ISO C99 and later support compound literals. As an extension, GCC supports compound literals also in C90 mode and in C++, although as explained below, the C++ semantics are somewhat different. Usually, the specified type of a compound literal is a structure. Assume that struct foo and structure are declared as shown: struct foo {int a; char b[2];} structure; Here is an example of constructing a struct foo with a compound literal: structure = ((struct foo) {x + y, 'a', 0}); This is equivalent to writing the following: { struct foo temp = {x + y, 'a', 0}; structure = temp; } You can also construct an array, though this is dangerous in C++, as explained below. If all the elements of the compound literal are (made up of) simple constant expressions suitable for use in initializers of objects of static storage duration, then the compound literal can be coerced to a pointer to its first element and used in such an initializer, as shown here: char **foo = (char *[]) { "x", "y", "z" }; Compound literals for scalar types and union types are also allowed. In the following example the variable i is initialized to the value 2, the result of incrementing the unnamed object created by the compound literal. int i = ++(int) { 1 }; As a GNU extension, GCC allows initialization of objects with static storage duration by compound literals (which is not possible in ISO C99 because the initializer is not a constant). It is handled as if the object were initialized only with the brace-enclosed list if the types of the compound literal and the object match. The elements of the compound literal must be constant. If the object being initialized has array type of unknown size, the size is determined by the size of the compound literal. static struct foo x = (struct foo) {1, 'a', 'b'}; static int y[] = (int []) {1, 2, 3}; static int z[] = (int [3]) {1}; The above lines are equivalent to the following: static struct foo x = {1, 'a', 'b'}; static int y[] = {1, 2, 3}; static int z[] = {1, 0, 0}; In C, a compound literal designates an unnamed object with static or automatic storage duration. In C++, a compound literal designates a temporary object that only lives until the end of its full-expression. As a result, well-defined C code that takes the address of a subobject of a compound literal can be undefined in C++, so G++ rejects the conversion of a temporary array to a pointer. For instance, if the array compound literal example above appeared inside a function, any subsequent use of foo in C++ would have undefined behavior because the lifetime of the array ends after the declaration of foo. As an optimization, G++ sometimes gives array compound literals longer lifetimes: when the array either appears outside a function or has a const-qualified type. If foo and its initializer had elements of type char *const rather than char *, or if foo were a global variable, the array would have static storage duration. But it is probably safest just to avoid the use of array compound literals in C++ code. ────────────────────────────────────────────────────────────────────────────── Next: Designated Inits, Previous: Initializers, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Case Ranges Link: prev: Compound Literals Next: Case Ranges, Previous: Compound Literals, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.29 Designated Initializers Standard C90 requires the elements of an initializer to appear in a fixed order, the same as the order of the elements in the array or structure being initialized. In ISO C99 you can give the elements in any order, specifying the array indices or structure field names they apply to, and GNU C allows this as an extension in C90 mode as well. This extension is not implemented in GNU C++. To specify an array index, write '[index] =' before the element value. For example, int a[6] = { [4] = 29, [2] = 15 }; is equivalent to int a[6] = { 0, 0, 15, 0, 29, 0 }; The index values must be constant expressions, even if the array being initialized is automatic. An alternative syntax for this that has been obsolete since GCC 2.5 but GCC still accepts is to write '[index]' before the element value, with no '='. To initialize a range of elements to the same value, write '[first ... last] = value'. This is a GNU extension. For example, int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 }; If the value in it has side effects, the side effects happen only once, not for each initialized field by the range initializer. Note that the length of the array is the highest value specified plus one. In a structure initializer, specify the name of a field to initialize with '.fieldname =' before the element value. For example, given the following structure, struct point { int x, y; }; the following initialization struct point p = { .y = yvalue, .x = xvalue }; is equivalent to struct point p = { xvalue, yvalue }; Another syntax that has the same meaning, obsolete since GCC 2.5, is 'fieldname:', as shown here: struct point p = { y: yvalue, x: xvalue }; Omitted fields are implicitly initialized the same as for objects that have static storage duration. The '[index]' or '.fieldname' is known as a designator. You can also use a designator (or the obsolete colon syntax) when initializing a union, to specify which element of the union should be used. For example, union foo { int i; double d; }; union foo f = { .d = 4 }; converts 4 to a double to store it in the union using the second element. By contrast, casting 4 to type union foo stores it into the union as the integer i, since it is an integer. See Cast to Union. You can combine this technique of naming elements with ordinary C initialization of successive elements. Each initializer element that does not have a designator applies to the next consecutive element of the array or structure. For example, int a[6] = { [1] = v1, v2, [4] = v4 }; is equivalent to int a[6] = { 0, v1, v2, 0, v4, 0 }; Labeling the elements of an array initializer is especially useful when the indices are characters or belong to an enum type. For example: int whitespace[256] = { [' '] = 1, ['\t'] = 1, ['\h'] = 1, ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 }; You can also write a series of '.fieldname' and '[index]' designators before an '=' to specify a nested subobject to initialize; the list is taken relative to the subobject corresponding to the closest surrounding brace pair. For example, with the 'struct point' declaration above: struct point ptarray[10] = { [2].y = yv2, [2].x = xv2, [0].x = xv0 }; If the same field is initialized multiple times, or overlapping fields of a union are initialized, the value from the last initialization is used. When a field of a union is itself a structure, the entire structure from the last field initialized is used. If any previous initializer has side effect, it is unspecified whether the side effect happens or not. Currently, GCC discards the side-effecting initializer expressions and issues a warning. ────────────────────────────────────────────────────────────────────────────── Next: Case Ranges, Previous: Compound Literals, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Cast to Union Link: prev: Designated Inits Next: Cast to Union, Previous: Designated Inits, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.30 Case Ranges You can specify a range of consecutive values in a single case label, like this: case low ... high: This has the same effect as the proper number of individual case labels, one for each integer value from low to high, inclusive. This feature is especially useful for ranges of ASCII character codes: case 'A' ... 'Z': Be careful: Write spaces around the ..., for otherwise it may be parsed wrong when you use it with integer values. For example, write this: case 1 ... 5: rather than this: case 1...5: ────────────────────────────────────────────────────────────────────────────── 6.31 Cast to a Union Type A cast to a union type is a C extension not available in C++. It looks just like ordinary casts with the constraint that the type specified is a union type. You can specify the type either with the union keyword or with a typedef name that refers to a union. The result of a cast to a union is a temporary rvalue of the union type with a member whose type matches that of the operand initialized to the value of the operand. The effect of a cast to a union is similar to a compound literal except that it yields an rvalue like standard casts do. See Compound Literals. Expressions that may be cast to the union type are those whose type matches at least one of the members of the union. Thus, given the following union and variables: union foo { int i; double d; }; int x; double y; union foo z; both x and y can be cast to type union foo and the following assignments z = (union foo) x; z = (union foo) y; are shorthand equivalents of these z = (union foo) { .i = x }; z = (union foo) { .d = y }; However, (union foo) FLT_MAX; is not a valid cast because the union has no member of type float. Using the cast as the right-hand side of an assignment to a variable of union type is equivalent to storing in a member of the union with the same type union foo u; /* … */ u = (union foo) x ≡ u.i = x u = (union foo) y ≡ u.d = y You can also use the union cast as a function argument: void hack (union foo); /* … */ hack ((union foo) x); ────────────────────────────────────────────────────────────────────────────── 6.32 Mixed Declarations and Code ISO C99 and ISO C++ allow declarations and code to be freely mixed within compound statements. As an extension, GNU C also allows this in C90 mode. For example, you could do: int i; /* … */ i++; int j = i + 2; ────────────────────────────────────────────────────────────────────────────── 6.33 Declaring Attributes of Functions In GNU C and C++, you can use function attributes to specify certain function properties that may help the compiler optimize calls or check code more carefully for correctness. For example, you can use attributes to specify that a function never returns (noreturn), returns a value depending only on the values of its arguments (const), or has printf-style arguments (format). You can also use attributes to control memory placement, code generation options or call/return conventions within the function being annotated. Many of these attributes are target-specific. For example, many targets support attributes for defining interrupt handler functions, which typically must follow special register usage and return conventions. Such attributes are described in the subsection for each target. However, a considerable number of attributes are supported by most, if not all targets. Those are described in the Common Function Attributes section. Function attributes are introduced by the __attribute__ keyword in the declaration of a function, followed by an attribute specification enclosed in double parentheses. You can specify multiple attributes in a declaration by separating them by commas within the double parentheses or by immediately following one attribute specification with another. See Attribute Syntax, for the exact rules on attribute syntax and placement. Compatible attribute specifications on distinct declarations of the same function are merged. An attribute specification that is not compatible with attributes already applied to a declaration of the same function is ignored with a warning. Some function attributes take one or more arguments that refer to the function's parameters by their positions within the function parameter list. Such attribute arguments are referred to as positional arguments. Unless specified otherwise, positional arguments that specify properties of parameters with pointer types can also specify the same properties of the implicit C++ this argument in non-static member functions, and of parameters of reference to a pointer type. For ordinary functions, position one refers to the first parameter on the list. In C++ non-static member functions, position one refers to the implicit this pointer. The same restrictions and effects apply to function attributes used with ordinary functions or C++ member functions. GCC also supports attributes on variable declarations (see Variable Attributes), labels (see Label Attributes), enumerators (see Enumerator Attributes), statements (see Statement Attributes), and types (see Type Attributes). There is some overlap between the purposes of attributes and pragmas (see Pragmas Accepted by GCC). It has been found convenient to use __attribute__ to achieve a natural attachment of attributes to their corresponding declarations, whereas #pragma is of use for compatibility with other compilers or constructs that do not naturally form part of the grammar. In addition to the attributes documented here, GCC plugins may provide their own attributes. ────────────────────────────────────────────────────────────────────────────── 6.33.1 Common Function Attributes The following attributes are supported on most targets. alias ("target") The alias attribute causes the declaration to be emitted as an alias for another symbol, which must be specified. For instance, void __f () { /* Do something. */; } void f () __attribute__ ((weak, alias ("__f"))); defines 'f' to be a weak alias for '__f'. In C++, the mangled name for the target must be used. It is an error if '__f' is not defined in the same translation unit. This attribute requires assembler and object file support, and may not be available on all targets. aligned aligned (alignment) The aligned attribute specifies a minimum alignment for the first instruction of the function, measured in bytes. When specified, alignment must be an integer constant power of 2. Specifying no alignment argument implies the ideal alignment for the target. The __alignof__ operator can be used to determine what that is (see Alignment). The attribute has no effect when a definition for the function is not provided in the same translation unit. The attribute cannot be used to decrease the alignment of a function previously declared with a more restrictive alignment; only to increase it. Attempts to do otherwise are diagnosed. Some targets specify a minimum default alignment for functions that is greater than 1. On such targets, specifying a less restrictive alignment is silently ignored. Using the attribute overrides the effect of the -falign-functions (see Optimize Options) option for this function. Note that the effectiveness of aligned attributes may be limited by inherent limitations in the system linker and/or object file format. On some systems, the linker is only able to arrange for functions to be aligned up to a certain maximum alignment. (For some linkers, the maximum supported alignment may be very very small.) See your linker documentation for further information. The aligned attribute can also be used for variables and fields (see Variable Attributes.) alloc_align (position) The alloc_align attribute may be applied to a function that returns a pointer and takes at least one argument of an integer or enumerated type. It indicates that the returned pointer is aligned on a boundary given by the function argument at position. Meaningful alignments are powers of 2 greater than one. GCC uses this information to improve pointer alignment analysis. The function parameter denoting the allocated alignment is specified by one constant integer argument whose number is the argument of the attribute. Argument numbering starts at one. For instance, void* my_memalign (size_t, size_t) __attribute__ ((alloc_align (1))); declares that my_memalign returns memory with minimum alignment given by parameter 1. alloc_size (position) alloc_size (position-1, position-2) The alloc_size attribute may be applied to a function that returns a pointer and takes at least one argument of an integer or enumerated type. It indicates that the returned pointer points to memory whose size is given by the function argument at position-1, or by the product of the arguments at position-1 and position-2. Meaningful sizes are positive values less than PTRDIFF_MAX. GCC uses this information to improve the results of __builtin_object_size. The function parameter(s) denoting the allocated size are specified by one or two integer arguments supplied to the attribute. The allocated size is either the value of the single function argument specified or the product of the two function arguments specified. Argument numbering starts at one for ordinary functions, and at two for C++ non-static member functions. For instance, void* my_calloc (size_t, size_t) __attribute__ ((alloc_size (1, 2))); void* my_realloc (void*, size_t) __attribute__ ((alloc_size (2))); declares that my_calloc returns memory of the size given by the product of parameter 1 and 2 and that my_realloc returns memory of the size given by parameter 2. always_inline Generally, functions are not inlined unless optimization is specified. For functions declared inline, this attribute inlines the function independent of any restrictions that otherwise apply to inlining. Failure to inline such a function is diagnosed as an error. Note that if such a function is called indirectly the compiler may or may not inline it depending on optimization level and a failure to inline an indirect call may or may not be diagnosed. artificial This attribute is useful for small inline wrappers that if possible should appear during debugging as a unit. Depending on the debug info format it either means marking the function as artificial or using the caller location for all instructions within the inlined body. assume_aligned (alignment) assume_aligned (alignment, offset) The assume_aligned attribute may be applied to a function that returns a pointer. It indicates that the returned pointer is aligned on a boundary given by alignment. If the attribute has two arguments, the second argument is misalignment offset. Meaningful values of alignment are powers of 2 greater than one. Meaningful values of offset are greater than zero and less than alignment. For instance void* my_alloc1 (size_t) __attribute__((assume_aligned (16))); void* my_alloc2 (size_t) __attribute__((assume_aligned (32, 8))); declares that my_alloc1 returns 16-byte aligned pointers and that my_alloc2 returns a pointer whose value modulo 32 is equal to 8. cold The cold attribute on functions is used to inform the compiler that the function is unlikely to be executed. The function is optimized for size rather than speed and on many targets it is placed into a special subsection of the text section so all cold functions appear close together, improving code locality of non-cold parts of program. The paths leading to calls of cold functions within code are marked as unlikely by the branch prediction mechanism. It is thus useful to mark functions used to handle unlikely conditions, such as perror, as cold to improve optimization of hot functions that do call marked functions in rare occasions. When profile feedback is available, via -fprofile-use, cold functions are automatically detected and this attribute is ignored. const Calls to functions whose return value is not affected by changes to the observable state of the program and that have no observable effects on such state other than to return a value may lend themselves to optimizations such as common subexpression elimination. Declaring such functions with the const attribute allows GCC to avoid emitting some calls in repeated invocations of the function with the same argument values. For example, int square (int) __attribute__ ((const)); tells GCC that subsequent calls to function square with the same argument value can be replaced by the result of the first call regardless of the statements in between. The const attribute prohibits a function from reading objects that affect its return value between successive invocations. However, functions declared with the attribute can safely read objects that do not change their return value, such as non-volatile constants. The const attribute imposes greater restrictions on a function's definition than the similar pure attribute. Declaring the same function with both the const and the pure attribute is diagnosed. Because a const function cannot have any observable side effects it does not make sense for it to return void. Declaring such a function is diagnosed. Note that a function that has pointer arguments and examines the data pointed to must not be declared const if the pointed-to data might change between successive invocations of the function. In general, since a function cannot distinguish data that might change from data that cannot, const functions should never take pointer or, in C++, reference arguments. Likewise, a function that calls a non-const function usually must not be const itself. constructor destructor constructor (priority) destructor (priority) The constructor attribute causes the function to be called automatically before execution enters main (). Similarly, the destructor attribute causes the function to be called automatically after main () completes or exit () is called. Functions with these attributes are useful for initializing data that is used implicitly during the execution of the program. On some targets the attributes also accept an integer argument to specify a priority to control the order in which constructor and destructor functions are run. A constructor with a smaller priority number runs before a constructor with a larger priority number; the opposite relationship holds for destructors. So, if you have a constructor that allocates a resource and a destructor that deallocates the same resource, both functions typically have the same priority. The priorities for constructor and destructor functions are the same as those specified for namespace-scope C++ objects (see C++ Attributes). However, at present, the order in which constructors for C++ objects with static storage duration and functions decorated with attribute constructor are invoked is unspecified. In mixed declarations, attribute init_priority can be used to impose a specific ordering. Using the argument forms of the constructor and destructor attributes on targets where the feature is not supported is rejected with an error. copy copy (function) The copy attribute applies the set of attributes with which function has been declared to the declaration of the function to which the attribute is applied. The attribute is designed for libraries that define aliases or function resolvers that are expected to specify the same set of attributes as their targets. The copy attribute can be used with functions, variables, or types. However, the kind of symbol to which the attribute is applied (either function or variable) must match the kind of symbol to which the argument refers. The copy attribute copies only syntactic and semantic attributes but not attributes that affect a symbol's linkage or visibility such as alias, visibility, or weak. The deprecated attribute is also not copied. See Common Type Attributes. See Common Variable Attributes. For example, the StrongAlias macro below makes use of the alias and copy attributes to define an alias named alloc for function allocate declared with attributes alloc_size, malloc, and nothrow. Thanks to the __typeof__ operator the alias has the same type as the target function. As a result of the copy attribute the alias also shares the same attributes as the target. #define StrongAlias(TagetFunc, AliasDecl) \ extern __typeof__ (TargetFunc) AliasDecl \ __attribute__ ((alias (#TargetFunc), copy (TargetFunc))); extern __attribute__ ((alloc_size (1), malloc, nothrow)) void* allocate (size_t); StrongAlias (allocate, alloc); deprecated deprecated (msg) The deprecated attribute results in a warning if the function is used anywhere in the source file. This is useful when identifying functions that are expected to be removed in a future version of a program. The warning also includes the location of the declaration of the deprecated function, to enable users to easily find further information about why the function is deprecated, or what they should do instead. Note that the warnings only occurs for uses: int old_fn () __attribute__ ((deprecated)); int old_fn (); int (*fn_ptr)() = old_fn; results in a warning on line 3 but not line 2. The optional msg argument, which must be a string, is printed in the warning if present. The deprecated attribute can also be used for variables and types (see Variable Attributes, see Type Attributes.) The message attached to the attribute is affected by the setting of the -fmessage-length option. error ("message") warning ("message") If the error or warning attribute is used on a function declaration and a call to such a function is not eliminated through dead code elimination or other optimizations, an error or warning (respectively) that includes message is diagnosed. This is useful for compile-time checking, especially together with __builtin_constant_p and inline functions where checking the inline function arguments is not possible through extern char [(condition) ? 1 : -1]; tricks. While it is possible to leave the function undefined and thus invoke a link failure (to define the function with a message in .gnu.warning* section), when using these attributes the problem is diagnosed earlier and with exact location of the call even in presence of inline functions or when not emitting debugging information. externally_visible This attribute, attached to a global variable or function, nullifies the effect of the -fwhole-program command-line option, so the object remains visible outside the current compilation unit. If -fwhole-program is used together with -flto and gold is used as the linker plugin, externally_visible attributes are automatically added to functions (not variable yet due to a current gold issue) that are accessed outside of LTO objects according to resolution file produced by gold. For other linkers that cannot generate resolution file, explicit externally_visible attributes are still necessary. flatten Generally, inlining into a function is limited. For a function marked with this attribute, every call inside this function is inlined, if possible. Functions declared with attribute noinline and similar are not inlined. Whether the function itself is considered for inlining depends on its size and the current inlining parameters. format (archetype, string-index, first-to-check) The format attribute specifies that a function takes printf, scanf, strftime or strfmon style arguments that should be type-checked against a format string. For example, the declaration: extern int my_printf (void *my_object, const char *my_format, ...) __attribute__ ((format (printf, 2, 3))); causes the compiler to check the arguments in calls to my_printf for consistency with the printf style format string argument my_format. The parameter archetype determines how the format string is interpreted, and should be printf, scanf, strftime, gnu_printf, gnu_scanf, gnu_strftime or strfmon. (You can also use __printf__, __scanf__, __strftime__ or __strfmon__.) On MinGW targets, ms_printf, ms_scanf, and ms_strftime are also present. archetype values such as printf refer to the formats accepted by the system's C runtime library, while values prefixed with 'gnu_' always refer to the formats accepted by the GNU C Library. On Microsoft Windows targets, values prefixed with 'ms_' refer to the formats accepted by the msvcrt.dll library. The parameter string-index specifies which argument is the format string argument (starting from 1), while first-to-check is the number of the first argument to check against the format string. For functions where the arguments are not available to be checked (such as vprintf), specify the third parameter as zero. In this case the compiler only checks the format string for consistency. For strftime formats, the third parameter is required to be zero. Since non-static C++ methods have an implicit this argument, the arguments of such methods should be counted from two, not one, when giving values for string-index and first-to-check. In the example above, the format string (my_format) is the second argument of the function my_print, and the arguments to check start with the third argument, so the correct parameters for the format attribute are 2 and 3. The format attribute allows you to identify your own functions that take format strings as arguments, so that GCC can check the calls to these functions for errors. The compiler always (unless -ffreestanding or -fno-builtin is used) checks formats for the standard library functions printf, fprintf, sprintf, scanf, fscanf, sscanf, strftime, vprintf, vfprintf and vsprintf whenever such warnings are requested (using -Wformat), so there is no need to modify the header file stdio.h. In C99 mode, the functions snprintf, vsnprintf, vscanf, vfscanf and vsscanf are also checked. Except in strictly conforming C standard modes, the X/Open function strfmon is also checked as are printf_unlocked and fprintf_unlocked. See Options Controlling C Dialect. For Objective-C dialects, NSString (or __NSString__) is recognized in the same context. Declarations including these format attributes are parsed for correct syntax, however the result of checking of such format strings is not yet defined, and is not carried out by this version of the compiler. The target may also provide additional types of format checks. See Format Checks Specific to Particular Target Machines. format_arg (string-index) The format_arg attribute specifies that a function takes one or more format strings for a printf, scanf, strftime or strfmon style function and modifies it (for example, to translate it into another language), so the result can be passed to a printf, scanf, strftime or strfmon style function (with the remaining arguments to the format function the same as they would have been for the unmodified string). Multiple format_arg attributes may be applied to the same function, each designating a distinct parameter as a format string. For example, the declaration: extern char * my_dgettext (char *my_domain, const char *my_format) __attribute__ ((format_arg (2))); causes the compiler to check the arguments in calls to a printf, scanf, strftime or strfmon type function, whose format string argument is a call to the my_dgettext function, for consistency with the format string argument my_format. If the format_arg attribute had not been specified, all the compiler could tell in such calls to format functions would be that the format string argument is not constant; this would generate a warning when -Wformat-nonliteral is used, but the calls could not be checked without the attribute. In calls to a function declared with more than one format_arg attribute, each with a distinct argument value, the corresponding actual function arguments are checked against all format strings designated by the attributes. This capability is designed to support the GNU ngettext family of functions. The parameter string-index specifies which argument is the format string argument (starting from one). Since non-static C++ methods have an implicit this argument, the arguments of such methods should be counted from two. The format_arg attribute allows you to identify your own functions that modify format strings, so that GCC can check the calls to printf, scanf, strftime or strfmon type function whose operands are a call to one of your own function. The compiler always treats gettext, dgettext, and dcgettext in this manner except when strict ISO C support is requested by -ansi or an appropriate -std option, or -ffreestanding or -fno-builtin is used. See Options Controlling C Dialect. For Objective-C dialects, the format-arg attribute may refer to an NSString reference for compatibility with the format attribute above. The target may also allow additional types in format-arg attributes. See Format Checks Specific to Particular Target Machines. gnu_inline This attribute should be used with a function that is also declared with the inline keyword. It directs GCC to treat the function as if it were defined in gnu90 mode even when compiling in C99 or gnu99 mode. If the function is declared extern, then this definition of the function is used only for inlining. In no case is the function compiled as a standalone function, not even if you take its address explicitly. Such an address becomes an external reference, as if you had only declared the function, and had not defined it. This has almost the effect of a macro. The way to use this is to put a function definition in a header file with this attribute, and put another copy of the function, without extern, in a library file. The definition in the header file causes most calls to the function to be inlined. If any uses of the function remain, they refer to the single copy in the library. Note that the two definitions of the functions need not be precisely the same, although if they do not have the same effect your program may behave oddly. In C, if the function is neither extern nor static, then the function is compiled as a standalone function, as well as being inlined where possible. This is how GCC traditionally handled functions declared inline. Since ISO C99 specifies a different semantics for inline, this function attribute is provided as a transition measure and as a useful feature in its own right. This attribute is available in GCC 4.1.3 and later. It is available if either of the preprocessor macros __GNUC_GNU_INLINE__ or __GNUC_STDC_INLINE__ are defined. See An Inline Function is As Fast As a Macro. In C++, this attribute does not depend on extern in any way, but it still requires the inline keyword to enable its special behavior. hot The hot attribute on a function is used to inform the compiler that the function is a hot spot of the compiled program. The function is optimized more aggressively and on many targets it is placed into a special subsection of the text section so all hot functions appear close together, improving locality. When profile feedback is available, via -fprofile-use, hot functions are automatically detected and this attribute is ignored. ifunc ("resolver") The ifunc attribute is used to mark a function as an indirect function using the STT_GNU_IFUNC symbol type extension to the ELF standard. This allows the resolution of the symbol value to be determined dynamically at load time, and an optimized version of the routine to be selected for the particular processor or other system characteristics determined then. To use this attribute, first define the implementation functions available, and a resolver function that returns a pointer to the selected implementation function. The implementation functions' declarations must match the API of the function being implemented. The resolver should be declared to be a function taking no arguments and returning a pointer to a function of the same type as the implementation. For example: void *my_memcpy (void *dst, const void *src, size_t len) { … return dst; } static void * (*resolve_memcpy (void))(void *, const void *, size_t) { return my_memcpy; // we will just always select this routine } The exported header file declaring the function the user calls would contain: extern void *memcpy (void *, const void *, size_t); allowing the user to call memcpy as a regular function, unaware of the actual implementation. Finally, the indirect function needs to be defined in the same translation unit as the resolver function: void *memcpy (void *, const void *, size_t) __attribute__ ((ifunc ("resolve_memcpy"))); In C++, the ifunc attribute takes a string that is the mangled name of the resolver function. A C++ resolver for a non-static member function of class C should be declared to return a pointer to a non-member function taking pointer to C as the first argument, followed by the same arguments as of the implementation function. G++ checks the signatures of the two functions and issues a -Wattribute-alias warning for mismatches. To suppress a warning for the necessary cast from a pointer to the implementation member function to the type of the corresponding non-member function use the -Wno-pmf-conversions option. For example: class S { private: int debug_impl (int); int optimized_impl (int); typedef int Func (S*, int); static Func* resolver (); public: int interface (int); }; int S::debug_impl (int) { /* … */ } int S::optimized_impl (int) { /* … */ } S::Func* S::resolver () { int (S::*pimpl) (int) = getenv ("DEBUG") ? &S::debug_impl : &S::optimized_impl; // Cast triggers -Wno-pmf-conversions. return reinterpret_cast(pimpl); } int S::interface (int) __attribute__ ((ifunc ("_ZN1S8resolverEv"))); Indirect functions cannot be weak. Binutils version 2.20.1 or higher and GNU C Library version 2.11.1 are required to use this feature. interrupt interrupt_handler Many GCC back ends support attributes to indicate that a function is an interrupt handler, which tells the compiler to generate function entry and exit sequences that differ from those from regular functions. The exact syntax and behavior are target-specific; refer to the following subsections for details. leaf Calls to external functions with this attribute must return to the current compilation unit only by return or by exception handling. In particular, a leaf function is not allowed to invoke callback functions passed to it from the current compilation unit, directly call functions exported by the unit, or longjmp into the unit. Leaf functions might still call functions from other compilation units and thus they are not necessarily leaf in the sense that they contain no function calls at all. The attribute is intended for library functions to improve dataflow analysis. The compiler takes the hint that any data not escaping the current compilation unit cannot be used or modified by the leaf function. For example, the sin function is a leaf function, but qsort is not. Note that leaf functions might indirectly run a signal handler defined in the current compilation unit that uses static variables. Similarly, when lazy symbol resolution is in effect, leaf functions might invoke indirect functions whose resolver function or implementation function is defined in the current compilation unit and uses static variables. There is no standard-compliant way to write such a signal handler, resolver function, or implementation function, and the best that you can do is to remove the leaf attribute or mark all such static variables volatile. Lastly, for ELF-based systems that support symbol interposition, care should be taken that functions defined in the current compilation unit do not unexpectedly interpose other symbols based on the defined standards mode and defined feature test macros; otherwise an inadvertent callback would be added. The attribute has no effect on functions defined within the current compilation unit. This is to allow easy merging of multiple compilation units into one, for example, by using the link-time optimization. For this reason the attribute is not allowed on types to annotate indirect calls. malloc This tells the compiler that a function is malloc-like, i.e., that the pointer P returned by the function cannot alias any other pointer valid when the function returns, and moreover no pointers to valid objects occur in any storage addressed by P. Using this attribute can improve optimization. Compiler predicts that a function with the attribute returns non-null in most cases. Functions like malloc and calloc have this property because they return a pointer to uninitialized or zeroed-out storage. However, functions like realloc do not have this property, as they can return a pointer to storage containing pointers. no_icf This function attribute prevents a functions from being merged with another semantically equivalent function. no_instrument_function If any of -finstrument-functions, -p, or -pg are given, profiling function calls are generated at entry and exit of most user-compiled functions. Functions with this attribute are not so instrumented. no_profile_instrument_function The no_profile_instrument_function attribute on functions is used to inform the compiler that it should not process any profile feedback based optimization code instrumentation. no_reorder Do not reorder functions or variables marked no_reorder against each other or top level assembler statements the executable. The actual order in the program will depend on the linker command line. Static variables marked like this are also not removed. This has a similar effect as the -fno-toplevel-reorder option, but only applies to the marked symbols. no_sanitize ("sanitize_option") The no_sanitize attribute on functions is used to inform the compiler that it should not do sanitization of all options mentioned in sanitize_option. A list of values acceptable by -fsanitize option can be provided. void __attribute__ ((no_sanitize ("alignment", "object-size"))) f () { /* Do something. */; } void __attribute__ ((no_sanitize ("alignment,object-size"))) g () { /* Do something. */; } no_sanitize_address no_address_safety_analysis The no_sanitize_address attribute on functions is used to inform the compiler that it should not instrument memory accesses in the function when compiling with the -fsanitize=address option. The no_address_safety_analysis is a deprecated alias of the no_sanitize_address attribute, new code should use no_sanitize_address. no_sanitize_thread The no_sanitize_thread attribute on functions is used to inform the compiler that it should not instrument memory accesses in the function when compiling with the -fsanitize=thread option. no_sanitize_undefined The no_sanitize_undefined attribute on functions is used to inform the compiler that it should not check for undefined behavior in the function when compiling with the -fsanitize=undefined option. no_split_stack If -fsplit-stack is given, functions have a small prologue which decides whether to split the stack. Functions with the no_split_stack attribute do not have that prologue, and thus may run with only a small amount of stack space available. no_stack_limit This attribute locally overrides the -fstack-limit-register and -fstack-limit-symbol command-line options; it has the effect of disabling stack limit checking in the function it applies to. noclone This function attribute prevents a function from being considered for cloning—a mechanism that produces specialized copies of functions and which is (currently) performed by interprocedural constant propagation. noinline This function attribute prevents a function from being considered for inlining. If the function does not have side effects, there are optimizations other than inlining that cause function calls to be optimized away, although the function call is live. To keep such calls from being optimized away, put asm (""); (see Extended Asm) in the called function, to serve as a special side effect. noipa Disable interprocedural optimizations between the function with this attribute and its callers, as if the body of the function is not available when optimizing callers and the callers are unavailable when optimizing the body. This attribute implies noinline, noclone and no_icf attributes. However, this attribute is not equivalent to a combination of other attributes, because its purpose is to suppress existing and future optimizations employing interprocedural analysis, including those that do not have an attribute suitable for disabling them individually. This attribute is supported mainly for the purpose of testing the compiler. nonnull nonnull (arg-index, …) The nonnull attribute may be applied to a function that takes at least one argument of a pointer type. It indicates that the referenced arguments must be non-null pointers. For instance, the declaration: extern void * my_memcpy (void *dest, const void *src, size_t len) __attribute__((nonnull (1, 2))); causes the compiler to check that, in calls to my_memcpy, arguments dest and src are non-null. If the compiler determines that a null pointer is passed in an argument slot marked as non-null, and the -Wnonnull option is enabled, a warning is issued. See Warning Options. Unless disabled by the -fno-delete-null-pointer-checks option the compiler may also perform optimizations based on the knowledge that certain function arguments cannot be null. In addition, the -fisolate-erroneous-paths-attribute option can be specified to have GCC transform calls with null arguments to non-null functions into traps. See Optimize Options. If no arg-index is given to the nonnull attribute, all pointer arguments are marked as non-null. To illustrate, the following declaration is equivalent to the previous example: extern void * my_memcpy (void *dest, const void *src, size_t len) __attribute__((nonnull)); noplt The noplt attribute is the counterpart to option -fno-plt. Calls to functions marked with this attribute in position-independent code do not use the PLT. /* Externally defined function foo. */ int foo () __attribute__ ((noplt)); int main (/* … */) { /* … */ foo (); /* … */ } The noplt attribute on function foo tells the compiler to assume that the function foo is externally defined and that the call to foo must avoid the PLT in position-independent code. In position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT to use the GOT instead. noreturn A few standard library functions, such as abort and exit, cannot return. GCC knows this automatically. Some programs define their own functions that never return. You can declare them noreturn to tell the compiler this fact. For example, void fatal () __attribute__ ((noreturn)); void fatal (/* … */) { /* … */ /* Print error message. */ /* … */ exit (1); } The noreturn keyword tells the compiler to assume that fatal cannot return. It can then optimize without regard to what would happen if fatal ever did return. This makes slightly better code. More importantly, it helps avoid spurious warnings of uninitialized variables. The noreturn keyword does not affect the exceptional path when that applies: a noreturn-marked function may still return to the caller by throwing an exception or calling longjmp. In order to preserve backtraces, GCC will never turn calls to noreturn functions into tail calls. Do not assume that registers saved by the calling function are restored before calling the noreturn function. It does not make sense for a noreturn function to have a return type other than void. nothrow The nothrow attribute is used to inform the compiler that a function cannot throw an exception. For example, most functions in the standard C library can be guaranteed not to throw an exception with the notable exceptions of qsort and bsearch that take function pointer arguments. optimize (level, …) optimize (string, …) The optimize attribute is used to specify that a function is to be compiled with different optimization options than specified on the command line. Valid arguments are constant non-negative integers and strings. Each numeric argument specifies an optimization level. Each string argument consists of one or more comma-separated substrings. Each substring that begins with the letter O refers to an optimization option such as -O0 or -Os. Other substrings are taken as suffixes to the -f prefix jointly forming the name of an optimization option. See Optimize Options. '#pragma GCC optimize' can be used to set optimization options for more than one function. See Function Specific Option Pragmas, for details about the pragma. Providing multiple strings as arguments separated by commas to specify multiple options is equivalent to separating the option suffixes with a comma (',') within a single string. Spaces are not permitted within the strings. Not every optimization option that starts with the -f prefix specified by the attribute necessarily has an effect on the function. The optimize attribute should be used for debugging purposes only. It is not suitable in production code. patchable_function_entry In case the target's text segment can be made writable at run time by any means, padding the function entry with a number of NOPs can be used to provide a universal tool for instrumentation. The patchable_function_entry function attribute can be used to change the number of NOPs to any desired value. The two-value syntax is the same as for the command-line switch -fpatchable-function-entry=N,M, generating N NOPs, with the function entry point before the Mth NOP instruction. M defaults to 0 if omitted e.g. function entry point is before the first NOP. If patchable function entries are enabled globally using the command-line option -fpatchable-function-entry=N,M, then you must disable instrumentation on all functions that are part of the instrumentation framework with the attribute patchable_function_entry (0) to prevent recursion. pure Calls to functions that have no observable effects on the state of the program other than to return a value may lend themselves to optimizations such as common subexpression elimination. Declaring such functions with the pure attribute allows GCC to avoid emitting some calls in repeated invocations of the function with the same argument values. The pure attribute prohibits a function from modifying the state of the program that is observable by means other than inspecting the function's return value. However, functions declared with the pure attribute can safely read any non-volatile objects, and modify the value of objects in a way that does not affect their return value or the observable state of the program. For example, int hash (char *) __attribute__ ((pure)); tells GCC that subsequent calls to the function hash with the same string can be replaced by the result of the first call provided the state of the program observable by hash, including the contents of the array itself, does not change in between. Even though hash takes a non-const pointer argument it must not modify the array it points to, or any other object whose value the rest of the program may depend on. However, the caller may safely change the contents of the array between successive calls to the function (doing so disables the optimization). The restriction also applies to member objects referenced by the this pointer in C++ non-static member functions. Some common examples of pure functions are strlen or memcmp. Interesting non-pure functions are functions with infinite loops or those depending on volatile memory or other system resource, that may change between consecutive calls (such as the standard C feof function in a multithreading environment). The pure attribute imposes similar but looser restrictions on a function's definition than the const attribute: pure allows the function to read any non-volatile memory, even if it changes in between successive invocations of the function. Declaring the same function with both the pure and the const attribute is diagnosed. Because a pure function cannot have any observable side effects it does not make sense for such a function to return void. Declaring such a function is diagnosed. returns_nonnull The returns_nonnull attribute specifies that the function return value should be a non-null pointer. For instance, the declaration: extern void * mymalloc (size_t len) __attribute__((returns_nonnull)); lets the compiler optimize callers based on the knowledge that the return value will never be null. returns_twice The returns_twice attribute tells the compiler that a function may return more than one time. The compiler ensures that all registers are dead before calling such a function and emits a warning about the variables that may be clobbered after the second return from the function. Examples of such functions are setjmp and vfork. The longjmp-like counterpart of such function, if any, might need to be marked with the noreturn attribute. section ("section-name") Normally, the compiler places the code it generates in the text section. Sometimes, however, you need additional sections, or you need certain particular functions to appear in special sections. The section attribute specifies that a function lives in a particular section. For example, the declaration: extern void foobar (void) __attribute__ ((section ("bar"))); puts the function foobar in the bar section. Some file formats do not support arbitrary sections so the section attribute is not available on all platforms. If you need to map the entire contents of a module to a particular section, consider using the facilities of the linker instead. sentinel sentinel (position) This function attribute indicates that an argument in a call to the function is expected to be an explicit NULL. The attribute is only valid on variadic functions. By default, the sentinel is expected to be the last argument of the function call. If the optional position argument is specified to the attribute, the sentinel must be located at position counting backwards from the end of the argument list. __attribute__ ((sentinel)) is equivalent to __attribute__ ((sentinel(0))) The attribute is automatically set with a position of 0 for the built-in functions execl and execlp. The built-in function execle has the attribute set with a position of 1. A valid NULL in this context is defined as zero with any object pointer type. If your system defines the NULL macro with an integer type then you need to add an explicit cast. During installation GCC replaces the system header with a copy that redefines NULL appropriately. The warnings for missing or incorrect sentinels are enabled with -Wformat. simd simd("mask") This attribute enables creation of one or more function versions that can process multiple arguments using SIMD instructions from a single invocation. Specifying this attribute allows compiler to assume that such versions are available at link time (provided in the same or another translation unit). Generated versions are target-dependent and described in the corresponding Vector ABI document. For x86_64 target this document can be found here. The optional argument mask may have the value notinbranch or inbranch, and instructs the compiler to generate non-masked or masked clones correspondingly. By default, all clones are generated. If the attribute is specified and #pragma omp declare simd is present on a declaration and the -fopenmp or -fopenmp-simd switch is specified, then the attribute is ignored. stack_protect This attribute adds stack protection code to the function if flags -fstack-protector, -fstack-protector-strong or -fstack-protector-explicit are set. target (string, …) Multiple target back ends implement the target attribute to specify that a function is to be compiled with different target options than specified on the command line. One or more strings can be provided as arguments. Each string consists of one or more comma-separated suffixes to the -m prefix jointly forming the name of a machine-dependent option. See Machine-Dependent Options. The target attribute can be used for instance to have a function compiled with a different ISA (instruction set architecture) than the default. '#pragma GCC target' can be used to specify target-specific options for more than one function. See Function Specific Option Pragmas, for details about the pragma. For instance, on an x86, you could declare one function with the target("sse4.1,arch=core2") attribute and another with target("sse4a,arch=amdfam10"). This is equivalent to compiling the first function with -msse4.1 and -march=core2 options, and the second function with -msse4a and -march=amdfam10 options. It is up to you to make sure that a function is only invoked on a machine that supports the particular ISA it is compiled for (for example by using cpuid on x86 to determine what feature bits and architecture family are used). int core2_func (void) __attribute__ ((__target__ ("arch=core2"))); int sse3_func (void) __attribute__ ((__target__ ("sse3"))); Providing multiple strings as arguments separated by commas to specify multiple options is equivalent to separating the option suffixes with a comma (',') within a single string. Spaces are not permitted within the strings. The options supported are specific to each target; refer to x86 Function Attributes, PowerPC Function Attributes, ARM Function Attributes, AArch64 Function Attributes, Nios II Function Attributes, and S/390 Function Attributes for details. target_clones (options) The target_clones attribute is used to specify that a function be cloned into multiple versions compiled with different target options than specified on the command line. The supported options and restrictions are the same as for target attribute. For instance, on an x86, you could compile a function with target_clones("sse4.1,avx"). GCC creates two function clones, one compiled with -msse4.1 and another with -mavx. On a PowerPC, you can compile a function with target_clones("cpu=power9,default"). GCC will create two function clones, one compiled with -mcpu=power9 and another with the default options. GCC must be configured to use GLIBC 2.23 or newer in order to use the target_clones attribute. It also creates a resolver function (see the ifunc attribute above) that dynamically selects a clone suitable for current architecture. The resolver is created only if there is a usage of a function with target_clones attribute. unused This attribute, attached to a function, means that the function is meant to be possibly unused. GCC does not produce a warning for this function. used This attribute, attached to a function, means that code must be emitted for the function even if it appears that the function is not referenced. This is useful, for example, when the function is referenced only in inline assembly. When applied to a member function of a C++ class template, the attribute also means that the function is instantiated if the class itself is instantiated. visibility ("visibility_type") This attribute affects the linkage of the declaration to which it is attached. It can be applied to variables (see Common Variable Attributes) and types (see Common Type Attributes) as well as functions. There are four supported visibility_type values: default, hidden, protected or internal visibility. void __attribute__ ((visibility ("protected"))) f () { /* Do something. */; } int i __attribute__ ((visibility ("hidden"))); The possible values of visibility_type correspond to the visibility settings in the ELF gABI. default Default visibility is the normal case for the object file format. This value is available for the visibility attribute to override other options that may change the assumed visibility of entities. On ELF, default visibility means that the declaration is visible to other modules and, in shared libraries, means that the declared entity may be overridden. On Darwin, default visibility means that the declaration is visible to other modules. Default visibility corresponds to “external linkage” in the language. hidden Hidden visibility indicates that the entity declared has a new form of linkage, which we call “hidden linkage”. Two declarations of an object with hidden linkage refer to the same object if they are in the same shared object. internal Internal visibility is like hidden visibility, but with additional processor specific semantics. Unless otherwise specified by the psABI, GCC defines internal visibility to mean that a function is never called from another module. Compare this with hidden functions which, while they cannot be referenced directly by other modules, can be referenced indirectly via function pointers. By indicating that a function cannot be called from outside the module, GCC may for instance omit the load of a PIC register since it is known that the calling function loaded the correct value. protected Protected visibility is like default visibility except that it indicates that references within the defining module bind to the definition in that module. That is, the declared entity cannot be overridden by another module. All visibilities are supported on many, but not all, ELF targets (supported when the assembler supports the '.visibility' pseudo-op). Default visibility is supported everywhere. Hidden visibility is supported on Darwin targets. The visibility attribute should be applied only to declarations that would otherwise have external linkage. The attribute should be applied consistently, so that the same entity should not be declared with different settings of the attribute. In C++, the visibility attribute applies to types as well as functions and objects, because in C++ types have linkage. A class must not have greater visibility than its non-static data member types and bases, and class members default to the visibility of their class. Also, a declaration without explicit visibility is limited to the visibility of its type. In C++, you can mark member functions and static member variables of a class with the visibility attribute. This is useful if you know a particular method or static member variable should only be used from one shared object; then you can mark it hidden while the rest of the class has default visibility. Care must be taken to avoid breaking the One Definition Rule; for example, it is usually not useful to mark an inline method as hidden without marking the whole class as hidden. A C++ namespace declaration can also have the visibility attribute. namespace nspace1 __attribute__ ((visibility ("protected"))) { /* Do something. */; } This attribute applies only to the particular namespace body, not to other definitions of the same namespace; it is equivalent to using '#pragma GCC visibility' before and after the namespace definition (see Visibility Pragmas). In C++, if a template argument has limited visibility, this restriction is implicitly propagated to the template instantiation. Otherwise, template instantiations and specializations default to the visibility of their template. If both the template and enclosing class have explicit visibility, the visibility from the template is used. warn_unused_result The warn_unused_result attribute causes a warning to be emitted if a caller of the function with this attribute does not use its return value. This is useful for functions where not checking the result is either a security problem or always a bug, such as realloc. int fn () __attribute__ ((warn_unused_result)); int foo () { if (fn () < 0) return -1; fn (); return 0; } results in warning on line 5. weak The weak attribute causes the declaration to be emitted as a weak symbol rather than a global. This is primarily useful in defining library functions that can be overridden in user code, though it can also be used with non-function declarations. Weak symbols are supported for ELF targets, and also for a.out targets when using the GNU assembler and linker. weakref weakref ("target") The weakref attribute marks a declaration as a weak reference. Without arguments, it should be accompanied by an alias attribute naming the target symbol. Optionally, the target may be given as an argument to weakref itself. In either case, weakref implicitly marks the declaration as weak. Without a target, given as an argument to weakref or to alias, weakref is equivalent to weak. static int x() __attribute__ ((weakref ("y"))); /* is equivalent to... */ static int x() __attribute__ ((weak, weakref, alias ("y"))); /* and to... */ static int x() __attribute__ ((weakref)); static int x() __attribute__ ((alias ("y"))); A weak reference is an alias that does not by itself require a definition to be given for the target symbol. If the target symbol is only referenced through weak references, then it becomes a weak undefined symbol. If it is directly referenced, however, then such strong references prevail, and a definition is required for the symbol, not necessarily in the same translation unit. The effect is equivalent to moving all references to the alias to a separate translation unit, renaming the alias to the aliased symbol, declaring it as weak, compiling the two separate translation units and performing a link with relocatable output (ie: ld -r) on them. At present, a declaration to which weakref is attached can only be static. ────────────────────────────────────────────────────────────────────────────── 6.33.3 AMD GCN Function Attributes These function attributes are supported by the AMD GCN back end: amdgpu_hsa_kernel This attribute indicates that the corresponding function should be compiled as a kernel function, that is an entry point that can be invoked from the host via the HSA runtime library. By default functions are only callable only from other GCN functions. This attribute is implicitly applied to any function named main, using default parameters. Kernel functions may return an integer value, which will be written to a conventional place within the HSA "kernargs" region. The attribute parameters configure what values are passed into the kernel function by the GPU drivers, via the initial register state. Some values are used by the compiler, and therefore forced on. Enabling other options may break assumptions in the compiler and/or run-time libraries. private_segment_buffer Set enable_sgpr_private_segment_buffer flag. Always on (required to locate the stack). dispatch_ptr Set enable_sgpr_dispatch_ptr flag. Always on (required to locate the launch dimensions). queue_ptr Set enable_sgpr_queue_ptr flag. Always on (required to convert address spaces). kernarg_segment_ptr Set enable_sgpr_kernarg_segment_ptr flag. Always on (required to locate the kernel arguments, "kernargs"). dispatch_id Set enable_sgpr_dispatch_id flag. flat_scratch_init Set enable_sgpr_flat_scratch_init flag. private_segment_size Set enable_sgpr_private_segment_size flag. grid_workgroup_count_X Set enable_sgpr_grid_workgroup_count_x flag. Always on (required to use OpenACC/OpenMP). grid_workgroup_count_Y Set enable_sgpr_grid_workgroup_count_y flag. grid_workgroup_count_Z Set enable_sgpr_grid_workgroup_count_z flag. workgroup_id_X Set enable_sgpr_workgroup_id_x flag. workgroup_id_Y Set enable_sgpr_workgroup_id_y flag. workgroup_id_Z Set enable_sgpr_workgroup_id_z flag. workgroup_info Set enable_sgpr_workgroup_info flag. private_segment_wave_offset Set enable_sgpr_private_segment_wave_byte_offset flag. Always on (required to locate the stack). work_item_id_X Set enable_vgpr_workitem_id parameter. Always on (can't be disabled). work_item_id_Y Set enable_vgpr_workitem_id parameter. Always on (required to enable vectorization.) work_item_id_Z Set enable_vgpr_workitem_id parameter. Always on (required to use OpenACC/OpenMP). ────────────────────────────────────────────────────────────────────────────── 6.33.9 C-SKY Function Attributes These function attributes are supported by the C-SKY back end: interrupt isr Use these attributes to indicate that the specified function is an interrupt handler. The compiler generates function entry and exit sequences suitable for use in an interrupt handler when either of these attributes are present. Use of these options requires the -mistack command-line option to enable support for the necessary interrupt stack instructions. They are ignored with a warning otherwise. See C-SKY Options. naked This attribute allows the compiler to construct the requisite function declaration, while allowing the body of the function to be assembly code. The specified function will not have prologue/epilogue sequences generated by the compiler. Only basic asm statements can safely be included in naked functions (see Basic Asm). While using extended asm or a mixture of basic asm and C code may appear to work, they cannot be depended upon to work reliably and are not supported. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Function Attributes Link: next: H8/300 Function Attributes Link: prev: C-SKY Function Attributes Next: H8/300 Function Attributes, Previous: C-SKY Function Attributes, Up: ────────────────────────────────────────────────────────────────────────────── 6.33.10 Epiphany Function Attributes These function attributes are supported by the Epiphany back end: disinterrupt This attribute causes the compiler to emit instructions to disable interrupts for the duration of the given function. forwarder_section This attribute modifies the behavior of an interrupt handler. The interrupt handler may be in external memory which cannot be reached by a branch instruction, so generate a local memory trampoline to transfer control. The single parameter identifies the section where the trampoline is placed. interrupt Use this attribute to indicate that the specified function is an interrupt handler. The compiler generates function entry and exit sequences suitable for use in an interrupt handler when this attribute is present. It may also generate a special section with code to initialize the interrupt vector table. On Epiphany targets one or more optional parameters can be added like this: void __attribute__ ((interrupt ("dma0, dma1"))) universal_dma_handler (); Permissible values for these parameters are: reset, software_exception, page_miss, timer0, timer1, message, dma0, dma1, wand and swi. Multiple parameters indicate that multiple entries in the interrupt vector table should be initialized for this function, i.e. for each parameter name, a jump to the function is emitted in the section ivt_entry_name. The parameter(s) may be omitted entirely, in which case no interrupt vector table entry is provided. Note that interrupts are enabled inside the function unless the disinterrupt attribute is also specified. The following examples are all valid uses of these attributes on Epiphany targets: void __attribute__ ((interrupt)) universal_handler (); void __attribute__ ((interrupt ("dma1"))) dma1_handler (); void __attribute__ ((interrupt ("dma0, dma1"))) universal_dma_handler (); void __attribute__ ((interrupt ("timer0"), disinterrupt)) fast_timer_handler (); void __attribute__ ((interrupt ("dma0, dma1"), forwarder_section ("tramp"))) external_dma_handler (); long_call short_call These attributes specify how a particular function is called. These attributes override the -mlong-calls (see Adapteva Epiphany Options) command-line switch and #pragma long_calls settings. ────────────────────────────────────────────────────────────────────────────── Next: H8/300 Function Attributes, Previous: C-SKY Function Attributes, Up: Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Function Attributes Link: next: V850 Function Attributes Link: prev: SPU Function Attributes Next: V850 Function Attributes, Previous: SPU Function Attributes, Up: ────────────────────────────────────────────────────────────────────────────── 6.33.32 Symbian OS Function Attributes See Microsoft Windows Function Attributes, for discussion of the dllexport and dllimport attributes. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Function Attributes Link: next: Xstormy16 Function Attributes Link: prev: Visium Function Attributes Next: Xstormy16 Function Attributes, Previous: Visium Function Attributes, ────────────────────────────────────────────────────────────────────────────── 6.33.35 x86 Function Attributes These function attributes are supported by the x86 back end: cdecl On the x86-32 targets, the cdecl attribute causes the compiler to assume that the calling function pops off the stack space used to pass arguments. This is useful to override the effects of the -mrtd switch. fastcall On x86-32 targets, the fastcall attribute causes the compiler to pass the first argument (if of integral type) in the register ECX and the second argument (if of integral type) in the register EDX. Subsequent and other typed arguments are passed on the stack. The called function pops the arguments off the stack. If the number of arguments is variable all arguments are pushed on the stack. thiscall On x86-32 targets, the thiscall attribute causes the compiler to pass the first argument (if of integral type) in the register ECX. Subsequent and other typed arguments are passed on the stack. The called function pops the arguments off the stack. If the number of arguments is variable all arguments are pushed on the stack. The thiscall attribute is intended for C++ non-static member functions. As a GCC extension, this calling convention can be used for C functions and for static member methods. ms_abi sysv_abi On 32-bit and 64-bit x86 targets, you can use an ABI attribute to indicate which calling convention should be used for a function. The ms_abi attribute tells the compiler to use the Microsoft ABI, while the sysv_abi attribute tells the compiler to use the ABI used on GNU/Linux and other systems. The default is to use the Microsoft ABI when targeting Windows. On all other systems, the default is the x86/AMD ABI. Note, the ms_abi attribute for Microsoft Windows 64-bit targets currently requires the -maccumulate-outgoing-args option. callee_pop_aggregate_return (number) On x86-32 targets, you can use this attribute to control how aggregates are returned in memory. If the caller is responsible for popping the hidden pointer together with the rest of the arguments, specify number equal to zero. If callee is responsible for popping the hidden pointer, specify number equal to one. The default x86-32 ABI assumes that the callee pops the stack for hidden pointer. However, on x86-32 Microsoft Windows targets, the compiler assumes that the caller pops the stack for hidden pointer. ms_hook_prologue On 32-bit and 64-bit x86 targets, you can use this function attribute to make GCC generate the “hot-patching” function prologue used in Win32 API functions in Microsoft Windows XP Service Pack 2 and newer. naked This attribute allows the compiler to construct the requisite function declaration, while allowing the body of the function to be assembly code. The specified function will not have prologue/epilogue sequences generated by the compiler. Only basic asm statements can safely be included in naked functions (see Basic Asm). While using extended asm or a mixture of basic asm and C code may appear to work, they cannot be depended upon to work reliably and are not supported. regparm (number) On x86-32 targets, the regparm attribute causes the compiler to pass arguments number one to number if they are of integral type in registers EAX, EDX, and ECX instead of on the stack. Functions that take a variable number of arguments continue to be passed all of their arguments on the stack. Beware that on some ELF systems this attribute is unsuitable for global functions in shared libraries with lazy binding (which is the default). Lazy binding sends the first call via resolving code in the loader, which might assume EAX, EDX and ECX can be clobbered, as per the standard calling conventions. Solaris 8 is affected by this. Systems with the GNU C Library version 2.1 or higher and FreeBSD are believed to be safe since the loaders there save EAX, EDX and ECX. (Lazy binding can be disabled with the linker or the loader if desired, to avoid the problem.) sseregparm On x86-32 targets with SSE support, the sseregparm attribute causes the compiler to pass up to 3 floating-point arguments in SSE registers instead of on the stack. Functions that take a variable number of arguments continue to pass all of their floating-point arguments on the stack. force_align_arg_pointer On x86 targets, the force_align_arg_pointer attribute may be applied to individual function definitions, generating an alternate prologue and epilogue that realigns the run-time stack if necessary. This supports mixing legacy codes that run with a 4-byte aligned stack with modern codes that keep a 16-byte stack for SSE compatibility. stdcall On x86-32 targets, the stdcall attribute causes the compiler to assume that the called function pops off the stack space used to pass arguments, unless it takes a variable number of arguments. no_caller_saved_registers Use this attribute to indicate that the specified function has no caller-saved registers. That is, all registers are callee-saved. For example, this attribute can be used for a function called from an interrupt handler. The compiler generates proper function entry and exit sequences to save and restore any modified registers, except for the EFLAGS register. Since GCC doesn't preserve SSE, MMX nor x87 states, the GCC option -mgeneral-regs-only should be used to compile functions with no_caller_saved_registers attribute. interrupt Use this attribute to indicate that the specified function is an interrupt handler or an exception handler (depending on parameters passed to the function, explained further). The compiler generates function entry and exit sequences suitable for use in an interrupt handler when this attribute is present. The IRET instruction, instead of the RET instruction, is used to return from interrupt handlers. All registers, except for the EFLAGS register which is restored by the IRET instruction, are preserved by the compiler. Since GCC doesn't preserve SSE, MMX nor x87 states, the GCC option -mgeneral-regs-only should be used to compile interrupt and exception handlers. Any interruptible-without-stack-switch code must be compiled with -mno-red-zone since interrupt handlers can and will, because of the hardware design, touch the red zone. An interrupt handler must be declared with a mandatory pointer argument: struct interrupt_frame; __attribute__ ((interrupt)) void f (struct interrupt_frame *frame) { } and you must define struct interrupt_frame as described in the processor's manual. Exception handlers differ from interrupt handlers because the system pushes an error code on the stack. An exception handler declaration is similar to that for an interrupt handler, but with a different mandatory function signature. The compiler arranges to pop the error code off the stack before the IRET instruction. #ifdef __x86_64__ typedef unsigned long long int uword_t; #else typedef unsigned int uword_t; #endif struct interrupt_frame; __attribute__ ((interrupt)) void f (struct interrupt_frame *frame, uword_t error_code) { ... } Exception handlers should only be used for exceptions that push an error code; you should use an interrupt handler in other cases. The system will crash if the wrong kind of handler is used. target (options) As discussed in Common Function Attributes, this attribute allows specification of target-specific compilation options. On the x86, the following options are allowed: '3dnow' 'no-3dnow' Enable/disable the generation of the 3DNow! instructions. '3dnowa' 'no-3dnowa' Enable/disable the generation of the enhanced 3DNow! instructions. 'abm' 'no-abm' Enable/disable the generation of the advanced bit instructions. 'adx' 'no-adx' Enable/disable the generation of the ADX instructions. 'aes' 'no-aes' Enable/disable the generation of the AES instructions. 'avx' 'no-avx' Enable/disable the generation of the AVX instructions. 'avx2' 'no-avx2' Enable/disable the generation of the AVX2 instructions. 'avx5124fmaps' 'no-avx5124fmaps' Enable/disable the generation of the AVX5124FMAPS instructions. 'avx5124vnniw' 'no-avx5124vnniw' Enable/disable the generation of the AVX5124VNNIW instructions. 'avx512bitalg' 'no-avx512bitalg' Enable/disable the generation of the AVX512BITALG instructions. 'avx512bw' 'no-avx512bw' Enable/disable the generation of the AVX512BW instructions. 'avx512cd' 'no-avx512cd' Enable/disable the generation of the AVX512CD instructions. 'avx512dq' 'no-avx512dq' Enable/disable the generation of the AVX512DQ instructions. 'avx512er' 'no-avx512er' Enable/disable the generation of the AVX512ER instructions. 'avx512f' 'no-avx512f' Enable/disable the generation of the AVX512F instructions. 'avx512ifma' 'no-avx512ifma' Enable/disable the generation of the AVX512IFMA instructions. 'avx512pf' 'no-avx512pf' Enable/disable the generation of the AVX512PF instructions. 'avx512vbmi' 'no-avx512vbmi' Enable/disable the generation of the AVX512VBMI instructions. 'avx512vbmi2' 'no-avx512vbmi2' Enable/disable the generation of the AVX512VBMI2 instructions. 'avx512vl' 'no-avx512vl' Enable/disable the generation of the AVX512VL instructions. 'avx512vnni' 'no-avx512vnni' Enable/disable the generation of the AVX512VNNI instructions. 'avx512vpopcntdq' 'no-avx512vpopcntdq' Enable/disable the generation of the AVX512VPOPCNTDQ instructions. 'bmi' 'no-bmi' Enable/disable the generation of the BMI instructions. 'bmi2' 'no-bmi2' Enable/disable the generation of the BMI2 instructions. 'cldemote' 'no-cldemote' Enable/disable the generation of the CLDEMOTE instructions. 'clflushopt' 'no-clflushopt' Enable/disable the generation of the CLFLUSHOPT instructions. 'clwb' 'no-clwb' Enable/disable the generation of the CLWB instructions. 'clzero' 'no-clzero' Enable/disable the generation of the CLZERO instructions. 'crc32' 'no-crc32' Enable/disable the generation of the CRC32 instructions. 'cx16' 'no-cx16' Enable/disable the generation of the CMPXCHG16B instructions. 'default' See Function Multiversioning, where it is used to specify the default function version. 'f16c' 'no-f16c' Enable/disable the generation of the F16C instructions. 'fma' 'no-fma' Enable/disable the generation of the FMA instructions. 'fma4' 'no-fma4' Enable/disable the generation of the FMA4 instructions. 'fsgsbase' 'no-fsgsbase' Enable/disable the generation of the FSGSBASE instructions. 'fxsr' 'no-fxsr' Enable/disable the generation of the FXSR instructions. 'gfni' 'no-gfni' Enable/disable the generation of the GFNI instructions. 'hle' 'no-hle' Enable/disable the generation of the HLE instruction prefixes. 'lwp' 'no-lwp' Enable/disable the generation of the LWP instructions. 'lzcnt' 'no-lzcnt' Enable/disable the generation of the LZCNT instructions. 'mmx' 'no-mmx' Enable/disable the generation of the MMX instructions. 'movbe' 'no-movbe' Enable/disable the generation of the MOVBE instructions. 'movdir64b' 'no-movdir64b' Enable/disable the generation of the MOVDIR64B instructions. 'movdiri' 'no-movdiri' Enable/disable the generation of the MOVDIRI instructions. 'mwaitx' 'no-mwaitx' Enable/disable the generation of the MWAITX instructions. 'pclmul' 'no-pclmul' Enable/disable the generation of the PCLMUL instructions. 'pconfig' 'no-pconfig' Enable/disable the generation of the PCONFIG instructions. 'pku' 'no-pku' Enable/disable the generation of the PKU instructions. 'popcnt' 'no-popcnt' Enable/disable the generation of the POPCNT instruction. 'prefetchwt1' 'no-prefetchwt1' Enable/disable the generation of the PREFETCHWT1 instructions. 'prfchw' 'no-prfchw' Enable/disable the generation of the PREFETCHW instruction. 'ptwrite' 'no-ptwrite' Enable/disable the generation of the PTWRITE instructions. 'rdpid' 'no-rdpid' Enable/disable the generation of the RDPID instructions. 'rdrnd' 'no-rdrnd' Enable/disable the generation of the RDRND instructions. 'rdseed' 'no-rdseed' Enable/disable the generation of the RDSEED instructions. 'rtm' 'no-rtm' Enable/disable the generation of the RTM instructions. 'sahf' 'no-sahf' Enable/disable the generation of the SAHF instructions. 'sgx' 'no-sgx' Enable/disable the generation of the SGX instructions. 'sha' 'no-sha' Enable/disable the generation of the SHA instructions. 'shstk' 'no-shstk' Enable/disable the shadow stack built-in functions from CET. 'sse' 'no-sse' Enable/disable the generation of the SSE instructions. 'sse2' 'no-sse2' Enable/disable the generation of the SSE2 instructions. 'sse3' 'no-sse3' Enable/disable the generation of the SSE3 instructions. 'sse4' 'no-sse4' Enable/disable the generation of the SSE4 instructions (both SSE4.1 and SSE4.2). 'sse4.1' 'no-sse4.1' Enable/disable the generation of the sse4.1 instructions. 'sse4.2' 'no-sse4.2' Enable/disable the generation of the sse4.2 instructions. 'sse4a' 'no-sse4a' Enable/disable the generation of the SSE4A instructions. 'ssse3' 'no-ssse3' Enable/disable the generation of the SSSE3 instructions. 'tbm' 'no-tbm' Enable/disable the generation of the TBM instructions. 'vaes' 'no-vaes' Enable/disable the generation of the VAES instructions. 'vpclmulqdq' 'no-vpclmulqdq' Enable/disable the generation of the VPCLMULQDQ instructions. 'waitpkg' 'no-waitpkg' Enable/disable the generation of the WAITPKG instructions. 'wbnoinvd' 'no-wbnoinvd' Enable/disable the generation of the WBNOINVD instructions. 'xop' 'no-xop' Enable/disable the generation of the XOP instructions. 'xsave' 'no-xsave' Enable/disable the generation of the XSAVE instructions. 'xsavec' 'no-xsavec' Enable/disable the generation of the XSAVEC instructions. 'xsaveopt' 'no-xsaveopt' Enable/disable the generation of the XSAVEOPT instructions. 'xsaves' 'no-xsaves' Enable/disable the generation of the XSAVES instructions. 'cld' 'no-cld' Enable/disable the generation of the CLD before string moves. 'fancy-math-387' 'no-fancy-math-387' Enable/disable the generation of the sin, cos, and sqrt instructions on the 387 floating-point unit. 'ieee-fp' 'no-ieee-fp' Enable/disable the generation of floating point that depends on IEEE arithmetic. 'inline-all-stringops' 'no-inline-all-stringops' Enable/disable inlining of string operations. 'inline-stringops-dynamically' 'no-inline-stringops-dynamically' Enable/disable the generation of the inline code to do small string operations and calling the library routines for large operations. 'align-stringops' 'no-align-stringops' Do/do not align destination of inlined string operations. 'recip' 'no-recip' Enable/disable the generation of RCPSS, RCPPS, RSQRTSS and RSQRTPS instructions followed an additional Newton-Raphson step instead of doing a floating-point division. 'arch=ARCH' Specify the architecture to generate code for in compiling the function. 'tune=TUNE' Specify the architecture to tune for in compiling the function. 'fpmath=FPMATH' Specify which floating-point unit to use. You must specify the target("fpmath=sse,387") option as target("fpmath=sse+387") because the comma would separate different options. 'indirect_branch("choice")' On x86 targets, the indirect_branch attribute causes the compiler to convert indirect call and jump with choice. 'keep' keeps indirect call and jump unmodified. 'thunk' converts indirect call and jump to call and return thunk. 'thunk-inline' converts indirect call and jump to inlined call and return thunk. 'thunk-extern' converts indirect call and jump to external call and return thunk provided in a separate object file. 'function_return("choice")' On x86 targets, the function_return attribute causes the compiler to convert function return with choice. 'keep' keeps function return unmodified. 'thunk' converts function return to call and return thunk. 'thunk-inline' converts function return to inlined call and return thunk. 'thunk-extern' converts function return to external call and return thunk provided in a separate object file. 'nocf_check' The nocf_check attribute on a function is used to inform the compiler that the function's prologue should not be instrumented when compiled with the -fcf-protection=branch option. The compiler assumes that the function's address is a valid target for a control-flow transfer. The nocf_check attribute on a type of pointer to function is used to inform the compiler that a call through the pointer should not be instrumented when compiled with the -fcf-protection=branch option. The compiler assumes that the function's address from the pointer is a valid target for a control-flow transfer. A direct function call through a function name is assumed to be a safe call thus direct calls are not instrumented by the compiler. The nocf_check attribute is applied to an object's type. In case of assignment of a function address or a function pointer to another pointer, the attribute is not carried over from the right-hand object's type; the type of left-hand object stays unchanged. The compiler checks for nocf_check attribute mismatch and reports a warning in case of mismatch. { int foo (void) __attribute__(nocf_check); void (*foo1)(void) __attribute__(nocf_check); void (*foo2)(void); /* foo's address is assumed to be valid. */ int foo (void) /* This call site is not checked for control-flow validity. */ (*foo1)(); /* A warning is issued about attribute mismatch. */ foo1 = foo2; /* This call site is still not checked. */ (*foo1)(); /* This call site is checked. */ (*foo2)(); /* A warning is issued about attribute mismatch. */ foo2 = foo1; /* This call site is still checked. */ (*foo2)(); return 0; } 'cf_check' The cf_check attribute on a function is used to inform the compiler that ENDBR instruction should be placed at the function entry when -fcf-protection=branch is enabled. 'indirect_return' The indirect_return attribute can be applied to a function, as well as variable or type of function pointer to inform the compiler that the function may return via indirect branch. 'fentry_name("name")' On x86 targets, the fentry_name attribute sets the function to call on function entry when function instrumentation is enabled with -pg -mfentry. When name is nop then a 5 byte nop sequence is generated. 'fentry_section("name")' On x86 targets, the fentry_section attribute sets the name of the section to record function entry instrumentation calls in when enabled with -pg -mrecord-mcount On the x86, the inliner does not inline a function that has different target options than the caller, unless the callee has a subset of the target options of the caller. For example a function declared with target("sse3") can inline a function with target("sse2"), since -msse3 implies -msse2. ────────────────────────────────────────────────────────────────────────────── Next: Xstormy16 Function Attributes, Previous: Visium Function Attributes, Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Common Variable Attributes Link: prev: Xstormy16 Function Attributes Next: Type Attributes, Previous: Function Attributes, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.34 Specifying Attributes of Variables The keyword __attribute__ allows you to specify special properties of variables, function parameters, or structure, union, and, in C++, class members. This __attribute__ keyword is followed by an attribute specification enclosed in double parentheses. Some attributes are currently defined generically for variables. Other attributes are defined for variables on particular target systems. Other attributes are available for functions (see Function Attributes), labels (see Label Attributes), enumerators (see Enumerator Attributes), statements (see Statement Attributes), and for types (see Type Attributes). Other front ends might define more attributes (see Extensions to the C++ Language). See Attribute Syntax, for details of the exact syntax for using attributes. • Common Variable Attributes: • ARC Variable Attributes: • AVR Variable Attributes: • Blackfin Variable Attributes: • H8/300 Variable Attributes: • IA-64 Variable Attributes: • M32R/D Variable Attributes: • MeP Variable Attributes: • Microsoft Windows Variable Attributes: • MSP430 Variable Attributes: • Nvidia PTX Variable Attributes: • PowerPC Variable Attributes: • RL78 Variable Attributes: • SPU Variable Attributes: • V850 Variable Attributes: • x86 Variable Attributes: • Xstormy16 Variable Attributes: ────────────────────────────────────────────────────────────────────────────── Next: Type Attributes, Previous: Function Attributes, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Variable Attributes Link: next: ARC Variable Attributes Link: prev: Variable Attributes ────────────────────────────────────────────────────────────────────────────── 6.34.1 Common Variable Attributes The following attributes are supported on most targets. alias ("target") The alias variable attribute causes the declaration to be emitted as an alias for another symbol known as an alias target. Except for top-level qualifiers the alias target must have the same type as the alias. For instance, the following int var_target; extern int __attribute__ ((alias ("var_target"))) var_alias; defines var_alias to be an alias for the var_target variable. It is an error if the alias target is not defined in the same translation unit as the alias. Note that in the absence of the attribute GCC assumes that distinct declarations with external linkage denote distinct objects. Using both the alias and the alias target to access the same object is undefined in a translation unit without a declaration of the alias with the attribute. This attribute requires assembler and object file support, and may not be available on all targets. aligned aligned (alignment) The aligned attribute specifies a minimum alignment for the variable or structure field, measured in bytes. When specified, alignment must be an integer constant power of 2. Specifying no alignment argument implies the maximum alignment for the target, which is often, but by no means always, 8 or 16 bytes. For example, the declaration: int x __attribute__ ((aligned (16))) = 0; causes the compiler to allocate the global variable x on a 16-byte boundary. On a 68040, this could be used in conjunction with an asm expression to access the move16 instruction which requires 16-byte aligned operands. You can also specify the alignment of structure fields. For example, to create a double-word aligned int pair, you could write: struct foo { int x[2] __attribute__ ((aligned (8))); }; This is an alternative to creating a union with a double member, which forces the union to be double-word aligned. As in the preceding examples, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given variable or structure field. Alternatively, you can leave out the alignment factor and just ask the compiler to align a variable or field to the default alignment for the target architecture you are compiling for. The default alignment is sufficient for all scalar types, but may not be enough for all vector types on a target that supports vector operations. The default alignment is fixed for a particular target ABI. GCC also provides a target specific macro __BIGGEST_ALIGNMENT__, which is the largest alignment ever used for any data type on the target machine you are compiling for. For example, you could write: short array[3] __attribute__ ((aligned (__BIGGEST_ALIGNMENT__))); The compiler automatically sets the alignment for the declared variable or field to __BIGGEST_ALIGNMENT__. Doing this can often make copy operations more efficient, because the compiler can use whatever instructions copy the biggest chunks of memory when performing copies to or from the variables or fields that you have aligned this way. Note that the value of __BIGGEST_ALIGNMENT__ may change depending on command-line options. When used on a struct, or struct member, the aligned attribute can only increase the alignment; in order to decrease it, the packed attribute must be specified as well. When used as part of a typedef, the aligned attribute can both increase and decrease alignment, and specifying the packed attribute generates a warning. Note that the effectiveness of aligned attributes for static variables may be limited by inherent limitations in the system linker and/or object file format. On some systems, the linker is only able to arrange for variables to be aligned up to a certain maximum alignment. (For some linkers, the maximum supported alignment may be very very small.) If your linker is only able to align variables up to a maximum of 8-byte alignment, then specifying aligned(16) in an __attribute__ still only provides you with 8-byte alignment. See your linker documentation for further information. Stack variables are not affected by linker restrictions; GCC can properly align them on any target. The aligned attribute can also be used for functions (see Common Function Attributes.) warn_if_not_aligned (alignment) This attribute specifies a threshold for the structure field, measured in bytes. If the structure field is aligned below the threshold, a warning will be issued. For example, the declaration: struct foo { int i1; int i2; unsigned long long x __attribute__ ((warn_if_not_aligned (16))); }; causes the compiler to issue an warning on struct foo, like 'warning: alignment 8 of 'struct foo' is less than 16'. The compiler also issues a warning, like 'warning: 'x' offset 8 in 'struct foo' isn't aligned to 16', when the structure field has the misaligned offset: struct __attribute__ ((aligned (16))) foo { int i1; int i2; unsigned long long x __attribute__ ((warn_if_not_aligned (16))); }; This warning can be disabled by -Wno-if-not-aligned. The warn_if_not_aligned attribute can also be used for types (see Common Type Attributes.) alloc_size (position) alloc_size (position-1, position-2) The alloc_size variable attribute may be applied to the declaration of a pointer to a function that returns a pointer and takes at least one argument of an integer type. It indicates that the returned pointer points to an object whose size is given by the function argument at position-1, or by the product of the arguments at position-1 and position-2. Meaningful sizes are positive values less than PTRDIFF_MAX. Other sizes are disagnosed when detected. GCC uses this information to improve the results of __builtin_object_size. For instance, the following declarations typedef __attribute__ ((alloc_size (1, 2))) void* (*calloc_ptr) (size_t, size_t); typedef __attribute__ ((alloc_size (1))) void* (*malloc_ptr) (size_t); specify that calloc_ptr is a pointer of a function that, like the standard C function calloc, returns an object whose size is given by the product of arguments 1 and 2, and similarly, that malloc_ptr, like the standard C function malloc, returns an object whose size is given by argument 1 to the function. cleanup (cleanup_function) The cleanup attribute runs a function when the variable goes out of scope. This attribute can only be applied to auto function scope variables; it may not be applied to parameters or variables with static storage duration. The function must take one parameter, a pointer to a type compatible with the variable. The return value of the function (if any) is ignored. If -fexceptions is enabled, then cleanup_function is run during the stack unwinding that happens during the processing of the exception. Note that the cleanup attribute does not allow the exception to be caught, only to perform an action. It is undefined what happens if cleanup_function does not return normally. common nocommon The common attribute requests GCC to place a variable in “common” storage. The nocommon attribute requests the opposite—to allocate space for it directly. These attributes override the default chosen by the -fno-common and -fcommon flags respectively. copy copy (variable) The copy attribute applies the set of attributes with which variable has been declared to the declaration of the variable to which the attribute is applied. The attribute is designed for libraries that define aliases that are expected to specify the same set of attributes as the aliased symbols. The copy attribute can be used with variables, functions or types. However, the kind of symbol to which the attribute is applied (either varible or function) must match the kind of symbol to which the argument refers. The copy attribute copies only syntactic and semantic attributes but not attributes that affect a symbol's linkage or visibility such as alias, visibility, or weak. The deprecated attribute is also not copied. See Common Function Attributes. See Common Type Attributes. deprecated deprecated (msg) The deprecated attribute results in a warning if the variable is used anywhere in the source file. This is useful when identifying variables that are expected to be removed in a future version of a program. The warning also includes the location of the declaration of the deprecated variable, to enable users to easily find further information about why the variable is deprecated, or what they should do instead. Note that the warning only occurs for uses: extern int old_var __attribute__ ((deprecated)); extern int old_var; int new_fn () { return old_var; } results in a warning on line 3 but not line 2. The optional msg argument, which must be a string, is printed in the warning if present. The deprecated attribute can also be used for functions and types (see Common Function Attributes, see Common Type Attributes). The message attached to the attribute is affected by the setting of the -fmessage-length option. mode (mode) This attribute specifies the data type for the declaration—whichever type corresponds to the mode mode. This in effect lets you request an integer or floating-point type according to its width. See Machine Modes in GNU Compiler Collection (GCC) Internals, for a list of the possible keywords for mode. You may also specify a mode of byte or __byte__ to indicate the mode corresponding to a one-byte integer, word or __word__ for the mode of a one-word integer, and pointer or __pointer__ for the mode used to represent pointers. nonstring The nonstring variable attribute specifies that an object or member declaration with type array of char, signed char, or unsigned char, or pointer to such a type is intended to store character arrays that do not necessarily contain a terminating NUL. This is useful in detecting uses of such arrays or pointers with functions that expect NUL-terminated strings, and to avoid warnings when such an array or pointer is used as an argument to a bounded string manipulation function such as strncpy. For example, without the attribute, GCC will issue a warning for the strncpy call below because it may truncate the copy without appending the terminating NUL character. Using the attribute makes it possible to suppress the warning. However, when the array is declared with the attribute the call to strlen is diagnosed because when the array doesn't contain a NUL-terminated string the call is undefined. To copy, compare, of search non-string character arrays use the memcpy, memcmp, memchr, and other functions that operate on arrays of bytes. In addition, calling strnlen and strndup with such arrays is safe provided a suitable bound is specified, and not diagnosed. struct Data { char name [32] __attribute__ ((nonstring)); }; int f (struct Data *pd, const char *s) { strncpy (pd->name, s, sizeof pd->name); … return strlen (pd->name); // unsafe, gets a warning } packed The packed attribute specifies that a structure member should have the smallest possible alignment—one bit for a bit-field and one byte otherwise, unless a larger value is specified with the aligned attribute. The attribute does not apply to non-member objects. For example in the structure below, the member array x is packed so that it immediately follows a with no intervening padding: struct foo { char a; int x[2] __attribute__ ((packed)); }; Note: The 4.1, 4.2 and 4.3 series of GCC ignore the packed attribute on bit-fields of type char. This has been fixed in GCC 4.4 but the change can lead to differences in the structure layout. See the documentation of -Wpacked-bitfield-compat for more information. section ("section-name") Normally, the compiler places the objects it generates in sections like data and bss. Sometimes, however, you need additional sections, or you need certain particular variables to appear in special sections, for example to map to special hardware. The section attribute specifies that a variable (or function) lives in a particular section. For example, this small program uses several specific section names: struct duart a __attribute__ ((section ("DUART_A"))) = { 0 }; struct duart b __attribute__ ((section ("DUART_B"))) = { 0 }; char stack[10000] __attribute__ ((section ("STACK"))) = { 0 }; int init_data __attribute__ ((section ("INITDATA"))); main() { /* Initialize stack pointer */ init_sp (stack + sizeof (stack)); /* Initialize initialized data */ memcpy (&init_data, &data, &edata - &data); /* Turn on the serial ports */ init_duart (&a); init_duart (&b); } Use the section attribute with global variables and not local variables, as shown in the example. You may use the section attribute with initialized or uninitialized global variables but the linker requires each object be defined once, with the exception that uninitialized variables tentatively go in the common (or bss) section and can be multiply “defined”. Using the section attribute changes what section the variable goes into and may cause the linker to issue an error if an uninitialized variable has multiple definitions. You can force a variable to be initialized with the -fno-common flag or the nocommon attribute. Some file formats do not support arbitrary sections so the section attribute is not available on all platforms. If you need to map the entire contents of a module to a particular section, consider using the facilities of the linker instead. tls_model ("tls_model") The tls_model attribute sets thread-local storage model (see Thread-Local) of a particular __thread variable, overriding -ftls-model= command-line switch on a per-variable basis. The tls_model argument should be one of global-dynamic, local-dynamic, initial-exec or local-exec. Not all targets support this attribute. unused This attribute, attached to a variable, means that the variable is meant to be possibly unused. GCC does not produce a warning for this variable. used This attribute, attached to a variable with static storage, means that the variable must be emitted even if it appears that the variable is not referenced. When applied to a static data member of a C++ class template, the attribute also means that the member is instantiated if the class itself is instantiated. vector_size (bytes) This attribute specifies the vector size for the type of the declared variable, measured in bytes. The type to which it applies is known as the base type. The bytes argument must be a positive power-of-two multiple of the base type size. For example, the declaration: int foo __attribute__ ((vector_size (16))); causes the compiler to set the mode for foo, to be 16 bytes, divided into int sized units. Assuming a 32-bit int, foo's type is a vector of four units of four bytes each, and the corresponding mode of foo is V4SI. See Vector Extensions for details of manipulating vector variables. This attribute is only applicable to integral and floating scalars, although arrays, pointers, and function return values are allowed in conjunction with this construct. Aggregates with this attribute are invalid, even if they are of the same size as a corresponding scalar. For example, the declaration: struct S { int a; }; struct S __attribute__ ((vector_size (16))) foo; is invalid even if the size of the structure is the same as the size of the int. visibility ("visibility_type") This attribute affects the linkage of the declaration to which it is attached. The visibility attribute is described in Common Function Attributes. weak The weak attribute is described in Common Function Attributes. ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Variable Attributes Link: next: Xstormy16 Variable Attributes Link: prev: V850 Variable Attributes Next: Xstormy16 Variable Attributes, Previous: V850 Variable Attributes, ────────────────────────────────────────────────────────────────────────────── 6.34.16 x86 Variable Attributes Two attributes are currently defined for x86 configurations: ms_struct and gcc_struct. ms_struct gcc_struct If packed is used on a structure, or if bit-fields are used, it may be that the Microsoft ABI lays out the structure differently than the way GCC normally does. Particularly when moving packed data between functions compiled with GCC and the native Microsoft compiler (either via function call or as data in a file), it may be necessary to access either format. The ms_struct and gcc_struct attributes correspond to the -mms-bitfields and -mno-ms-bitfields command-line options, respectively; see x86 Options, for details of how structure layout is affected. See x86 Type Attributes, for information about the corresponding attributes on types. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Common Type Attributes Link: prev: Xstormy16 Variable Attributes Next: Label Attributes, Previous: Variable Attributes, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.35 Specifying Attributes of Types The keyword __attribute__ allows you to specify various special properties of types. Some type attributes apply only to structure and union types, and in C++, also class types, while others can apply to any type defined via a typedef declaration. Unless otherwise specified, the same restrictions and effects apply to attributes regardless of whether a type is a trivial structure or a C++ class with user-defined constructors, destructors, or a copy assignment. Other attributes are defined for functions (see Function Attributes), labels (see Label Attributes), enumerators (see Enumerator Attributes), statements (see Statement Attributes), and for variables (see Variable Attributes). The __attribute__ keyword is followed by an attribute specification enclosed in double parentheses. You may specify type attributes in an enum, struct or union type declaration or definition by placing them immediately after the struct, union or enum keyword. You can also place them just past the closing curly brace of the definition, but this is less preferred because logically the type should be fully defined at the closing brace. You can also include type attributes in a typedef declaration. See Attribute Syntax, for details of the exact syntax for using attributes. ────────────────────────────────────────────────────────────────────────────── 6.35.1 Common Type Attributes The following type attributes are supported on most targets. aligned aligned (alignment) The aligned attribute specifies a minimum alignment (in bytes) for variables of the specified type. When specified, alignment must be a power of 2. Specifying no alignment argument implies the maximum alignment for the target, which is often, but by no means always, 8 or 16 bytes. For example, the declarations: struct __attribute__ ((aligned (8))) S { short f[3]; }; typedef int more_aligned_int __attribute__ ((aligned (8))); force the compiler to ensure (as far as it can) that each variable whose type is struct S or more_aligned_int is allocated and aligned at least on a 8-byte boundary. On a SPARC, having all variables of type struct S aligned to 8-byte boundaries allows the compiler to use the ldd and std (doubleword load and store) instructions when copying one variable of type struct S to another, thus improving run-time efficiency. Note that the alignment of any given struct or union type is required by the ISO C standard to be at least a perfect multiple of the lowest common multiple of the alignments of all of the members of the struct or union in question. This means that you can effectively adjust the alignment of a struct or union type by attaching an aligned attribute to any one of the members of such a type, but the notation illustrated in the example above is a more obvious, intuitive, and readable way to request the compiler to adjust the alignment of an entire struct or union type. As in the preceding example, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given struct or union type. Alternatively, you can leave out the alignment factor and just ask the compiler to align a type to the maximum useful alignment for the target machine you are compiling for. For example, you could write: struct __attribute__ ((aligned)) S { short f[3]; }; Whenever you leave out the alignment factor in an aligned attribute specification, the compiler automatically sets the alignment for the type to the largest alignment that is ever used for any data type on the target machine you are compiling for. Doing this can often make copy operations more efficient, because the compiler can use whatever instructions copy the biggest chunks of memory when performing copies to or from the variables that have types that you have aligned this way. In the example above, if the size of each short is 2 bytes, then the size of the entire struct S type is 6 bytes. The smallest power of two that is greater than or equal to that is 8, so the compiler sets the alignment for the entire struct S type to 8 bytes. Note that although you can ask the compiler to select a time-efficient alignment for a given type and then declare only individual stand-alone objects of that type, the compiler's ability to select a time-efficient alignment is primarily useful only when you plan to create arrays of variables having the relevant (efficiently aligned) type. If you declare or use arrays of variables of an efficiently-aligned type, then it is likely that your program also does pointer arithmetic (or subscripting, which amounts to the same thing) on pointers to the relevant type, and the code that the compiler generates for these pointer arithmetic operations is often more efficient for efficiently-aligned types than for other types. Note that the effectiveness of aligned attributes may be limited by inherent limitations in your linker. On many systems, the linker is only able to arrange for variables to be aligned up to a certain maximum alignment. (For some linkers, the maximum supported alignment may be very very small.) If your linker is only able to align variables up to a maximum of 8-byte alignment, then specifying aligned (16) in an __attribute__ still only provides you with 8-byte alignment. See your linker documentation for further information. When used on a struct, or struct member, the aligned attribute can only increase the alignment; in order to decrease it, the packed attribute must be specified as well. When used as part of a typedef, the aligned attribute can both increase and decrease alignment, and specifying the packed attribute generates a warning. warn_if_not_aligned (alignment) This attribute specifies a threshold for the structure field, measured in bytes. If the structure field is aligned below the threshold, a warning will be issued. For example, the declaration: typedef unsigned long long __u64 __attribute__((aligned (4), warn_if_not_aligned (8))); struct foo { int i1; int i2; __u64 x; }; causes the compiler to issue an warning on struct foo, like 'warning: alignment 4 of 'struct foo' is less than 8'. It is used to define struct foo in such a way that struct foo has the same layout and the structure field x has the same alignment when __u64 is aligned at either 4 or 8 bytes. Align struct foo to 8 bytes: struct __attribute__ ((aligned (8))) foo { int i1; int i2; __u64 x; }; silences the warning. The compiler also issues a warning, like 'warning: 'x' offset 12 in 'struct foo' isn't aligned to 8', when the structure field has the misaligned offset: struct __attribute__ ((aligned (8))) foo { int i1; int i2; int i3; __u64 x; }; This warning can be disabled by -Wno-if-not-aligned. alloc_size (position) alloc_size (position-1, position-2) The alloc_size type attribute may be applied to the definition of a type of a function that returns a pointer and takes at least one argument of an integer type. It indicates that the returned pointer points to an object whose size is given by the function argument at position-1, or by the product of the arguments at position-1 and position-2. Meaningful sizes are positive values less than PTRDIFF_MAX. Other sizes are disagnosed when detected. GCC uses this information to improve the results of __builtin_object_size. For instance, the following declarations typedef __attribute__ ((alloc_size (1, 2))) void* calloc_type (size_t, size_t); typedef __attribute__ ((alloc_size (1))) void* malloc_type (size_t); specify that calloc_type is a type of a function that, like the standard C function calloc, returns an object whose size is given by the product of arguments 1 and 2, and that malloc_type, like the standard C function malloc, returns an object whose size is given by argument 1 to the function. copy copy (expression) The copy attribute applies the set of attributes with which the type of the expression has been declared to the declaration of the type to which the attribute is applied. The attribute is designed for libraries that define aliases that are expected to specify the same set of attributes as the aliased symbols. The copy attribute can be used with types, variables, or functions. However, the kind of symbol to which the attribute is applied (either varible or function) must match the kind of symbol to which the argument refers. The copy attribute copies only syntactic and semantic attributes but not attributes that affect a symbol's linkage or visibility such as alias, visibility, or weak. The deprecated attribute is also not copied. See Common Function Attributes. See Common Variable Attributes. For example, suppose struct A below is defined in some third party library header to have the alignment requirement N and to force a warning whenever a variable of the type is not so aligned due to attribute packed. Specifying the copy attribute on the definition on the unrelated struct B has the effect of copying all relevant attributes from the type referenced by the pointer expression to struct B. struct __attribute__ ((aligned (N), warn_if_not_aligned (N))) A { /* … */ }; struct __attribute__ ((copy ( (struct A *)0)) B { /* … */ }; deprecated deprecated (msg) The deprecated attribute results in a warning if the type is used anywhere in the source file. This is useful when identifying types that are expected to be removed in a future version of a program. If possible, the warning also includes the location of the declaration of the deprecated type, to enable users to easily find further information about why the type is deprecated, or what they should do instead. Note that the warnings only occur for uses and then only if the type is being applied to an identifier that itself is not being declared as deprecated. typedef int T1 __attribute__ ((deprecated)); T1 x; typedef T1 T2; T2 y; typedef T1 T3 __attribute__ ((deprecated)); T3 z __attribute__ ((deprecated)); results in a warning on line 2 and 3 but not lines 4, 5, or 6. No warning is issued for line 4 because T2 is not explicitly deprecated. Line 5 has no warning because T3 is explicitly deprecated. Similarly for line 6. The optional msg argument, which must be a string, is printed in the warning if present. Control characters in the string will be replaced with escape sequences, and if the -fmessage-length option is set to 0 (its default value) then any newline characters will be ignored. The deprecated attribute can also be used for functions and variables (see Function Attributes, see Variable Attributes.) The message attached to the attribute is affected by the setting of the -fmessage-length option. designated_init This attribute may only be applied to structure types. It indicates that any initialization of an object of this type must use designated initializers rather than positional initializers. The intent of this attribute is to allow the programmer to indicate that a structure's layout may change, and that therefore relying on positional initialization will result in future breakage. GCC emits warnings based on this attribute by default; use -Wno-designated-init to suppress them. may_alias Accesses through pointers to types with this attribute are not subject to type-based alias analysis, but are instead assumed to be able to alias any other type of objects. In the context of section 6.5 paragraph 7 of the C99 standard, an lvalue expression dereferencing such a pointer is treated like having a character type. See -fstrict-aliasing for more information on aliasing issues. This extension exists to support some vector APIs, in which pointers to one vector type are permitted to alias pointers to a different vector type. Note that an object of a type with this attribute does not have any special semantics. Example of use: typedef short __attribute__ ((__may_alias__)) short_a; int main (void) { int a = 0x12345678; short_a *b = (short_a *) &a; b[1] = 0; if (a == 0x12345678) abort(); exit(0); } If you replaced short_a with short in the variable declaration, the above program would abort when compiled with -fstrict-aliasing, which is on by default at -O2 or above. mode (mode) This attribute specifies the data type for the declaration—whichever type corresponds to the mode mode. This in effect lets you request an integer or floating-point type according to its width. See Machine Modes in GNU Compiler Collection (GCC) Internals, for a list of the possible keywords for mode. You may also specify a mode of byte or __byte__ to indicate the mode corresponding to a one-byte integer, word or __word__ for the mode of a one-word integer, and pointer or __pointer__ for the mode used to represent pointers. packed This attribute, attached to a struct, union, or C++ class type definition, specifies that each of its members (other than zero-width bit-fields) is placed to minimize the memory required. This is equivalent to specifying the packed attribute on each of the members. When attached to an enum definition, the packed attribute indicates that the smallest integral type should be used. Specifying the -fshort-enums flag on the command line is equivalent to specifying the packed attribute on all enum definitions. In the following example struct my_packed_struct's members are packed closely together, but the internal layout of its s member is not packed—to do that, struct my_unpacked_struct needs to be packed too. struct my_unpacked_struct { char c; int i; }; struct __attribute__ ((__packed__)) my_packed_struct { char c; int i; struct my_unpacked_struct s; }; You may only specify the packed attribute on the definition of an enum, struct, union, or class, not on a typedef that does not also define the enumerated type, structure, union, or class. scalar_storage_order ("endianness") When attached to a union or a struct, this attribute sets the storage order, aka endianness, of the scalar fields of the type, as well as the array fields whose component is scalar. The supported endiannesses are big-endian and little-endian. The attribute has no effects on fields which are themselves a union, a struct or an array whose component is a union or a struct, and it is possible for these fields to have a different scalar storage order than the enclosing type. This attribute is supported only for targets that use a uniform default scalar storage order (fortunately, most of them), i.e. targets that store the scalars either all in big-endian or all in little-endian. Additional restrictions are enforced for types with the reverse scalar storage order with regard to the scalar storage order of the target: * Taking the address of a scalar field of a union or a struct with reverse scalar storage order is not permitted and yields an error. * Taking the address of an array field, whose component is scalar, of a union or a struct with reverse scalar storage order is permitted but yields a warning, unless -Wno-scalar-storage-order is specified. * Taking the address of a union or a struct with reverse scalar storage order is permitted. These restrictions exist because the storage order attribute is lost when the address of a scalar or the address of an array with scalar component is taken, so storing indirectly through this address generally does not work. The second case is nevertheless allowed to be able to perform a block copy from or to the array. Moreover, the use of type punning or aliasing to toggle the storage order is not supported; that is to say, a given scalar object cannot be accessed through distinct types that assign a different storage order to it. transparent_union This attribute, attached to a union type definition, indicates that any function parameter having that union type causes calls to that function to be treated in a special way. First, the argument corresponding to a transparent union type can be of any type in the union; no cast is required. Also, if the union contains a pointer type, the corresponding argument can be a null pointer constant or a void pointer expression; and if the union contains a void pointer type, the corresponding argument can be any pointer expression. If the union member type is a pointer, qualifiers like const on the referenced type must be respected, just as with normal pointer conversions. Second, the argument is passed to the function using the calling conventions of the first member of the transparent union, not the calling conventions of the union itself. All members of the union must have the same machine representation; this is necessary for this argument passing to work properly. Transparent unions are designed for library functions that have multiple interfaces for compatibility reasons. For example, suppose the wait function must accept either a value of type int * to comply with POSIX, or a value of type union wait * to comply with the 4.1BSD interface. If wait's parameter were void *, wait would accept both kinds of arguments, but it would also accept any other pointer type and this would make argument type checking less useful. Instead, might define the interface as follows: typedef union __attribute__ ((__transparent_union__)) { int *__ip; union wait *__up; } wait_status_ptr_t; pid_t wait (wait_status_ptr_t); This interface allows either int * or union wait * arguments to be passed, using the int * calling convention. The program can call wait with arguments of either type: int w1 () { int w; return wait (&w); } int w2 () { union wait w; return wait (&w); } With this interface, wait's implementation might look like this: pid_t wait (wait_status_ptr_t p) { return waitpid (-1, p.__ip, 0); } unused When attached to a type (including a union or a struct), this attribute means that variables of that type are meant to appear possibly unused. GCC does not produce a warning for any variables of that type, even if the variable appears to do nothing. This is often the case with lock or thread classes, which are usually defined and then not referenced, but contain constructors and destructors that have nontrivial bookkeeping functions. vector_size (bytes) This attribute specifies the vector size for the type, measured in bytes. The type to which it applies is known as the base type. The bytes argument must be a positive power-of-two multiple of the base type size. For example, the following declarations: typedef __attribute__ ((vector_size (32))) int int_vec32_t ; typedef __attribute__ ((vector_size (32))) int* int_vec32_ptr_t; typedef __attribute__ ((vector_size (32))) int int_vec32_arr3_t[3]; define int_vec32_t to be a 32-byte vector type composed of int sized units. With int having a size of 4 bytes, the type defines a vector of eight units, four bytes each. The mode of variables of type int_vec32_t is V8SI. int_vec32_ptr_t is then defined to be a pointer to such a vector type, and int_vec32_arr3_t to be an array of three such vectors. See Vector Extensions for details of manipulating objects of vector types. This attribute is only applicable to integral and floating scalar types. In function declarations the attribute applies to the function return type. For example, the following: __attribute__ ((vector_size (16))) float get_flt_vec16 (void); declares get_flt_vec16 to be a function returning a 16-byte vector with the base type float. visibility In C++, attribute visibility (see Function Attributes) can also be applied to class, struct, union and enum types. Unlike other type attributes, the attribute must appear between the initial keyword and the name of the type; it cannot appear after the body of the type. Note that the type visibility is applied to vague linkage entities associated with the class (vtable, typeinfo node, etc.). In particular, if a class is thrown as an exception in one shared object and caught in another, the class must have default visibility. Otherwise the two shared objects are unable to use the same typeinfo node and exception handling will break. To specify multiple attributes, separate them by commas within the double parentheses: for example, '__attribute__ ((aligned (16), packed))'. ────────────────────────────────────────────────────────────────────────────── 6.35.7 x86 Type Attributes Two attributes are currently defined for x86 configurations: ms_struct and gcc_struct. ms_struct gcc_struct If packed is used on a structure, or if bit-fields are used it may be that the Microsoft ABI packs them differently than GCC normally packs them. Particularly when moving packed data between functions compiled with GCC and the native Microsoft compiler (either via function call or as data in a file), it may be necessary to access either format. The ms_struct and gcc_struct attributes correspond to the -mms-bitfields and -mno-ms-bitfields command-line options, respectively; see x86 Options, for details of how structure layout is affected. See x86 Variable Attributes, for information about the corresponding attributes on variables. ────────────────────────────────────────────────────────────────────────────── 6.36 Label Attributes GCC allows attributes to be set on C labels. See Attribute Syntax, for details of the exact syntax for using attributes. Other attributes are available for functions (see Function Attributes), variables (see Variable Attributes), enumerators (see Enumerator Attributes), statements (see Statement Attributes), and for types (see Type Attributes). This example uses the cold label attribute to indicate the ErrorHandling branch is unlikely to be taken and that the ErrorHandling label is unused: asm goto ("some asm" : : : : NoError); /* This branch (the fall-through from the asm) is less commonly used */ ErrorHandling: __attribute__((cold, unused)); /* Semi-colon is required here */ printf("error\n"); return 0; NoError: printf("no error\n"); return 1; unused This feature is intended for program-generated code that may contain unused labels, but which is compiled with -Wall. It is not normally appropriate to use in it human-written code, though it could be useful in cases where the code that jumps to the label is contained within an #ifdef conditional. hot The hot attribute on a label is used to inform the compiler that the path following the label is more likely than paths that are not so annotated. This attribute is used in cases where __builtin_expect cannot be used, for instance with computed goto or asm goto. cold The cold attribute on labels is used to inform the compiler that the path following the label is unlikely to be executed. This attribute is used in cases where __builtin_expect cannot be used, for instance with computed goto or asm goto. ────────────────────────────────────────────────────────────────────────────── 6.37 Enumerator Attributes GCC allows attributes to be set on enumerators. See Attribute Syntax, for details of the exact syntax for using attributes. Other attributes are available for functions (see Function Attributes), variables (see Variable Attributes), labels (see Label Attributes), statements (see Statement Attributes), and for types (see Type Attributes). This example uses the deprecated enumerator attribute to indicate the oldval enumerator is deprecated: enum E { oldval __attribute__((deprecated)), newval }; int fn (void) { return oldval; } deprecated The deprecated attribute results in a warning if the enumerator is used anywhere in the source file. This is useful when identifying enumerators that are expected to be removed in a future version of a program. The warning also includes the location of the declaration of the deprecated enumerator, to enable users to easily find further information about why the enumerator is deprecated, or what they should do instead. Note that the warnings only occurs for uses. ────────────────────────────────────────────────────────────────────────────── 6.38 Statement Attributes GCC allows attributes to be set on null statements. See Attribute Syntax, for details of the exact syntax for using attributes. Other attributes are available for functions (see Function Attributes), variables (see Variable Attributes), labels (see Label Attributes), enumerators (see Enumerator Attributes), and for types (see Type Attributes). This example uses the fallthrough statement attribute to indicate that the -Wimplicit-fallthrough warning should not be emitted: switch (cond) { case 1: bar (1); __attribute__((fallthrough)); case 2: … } fallthrough The fallthrough attribute with a null statement serves as a fallthrough statement. It hints to the compiler that a statement that falls through to another case label, or user-defined label in a switch statement is intentional and thus the -Wimplicit-fallthrough warning must not trigger. The fallthrough attribute may appear at most once in each attribute list, and may not be mixed with other attributes. It can only be used in a switch statement (the compiler will issue an error otherwise), after a preceding statement and before a logically succeeding case label, or user-defined label. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Function Prototypes Link: prev: Statement Attributes Next: Function Prototypes, Previous: Statement Attributes, Up: C ────────────────────────────────────────────────────────────────────────────── 6.39 Attribute Syntax This section describes the syntax with which __attribute__ may be used, and the constructs to which attribute specifiers bind, for the C language. Some details may vary for C++ and Objective-C. Because of infelicities in the grammar for attributes, some forms described here may not be successfully parsed in all cases. There are some problems with the semantics of attributes in C++. For example, there are no manglings for attributes, although they may affect code generation, so problems may arise when attributed types are used in conjunction with templates or overloading. Similarly, typeid does not distinguish between types with different attributes. Support for attributes in C++ may be restricted in future to attributes on declarations only, but not on nested declarators. See Function Attributes, for details of the semantics of attributes applying to functions. See Variable Attributes, for details of the semantics of attributes applying to variables. See Type Attributes, for details of the semantics of attributes applying to structure, union and enumerated types. See Label Attributes, for details of the semantics of attributes applying to labels. See Enumerator Attributes, for details of the semantics of attributes applying to enumerators. See Statement Attributes, for details of the semantics of attributes applying to statements. An attribute specifier is of the form __attribute__ ((attribute-list)). An attribute list is a possibly empty comma-separated sequence of attributes, where each attribute is one of the following: * Empty. Empty attributes are ignored. * An attribute name (which may be an identifier such as unused, or a reserved word such as const). * An attribute name followed by a parenthesized list of parameters for the attribute. These parameters take one of the following forms: * An identifier. For example, mode attributes use this form. * An identifier followed by a comma and a non-empty comma-separated list of expressions. For example, format attributes use this form. * A possibly empty comma-separated list of expressions. For example, format_arg attributes use this form with the list being a single integer constant expression, and alias attributes use this form with the list being a single string constant. An attribute specifier list is a sequence of one or more attribute specifiers, not separated by any other tokens. You may optionally specify attribute names with '__' preceding and following the name. This allows you to use them in header files without being concerned about a possible macro of the same name. For example, you may use the attribute name __noreturn__ instead of noreturn. Label Attributes In GNU C, an attribute specifier list may appear after the colon following a label, other than a case or default label. GNU C++ only permits attributes on labels if the attribute specifier is immediately followed by a semicolon (i.e., the label applies to an empty statement). If the semicolon is missing, C++ label attributes are ambiguous, as it is permissible for a declaration, which could begin with an attribute list, to be labelled in C++. Declarations cannot be labelled in C90 or C99, so the ambiguity does not arise there. Enumerator Attributes In GNU C, an attribute specifier list may appear as part of an enumerator. The attribute goes after the enumeration constant, before =, if present. The optional attribute in the enumerator appertains to the enumeration constant. It is not possible to place the attribute after the constant expression, if present. Statement Attributes In GNU C, an attribute specifier list may appear as part of a null statement. The attribute goes before the semicolon. Type Attributes An attribute specifier list may appear as part of a struct, union or enum specifier. It may go either immediately after the struct, union or enum keyword, or after the closing brace. The former syntax is preferred. Where attribute specifiers follow the closing brace, they are considered to relate to the structure, union or enumerated type defined, not to any enclosing declaration the type specifier appears in, and the type defined is not complete until after the attribute specifiers. All other attributes Otherwise, an attribute specifier appears as part of a declaration, counting declarations of unnamed parameters and type names, and relates to that declaration (which may be nested in another declaration, for example in the case of a parameter declaration), or to a particular declarator within a declaration. Where an attribute specifier is applied to a parameter declared as a function or an array, it should apply to the function or array rather than the pointer to which the parameter is implicitly converted, but this is not yet correctly implemented. Any list of specifiers and qualifiers at the start of a declaration may contain attribute specifiers, whether or not such a list may in that context contain storage class specifiers. (Some attributes, however, are essentially in the nature of storage class specifiers, and only make sense where storage class specifiers may be used; for example, section.) There is one necessary limitation to this syntax: the first old-style parameter declaration in a function definition cannot begin with an attribute specifier, because such an attribute applies to the function instead by syntax described below (which, however, is not yet implemented in this case). In some other cases, attribute specifiers are permitted by this grammar but not yet supported by the compiler. All attribute specifiers in this place relate to the declaration as a whole. In the obsolescent usage where a type of int is implied by the absence of type specifiers, such a list of specifiers and qualifiers may be an attribute specifier list with no other specifiers or qualifiers. At present, the first parameter in a function prototype must have some type specifier that is not an attribute specifier; this resolves an ambiguity in the interpretation of void f(int (__attribute__((foo)) x)), but is subject to change. At present, if the parentheses of a function declarator contain only attributes then those attributes are ignored, rather than yielding an error or warning or implying a single parameter of type int, but this is subject to change. An attribute specifier list may appear immediately before a declarator (other than the first) in a comma-separated list of declarators in a declaration of more than one identifier using a single list of specifiers and qualifiers. Such attribute specifiers apply only to the identifier before whose declarator they appear. For example, in __attribute__((noreturn)) void d0 (void), __attribute__((format(printf, 1, 2))) d1 (const char *, ...), d2 (void); the noreturn attribute applies to all the functions declared; the format attribute only applies to d1. An attribute specifier list may appear immediately before the comma, = or semicolon terminating the declaration of an identifier other than a function definition. Such attribute specifiers apply to the declared object or function. Where an assembler name for an object or function is specified (see Asm Labels), the attribute must follow the asm specification. An attribute specifier list may, in future, be permitted to appear after the declarator in a function definition (before any old-style parameter declarations or the function body). Attribute specifiers may be mixed with type qualifiers appearing inside the [] of a parameter array declarator, in the C99 construct by which such qualifiers are applied to the pointer to which the array is implicitly converted. Such attribute specifiers apply to the pointer, not to the array, but at present this is not implemented and they are ignored. An attribute specifier list may appear at the start of a nested declarator. At present, there are some limitations in this usage: the attributes correctly apply to the declarator, but for most individual attributes the semantics this implies are not implemented. When attribute specifiers follow the * of a pointer declarator, they may be mixed with any type qualifiers present. The following describes the formal semantics of this syntax. It makes the most sense if you are familiar with the formal specification of declarators in the ISO C standard. Consider (as in C99 subclause 6.7.5 paragraph 4) a declaration T D1, where T contains declaration specifiers that specify a type Type (such as int) and D1 is a declarator that contains an identifier ident. The type specified for ident for derived declarators whose type does not include an attribute specifier is as in the ISO C standard. If D1 has the form ( attribute-specifier-list D ), and the declaration T D specifies the type “derived-declarator-type-list Type” for ident, then T D1 specifies the type “derived-declarator-type-list attribute-specifier-list Type” for ident. If D1 has the form * type-qualifier-and-attribute-specifier-list D, and the declaration T D specifies the type “derived-declarator-type-list Type” for ident, then T D1 specifies the type “derived-declarator-type-list type-qualifier-and-attribute-specifier-list pointer to Type” for ident. For example, void (__attribute__((noreturn)) ****f) (void); specifies the type “pointer to pointer to pointer to pointer to non-returning function returning void”. As another example, char *__attribute__((aligned(8))) *f; specifies the type “pointer to 8-byte-aligned pointer to char”. Note again that this does not work with most attributes; for example, the usage of 'aligned' and 'noreturn' attributes given above is not yet supported. For compatibility with existing code written for compiler versions that did not implement attributes on nested declarators, some laxity is allowed in the placing of attributes. If an attribute that only applies to types is applied to a declaration, it is treated as applying to the type of that declaration. If an attribute that only applies to declarations is applied to the type of a declaration, it is treated as applying to that declaration; and, for compatibility with code placing the attributes immediately before the identifier declared, such an attribute applied to a function return type is treated as applying to the function type, and such an attribute applied to an array element type is treated as applying to the array type. If an attribute that only applies to function types is applied to a pointer-to-function type, it is treated as applying to the pointer target type; if such an attribute is applied to a function return type that is not a pointer-to-function type, it is treated as applying to the function type. ────────────────────────────────────────────────────────────────────────────── Next: Function Prototypes, Previous: Statement Attributes, Up: C Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: C++ Comments Link: prev: Attribute Syntax Next: C++ Comments, Previous: Attribute Syntax, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.40 Prototypes and Old-Style Function Definitions GNU C extends ISO C to allow a function prototype to override a later old-style non-prototype definition. Consider the following example: /* Use prototypes unless the compiler is old-fashioned. */ #ifdef __STDC__ #define P(x) x #else #define P(x) () #endif /* Prototype function declaration. */ int isroot P((uid_t)); /* Old-style function definition. */ int isroot (x) /* ??? lossage here ??? */ uid_t x; { return x == 0; } Suppose the type uid_t happens to be short. ISO C does not allow this example, because subword arguments in old-style non-prototype definitions are promoted. Therefore in this example the function definition's argument is really an int, which does not match the prototype argument type of short. This restriction of ISO C makes it hard to write code that is portable to traditional C compilers, because the programmer does not know whether the uid_t type is short, int, or long. Therefore, in cases like these GNU C allows a prototype to override a later old-style definition. More precisely, in GNU C, a function prototype argument type overrides the argument type specified by a later old-style definition if the former type is the same as the latter type before promotion. Thus in GNU C the above example is equivalent to the following: int isroot (uid_t); int isroot (uid_t x) { return x == 0; } GNU C++ does not support old-style function definitions, so this extension is irrelevant. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Dollar Signs Link: prev: Function Prototypes Next: Dollar Signs, Previous: Function Prototypes, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.41 C++ Style Comments In GNU C, you may use C++ style comments, which start with '//' and continue until the end of the line. Many other C implementations allow such comments, and they are included in the 1999 C standard. However, C++ style comments are not recognized if you specify an -std option specifying a version of ISO C before C99, or -ansi (equivalent to -std=c90). Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Character Escapes Link: prev: C++ Comments Next: Character Escapes, Previous: C++ Comments, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.42 Dollar Signs in Identifier Names In GNU C, you may normally use dollar signs in identifier names. This is because many traditional C implementations allow such identifiers. However, dollar signs in identifiers are not supported on a few target machines, typically because the target assembler does not allow them. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Alignment Link: prev: Dollar Signs Next: Alignment, Previous: Dollar Signs, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.43 The Character ESC in Constants You can use the sequence '\e' in a string or character constant to stand for the ASCII character ESC. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Inline Link: prev: Character Escapes Next: Inline, Previous: Character Escapes, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.44 Determining the Alignment of Functions, Types or Variables The keyword __alignof__ determines the alignment requirement of a function, object, or a type, or the minimum alignment usually required by a type. Its syntax is just like sizeof and C11 _Alignof. For example, if the target machine requires a double value to be aligned on an 8-byte boundary, then __alignof__ (double) is 8. This is true on many RISC machines. On more traditional machine designs, __alignof__ (double) is 4 or even 2. Some machines never actually require alignment; they allow references to any data type even at an odd address. For these machines, __alignof__ reports the smallest alignment that GCC gives the data type, usually as mandated by the target ABI. If the operand of __alignof__ is an lvalue rather than a type, its value is the required alignment for its type, taking into account any minimum alignment specified by attribute aligned (see Common Variable Attributes). For example, after this declaration: struct foo { int x; char y; } foo1; the value of __alignof__ (foo1.y) is 1, even though its actual alignment is probably 2 or 4, the same as __alignof__ (int). It is an error to ask for the alignment of an incomplete type other than void. If the operand of the __alignof__ expression is a function, the expression evaluates to the alignment of the function which may be specified by attribute aligned (see Common Function Attributes). Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Volatiles Link: prev: Alignment ────────────────────────────────────────────────────────────────────────────── 6.45 An Inline Function is As Fast As a Macro By declaring a function inline, you can direct GCC to make calls to that function faster. One way GCC can achieve this is to integrate that function's code into the code for its callers. This makes execution faster by eliminating the function-call overhead; in addition, if any of the actual argument values are constant, their known values may permit simplifications at compile time so that not all of the inline function's code needs to be included. The effect on code size is less predictable; object code may be larger or smaller with function inlining, depending on the particular case. You can also direct GCC to try to integrate all “simple enough” functions into their callers with the option -finline-functions. GCC implements three different semantics of declaring a function inline. One is available with -std=gnu89 or -fgnu89-inline or when gnu_inline attribute is present on all inline declarations, another when -std=c99, -std=gnu99 or an option for a later C version is used (without -fgnu89-inline), and the third is used when compiling C++. To declare a function inline, use the inline keyword in its declaration, like this: static inline int inc (int *a) { return (*a)++; } If you are writing a header file to be included in ISO C90 programs, write __inline__ instead of inline. See Alternate Keywords. The three types of inlining behave similarly in two important cases: when the inline keyword is used on a static function, like the example above, and when a function is first declared without using the inline keyword and then is defined with inline, like this: extern int inc (int *a); inline int inc (int *a) { return (*a)++; } In both of these common cases, the program behaves the same as if you had not used the inline keyword, except for its speed. When a function is both inline and static, if all calls to the function are integrated into the caller, and the function's address is never used, then the function's own assembler code is never referenced. In this case, GCC does not actually output assembler code for the function, unless you specify the option -fkeep-inline-functions. If there is a nonintegrated call, then the function is compiled to assembler code as usual. The function must also be compiled as usual if the program refers to its address, because that cannot be inlined. Note that certain usages in a function definition can make it unsuitable for inline substitution. Among these usages are: variadic functions, use of alloca, use of computed goto (see Labels as Values), use of nonlocal goto, use of nested functions, use of setjmp, use of __builtin_longjmp and use of __builtin_return or __builtin_apply_args. Using -Winline warns when a function marked inline could not be substituted, and gives the reason for the failure. As required by ISO C++, GCC considers member functions defined within the body of a class to be marked inline even if they are not explicitly declared with the inline keyword. You can override this with -fno-default-inline; see Options Controlling C++ Dialect. GCC does not inline any functions when not optimizing unless you specify the 'always_inline' attribute for the function, like this: /* Prototype. */ inline void foo (const char) __attribute__((always_inline)); The remainder of this section is specific to GNU C90 inlining. When an inline function is not static, then the compiler must assume that there may be calls from other source files; since a global symbol can be defined only once in any program, the function must not be defined in the other source files, so the calls therein cannot be integrated. Therefore, a non-static inline function is always compiled on its own in the usual fashion. If you specify both inline and extern in the function definition, then the definition is used only for inlining. In no case is the function compiled on its own, not even if you refer to its address explicitly. Such an address becomes an external reference, as if you had only declared the function, and had not defined it. This combination of inline and extern has almost the effect of a macro. The way to use it is to put a function definition in a header file with these keywords, and put another copy of the definition (lacking inline and extern) in a library file. The definition in the header file causes most calls to the function to be inlined. If any uses of the function remain, they refer to the single copy in the library. ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Using Assembly Language with C Link: prev: Inline Next: Using Assembly Language with C, Previous: Inline, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.46 When is a Volatile Object Accessed? C has the concept of volatile objects. These are normally accessed by pointers and used for accessing hardware or inter-thread communication. The standard encourages compilers to refrain from optimizations concerning accesses to volatile objects, but leaves it implementation defined as to what constitutes a volatile access. The minimum requirement is that at a sequence point all previous accesses to volatile objects have stabilized and no subsequent accesses have occurred. Thus an implementation is free to reorder and combine volatile accesses that occur between sequence points, but cannot do so for accesses across a sequence point. The use of volatile does not allow you to violate the restriction on updating objects multiple times between two sequence points. Accesses to non-volatile objects are not ordered with respect to volatile accesses. You cannot use a volatile object as a memory barrier to order a sequence of writes to non-volatile memory. For instance: int *ptr = something; volatile int vobj; *ptr = something; vobj = 1; Unless *ptr and vobj can be aliased, it is not guaranteed that the write to *ptr occurs by the time the update of vobj happens. If you need this guarantee, you must use a stronger memory barrier such as: int *ptr = something; volatile int vobj; *ptr = something; asm volatile ("" : : : "memory"); vobj = 1; A scalar volatile object is read when it is accessed in a void context: volatile int *src = somevalue; *src; Such expressions are rvalues, and GCC implements this as a read of the volatile object being pointed to. Assignments are also expressions and have an rvalue. However when assigning to a scalar volatile, the volatile object is not reread, regardless of whether the assignment expression's rvalue is used or not. If the assignment's rvalue is used, the value is that assigned to the volatile object. For instance, there is no read of vobj in all the following cases: int obj; volatile int vobj; vobj = something; obj = vobj = something; obj ? vobj = onething : vobj = anotherthing; obj = (something, vobj = anotherthing); If you need to read the volatile object after an assignment has occurred, you must use a separate expression with an intervening sequence point. As bit-fields are not individually addressable, volatile bit-fields may be implicitly read when written to, or when adjacent bit-fields are accessed. Bit-field operations may be optimized such that adjacent bit-fields are only partially accessed, if they straddle a storage unit boundary. For these reasons it is unwise to use volatile bit-fields to access hardware. ────────────────────────────────────────────────────────────────────────────── Next: Using Assembly Language with C, Previous: Inline, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Basic Asm Link: prev: Volatiles Next: Alternate Keywords, Previous: Volatiles, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.47 How to Use Inline Assembly Language in C Code The asm keyword allows you to embed assembler instructions within C code. GCC provides two forms of inline asm statements. A basic asm statement is one with no operands (see Basic Asm), while an extended asm statement (see Extended Asm) includes one or more operands. The extended form is preferred for mixing C and assembly language within a function, but to include assembly language at top level you must use basic asm. You can also use the asm keyword to override the assembler name for a C symbol, or to place a C variable in a specific register. • Basic Asm: Inline assembler without operands. • Extended Asm: Inline assembler with operands. • Constraints: Constraints for asm operands • Asm Labels: Specifying the assembler name to use for a C symbol. • Explicit Register Variables: Defining variables residing in specified registers. • Size of an asm: How GCC calculates the size of an asm block. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Using Assembly Language with C Link: next: Extended Asm Link: prev: Using Assembly Language with C ────────────────────────────────────────────────────────────────────────────── 6.47.1 Basic Asm — Assembler Instructions Without Operands A basic asm statement has the following syntax: asm asm-qualifiers ( AssemblerInstructions ) The asm keyword is a GNU extension. When writing code that can be compiled with -ansi and the various -std options, use __asm__ instead of asm (see Alternate Keywords). Qualifiers volatile The optional volatile qualifier has no effect. All basic asm blocks are implicitly volatile. inline If you use the inline qualifier, then for inlining purposes the size of the asm statement is taken as the smallest size possible (see Size of an asm). Parameters AssemblerInstructions This is a literal string that specifies the assembler code. The string can contain any instructions recognized by the assembler, including directives. GCC does not parse the assembler instructions themselves and does not know what they mean or even whether they are valid assembler input. You may place multiple assembler instructions together in a single asm string, separated by the characters normally used in assembly code for the system. A combination that works in most places is a newline to break the line, plus a tab character (written as '\n\t'). Some assemblers allow semicolons as a line separator. However, note that some assembler dialects use semicolons to start a comment. Remarks Using extended asm (see Extended Asm) typically produces smaller, safer, and more efficient code, and in most cases it is a better solution than basic asm. However, there are two situations where only basic asm can be used: * Extended asm statements have to be inside a C function, so to write inline assembly language at file scope (“top-level”), outside of C functions, you must use basic asm. You can use this technique to emit assembler directives, define assembly language macros that can be invoked elsewhere in the file, or write entire functions in assembly language. Basic asm statements outside of functions may not use any qualifiers. * Functions declared with the naked attribute also require basic asm (see Function Attributes). Safely accessing C data and calling functions from basic asm is more complex than it may appear. To access C data, it is better to use extended asm. Do not expect a sequence of asm statements to remain perfectly consecutive after compilation. If certain instructions need to remain consecutive in the output, put them in a single multi-instruction asm statement. Note that GCC's optimizers can move asm statements relative to other code, including across jumps. asm statements may not perform jumps into other asm statements. GCC does not know about these jumps, and therefore cannot take account of them when deciding how to optimize. Jumps from asm to C labels are only supported in extended asm. Under certain circumstances, GCC may duplicate (or remove duplicates of) your assembly code when optimizing. This can lead to unexpected duplicate symbol errors during compilation if your assembly code defines symbols or labels. Warning: The C standards do not specify semantics for asm, making it a potential source of incompatibilities between compilers. These incompatibilities may not produce compiler warnings/errors. GCC does not parse basic asm's AssemblerInstructions, which means there is no way to communicate to the compiler what is happening inside them. GCC has no visibility of symbols in the asm and may discard them as unreferenced. It also does not know about side effects of the assembler code, such as modifications to memory or registers. Unlike some compilers, GCC assumes that no changes to general purpose registers occur. This assumption may change in a future release. To avoid complications from future changes to the semantics and the compatibility issues between compilers, consider replacing basic asm with extended asm. See How to convert from basic asm to extended asm for information about how to perform this conversion. The compiler copies the assembler instructions in a basic asm verbatim to the assembly language output file, without processing dialects or any of the '%' operators that are available with extended asm. This results in minor differences between basic asm strings and extended asm templates. For example, to refer to registers you might use '%eax' in basic asm and '%%eax' in extended asm. On targets such as x86 that support multiple assembler dialects, all basic asm blocks use the assembler dialect specified by the -masm command-line option (see x86 Options). Basic asm provides no mechanism to provide different assembler strings for different dialects. For basic asm with non-empty assembler string GCC assumes the assembler block does not change any general purpose registers, but it may read or write any globally accessible variable. Here is an example of basic asm for i386: /* Note that this code will not compile with -masm=intel */ #define DebugBreak() asm("int $3") ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Using Assembly Language with C Link: next: Constraints Link: prev: Basic Asm Next: Constraints, Previous: Basic Asm, Up: Using Assembly Language with C ────────────────────────────────────────────────────────────────────────────── 6.47.2 Extended Asm - Assembler Instructions with C Expression Operands With extended asm you can read and write C variables from assembler and perform jumps from assembler code to C labels. Extended asm syntax uses colons (':') to delimit the operand parameters after the assembler template: asm asm-qualifiers ( AssemblerTemplate : OutputOperands [ : InputOperands [ : Clobbers ] ]) asm asm-qualifiers ( AssemblerTemplate : : InputOperands : Clobbers : GotoLabels) where in the last form, asm-qualifiers contains goto (and in the first form, not). The asm keyword is a GNU extension. When writing code that can be compiled with -ansi and the various -std options, use __asm__ instead of asm (see Alternate Keywords). Qualifiers volatile The typical use of extended asm statements is to manipulate input values to produce output values. However, your asm statements may also produce side effects. If so, you may need to use the volatile qualifier to disable certain optimizations. See Volatile. inline If you use the inline qualifier, then for inlining purposes the size of the asm statement is taken as the smallest size possible (see Size of an asm). goto This qualifier informs the compiler that the asm statement may perform a jump to one of the labels listed in the GotoLabels. See GotoLabels. Parameters AssemblerTemplate This is a literal string that is the template for the assembler code. It is a combination of fixed text and tokens that refer to the input, output, and goto parameters. See AssemblerTemplate. OutputOperands A comma-separated list of the C variables modified by the instructions in the AssemblerTemplate. An empty list is permitted. See OutputOperands. InputOperands A comma-separated list of C expressions read by the instructions in the AssemblerTemplate. An empty list is permitted. See InputOperands. Clobbers A comma-separated list of registers or other values changed by the AssemblerTemplate, beyond those listed as outputs. An empty list is permitted. See Clobbers and Scratch Registers. GotoLabels When you are using the goto form of asm, this section contains the list of all C labels to which the code in the AssemblerTemplate may jump. See GotoLabels. asm statements may not perform jumps into other asm statements, only to the listed GotoLabels. GCC's optimizers do not know about other jumps; therefore they cannot take account of them when deciding how to optimize. The total number of input + output + goto operands is limited to 30. Remarks The asm statement allows you to include assembly instructions directly within C code. This may help you to maximize performance in time-sensitive code or to access assembly instructions that are not readily available to C programs. Note that extended asm statements must be inside a function. Only basic asm may be outside functions (see Basic Asm). Functions declared with the naked attribute also require basic asm (see Function Attributes). While the uses of asm are many and varied, it may help to think of an asm statement as a series of low-level instructions that convert input parameters to output parameters. So a simple (if not particularly useful) example for i386 using asm might look like this: int src = 1; int dst; asm ("mov %1, %0\n\t" "add $1, %0" : "=r" (dst) : "r" (src)); printf("%d\n", dst); This code copies src to dst and add 1 to dst. 6.47.2.1 Volatile GCC's optimizers sometimes discard asm statements if they determine there is no need for the output variables. Also, the optimizers may move code out of loops if they believe that the code will always return the same result (i.e. none of its input values change between calls). Using the volatile qualifier disables these optimizations. asm statements that have no output operands, including asm goto statements, are implicitly volatile. This i386 code demonstrates a case that does not use (or require) the volatile qualifier. If it is performing assertion checking, this code uses asm to perform the validation. Otherwise, dwRes is unreferenced by any code. As a result, the optimizers can discard the asm statement, which in turn removes the need for the entire DoCheck routine. By omitting the volatile qualifier when it isn't needed you allow the optimizers to produce the most efficient code possible. void DoCheck(uint32_t dwSomeValue) { uint32_t dwRes; // Assumes dwSomeValue is not zero. asm ("bsfl %1,%0" : "=r" (dwRes) : "r" (dwSomeValue) : "cc"); assert(dwRes > 3); } The next example shows a case where the optimizers can recognize that the input (dwSomeValue) never changes during the execution of the function and can therefore move the asm outside the loop to produce more efficient code. Again, using the volatile qualifier disables this type of optimization. void do_print(uint32_t dwSomeValue) { uint32_t dwRes; for (uint32_t x=0; x < 5; x++) { // Assumes dwSomeValue is not zero. asm ("bsfl %1,%0" : "=r" (dwRes) : "r" (dwSomeValue) : "cc"); printf("%u: %u %u\n", x, dwSomeValue, dwRes); } } The following example demonstrates a case where you need to use the volatile qualifier. It uses the x86 rdtsc instruction, which reads the computer's time-stamp counter. Without the volatile qualifier, the optimizers might assume that the asm block will always return the same value and therefore optimize away the second call. uint64_t msr; asm volatile ( "rdtsc\n\t" // Returns the time in EDX:EAX. "shl $32, %%rdx\n\t" // Shift the upper bits left. "or %%rdx, %0" // 'Or' in the lower bits. : "=a" (msr) : : "rdx"); printf("msr: %llx\n", msr); // Do other work... // Reprint the timestamp asm volatile ( "rdtsc\n\t" // Returns the time in EDX:EAX. "shl $32, %%rdx\n\t" // Shift the upper bits left. "or %%rdx, %0" // 'Or' in the lower bits. : "=a" (msr) : : "rdx"); printf("msr: %llx\n", msr); GCC's optimizers do not treat this code like the non-volatile code in the earlier examples. They do not move it out of loops or omit it on the assumption that the result from a previous call is still valid. Note that the compiler can move even volatile asm instructions relative to other code, including across jump instructions. For example, on many targets there is a system register that controls the rounding mode of floating-point operations. Setting it with a volatile asm statement, as in the following PowerPC example, does not work reliably. asm volatile("mtfsf 255, %0" : : "f" (fpenv)); sum = x + y; The compiler may move the addition back before the volatile asm statement. To make it work as expected, add an artificial dependency to the asm by referencing a variable in the subsequent code, for example: asm volatile ("mtfsf 255,%1" : "=X" (sum) : "f" (fpenv)); sum = x + y; Under certain circumstances, GCC may duplicate (or remove duplicates of) your assembly code when optimizing. This can lead to unexpected duplicate symbol errors during compilation if your asm code defines symbols or labels. Using '%=' (see AssemblerTemplate) may help resolve this problem. 6.47.2.2 Assembler Template An assembler template is a literal string containing assembler instructions. The compiler replaces tokens in the template that refer to inputs, outputs, and goto labels, and then outputs the resulting string to the assembler. The string can contain any instructions recognized by the assembler, including directives. GCC does not parse the assembler instructions themselves and does not know what they mean or even whether they are valid assembler input. However, it does count the statements (see Size of an asm). You may place multiple assembler instructions together in a single asm string, separated by the characters normally used in assembly code for the system. A combination that works in most places is a newline to break the line, plus a tab character to move to the instruction field (written as '\n\t'). Some assemblers allow semicolons as a line separator. However, note that some assembler dialects use semicolons to start a comment. Do not expect a sequence of asm statements to remain perfectly consecutive after compilation, even when you are using the volatile qualifier. If certain instructions need to remain consecutive in the output, put them in a single multi-instruction asm statement. Accessing data from C programs without using input/output operands (such as by using global symbols directly from the assembler template) may not work as expected. Similarly, calling functions directly from an assembler template requires a detailed understanding of the target assembler and ABI. Since GCC does not parse the assembler template, it has no visibility of any symbols it references. This may result in GCC discarding those symbols as unreferenced unless they are also listed as input, output, or goto operands. Special format strings In addition to the tokens described by the input, output, and goto operands, these tokens have special meanings in the assembler template: '%%' Outputs a single '%' into the assembler code. '%=' Outputs a number that is unique to each instance of the asm statement in the entire compilation. This option is useful when creating local labels and referring to them multiple times in a single template that generates multiple assembler instructions. '%{' '%|' '%}' Outputs '{', '|', and '}' characters (respectively) into the assembler code. When unescaped, these characters have special meaning to indicate multiple assembler dialects, as described below. Multiple assembler dialects in asm templates On targets such as x86, GCC supports multiple assembler dialects. The -masm option controls which dialect GCC uses as its default for inline assembler. The target-specific documentation for the -masm option contains the list of supported dialects, as well as the default dialect if the option is not specified. This information may be important to understand, since assembler code that works correctly when compiled using one dialect will likely fail if compiled using another. See x86 Options. If your code needs to support multiple assembler dialects (for example, if you are writing public headers that need to support a variety of compilation options), use constructs of this form: { dialect0 | dialect1 | dialect2... } This construct outputs dialect0 when using dialect #0 to compile the code, dialect1 for dialect #1, etc. If there are fewer alternatives within the braces than the number of dialects the compiler supports, the construct outputs nothing. For example, if an x86 compiler supports two dialects ('att', 'intel'), an assembler template such as this: "bt{l %[Offset],%[Base] | %[Base],%[Offset]}; jc %l2" is equivalent to one of "btl %[Offset],%[Base] ; jc %l2" /* att dialect */ "bt %[Base],%[Offset]; jc %l2" /* intel dialect */ Using that same compiler, this code: "xchg{l}\t{%%}ebx, %1" corresponds to either "xchgl\t%%ebx, %1" /* att dialect */ "xchg\tebx, %1" /* intel dialect */ There is no support for nesting dialect alternatives. 6.47.2.3 Output Operands An asm statement has zero or more output operands indicating the names of C variables modified by the assembler code. In this i386 example, old (referred to in the template string as %0) and *Base (as %1) are outputs and Offset (%2) is an input: bool old; __asm__ ("btsl %2,%1\n\t" // Turn on zero-based bit #Offset in Base. "sbb %0,%0" // Use the CF to calculate old. : "=r" (old), "+rm" (*Base) : "Ir" (Offset) : "cc"); return old; Operands are separated by commas. Each operand has this format: [ [asmSymbolicName] ] constraint (cvariablename) asmSymbolicName Specifies a symbolic name for the operand. Reference the name in the assembler template by enclosing it in square brackets (i.e. '%[Value]'). The scope of the name is the asm statement that contains the definition. Any valid C variable name is acceptable, including names already defined in the surrounding code. No two operands within the same asm statement can use the same symbolic name. When not using an asmSymbolicName, use the (zero-based) position of the operand in the list of operands in the assembler template. For example if there are three output operands, use '%0' in the template to refer to the first, '%1' for the second, and '%2' for the third. constraint A string constant specifying constraints on the placement of the operand; See Constraints, for details. Output constraints must begin with either '=' (a variable overwriting an existing value) or '+' (when reading and writing). When using '=', do not assume the location contains the existing value on entry to the asm, except when the operand is tied to an input; see Input Operands. After the prefix, there must be one or more additional constraints (see Constraints) that describe where the value resides. Common constraints include 'r' for register and 'm' for memory. When you list more than one possible location (for example, "=rm"), the compiler chooses the most efficient one based on the current context. If you list as many alternates as the asm statement allows, you permit the optimizers to produce the best possible code. If you must use a specific register, but your Machine Constraints do not provide sufficient control to select the specific register you want, local register variables may provide a solution (see Local Register Variables). cvariablename Specifies a C lvalue expression to hold the output, typically a variable name. The enclosing parentheses are a required part of the syntax. When the compiler selects the registers to use to represent the output operands, it does not use any of the clobbered registers (see Clobbers and Scratch Registers). Output operand expressions must be lvalues. The compiler cannot check whether the operands have data types that are reasonable for the instruction being executed. For output expressions that are not directly addressable (for example a bit-field), the constraint must allow a register. In that case, GCC uses the register as the output of the asm, and then stores that register into the output. Operands using the '+' constraint modifier count as two operands (that is, both as input and output) towards the total maximum of 30 operands per asm statement. Use the '&' constraint modifier (see Modifiers) on all output operands that must not overlap an input. Otherwise, GCC may allocate the output operand in the same register as an unrelated input operand, on the assumption that the assembler code consumes its inputs before producing outputs. This assumption may be false if the assembler code actually consists of more than one instruction. The same problem can occur if one output parameter (a) allows a register constraint and another output parameter (b) allows a memory constraint. The code generated by GCC to access the memory address in b can contain registers which might be shared by a, and GCC considers those registers to be inputs to the asm. As above, GCC assumes that such input registers are consumed before any outputs are written. This assumption may result in incorrect behavior if the asm statement writes to a before using b. Combining the '&' modifier with the register constraint on a ensures that modifying a does not affect the address referenced by b. Otherwise, the location of b is undefined if a is modified before using b. asm supports operand modifiers on operands (for example '%k2' instead of simply '%2'). Typically these qualifiers are hardware dependent. The list of supported modifiers for x86 is found at x86 Operand modifiers. If the C code that follows the asm makes no use of any of the output operands, use volatile for the asm statement to prevent the optimizers from discarding the asm statement as unneeded (see Volatile). This code makes no use of the optional asmSymbolicName. Therefore it references the first output operand as %0 (were there a second, it would be %1, etc). The number of the first input operand is one greater than that of the last output operand. In this i386 example, that makes Mask referenced as %1: uint32_t Mask = 1234; uint32_t Index; asm ("bsfl %1, %0" : "=r" (Index) : "r" (Mask) : "cc"); That code overwrites the variable Index ('='), placing the value in a register ('r'). Using the generic 'r' constraint instead of a constraint for a specific register allows the compiler to pick the register to use, which can result in more efficient code. This may not be possible if an assembler instruction requires a specific register. The following i386 example uses the asmSymbolicName syntax. It produces the same result as the code above, but some may consider it more readable or more maintainable since reordering index numbers is not necessary when adding or removing operands. The names aIndex and aMask are only used in this example to emphasize which names get used where. It is acceptable to reuse the names Index and Mask. uint32_t Mask = 1234; uint32_t Index; asm ("bsfl %[aMask], %[aIndex]" : [aIndex] "=r" (Index) : [aMask] "r" (Mask) : "cc"); Here are some more examples of output operands. uint32_t c = 1; uint32_t d; uint32_t *e = &c; asm ("mov %[e], %[d]" : [d] "=rm" (d) : [e] "rm" (*e)); Here, d may either be in a register or in memory. Since the compiler might already have the current value of the uint32_t location pointed to by e in a register, you can enable it to choose the best location for d by specifying both constraints. 6.47.2.4 Flag Output Operands Some targets have a special register that holds the “flags” for the result of an operation or comparison. Normally, the contents of that register are either unmodifed by the asm, or the asm statement is considered to clobber the contents. On some targets, a special form of output operand exists by which conditions in the flags register may be outputs of the asm. The set of conditions supported are target specific, but the general rule is that the output variable must be a scalar integer, and the value is boolean. When supported, the target defines the preprocessor symbol __GCC_ASM_FLAG_OUTPUTS__. Because of the special nature of the flag output operands, the constraint may not include alternatives. Most often, the target has only one flags register, and thus is an implied operand of many instructions. In this case, the operand should not be referenced within the assembler template via %0 etc, as there's no corresponding text in the assembly language. x86 family The flag output constraints for the x86 family are of the form '=@cccond' where cond is one of the standard conditions defined in the ISA manual for jcc or setcc. a “above” or unsigned greater than ae “above or equal” or unsigned greater than or equal b “below” or unsigned less than be “below or equal” or unsigned less than or equal c carry flag set e z “equal” or zero flag set g signed greater than ge signed greater than or equal l signed less than le signed less than or equal o overflow flag set p parity flag set s sign flag set na nae nb nbe nc ne ng nge nl nle no np ns nz “not” flag, or inverted versions of those above 6.47.2.5 Input Operands Input operands make values from C variables and expressions available to the assembly code. Operands are separated by commas. Each operand has this format: [ [asmSymbolicName] ] constraint (cexpression) asmSymbolicName Specifies a symbolic name for the operand. Reference the name in the assembler template by enclosing it in square brackets (i.e. '%[Value]'). The scope of the name is the asm statement that contains the definition. Any valid C variable name is acceptable, including names already defined in the surrounding code. No two operands within the same asm statement can use the same symbolic name. When not using an asmSymbolicName, use the (zero-based) position of the operand in the list of operands in the assembler template. For example if there are two output operands and three inputs, use '%2' in the template to refer to the first input operand, '%3' for the second, and '%4' for the third. constraint A string constant specifying constraints on the placement of the operand; See Constraints, for details. Input constraint strings may not begin with either '=' or '+'. When you list more than one possible location (for example, '"irm"'), the compiler chooses the most efficient one based on the current context. If you must use a specific register, but your Machine Constraints do not provide sufficient control to select the specific register you want, local register variables may provide a solution (see Local Register Variables). Input constraints can also be digits (for example, "0"). This indicates that the specified input must be in the same place as the output constraint at the (zero-based) index in the output constraint list. When using asmSymbolicName syntax for the output operands, you may use these names (enclosed in brackets '[]') instead of digits. cexpression This is the C variable or expression being passed to the asm statement as input. The enclosing parentheses are a required part of the syntax. When the compiler selects the registers to use to represent the input operands, it does not use any of the clobbered registers (see Clobbers and Scratch Registers). If there are no output operands but there are input operands, place two consecutive colons where the output operands would go: __asm__ ("some instructions" : /* No outputs. */ : "r" (Offset / 8)); Warning: Do not modify the contents of input-only operands (except for inputs tied to outputs). The compiler assumes that on exit from the asm statement these operands contain the same values as they had before executing the statement. It is not possible to use clobbers to inform the compiler that the values in these inputs are changing. One common work-around is to tie the changing input variable to an output variable that never gets used. Note, however, that if the code that follows the asm statement makes no use of any of the output operands, the GCC optimizers may discard the asm statement as unneeded (see Volatile). asm supports operand modifiers on operands (for example '%k2' instead of simply '%2'). Typically these qualifiers are hardware dependent. The list of supported modifiers for x86 is found at x86 Operand modifiers. In this example using the fictitious combine instruction, the constraint "0" for input operand 1 says that it must occupy the same location as output operand 0. Only input operands may use numbers in constraints, and they must each refer to an output operand. Only a number (or the symbolic assembler name) in the constraint can guarantee that one operand is in the same place as another. The mere fact that foo is the value of both operands is not enough to guarantee that they are in the same place in the generated assembler code. asm ("combine %2, %0" : "=r" (foo) : "0" (foo), "g" (bar)); Here is an example using symbolic names. asm ("cmoveq %1, %2, %[result]" : [result] "=r"(result) : "r" (test), "r" (new), "[result]" (old)); 6.47.2.6 Clobbers and Scratch Registers While the compiler is aware of changes to entries listed in the output operands, the inline asm code may modify more than just the outputs. For example, calculations may require additional registers, or the processor may overwrite a register as a side effect of a particular assembler instruction. In order to inform the compiler of these changes, list them in the clobber list. Clobber list items are either register names or the special clobbers (listed below). Each clobber list item is a string constant enclosed in double quotes and separated by commas. Clobber descriptions may not in any way overlap with an input or output operand. For example, you may not have an operand describing a register class with one member when listing that register in the clobber list. Variables declared to live in specific registers (see Explicit Register Variables) and used as asm input or output operands must have no part mentioned in the clobber description. In particular, there is no way to specify that input operands get modified without also specifying them as output operands. When the compiler selects which registers to use to represent input and output operands, it does not use any of the clobbered registers. As a result, clobbered registers are available for any use in the assembler code. Another restriction is that the clobber list should not contain the stack pointer register. This is because the compiler requires the value of the stack pointer to be the same after an asm statement as it was on entry to the statement. However, previous versions of GCC did not enforce this rule and allowed the stack pointer to appear in the list, with unclear semantics. This behavior is deprecated and listing the stack pointer may become an error in future versions of GCC. Here is a realistic example for the VAX showing the use of clobbered registers: asm volatile ("movc3 %0, %1, %2" : /* No outputs. */ : "g" (from), "g" (to), "g" (count) : "r0", "r1", "r2", "r3", "r4", "r5", "memory"); Also, there are two special clobber arguments: "cc" The "cc" clobber indicates that the assembler code modifies the flags register. On some machines, GCC represents the condition codes as a specific hardware register; "cc" serves to name this register. On other machines, condition code handling is different, and specifying "cc" has no effect. But it is valid no matter what the target. "memory" The "memory" clobber tells the compiler that the assembly code performs memory reads or writes to items other than those listed in the input and output operands (for example, accessing the memory pointed to by one of the input parameters). To ensure memory contains correct values, GCC may need to flush specific register values to memory before executing the asm. Further, the compiler does not assume that any values read from memory before an asm remain unchanged after that asm; it reloads them as needed. Using the "memory" clobber effectively forms a read/write memory barrier for the compiler. Note that this clobber does not prevent the processor from doing speculative reads past the asm statement. To prevent that, you need processor-specific fence instructions. Flushing registers to memory has performance implications and may be an issue for time-sensitive code. You can provide better information to GCC to avoid this, as shown in the following examples. At a minimum, aliasing rules allow GCC to know what memory doesn't need to be flushed. Here is a fictitious sum of squares instruction, that takes two pointers to floating point values in memory and produces a floating point register output. Notice that x, and y both appear twice in the asm parameters, once to specify memory accessed, and once to specify a base register used by the asm. You won't normally be wasting a register by doing this as GCC can use the same register for both purposes. However, it would be foolish to use both %1 and %3 for x in this asm and expect them to be the same. In fact, %3 may well not be a register. It might be a symbolic memory reference to the object pointed to by x. asm ("sumsq %0, %1, %2" : "+f" (result) : "r" (x), "r" (y), "m" (*x), "m" (*y)); Here is a fictitious *z++ = *x++ * *y++ instruction. Notice that the x, y and z pointer registers must be specified as input/output because the asm modifies them. asm ("vecmul %0, %1, %2" : "+r" (z), "+r" (x), "+r" (y), "=m" (*z) : "m" (*x), "m" (*y)); An x86 example where the string memory argument is of unknown length. asm("repne scasb" : "=c" (count), "+D" (p) : "m" (*(const char (*)[]) p), "0" (-1), "a" (0)); If you know the above will only be reading a ten byte array then you could instead use a memory input like: "m" (*(const char (*)[10]) p). Here is an example of a PowerPC vector scale implemented in assembly, complete with vector and condition code clobbers, and some initialized offset registers that are unchanged by the asm. void dscal (size_t n, double *x, double alpha) { asm ("/* lots of asm here */" : "+m" (*(double (*)[n]) x), "+&r" (n), "+b" (x) : "d" (alpha), "b" (32), "b" (48), "b" (64), "b" (80), "b" (96), "b" (112) : "cr0", "vs32","vs33","vs34","vs35","vs36","vs37","vs38","vs39", "vs40","vs41","vs42","vs43","vs44","vs45","vs46","vs47"); } Rather than allocating fixed registers via clobbers to provide scratch registers for an asm statement, an alternative is to define a variable and make it an early-clobber output as with a2 and a3 in the example below. This gives the compiler register allocator more freedom. You can also define a variable and make it an output tied to an input as with a0 and a1, tied respectively to ap and lda. Of course, with tied outputs your asm can't use the input value after modifying the output register since they are one and the same register. What's more, if you omit the early-clobber on the output, it is possible that GCC might allocate the same register to another of the inputs if GCC could prove they had the same value on entry to the asm. This is why a1 has an early-clobber. Its tied input, lda might conceivably be known to have the value 16 and without an early-clobber share the same register as %11. On the other hand, ap can't be the same as any of the other inputs, so an early-clobber on a0 is not needed. It is also not desirable in this case. An early-clobber on a0 would cause GCC to allocate a separate register for the "m" (*(const double (*)[]) ap) input. Note that tying an input to an output is the way to set up an initialized temporary register modified by an asm statement. An input not tied to an output is assumed by GCC to be unchanged, for example "b" (16) below sets up %11 to 16, and GCC might use that register in following code if the value 16 happened to be needed. You can even use a normal asm output for a scratch if all inputs that might share the same register are consumed before the scratch is used. The VSX registers clobbered by the asm statement could have used this technique except for GCC's limit on the number of asm parameters. static void dgemv_kernel_4x4 (long n, const double *ap, long lda, const double *x, double *y, double alpha) { double *a0; double *a1; double *a2; double *a3; __asm__ ( /* lots of asm here */ "#n=%1 ap=%8=%12 lda=%13 x=%7=%10 y=%0=%2 alpha=%9 o16=%11\n" "#a0=%3 a1=%4 a2=%5 a3=%6" : "+m" (*(double (*)[n]) y), "+&r" (n), // 1 "+b" (y), // 2 "=b" (a0), // 3 "=&b" (a1), // 4 "=&b" (a2), // 5 "=&b" (a3) // 6 : "m" (*(const double (*)[n]) x), "m" (*(const double (*)[]) ap), "d" (alpha), // 9 "r" (x), // 10 "b" (16), // 11 "3" (ap), // 12 "4" (lda) // 13 : "cr0", "vs32","vs33","vs34","vs35","vs36","vs37", "vs40","vs41","vs42","vs43","vs44","vs45","vs46","vs47" ); } 6.47.2.7 Goto Labels asm goto allows assembly code to jump to one or more C labels. The GotoLabels section in an asm goto statement contains a comma-separated list of all C labels to which the assembler code may jump. GCC assumes that asm execution falls through to the next statement (if this is not the case, consider using the __builtin_unreachable intrinsic after the asm statement). Optimization of asm goto may be improved by using the hot and cold label attributes (see Label Attributes). An asm goto statement cannot have outputs. This is due to an internal restriction of the compiler: control transfer instructions cannot have outputs. If the assembler code does modify anything, use the "memory" clobber to force the optimizers to flush all register values to memory and reload them if necessary after the asm statement. Also note that an asm goto statement is always implicitly considered volatile. To reference a label in the assembler template, prefix it with '%l' (lowercase 'L') followed by its (zero-based) position in GotoLabels plus the number of input operands. For example, if the asm has three inputs and references two labels, refer to the first label as '%l3' and the second as '%l4'). Alternately, you can reference labels using the actual C label name enclosed in brackets. For example, to reference a label named carry, you can use '%l[carry]'. The label must still be listed in the GotoLabels section when using this approach. Here is an example of asm goto for i386: asm goto ( "btl %1, %0\n\t" "jc %l2" : /* No outputs. */ : "r" (p1), "r" (p2) : "cc" : carry); return 0; carry: return 1; The following example shows an asm goto that uses a memory clobber. int frob(int x) { int y; asm goto ("frob %%r5, %1; jc %l[error]; mov (%2), %%r5" : /* No outputs. */ : "r"(x), "r"(&y) : "r5", "memory" : error); return y; error: return -1; } 6.47.2.8 x86 Operand Modifiers References to input, output, and goto operands in the assembler template of extended asm statements can use modifiers to affect the way the operands are formatted in the code output to the assembler. For example, the following code uses the 'h' and 'b' modifiers for x86: uint16_t num; asm volatile ("xchg %h0, %b0" : "+a" (num) ); These modifiers generate this assembler code: xchg %ah, %al The rest of this discussion uses the following code for illustrative purposes. int main() { int iInt = 1; top: asm volatile goto ("some assembler instructions here" : /* No outputs. */ : "q" (iInt), "X" (sizeof(unsigned char) + 1), "i" (42) : /* No clobbers. */ : top); } With no modifiers, this is what the output from the operands would be for the 'att' and 'intel' dialects of assembler: Operand 'att' 'intel' %0 %eax eax %1 $2 2 %3 $.L3 OFFSET FLAT:.L3 The table below shows the list of supported modifiers and their effects. Modifier Description Operand 'att' 'intel' a Print an absolute memory reference. %A0 *%rax rax b Print the QImode name of the register. %b0 %al al c Require a constant operand and print the %c1 2 2 constant expression with no punctuation. Print the address in Double Integer E (DImode) mode (8 bytes) when the target %E1 %(rax) [rax] is 64-bit. Otherwise mode is unspecified (VOIDmode). h Print the QImode name for a “high” %h0 %ah ah register. Add 8 bytes to an offsettable memory H reference. Useful when accessing the high %H0 8(%rax) 8[rax] 8 bytes of SSE values. For a memref in (%rax), it generates k Print the SImode name of the register. %k0 %eax eax l Print the label name with no punctuation. %l3 .L3 .L3 p Print raw symbol name (without %p2 42 42 syntax-specific prefixes). If used for a function, print the PLT suffix and generate PIC code. For example, emit foo@PLT instead of 'foo' P for the function foo(). If used for a constant, drop all syntax-specific prefixes and issue the bare constant. See p above. q Print the DImode name of the register. %q0 %rax rax w Print the HImode name of the register. %w0 %ax ax Print the opcode suffix for the size of z the current integer operand (one of %z0 l b/w/l/q). V is a special modifier which prints the name of the full integer register without %. 6.47.2.9 x86 Floating-Point asm Operands On x86 targets, there are several rules on the usage of stack-like registers in the operands of an asm. These rules apply only to the operands that are stack-like registers: 1. Given a set of input registers that die in an asm, it is necessary to know which are implicitly popped by the asm, and which must be explicitly popped by GCC. An input register that is implicitly popped by the asm must be explicitly clobbered, unless it is constrained to match an output operand. 2. For any input register that is implicitly popped by an asm, it is necessary to know how to adjust the stack to compensate for the pop. If any non-popped input is closer to the top of the reg-stack than the implicitly popped register, it would not be possible to know what the stack looked like—it's not clear how the rest of the stack “slides up”. All implicitly popped input registers must be closer to the top of the reg-stack than any input that is not implicitly popped. It is possible that if an input dies in an asm, the compiler might use the input register for an output reload. Consider this example: asm ("foo" : "=t" (a) : "f" (b)); This code says that input b is not popped by the asm, and that the asm pushes a result onto the reg-stack, i.e., the stack is one deeper after the asm than it was before. But, it is possible that reload may think that it can use the same register for both the input and the output. To prevent this from happening, if any input operand uses the 'f' constraint, all output register constraints must use the '&' early-clobber modifier. The example above is correctly written as: asm ("foo" : "=&t" (a) : "f" (b)); 3. Some operands need to be in particular places on the stack. All output operands fall in this category—GCC has no other way to know which registers the outputs appear in unless you indicate this in the constraints. Output operands must specifically indicate which register an output appears in after an asm. '=f' is not allowed: the operand constraints must select a class with a single register. 4. Output operands may not be “inserted” between existing stack registers. Since no 387 opcode uses a read/write operand, all output operands are dead before the asm, and are pushed by the asm. It makes no sense to push anywhere but the top of the reg-stack. Output operands must start at the top of the reg-stack: output operands may not “skip” a register. 5. Some asm statements may need extra stack space for internal calculations. This can be guaranteed by clobbering stack registers unrelated to the inputs and outputs. This asm takes one input, which is internally popped, and produces two outputs. asm ("fsincos" : "=t" (cos), "=u" (sin) : "0" (inp)); This asm takes two inputs, which are popped by the fyl2xp1 opcode, and replaces them with one output. The st(1) clobber is necessary for the compiler to know that fyl2xp1 pops both inputs. asm ("fyl2xp1" : "=t" (result) : "0" (x), "u" (y) : "st(1)"); ────────────────────────────────────────────────────────────────────────────── Next: Constraints, Previous: Basic Asm, Up: Using Assembly Language with C Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Using Assembly Language with C Link: next: Simple Constraints Link: prev: Extended Asm Next: Asm Labels, Previous: Extended Asm, Up: Using Assembly Language with ────────────────────────────────────────────────────────────────────────────── 6.47.3 Constraints for asm Operands Here are specific details on what constraint letters you can use with asm operands. Constraints can say whether an operand may be in a register, and which kinds of register; whether the operand can be a memory reference, and which kinds of address; whether the operand may be an immediate constant, and which possible values it may have. Constraints can also require two operands to match. Side-effects aren't allowed in operands of inline asm, unless '<' or '>' constraints are used, because there is no guarantee that the side effects will happen exactly once in an instruction that can update the addressing register. • Simple Constraints: Basic use of constraints. • Multi-Alternative: When an insn has two alternative constraint-patterns. • Modifiers: More precise control over effects of constraints. • Machine Constraints: Special constraints for some particular machines. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Constraints Link: next: Multi-Alternative Link: prev: Constraints ────────────────────────────────────────────────────────────────────────────── 6.47.3.1 Simple Constraints The simplest kind of constraint is a string full of letters, each of which describes one kind of operand that is permitted. Here are the letters that are allowed: whitespace Whitespace characters are ignored and can be inserted at any position except the first. This enables each alternative for different operands to be visually aligned in the machine description even if they have different number of constraints and modifiers. 'm' A memory operand is allowed, with any kind of address that the machine supports in general. Note that the letter used for the general memory constraint can be re-defined by a back end using the TARGET_MEM_CONSTRAINT macro. 'o' A memory operand is allowed, but only if the address is offsettable. This means that adding a small integer (actually, the width in bytes of the operand, as determined by its machine mode) may be added to the address and the result is also a valid memory address. For example, an address which is constant is offsettable; so is an address that is the sum of a register and a constant (as long as a slightly larger constant is also within the range of address-offsets supported by the machine); but an autoincrement or autodecrement address is not offsettable. More complicated indirect/indexed addresses may or may not be offsettable depending on the other addressing modes that the machine supports. Note that in an output operand which can be matched by another operand, the constraint letter 'o' is valid only when accompanied by both '<' (if the target machine has predecrement addressing) and '>' (if the target machine has preincrement addressing). 'V' A memory operand that is not offsettable. In other words, anything that would fit the 'm' constraint but not the 'o' constraint. '<' A memory operand with autodecrement addressing (either predecrement or postdecrement) is allowed. In inline asm this constraint is only allowed if the operand is used exactly once in an instruction that can handle the side effects. Not using an operand with '<' in constraint string in the inline asm pattern at all or using it in multiple instructions isn't valid, because the side effects wouldn't be performed or would be performed more than once. Furthermore, on some targets the operand with '<' in constraint string must be accompanied by special instruction suffixes like %U0 instruction suffix on PowerPC or %P0 on IA-64. '>' A memory operand with autoincrement addressing (either preincrement or postincrement) is allowed. In inline asm the same restrictions as for '<' apply. 'r' A register operand is allowed provided that it is in a general register. 'i' An immediate integer operand (one with constant value) is allowed. This includes symbolic constants whose values will be known only at assembly time or later. 'n' An immediate integer operand with a known numeric value is allowed. Many systems cannot support assembly-time constants for operands less than a word wide. Constraints for these operands should use 'n' rather than 'i'. 'I', 'J', 'K', … 'P' Other letters in the range 'I' through 'P' may be defined in a machine-dependent fashion to permit immediate integer operands with explicit integer values in specified ranges. For example, on the 68000, 'I' is defined to stand for the range of values 1 to 8. This is the range permitted as a shift count in the shift instructions. 'E' An immediate floating operand (expression code const_double) is allowed, but only if the target floating point format is the same as that of the host machine (on which the compiler is running). 'F' An immediate floating operand (expression code const_double or const_vector) is allowed. 'G', 'H' 'G' and 'H' may be defined in a machine-dependent fashion to permit immediate floating operands in particular ranges of values. 's' An immediate integer operand whose value is not an explicit integer is allowed. This might appear strange; if an insn allows a constant operand with a value not known at compile time, it certainly must allow any known value. So why use 's' instead of 'i'? Sometimes it allows better code to be generated. For example, on the 68000 in a fullword instruction it is possible to use an immediate operand; but if the immediate value is between -128 and 127, better code results from loading the value into a register and using the register. This is because the load into the register can be done with a 'moveq' instruction. We arrange for this to happen by defining the letter 'K' to mean “any integer outside the range -128 to 127”, and then specifying 'Ks' in the operand constraints. 'g' Any register, memory or immediate integer operand is allowed, except for registers that are not general registers. 'X' Any operand whatsoever is allowed. '0', '1', '2', … '9' An operand that matches the specified operand number is allowed. If a digit is used together with letters within the same alternative, the digit should come last. This number is allowed to be more than a single digit. If multiple digits are encountered consecutively, they are interpreted as a single decimal integer. There is scant chance for ambiguity, since to-date it has never been desirable that '10' be interpreted as matching either operand 1 or operand 0. Should this be desired, one can use multiple alternatives instead. This is called a matching constraint and what it really means is that the assembler has only a single operand that fills two roles which asm distinguishes. For example, an add instruction uses two input operands and an output operand, but on most CISC machines an add instruction really has only two operands, one of them an input-output operand: addl #35,r12 Matching constraints are used in these circumstances. More precisely, the two operands that match must include one input-only operand and one output-only operand. Moreover, the digit must be a smaller number than the number of the operand that uses it in the constraint. 'p' An operand that is a valid memory address is allowed. This is for “load address” and “push address” instructions. 'p' in the constraint must be accompanied by address_operand as the predicate in the match_operand. This predicate interprets the mode specified in the match_operand as the mode of the memory reference for which the address would be valid. other-letters Other letters can be defined in machine-dependent fashion to stand for particular classes of registers or other arbitrary operand types. 'd', 'a' and 'f' are defined on the 68000/68020 to stand for data, address and floating point registers. ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Constraints Link: next: Modifiers Link: prev: Simple Constraints Next: Modifiers, Previous: Simple Constraints, Up: Constraints ────────────────────────────────────────────────────────────────────────────── 6.47.3.2 Multiple Alternative Constraints Sometimes a single instruction has multiple alternative sets of possible operands. For example, on the 68000, a logical-or instruction can combine register or an immediate value into memory, or it can combine any kind of operand into a register; but it cannot combine one memory location into another. These constraints are represented as multiple alternatives. An alternative can be described by a series of letters for each operand. The overall constraint for an operand is made from the letters for this operand from the first alternative, a comma, the letters for this operand from the second alternative, a comma, and so on until the last alternative. All operands for a single instruction must have the same number of alternatives. So the first alternative for the 68000's logical-or could be written as "+m" (output) : "ir" (input). The second could be "+r" (output): "irm" (input). However, the fact that two memory locations cannot be used in a single instruction prevents simply using "+rm" (output) : "irm" (input). Using multi-alternatives, this might be written as "+m,r" (output) : "ir,irm" (input). This describes all the available alternatives to the compiler, allowing it to choose the most efficient one for the current conditions. There is no way within the template to determine which alternative was chosen. However you may be able to wrap your asm statements with builtins such as __builtin_constant_p to achieve the desired results. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Constraints Link: next: Machine Constraints Link: prev: Multi-Alternative Next: Machine Constraints, Previous: Multi-Alternative, Up: Constraints ────────────────────────────────────────────────────────────────────────────── 6.47.3.3 Constraint Modifier Characters Here are constraint modifier characters. '=' Means that this operand is written to by this instruction: the previous value is discarded and replaced by new data. '+' Means that this operand is both read and written by the instruction. When the compiler fixes up the operands to satisfy the constraints, it needs to know which operands are read by the instruction and which are written by it. '=' identifies an operand which is only written; '+' identifies an operand that is both read and written; all other operands are assumed to only be read. If you specify '=' or '+' in a constraint, you put it in the first character of the constraint string. '&' Means (in a particular alternative) that this operand is an earlyclobber operand, which is written before the instruction is finished using the input operands. Therefore, this operand may not lie in a register that is read by the instruction or as part of any memory address. '&' applies only to the alternative in which it is written. In constraints with multiple alternatives, sometimes one alternative requires '&' while others do not. See, for example, the 'movdf' insn of the 68000. A operand which is read by the instruction can be tied to an earlyclobber operand if its only use as an input occurs before the early result is written. Adding alternatives of this form often allows GCC to produce better code when only some of the read operands can be affected by the earlyclobber. See, for example, the 'mulsi3' insn of the ARM. Furthermore, if the earlyclobber operand is also a read/write operand, then that operand is written only after it's used. '&' does not obviate the need to write '=' or '+'. As earlyclobber operands are always written, a read-only earlyclobber operand is ill-formed and will be rejected by the compiler. '%' Declares the instruction to be commutative for this operand and the following operand. This means that the compiler may interchange the two operands if that is the cheapest way to make all operands fit the constraints. '%' applies to all alternatives and must appear as the first character in the constraint. Only read-only operands can use '%'. GCC can only handle one commutative pair in an asm; if you use more, the compiler may fail. Note that you need not use the modifier if the two alternatives are strictly identical; this would only waste time in the reload pass. ────────────────────────────────────────────────────────────────────────────── Next: Machine Constraints, Previous: Multi-Alternative, Up: Constraints Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Constraints Link: next: Asm Labels Link: prev: Modifiers ────────────────────────────────────────────────────────────────────────────── 6.47.3.4 Constraints for Particular Machines Whenever possible, you should use the general-purpose constraint letters in asm arguments, since they will convey meaning more readily to people reading your code. Failing that, use the constraint letters that usually have very similar meanings across architectures. The most commonly used constraints are 'm' and 'r' (for memory and general-purpose registers respectively; see Simple Constraints), and 'I', usually the letter indicating the most common immediate-constant format. Each architecture defines additional constraints. These constraints are used by the compiler itself for instruction generation, as well as for asm statements; therefore, some of the constraints are not particularly useful for asm. Here is a summary of some of the machine-dependent constraints available on some particular machines; it includes both constraints that are useful for asm and constraints that aren't. The compiler source file mentioned in the table heading for each architecture is the definitive reference for the meanings of that architecture's constraints. AArch64 family—config/aarch64/constraints.md k The stack pointer register (SP) w Floating point register, Advanced SIMD vector register or SVE vector register Upl One of the low eight SVE predicate registers (P0 to P7) Upa Any of the SVE predicate registers (P0 to P15) I Integer constant that is valid as an immediate operand in an ADD instruction J Integer constant that is valid as an immediate operand in a SUB instruction (once negated) K Integer constant that can be used with a 32-bit logical instruction L Integer constant that can be used with a 64-bit logical instruction M Integer constant that is valid as an immediate operand in a 32-bit MOV pseudo instruction. The MOV may be assembled to one of several different machine instructions depending on the value N Integer constant that is valid as an immediate operand in a 64-bit MOV pseudo instruction S An absolute symbolic address or a label reference Y Floating point constant zero Z Integer constant zero Ush The high part (bits 12 and upwards) of the pc-relative address of a symbol within 4GB of the instruction Q A memory address which uses a single base register with no offset Ump A memory address suitable for a load/store pair instruction in SI, DI, SF and DF modes AMD GCN —config/gcn/constraints.md I Immediate integer in the range -16 to 64 J Immediate 16-bit signed integer Kf Immediate constant -1 L Immediate 15-bit unsigned integer A Immediate constant that can be inlined in an instruction encoding: integer -16..64, or float 0.0, +/-0.5, +/-1.0, +/-2.0, +/-4.0, 1.0/(2.0*PI) B Immediate 32-bit signed integer that can be attached to an instruction encoding C Immediate 32-bit integer in range -16..4294967295 (i.e. 32-bit unsigned integer or 'A' constraint) DA Immediate 64-bit constant that can be split into two 'A' constants DB Immediate 64-bit constant that can be split into two 'B' constants U Any unspec Y Any symbol_ref or label_ref v VGPR register Sg SGPR register SD SGPR registers valid for instruction destinations, including VCC, M0 and EXEC SS SGPR registers valid for instruction sources, including VCC, M0, EXEC and SCC Sm SGPR registers valid as a source for scalar memory instructions (excludes M0 and EXEC) Sv SGPR registers valid as a source or destination for vector instructions (excludes EXEC) ca All condition registers: SCC, VCCZ, EXECZ cs Scalar condition register: SCC cV Vector condition register: VCC, VCC_LO, VCC_HI e EXEC register (EXEC_LO and EXEC_HI) RB Memory operand with address space suitable for buffer_* instructions RF Memory operand with address space suitable for flat_* instructions RS Memory operand with address space suitable for s_* instructions RL Memory operand with address space suitable for ds_* LDS instructions RG Memory operand with address space suitable for ds_* GDS instructions RD Memory operand with address space suitable for any ds_* instructions RM Memory operand with address space suitable for global_* instructions ARC —config/arc/constraints.md q Registers usable in ARCompact 16-bit instructions: r0-r3, r12-r15. This constraint can only match when the -mq option is in effect. e Registers usable as base-regs of memory addresses in ARCompact 16-bit memory instructions: r0-r3, r12-r15, sp. This constraint can only match when the -mq option is in effect. D ARC FPX (dpfp) 64-bit registers. D0, D1. I A signed 12-bit integer constant. Cal constant for arithmetic/logical operations. This might be any constant that can be put into a long immediate by the assmbler or linker without involving a PIC relocation. K A 3-bit unsigned integer constant. L A 6-bit unsigned integer constant. CnL One's complement of a 6-bit unsigned integer constant. CmL Two's complement of a 6-bit unsigned integer constant. M A 5-bit unsigned integer constant. O A 7-bit unsigned integer constant. P A 8-bit unsigned integer constant. H Any const_double value. ARM family—config/arm/constraints.md h In Thumb state, the core registers r8-r15. k The stack pointer register. l In Thumb State the core registers r0-r7. In ARM state this is an alias for the r constraint. t VFP floating-point registers s0-s31. Used for 32 bit values. w VFP floating-point registers d0-d31 and the appropriate subset d0-d15 based on command line options. Used for 64 bit values only. Not valid for Thumb1. y The iWMMX co-processor registers. z The iWMMX GR registers. G The floating-point constant 0.0 I Integer that is valid as an immediate operand in a data processing instruction. That is, an integer in the range 0 to 255 rotated by a multiple of 2 J Integer in the range -4095 to 4095 K Integer that satisfies constraint 'I' when inverted (ones complement) L Integer that satisfies constraint 'I' when negated (twos complement) M Integer in the range 0 to 32 Q A memory reference where the exact address is in a single register (''m'' is preferable for asm statements) R An item in the constant pool S A symbol in the text segment of the current file Uv A memory reference suitable for VFP load/store insns (reg+constant offset) Uy A memory reference suitable for iWMMXt load/store instructions. Uq A memory reference suitable for the ARMv4 ldrsb instruction. AVR family—config/avr/constraints.md l Registers from r0 to r15 a Registers from r16 to r23 d Registers from r16 to r31 w Registers from r24 to r31. These registers can be used in 'adiw' command e Pointer register (r26–r31) b Base pointer register (r28–r31) q Stack pointer register (SPH:SPL) t Temporary register r0 x Register pair X (r27:r26) y Register pair Y (r29:r28) z Register pair Z (r31:r30) I Constant greater than -1, less than 64 J Constant greater than -64, less than 1 K Constant integer 2 L Constant integer 0 M Constant that fits in 8 bits N Constant integer -1 O Constant integer 8, 16, or 24 P Constant integer 1 G A floating point constant 0.0 Q A memory address based on Y or Z pointer with displacement. Blackfin family—config/bfin/constraints.md a P register d D register z A call clobbered P register. qn A single register. If n is in the range 0 to 7, the corresponding D register. If it is A, then the register P0. D Even-numbered D register W Odd-numbered D register e Accumulator register. A Even-numbered accumulator register. B Odd-numbered accumulator register. b I register v B register f M register c Registers used for circular buffering, i.e. I, B, or L registers. C The CC register. t LT0 or LT1. k LC0 or LC1. u LB0 or LB1. x Any D, P, B, M, I or L register. y Additional registers typically used only in prologues and epilogues: RETS, RETN, RETI, RETX, RETE, ASTAT, SEQSTAT and USP. w Any register except accumulators or CC. Ksh Signed 16 bit integer (in the range -32768 to 32767) Kuh Unsigned 16 bit integer (in the range 0 to 65535) Ks7 Signed 7 bit integer (in the range -64 to 63) Ku7 Unsigned 7 bit integer (in the range 0 to 127) Ku5 Unsigned 5 bit integer (in the range 0 to 31) Ks4 Signed 4 bit integer (in the range -8 to 7) Ks3 Signed 3 bit integer (in the range -3 to 4) Ku3 Unsigned 3 bit integer (in the range 0 to 7) Pn Constant n, where n is a single-digit constant in the range 0 to 4. PA An integer equal to one of the MACFLAG_XXX constants that is suitable for use with either accumulator. PB An integer equal to one of the MACFLAG_XXX constants that is suitable for use only with accumulator A1. M1 Constant 255. M2 Constant 65535. J An integer constant with exactly a single bit set. L An integer constant with all bits set except exactly one. H Q Any SYMBOL_REF. CR16 Architecture—config/cr16/cr16.h b Registers from r0 to r14 (registers without stack pointer) t Register from r0 to r11 (all 16-bit registers) p Register from r12 to r15 (all 32-bit registers) I Signed constant that fits in 4 bits J Signed constant that fits in 5 bits K Signed constant that fits in 6 bits L Unsigned constant that fits in 4 bits M Signed constant that fits in 32 bits N Check for 64 bits wide constants for add/sub instructions G Floating point constant that is legal for store immediate C-SKY—config/csky/constraints.md a The mini registers r0 - r7. b The low registers r0 - r15. c C register. y HI and LO registers. l LO register. h HI register. v Vector registers. z Stack pointer register (SP). Epiphany—config/epiphany/constraints.md U16 An unsigned 16-bit constant. K An unsigned 5-bit constant. L A signed 11-bit constant. Cm1 A signed 11-bit constant added to -1. Can only match when the -m1reg-reg option is active. Cl1 Left-shift of -1, i.e., a bit mask with a block of leading ones, the rest being a block of trailing zeroes. Can only match when the -m1reg-reg option is active. Cr1 Right-shift of -1, i.e., a bit mask with a trailing block of ones, the rest being zeroes. Or to put it another way, one less than a power of two. Can only match when the -m1reg-reg option is active. Cal Constant for arithmetic/logical operations. This is like i, except that for position independent code, no symbols / expressions needing relocations are allowed. Csy Symbolic constant for call/jump instruction. Rcs The register class usable in short insns. This is a register class constraint, and can thus drive register allocation. This constraint won't match unless -mprefer-short-insn-regs is in effect. Rsc The the register class of registers that can be used to hold a sibcall call address. I.e., a caller-saved register. Rct Core control register class. Rgs The register group usable in short insns. This constraint does not use a register class, so that it only passively matches suitable registers, and doesn't drive register allocation. Rra Matches the return address if it can be replaced with the link register. Rcc Matches the integer condition code register. Sra Matches the return address if it is in a stack slot. Cfm Matches control register values to switch fp mode, which are encapsulated in UNSPEC_FP_MODE. FRV—config/frv/frv.h a Register in the class ACC_REGS (acc0 to acc7). b Register in the class EVEN_ACC_REGS (acc0 to acc7). c Register in the class CC_REGS (fcc0 to fcc3 and icc0 to icc3). d Register in the class GPR_REGS (gr0 to gr63). e Register in the class EVEN_REGS (gr0 to gr63). Odd registers are excluded not in the class but through the use of a machine mode larger than 4 bytes. f Register in the class FPR_REGS (fr0 to fr63). h Register in the class FEVEN_REGS (fr0 to fr63). Odd registers are excluded not in the class but through the use of a machine mode larger than 4 bytes. l Register in the class LR_REG (the lr register). q Register in the class QUAD_REGS (gr2 to gr63). Register numbers not divisible by 4 are excluded not in the class but through the use of a machine mode larger than 8 bytes. t Register in the class ICC_REGS (icc0 to icc3). u Register in the class FCC_REGS (fcc0 to fcc3). v Register in the class ICR_REGS (cc4 to cc7). w Register in the class FCR_REGS (cc0 to cc3). x Register in the class QUAD_FPR_REGS (fr0 to fr63). Register numbers not divisible by 4 are excluded not in the class but through the use of a machine mode larger than 8 bytes. z Register in the class SPR_REGS (lcr and lr). A Register in the class QUAD_ACC_REGS (acc0 to acc7). B Register in the class ACCG_REGS (accg0 to accg7). C Register in the class CR_REGS (cc0 to cc7). G Floating point constant zero I 6-bit signed integer constant J 10-bit signed integer constant L 16-bit signed integer constant M 16-bit unsigned integer constant N 12-bit signed integer constant that is negative—i.e. in the range of -2048 to -1 O Constant zero P 12-bit signed integer constant that is greater than zero—i.e. in the range of 1 to 2047. FT32—config/ft32/constraints.md A An absolute address B An offset address W A register indirect memory operand e An offset address. f An offset address. O The constant zero or one I A 16-bit signed constant (-32768 … 32767) w A bitfield mask suitable for bext or bins x An inverted bitfield mask suitable for bext or bins L A 16-bit unsigned constant, multiple of 4 (0 … 65532) S A 20-bit signed constant (-524288 … 524287) b A constant for a bitfield width (1 … 16) KA A 10-bit signed constant (-512 … 511) Hewlett-Packard PA-RISC—config/pa/pa.h a General register 1 f Floating point register q Shift amount register x Floating point register (deprecated) y Upper floating point register (32-bit), floating point register (64-bit) Z Any register I Signed 11-bit integer constant J Signed 14-bit integer constant K Integer constant that can be deposited with a zdepi instruction L Signed 5-bit integer constant M Integer constant 0 N Integer constant that can be loaded with a ldil instruction O Integer constant whose value plus one is a power of 2 P Integer constant that can be used for and operations in depi and extru instructions S Integer constant 31 U Integer constant 63 G Floating-point constant 0.0 A A lo_sum data-linkage-table memory operand Q A memory operand that can be used as the destination operand of an integer store instruction R A scaled or unscaled indexed memory operand T A memory operand for floating-point loads and stores W A register indirect memory operand Intel IA-64—config/ia64/ia64.h a General register r0 to r3 for addl instruction b Branch register c Predicate register ('c' as in “conditional”) d Application register residing in M-unit e Application register residing in I-unit f Floating-point register m Memory operand. If used together with '<' or '>', the operand can have postincrement and postdecrement which require printing with '%Pn' on IA-64. G Floating-point constant 0.0 or 1.0 I 14-bit signed integer constant J 22-bit signed integer constant K 8-bit signed integer constant for logical instructions L 8-bit adjusted signed integer constant for compare pseudo-ops M 6-bit unsigned integer constant for shift counts N 9-bit signed integer constant for load and store postincrements O The constant zero P 0 or -1 for dep instruction Q Non-volatile memory for floating-point loads and stores R Integer constant in the range 1 to 4 for shladd instruction S Memory operand except postincrement and postdecrement. This is now roughly the same as 'm' when not used together with '<' or '>'. M32C—config/m32c/m32c.c Rsp Rfb Rsb '$sp', '$fb', '$sb'. Rcr Any control register, when they're 16 bits wide (nothing if control registers are 24 bits wide) Rcl Any control register, when they're 24 bits wide. R0w R1w R2w R3w $r0, $r1, $r2, $r3. R02 $r0 or $r2, or $r2r0 for 32 bit values. R13 $r1 or $r3, or $r3r1 for 32 bit values. Rdi A register that can hold a 64 bit value. Rhl $r0 or $r1 (registers with addressable high/low bytes) R23 $r2 or $r3 Raa Address registers Raw Address registers when they're 16 bits wide. Ral Address registers when they're 24 bits wide. Rqi Registers that can hold QI values. Rad Registers that can be used with displacements ($a0, $a1, $sb). Rsi Registers that can hold 32 bit values. Rhi Registers that can hold 16 bit values. Rhc Registers chat can hold 16 bit values, including all control registers. Rra $r0 through R1, plus $a0 and $a1. Rfl The flags register. Rmm The memory-based pseudo-registers $mem0 through $mem15. Rpi Registers that can hold pointers (16 bit registers for r8c, m16c; 24 bit registers for m32cm, m32c). Rpa Matches multiple registers in a PARALLEL to form a larger register. Used to match function return values. Is3 -8 … 7 IS1 -128 … 127 IS2 -32768 … 32767 IU2 0 … 65535 In4 -8 … -1 or 1 … 8 In5 -16 … -1 or 1 … 16 In6 -32 … -1 or 1 … 32 IM2 -65536 … -1 Ilb An 8 bit value with exactly one bit set. Ilw A 16 bit value with exactly one bit set. Sd The common src/dest memory addressing modes. Sa Memory addressed using $a0 or $a1. Si Memory addressed with immediate addresses. Ss Memory addressed using the stack pointer ($sp). Sf Memory addressed using the frame base register ($fb). Ss Memory addressed using the small base register ($sb). S1 $r1h MicroBlaze—config/microblaze/constraints.md d A general register (r0 to r31). z A status register (rmsr, $fcc1 to $fcc7). MIPS—config/mips/constraints.md d A general-purpose register. This is equivalent to r unless generating MIPS16 code, in which case the MIPS16 register set is used. f A floating-point register (if available). h Formerly the hi register. This constraint is no longer supported. l The lo register. Use this register to store values that are no bigger than a word. x The concatenated hi and lo registers. Use this register to store doubleword values. c A register suitable for use in an indirect jump. This will always be $25 for -mabicalls. v Register $3. Do not use this constraint in new code; it is retained only for compatibility with glibc. y Equivalent to r; retained for backwards compatibility. z A floating-point condition code register. I A signed 16-bit constant (for arithmetic instructions). J Integer zero. K An unsigned 16-bit constant (for logic instructions). L A signed 32-bit constant in which the lower 16 bits are zero. Such constants can be loaded using lui. M A constant that cannot be loaded using lui, addiu or ori. N A constant in the range -65535 to -1 (inclusive). O A signed 15-bit constant. P A constant in the range 1 to 65535 (inclusive). G Floating-point zero. R An address that can be used in a non-macro load or store. ZC A memory operand whose address is formed by a base register and offset that is suitable for use in instructions with the same addressing mode as ll and sc. ZD An address suitable for a prefetch instruction, or for any other instruction with the same addressing mode as prefetch. Motorola 680x0—config/m68k/constraints.md a Address register d Data register f 68881 floating-point register, if available I Integer in the range 1 to 8 J 16-bit signed number K Signed number whose magnitude is greater than 0x80 L Integer in the range -8 to -1 M Signed number whose magnitude is greater than 0x100 N Range 24 to 31, rotatert:SI 8 to 1 expressed as rotate O 16 (for rotate using swap) P Range 8 to 15, rotatert:HI 8 to 1 expressed as rotate R Numbers that mov3q can handle G Floating point constant that is not a 68881 constant S Operands that satisfy 'm' when -mpcrel is in effect T Operands that satisfy 's' when -mpcrel is not in effect Q Address register indirect addressing mode U Register offset addressing W const_call_operand Cs symbol_ref or const Ci const_int C0 const_int 0 Cj Range of signed numbers that don't fit in 16 bits Cmvq Integers valid for mvq Capsw Integers valid for a moveq followed by a swap Cmvz Integers valid for mvz Cmvs Integers valid for mvs Ap push_operand Ac Non-register operands allowed in clr Moxie—config/moxie/constraints.md A An absolute address B An offset address W A register indirect memory operand I A constant in the range of 0 to 255. N A constant in the range of 0 to -255. MSP430–config/msp430/constraints.md R12 Register R12. R13 Register R13. K Integer constant 1. L Integer constant -1^20..1^19. M Integer constant 1-4. Ya Memory references which do not require an extended MOVX instruction. Yl Memory reference, labels only. Ys Memory reference, stack only. NDS32—config/nds32/constraints.md w LOW register class $r0 to $r7 constraint for V3/V3M ISA. l LOW register class $r0 to $r7. d MIDDLE register class $r0 to $r11, $r16 to $r19. h HIGH register class $r12 to $r14, $r20 to $r31. t Temporary assist register $ta (i.e. $r15). k Stack register $sp. Iu03 Unsigned immediate 3-bit value. In03 Negative immediate 3-bit value in the range of -7–0. Iu04 Unsigned immediate 4-bit value. Is05 Signed immediate 5-bit value. Iu05 Unsigned immediate 5-bit value. In05 Negative immediate 5-bit value in the range of -31–0. Ip05 Unsigned immediate 5-bit value for movpi45 instruction with range 16–47. Iu06 Unsigned immediate 6-bit value constraint for addri36.sp instruction. Iu08 Unsigned immediate 8-bit value. Iu09 Unsigned immediate 9-bit value. Is10 Signed immediate 10-bit value. Is11 Signed immediate 11-bit value. Is15 Signed immediate 15-bit value. Iu15 Unsigned immediate 15-bit value. Ic15 A constant which is not in the range of imm15u but ok for bclr instruction. Ie15 A constant which is not in the range of imm15u but ok for bset instruction. It15 A constant which is not in the range of imm15u but ok for btgl instruction. Ii15 A constant whose compliment value is in the range of imm15u and ok for bitci instruction. Is16 Signed immediate 16-bit value. Is17 Signed immediate 17-bit value. Is19 Signed immediate 19-bit value. Is20 Signed immediate 20-bit value. Ihig The immediate value that can be simply set high 20-bit. Izeb The immediate value 0xff. Izeh The immediate value 0xffff. Ixls The immediate value 0x01. Ix11 The immediate value 0x7ff. Ibms The immediate value with power of 2. Ifex The immediate value with power of 2 minus 1. U33 Memory constraint for 333 format. U45 Memory constraint for 45 format. U37 Memory constraint for 37 format. Nios II family—config/nios2/constraints.md I Integer that is valid as an immediate operand in an instruction taking a signed 16-bit number. Range -32768 to 32767. J Integer that is valid as an immediate operand in an instruction taking an unsigned 16-bit number. Range 0 to 65535. K Integer that is valid as an immediate operand in an instruction taking only the upper 16-bits of a 32-bit number. Range 32-bit numbers with the lower 16-bits being 0. L Integer that is valid as an immediate operand for a shift instruction. Range 0 to 31. M Integer that is valid as an immediate operand for only the value 0. Can be used in conjunction with the format modifier z to use r0 instead of 0 in the assembly output. N Integer that is valid as an immediate operand for a custom instruction opcode. Range 0 to 255. P An immediate operand for R2 andchi/andci instructions. S Matches immediates which are addresses in the small data section and therefore can be added to gp as a 16-bit immediate to re-create their 32-bit value. U Matches constants suitable as an operand for the rdprs and cache instructions. v A memory operand suitable for Nios II R2 load/store exclusive instructions. w A memory operand suitable for load/store IO and cache instructions. OpenRISC—config/or1k/constraints.md I Integer that is valid as an immediate operand in an instruction taking a signed 16-bit number. Range -32768 to 32767. K Integer that is valid as an immediate operand in an instruction taking an unsigned 16-bit number. Range 0 to 65535. M Signed 16-bit constant shifted left 16 bits. (Used with l.movhi) O Zero PDP-11—config/pdp11/constraints.md a Floating point registers AC0 through AC3. These can be loaded from/to memory with a single instruction. d Odd numbered general registers (R1, R3, R5). These are used for 16-bit multiply operations. D A memory reference that is encoded within the opcode, but not auto-increment or auto-decrement. f Any of the floating point registers (AC0 through AC5). G Floating point constant 0. h Floating point registers AC4 and AC5. These cannot be loaded from/to memory with a single instruction. I An integer constant that fits in 16 bits. J An integer constant whose low order 16 bits are zero. K An integer constant that does not meet the constraints for codes 'I' or 'J'. L The integer constant 1. M The integer constant -1. N The integer constant 0. O Integer constants 0 through 3; shifts by these amounts are handled as multiple single-bit shifts rather than a single variable-length shift. Q A memory reference which requires an additional word (address or offset) after the opcode. R A memory reference that is encoded within the opcode. PowerPC and IBM RS6000—config/rs6000/constraints.md b Address base register d Floating point register (containing 64-bit value) f Floating point register (containing 32-bit value) v Altivec vector register wa Any VSX register if the -mvsx option was used or NO_REGS. When using any of the register constraints (wa, wd, wf, wg, wh, wi, wj, wk, wl, wm, wo, wp, wq, ws, wt, wu, wv, ww, or wy) that take VSX registers, you must use %x in the template so that the correct register is used. Otherwise the register number output in the assembly file will be incorrect if an Altivec register is an operand of a VSX instruction that expects VSX register numbering. asm ("xvadddp %x0,%x1,%x2" : "=wa" (v1) : "wa" (v2), "wa" (v3)); is correct, but: asm ("xvadddp %0,%1,%2" : "=wa" (v1) : "wa" (v2), "wa" (v3)); is not correct. If an instruction only takes Altivec registers, you do not want to use %x. asm ("xsaddqp %0,%1,%2" : "=v" (v1) : "v" (v2), "v" (v3)); is correct because the xsaddqp instruction only takes Altivec registers, while: asm ("xsaddqp %x0,%x1,%x2" : "=v" (v1) : "v" (v2), "v" (v3)); is incorrect. wb Altivec register if -mcpu=power9 is used or NO_REGS. wd VSX vector register to hold vector double data or NO_REGS. we VSX register if the -mcpu=power9 and -m64 options were used or NO_REGS. wf VSX vector register to hold vector float data or NO_REGS. wg If -mmfpgpr was used, a floating point register or NO_REGS. wh Floating point register if direct moves are available, or NO_REGS. wi FP or VSX register to hold 64-bit integers for VSX insns or NO_REGS. wj FP or VSX register to hold 64-bit integers for direct moves or NO_REGS. wk FP or VSX register to hold 64-bit doubles for direct moves or NO_REGS. wl Floating point register if the LFIWAX instruction is enabled or NO_REGS. wm VSX register if direct move instructions are enabled, or NO_REGS. wn No register (NO_REGS). wo VSX register to use for ISA 3.0 vector instructions, or NO_REGS. wp VSX register to use for IEEE 128-bit floating point TFmode, or NO_REGS. wq VSX register to use for IEEE 128-bit floating point, or NO_REGS. wr General purpose register if 64-bit instructions are enabled or NO_REGS. ws VSX vector register to hold scalar double values or NO_REGS. wt VSX vector register to hold 128 bit integer or NO_REGS. wu Altivec register to use for float/32-bit int loads/stores or NO_REGS. wv Altivec register to use for double loads/stores or NO_REGS. ww FP or VSX register to perform float operations under -mvsx or NO_REGS. wx Floating point register if the STFIWX instruction is enabled or NO_REGS. wy FP or VSX register to perform ISA 2.07 float ops or NO_REGS. wz Floating point register if the LFIWZX instruction is enabled or NO_REGS. wA Address base register if 64-bit instructions are enabled or NO_REGS. wB Signed 5-bit constant integer that can be loaded into an altivec register. wD Int constant that is the element number of the 64-bit scalar in a vector. wE Vector constant that can be loaded with the XXSPLTIB instruction. wF Memory operand suitable for power8 GPR load fusion wG Memory operand suitable for TOC fusion memory references. wH Altivec register if -mvsx-small-integer. wI Floating point register if -mvsx-small-integer. wJ FP register if -mvsx-small-integer and -mpower9-vector. wK Altivec register if -mvsx-small-integer and -mpower9-vector. wL Int constant that is the element number that the MFVSRLD instruction. targets. wM Match vector constant with all 1's if the XXLORC instruction is available. wO A memory operand suitable for the ISA 3.0 vector d-form instructions. wQ A memory address that will work with the lq and stq instructions. wS Vector constant that can be loaded with XXSPLTIB & sign extension. h 'VRSAVE', 'CTR', or 'LINK' register c 'CTR' register l 'LINK' register x 'CR' register (condition register) number 0 y 'CR' register (condition register) z 'XER[CA]' carry bit (part of the XER register) I Signed 16-bit constant J Unsigned 16-bit constant shifted left 16 bits (use 'L' instead for SImode constants) K Unsigned 16-bit constant L Signed 16-bit constant shifted left 16 bits M Constant larger than 31 N Exact power of 2 O Zero P Constant whose negation is a signed 16-bit constant G Floating point constant that can be loaded into a register with one instruction per word H Integer/Floating point constant that can be loaded into a register using three instructions m Memory operand. Normally, m does not allow addresses that update the base register. If '<' or '>' constraint is also used, they are allowed and therefore on PowerPC targets in that case it is only safe to use 'm<>' in an asm statement if that asm statement accesses the operand exactly once. The asm statement must also use '%U' as a placeholder for the “update” flag in the corresponding load or store instruction. For example: asm ("st%U0 %1,%0" : "=m<>" (mem) : "r" (val)); is correct but: asm ("st %1,%0" : "=m<>" (mem) : "r" (val)); is not. es A “stable” memory operand; that is, one which does not include any automodification of the base register. This used to be useful when 'm' allowed automodification of the base register, but as those are now only allowed when '<' or '>' is used, 'es' is basically the same as 'm' without '<' and '>'. Q Memory operand that is an offset from a register (it is usually better to use 'm' or 'es' in asm statements) Z Memory operand that is an indexed or indirect from a register (it is usually better to use 'm' or 'es' in asm statements) R AIX TOC entry a Address operand that is an indexed or indirect from a register ('p' is preferable for asm statements) U System V Release 4 small data area reference W Vector constant that does not require memory j Vector constant that is all zeros. RL78—config/rl78/constraints.md Int3 An integer constant in the range 1 … 7. Int8 An integer constant in the range 0 … 255. J An integer constant in the range -255 … 0 K The integer constant 1. L The integer constant -1. M The integer constant 0. N The integer constant 2. O The integer constant -2. P An integer constant in the range 1 … 15. Qbi The built-in compare types–eq, ne, gtu, ltu, geu, and leu. Qsc The synthetic compare types–gt, lt, ge, and le. Wab A memory reference with an absolute address. Wbc A memory reference using BC as a base register, with an optional offset. Wca A memory reference using AX, BC, DE, or HL for the address, for calls. Wcv A memory reference using any 16-bit register pair for the address, for calls. Wd2 A memory reference using DE as a base register, with an optional offset. Wde A memory reference using DE as a base register, without any offset. Wfr Any memory reference to an address in the far address space. Wh1 A memory reference using HL as a base register, with an optional one-byte offset. Whb A memory reference using HL as a base register, with B or C as the index register. Whl A memory reference using HL as a base register, without any offset. Ws1 A memory reference using SP as a base register, with an optional one-byte offset. Y Any memory reference to an address in the near address space. A The AX register. B The BC register. D The DE register. R A through L registers. S The SP register. T The HL register. Z08W The 16-bit R8 register. Z10W The 16-bit R10 register. Zint The registers reserved for interrupts (R24 to R31). a The A register. b The B register. c The C register. d The D register. e The E register. h The H register. l The L register. v The virtual registers. w The PSW register. x The X register. RISC-V—config/riscv/constraints.md f A floating-point register (if available). I An I-type 12-bit signed immediate. J Integer zero. K A 5-bit unsigned immediate for CSR access instructions. A An address that is held in a general-purpose register. RX—config/rx/constraints.md Q An address which does not involve register indirect addressing or pre/post increment/decrement addressing. Symbol A symbol reference. Int08 A constant in the range -256 to 255, inclusive. Sint08 A constant in the range -128 to 127, inclusive. Sint16 A constant in the range -32768 to 32767, inclusive. Sint24 A constant in the range -8388608 to 8388607, inclusive. Uint04 A constant in the range 0 to 15, inclusive. S/390 and zSeries—config/s390/s390.h a Address register (general purpose register except r0) c Condition code register d Data register (arbitrary general purpose register) f Floating-point register I Unsigned 8-bit constant (0–255) J Unsigned 12-bit constant (0–4095) K Signed 16-bit constant (-32768–32767) L Value appropriate as displacement. (0..4095) for short displacement (-524288..524287) for long displacement M Constant integer with a value of 0x7fffffff. N Multiple letter constraint followed by 4 parameter letters. 0..9: number of the part counting from most to least significant H,Q: mode of the part D,S,H: mode of the containing operand 0,F: value of the other parts (F—all bits set) The constraint matches if the specified part of a constant has a value different from its other parts. Q Memory reference without index register and with short displacement. R Memory reference with index register and short displacement. S Memory reference without index register but with long displacement. T Memory reference with index register and long displacement. U Pointer with short displacement. W Pointer with long displacement. Y Shift count operand. SPARC—config/sparc/sparc.h f Floating-point register on the SPARC-V8 architecture and lower floating-point register on the SPARC-V9 architecture. e Floating-point register. It is equivalent to 'f' on the SPARC-V8 architecture and contains both lower and upper floating-point registers on the SPARC-V9 architecture. c Floating-point condition code register. d Lower floating-point register. It is only valid on the SPARC-V9 architecture when the Visual Instruction Set is available. b Floating-point register. It is only valid on the SPARC-V9 architecture when the Visual Instruction Set is available. h 64-bit global or out register for the SPARC-V8+ architecture. C The constant all-ones, for floating-point. A Signed 5-bit constant D A vector constant I Signed 13-bit constant J Zero K 32-bit constant with the low 12 bits clear (a constant that can be loaded with the sethi instruction) L A constant in the range supported by movcc instructions (11-bit signed immediate) M A constant in the range supported by movrcc instructions (10-bit signed immediate) N Same as 'K', except that it verifies that bits that are not in the lower 32-bit range are all zero. Must be used instead of 'K' for modes wider than SImode O The constant 4096 G Floating-point zero H Signed 13-bit constant, sign-extended to 32 or 64 bits P The constant -1 Q Floating-point constant whose integral representation can be moved into an integer register using a single sethi instruction R Floating-point constant whose integral representation can be moved into an integer register using a single mov instruction S Floating-point constant whose integral representation can be moved into an integer register using a high/lo_sum instruction sequence T Memory address aligned to an 8-byte boundary U Even register W Memory address for 'e' constraint registers w Memory address with only a base register Y Vector zero SPU—config/spu/spu.h a An immediate which can be loaded with the il/ila/ilh/ilhu instructions. const_int is treated as a 64 bit value. c An immediate for and/xor/or instructions. const_int is treated as a 64 bit value. d An immediate for the iohl instruction. const_int is treated as a 64 bit value. f An immediate which can be loaded with fsmbi. A An immediate which can be loaded with the il/ila/ilh/ilhu instructions. const_int is treated as a 32 bit value. B An immediate for most arithmetic instructions. const_int is treated as a 32 bit value. C An immediate for and/xor/or instructions. const_int is treated as a 32 bit value. D An immediate for the iohl instruction. const_int is treated as a 32 bit value. I A constant in the range [-64, 63] for shift/rotate instructions. J An unsigned 7-bit constant for conversion/nop/channel instructions. K A signed 10-bit constant for most arithmetic instructions. M A signed 16 bit immediate for stop. N An unsigned 16-bit constant for iohl and fsmbi. O An unsigned 7-bit constant whose 3 least significant bits are 0. P An unsigned 3-bit constant for 16-byte rotates and shifts R Call operand, reg, for indirect calls S Call operand, symbol, for relative calls. T Call operand, const_int, for absolute calls. U An immediate which can be loaded with the il/ila/ilh/ilhu instructions. const_int is sign extended to 128 bit. W An immediate for shift and rotate instructions. const_int is treated as a 32 bit value. Y An immediate for and/xor/or instructions. const_int is sign extended as a 128 bit. Z An immediate for the iohl instruction. const_int is sign extended to 128 bit. TI C6X family—config/c6x/constraints.md a Register file A (A0–A31). b Register file B (B0–B31). A Predicate registers in register file A (A0–A2 on C64X and higher, A1 and A2 otherwise). B Predicate registers in register file B (B0–B2). C A call-used register in register file B (B0–B9, B16–B31). Da Register file A, excluding predicate registers (A3–A31, plus A0 if not C64X or higher). Db Register file B, excluding predicate registers (B3–B31). Iu4 Integer constant in the range 0 … 15. Iu5 Integer constant in the range 0 … 31. In5 Integer constant in the range -31 … 0. Is5 Integer constant in the range -16 … 15. I5x Integer constant that can be the operand of an ADDA or a SUBA insn. IuB Integer constant in the range 0 … 65535. IsB Integer constant in the range -32768 … 32767. IsC Integer constant in the range -2^{20} … 2^{20} - 1. Jc Integer constant that is a valid mask for the clr instruction. Js Integer constant that is a valid mask for the set instruction. Q Memory location with A base register. R Memory location with B base register. Z Register B14 (aka DP). TILE-Gx—config/tilegx/constraints.md R00 R01 R02 R03 R04 R05 R06 R07 R08 R09 R10 Each of these represents a register constraint for an individual register, from r0 to r10. I Signed 8-bit integer constant. J Signed 16-bit integer constant. K Unsigned 16-bit integer constant. L Integer constant that fits in one signed byte when incremented by one (-129 … 126). m Memory operand. If used together with '<' or '>', the operand can have postincrement which requires printing with '%In' and '%in' on TILE-Gx. For example: asm ("st_add %I0,%1,%i0" : "=m<>" (*mem) : "r" (val)); M A bit mask suitable for the BFINS instruction. N Integer constant that is a byte tiled out eight times. O The integer zero constant. P Integer constant that is a sign-extended byte tiled out as four shorts. Q Integer constant that fits in one signed byte when incremented (-129 … 126), but excluding -1. S Integer constant that has all 1 bits consecutive and starting at bit 0. T A 16-bit fragment of a got, tls, or pc-relative reference. U Memory operand except postincrement. This is roughly the same as 'm' when not used together with '<' or '>'. W An 8-element vector constant with identical elements. Y A 4-element vector constant with identical elements. Z0 The integer constant 0xffffffff. Z1 The integer constant 0xffffffff00000000. TILEPro—config/tilepro/constraints.md R00 R01 R02 R03 R04 R05 R06 R07 R08 R09 R10 Each of these represents a register constraint for an individual register, from r0 to r10. I Signed 8-bit integer constant. J Signed 16-bit integer constant. K Nonzero integer constant with low 16 bits zero. L Integer constant that fits in one signed byte when incremented by one (-129 … 126). m Memory operand. If used together with '<' or '>', the operand can have postincrement which requires printing with '%In' and '%in' on TILEPro. For example: asm ("swadd %I0,%1,%i0" : "=m<>" (mem) : "r" (val)); M A bit mask suitable for the MM instruction. N Integer constant that is a byte tiled out four times. O The integer zero constant. P Integer constant that is a sign-extended byte tiled out as two shorts. Q Integer constant that fits in one signed byte when incremented (-129 … 126), but excluding -1. T A symbolic operand, or a 16-bit fragment of a got, tls, or pc-relative reference. U Memory operand except postincrement. This is roughly the same as 'm' when not used together with '<' or '>'. W A 4-element vector constant with identical elements. Y A 2-element vector constant with identical elements. Visium—config/visium/constraints.md b EAM register mdb c EAM register mdc f Floating point register l General register, but not r29, r30 and r31 t Register r1 u Register r2 v Register r3 G Floating-point constant 0.0 J Integer constant in the range 0 .. 65535 (16-bit immediate) K Integer constant in the range 1 .. 31 (5-bit immediate) L Integer constant in the range -65535 .. -1 (16-bit negative immediate) M Integer constant -1 O Integer constant 0 P Integer constant 32 x86 family—config/i386/constraints.md R Legacy register—the eight integer registers available on all i386 processors (a, b, c, d, si, di, bp, sp). q Any register accessible as rl. In 32-bit mode, a, b, c, and d; in 64-bit mode, any integer register. Q Any register accessible as rh: a, b, c, and d. a The a register. b The b register. c The c register. d The d register. S The si register. D The di register. A The a and d registers. This class is used for instructions that return double word results in the ax:dx register pair. Single word values will be allocated either in ax or dx. For example on i386 the following implements rdtsc: unsigned long long rdtsc (void) { unsigned long long tick; __asm__ __volatile__("rdtsc":"=A"(tick)); return tick; } This is not correct on x86-64 as it would allocate tick in either ax or dx. You have to use the following variant instead: unsigned long long rdtsc (void) { unsigned int tickl, tickh; __asm__ __volatile__("rdtsc":"=a"(tickl),"=d"(tickh)); return ((unsigned long long)tickh << 32)|tickl; } U The call-clobbered integer registers. f Any 80387 floating-point (stack) register. t Top of 80387 floating-point stack (%st(0)). u Second from top of 80387 floating-point stack (%st(1)). y Any MMX register. x Any SSE register. v Any EVEX encodable SSE register (%xmm0-%xmm31). Yz First SSE register (%xmm0). I Integer constant in the range 0 … 31, for 32-bit shifts. J Integer constant in the range 0 … 63, for 64-bit shifts. K Signed 8-bit integer constant. L 0xFF or 0xFFFF, for andsi as a zero-extending move. M 0, 1, 2, or 3 (shifts for the lea instruction). N Unsigned 8-bit integer constant (for in and out instructions). G Standard 80387 floating point constant. C SSE constant zero operand. e 32-bit signed integer constant, or a symbolic reference known to fit that range (for immediate operands in sign-extending x86-64 instructions). We 32-bit signed integer constant, or a symbolic reference known to fit that range (for sign-extending conversion operations that require non-VOIDmode immediate operands). Wz 32-bit unsigned integer constant, or a symbolic reference known to fit that range (for zero-extending conversion operations that require non-VOIDmode immediate operands). Wd 128-bit integer constant where both the high and low 64-bit word satisfy the e constraint. Z 32-bit unsigned integer constant, or a symbolic reference known to fit that range (for immediate operands in zero-extending x86-64 instructions). Tv VSIB address operand. Ts Address operand without segment register. Xstormy16—config/stormy16/stormy16.h a Register r0. b Register r1. c Register r2. d Register r8. e Registers r0 through r7. t Registers r0 and r1. y The carry register. z Registers r8 and r9. I A constant between 0 and 3 inclusive. J A constant that has exactly one bit set. K A constant that has exactly one bit clear. L A constant between 0 and 255 inclusive. M A constant between -255 and 0 inclusive. N A constant between -3 and 0 inclusive. O A constant between 1 and 4 inclusive. P A constant between -4 and -1 inclusive. Q A memory reference that is a stack push. R A memory reference that is a stack pop. S A memory reference that refers to a constant address of known value. T The register indicated by Rx (not implemented yet). U A constant that is not between 2 and 15 inclusive. Z The constant 0. Xtensa—config/xtensa/constraints.md a General-purpose 32-bit register b One-bit boolean register A MAC16 40-bit accumulator register I Signed 12-bit integer constant, for use in MOVI instructions J Signed 8-bit integer constant, for use in ADDI instructions K Integer constant valid for BccI instructions L Unsigned constant valid for BccUI instructions ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Using Assembly Language with C Link: next: Explicit Register Variables Link: prev: Machine Constraints Next: Explicit Register Variables, Previous: Constraints, Up: Using ────────────────────────────────────────────────────────────────────────────── 6.47.4 Controlling Names Used in Assembler Code You can specify the name to be used in the assembler code for a C function or variable by writing the asm (or __asm__) keyword after the declarator. It is up to you to make sure that the assembler names you choose do not conflict with any other assembler symbols, or reference registers. Assembler names for data: This sample shows how to specify the assembler name for data: int foo asm ("myfoo") = 2; This specifies that the name to be used for the variable foo in the assembler code should be 'myfoo' rather than the usual '_foo'. On systems where an underscore is normally prepended to the name of a C variable, this feature allows you to define names for the linker that do not start with an underscore. GCC does not support using this feature with a non-static local variable since such variables do not have assembler names. If you are trying to put the variable in a particular register, see Explicit Register Variables. Assembler names for functions: To specify the assembler name for functions, write a declaration for the function before its definition and put asm there, like this: int func (int x, int y) asm ("MYFUNC"); int func (int x, int y) { /* … */ This specifies that the name to be used for the function func in the assembler code should be MYFUNC. ────────────────────────────────────────────────────────────────────────────── Next: Explicit Register Variables, Previous: Constraints, Up: Using Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Using Assembly Language with C Link: next: Global Register Variables Link: prev: Asm Labels Next: Size of an asm, Previous: Asm Labels, Up: Using Assembly Language ────────────────────────────────────────────────────────────────────────────── 6.47.5 Variables in Specified Registers GNU C allows you to associate specific hardware registers with C variables. In almost all cases, allowing the compiler to assign registers produces the best code. However under certain unusual circumstances, more precise control over the variable storage is required. Both global and local variables can be associated with a register. The consequences of performing this association are very different between the two, as explained in the sections below. • Global Register Variables: Variables declared at global scope. • Local Register Variables: Variables declared within a function. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Explicit Register Variables Link: next: Local Register Variables Link: prev: Explicit Register Variables Next: Local Register Variables, Up: Explicit Register Variables ────────────────────────────────────────────────────────────────────────────── 6.47.5.1 Defining Global Register Variables You can define a global register variable and associate it with a specified register like this: register int *foo asm ("r12"); Here r12 is the name of the register that should be used. Note that this is the same syntax used for defining local register variables, but for a global variable the declaration appears outside a function. The register keyword is required, and cannot be combined with static. The register name must be a valid register name for the target platform. Do not use type qualifiers such as const and volatile, as the outcome may be contrary to expectations. In particular, using the volatile qualifier does not fully prevent the compiler from optimizing accesses to the register. Registers are a scarce resource on most systems and allowing the compiler to manage their usage usually results in the best code. However, under special circumstances it can make sense to reserve some globally. For example this may be useful in programs such as programming language interpreters that have a couple of global variables that are accessed very often. After defining a global register variable, for the current compilation unit: * If the register is a call-saved register, call ABI is affected: the register will not be restored in function epilogue sequences after the variable has been assigned. Therefore, functions cannot safely return to callers that assume standard ABI. * Conversely, if the register is a call-clobbered register, making calls to functions that use standard ABI may lose contents of the variable. Such calls may be created by the compiler even if none are evident in the original program, for example when libgcc functions are used to make up for unavailable instructions. * Accesses to the variable may be optimized as usual and the register remains available for allocation and use in any computations, provided that observable values of the variable are not affected. * If the variable is referenced in inline assembly, the type of access must be provided to the compiler via constraints (see Constraints). Accesses from basic asms are not supported. Note that these points only apply to code that is compiled with the definition. The behavior of code that is merely linked in (for example code from libraries) is not affected. If you want to recompile source files that do not actually use your global register variable so they do not use the specified register for any other purpose, you need not actually add the global register declaration to their source code. It suffices to specify the compiler option -ffixed-reg (see Code Gen Options) to reserve the register. Declaring the variable Global register variables cannot have initial values, because an executable file has no means to supply initial contents for a register. When selecting a register, choose one that is normally saved and restored by function calls on your machine. This ensures that code which is unaware of this reservation (such as library routines) will restore it before returning. On machines with register windows, be sure to choose a global register that is not affected magically by the function call mechanism. Using the variable When calling routines that are not aware of the reservation, be cautious if those routines call back into code which uses them. As an example, if you call the system library version of qsort, it may clobber your registers during execution, but (if you have selected appropriate registers) it will restore them before returning. However it will not restore them before calling qsort's comparison function. As a result, global values will not reliably be available to the comparison function unless the qsort function itself is rebuilt. Similarly, it is not safe to access the global register variables from signal handlers or from more than one thread of control. Unless you recompile them specially for the task at hand, the system library routines may temporarily use the register for other things. Furthermore, since the register is not reserved exclusively for the variable, accessing it from handlers of asynchronous signals may observe unrelated temporary values residing in the register. On most machines, longjmp restores to each global register variable the value it had at the time of the setjmp. On some machines, however, longjmp does not change the value of global register variables. To be portable, the function that called setjmp should make other arrangements to save the values of the global register variables, and to restore them in a longjmp. This way, the same thing happens regardless of what longjmp does. ────────────────────────────────────────────────────────────────────────────── Next: Local Register Variables, Up: Explicit Register Variables Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Explicit Register Variables Link: next: Size of an asm Link: prev: Global Register Variables Previous: Global Register Variables, Up: Explicit Register Variables ────────────────────────────────────────────────────────────────────────────── 6.47.5.2 Specifying Registers for Local Variables You can define a local register variable and associate it with a specified register like this: register int *foo asm ("r12"); Here r12 is the name of the register that should be used. Note that this is the same syntax used for defining global register variables, but for a local variable the declaration appears within a function. The register keyword is required, and cannot be combined with static. The register name must be a valid register name for the target platform. Do not use type qualifiers such as const and volatile, as the outcome may be contrary to expectations. In particular, when the const qualifier is used, the compiler may substitute the variable with its initializer in asm statements, which may cause the corresponding operand to appear in a different register. As with global register variables, it is recommended that you choose a register that is normally saved and restored by function calls on your machine, so that calls to library routines will not clobber it. The only supported use for this feature is to specify registers for input and output operands when calling Extended asm (see Extended Asm). This may be necessary if the constraints for a particular machine don't provide sufficient control to select the desired register. To force an operand into a register, create a local variable and specify the register name after the variable's declaration. Then use the local variable for the asm operand and specify any constraint letter that matches the register: register int *p1 asm ("r0") = …; register int *p2 asm ("r1") = …; register int *result asm ("r0"); asm ("sysint" : "=r" (result) : "0" (p1), "r" (p2)); Warning: In the above example, be aware that a register (for example r0) can be call-clobbered by subsequent code, including function calls and library calls for arithmetic operators on other variables (for example the initialization of p2). In this case, use temporary variables for expressions between the register assignments: int t1 = …; register int *p1 asm ("r0") = …; register int *p2 asm ("r1") = t1; register int *result asm ("r0"); asm ("sysint" : "=r" (result) : "0" (p1), "r" (p2)); Defining a register variable does not reserve the register. Other than when invoking the Extended asm, the contents of the specified register are not guaranteed. For this reason, the following uses are explicitly not supported. If they appear to work, it is only happenstance, and may stop working as intended due to (seemingly) unrelated changes in surrounding code, or even minor changes in the optimization of a future version of gcc: * Passing parameters to or from Basic asm * Passing parameters to or from Extended asm without using input or output operands. * Passing parameters to or from routines written in assembler (or other languages) using non-standard calling conventions. Some developers use Local Register Variables in an attempt to improve gcc's allocation of registers, especially in large functions. In this case the register name is essentially a hint to the register allocator. While in some instances this can generate better code, improvements are subject to the whims of the allocator/optimizers. Since there are no guarantees that your improvements won't be lost, this usage of Local Register Variables is discouraged. On the MIPS platform, there is related use for local register variables with slightly different characteristics (see Defining coprocessor specifics for MIPS targets in GNU Compiler Collection (GCC) Internals). ────────────────────────────────────────────────────────────────────────────── Previous: Global Register Variables, Up: Explicit Register Variables Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Using Assembly Language with C Link: next: Alternate Keywords Link: prev: Local Register Variables Previous: Explicit Register Variables, Up: Using Assembly Language with C ────────────────────────────────────────────────────────────────────────────── 6.47.6 Size of an asm Some targets require that GCC track the size of each instruction used in order to generate correct code. Because the final length of the code produced by an asm statement is only known by the assembler, GCC must make an estimate as to how big it will be. It does this by counting the number of instructions in the pattern of the asm and multiplying that by the length of the longest instruction supported by that processor. (When working out the number of instructions, it assumes that any occurrence of a newline or of whatever statement separator character is supported by the assembler — typically ';' — indicates the end of an instruction.) Normally, GCC's estimate is adequate to ensure that correct code is generated, but it is possible to confuse the compiler if you use pseudo instructions or assembler macros that expand into multiple real instructions, or if you use assembler directives that expand to more space in the object file than is needed for a single instruction. If this happens then the assembler may produce a diagnostic saying that a label is unreachable. This size is also used for inlining decisions. If you use asm inline instead of just asm, then for inlining purposes the size of the asm is taken as the minimum size, ignoring how many instructions GCC thinks it is. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Incomplete Enums Link: prev: Size of an asm Next: Incomplete Enums, Previous: Using Assembly Language with C, Up: C ────────────────────────────────────────────────────────────────────────────── 6.48 Alternate Keywords -ansi and the various -std options disable certain keywords. This causes trouble when you want to use GNU C extensions, or a general-purpose header file that should be usable by all programs, including ISO C programs. The keywords asm, typeof and inline are not available in programs compiled with -ansi or -std (although inline can be used in a program compiled with -std=c99 or -std=c11). The ISO C99 keyword restrict is only available when -std=gnu99 (which will eventually be the default) or -std=c99 (or the equivalent -std=iso9899:1999), or an option for a later standard version, is used. The way to solve these problems is to put '__' at the beginning and end of each problematical keyword. For example, use __asm__ instead of asm, and __inline__ instead of inline. Other C compilers won't accept these alternative keywords; if you want to compile with another compiler, you can define the alternate keywords as macros to replace them with the customary keywords. It looks like this: #ifndef __GNUC__ #define __asm__ asm #endif -pedantic and other options cause warnings for many GNU C extensions. You can prevent such warnings within one expression by writing __extension__ before the expression. __extension__ has no effect aside from this. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Function Names Link: prev: Alternate Keywords Next: Function Names, Previous: Alternate Keywords, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.49 Incomplete enum Types You can define an enum tag without specifying its possible values. This results in an incomplete type, much like what you get if you write struct foo without describing the elements. A later declaration that does specify the possible values completes the type. You cannot allocate variables or storage using the type while it is incomplete. However, you can work with pointers to that type. This extension may not be very useful, but it makes the handling of enum more consistent with the way struct and union are handled. This extension is not supported by GNU C++. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Return Address Link: prev: Incomplete Enums Next: Return Address, Previous: Incomplete Enums, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.50 Function Names as Strings GCC provides three magic constants that hold the name of the current function as a string. In C++11 and later modes, all three are treated as constant expressions and can be used in constexpr constexts. The first of these constants is __func__, which is part of the C99 standard: The identifier __func__ is implicitly declared by the translator as if, immediately following the opening brace of each function definition, the declaration static const char __func__[] = "function-name"; appeared, where function-name is the name of the lexically-enclosing function. This name is the unadorned name of the function. As an extension, at file (or, in C++, namespace scope), __func__ evaluates to the empty string. __FUNCTION__ is another name for __func__, provided for backward compatibility with old versions of GCC. In C, __PRETTY_FUNCTION__ is yet another name for __func__, except that at file (or, in C++, namespace scope), it evaluates to the string "top level". In addition, in C++, __PRETTY_FUNCTION__ contains the signature of the function as well as its bare name. For example, this program: extern "C" int printf (const char *, ...); class a { public: void sub (int i) { printf ("__FUNCTION__ = %s\n", __FUNCTION__); printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__); } }; int main (void) { a ax; ax.sub (0); return 0; } gives this output: __FUNCTION__ = sub __PRETTY_FUNCTION__ = void a::sub(int) These identifiers are variables, not preprocessor macros, and may not be used to initialize char arrays or be concatenated with string literals. ────────────────────────────────────────────────────────────────────────────── Next: Return Address, Previous: Incomplete Enums, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Vector Extensions Link: prev: Function Names Next: Vector Extensions, Previous: Function Names, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.51 Getting the Return or Frame Address of a Function These functions may be used to get information about the callers of a function. Built-in Function: void * __builtin_return_address (unsigned int level) This function returns the return address of the current function, or of one of its callers. The level argument is number of frames to scan up the call stack. A value of 0 yields the return address of the current function, a value of 1 yields the return address of the caller of the current function, and so forth. When inlining the expected behavior is that the function returns the address of the function that is returned to. To work around this behavior use the noinline function attribute. The level argument must be a constant integer. On some machines it may be impossible to determine the return address of any function other than the current one; in such cases, or when the top of the stack has been reached, this function returns 0 or a random value. In addition, __builtin_frame_address may be used to determine if the top of the stack has been reached. Additional post-processing of the returned value may be needed, see __builtin_extract_return_addr. Calling this function with a nonzero argument can have unpredictable effects, including crashing the calling program. As a result, calls that are considered unsafe are diagnosed when the -Wframe-address option is in effect. Such calls should only be made in debugging situations. Built-in Function: void * __builtin_extract_return_addr (void *addr) The address as returned by __builtin_return_address may have to be fed through this function to get the actual encoded address. For example, on the 31-bit S/390 platform the highest bit has to be masked out, or on SPARC platforms an offset has to be added for the true next instruction to be executed. If no fixup is needed, this function simply passes through addr. Built-in Function: void * __builtin_frob_return_address (void *addr) This function does the reverse of __builtin_extract_return_addr. Built-in Function: void * __builtin_frame_address (unsigned int level) This function is similar to __builtin_return_address, but it returns the address of the function frame rather than the return address of the function. Calling __builtin_frame_address with a value of 0 yields the frame address of the current function, a value of 1 yields the frame address of the caller of the current function, and so forth. The frame is the area on the stack that holds local variables and saved registers. The frame address is normally the address of the first word pushed on to the stack by the function. However, the exact definition depends upon the processor and the calling convention. If the processor has a dedicated frame pointer register, and the function has a frame, then __builtin_frame_address returns the value of the frame pointer register. On some machines it may be impossible to determine the frame address of any function other than the current one; in such cases, or when the top of the stack has been reached, this function returns 0 if the first frame pointer is properly initialized by the startup code. Calling this function with a nonzero argument can have unpredictable effects, including crashing the calling program. As a result, calls that are considered unsafe are diagnosed when the -Wframe-address option is in effect. Such calls should only be made in debugging situations. ────────────────────────────────────────────────────────────────────────────── Next: Vector Extensions, Previous: Function Names, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Offsetof Link: prev: Return Address Next: Offsetof, Previous: Return Address, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.52 Using Vector Instructions through Built-in Functions On some targets, the instruction set contains SIMD vector instructions which operate on multiple values contained in one large register at the same time. For example, on the x86 the MMX, 3DNow! and SSE extensions can be used this way. The first step in using these extensions is to provide the necessary data types. This should be done using an appropriate typedef: typedef int v4si __attribute__ ((vector_size (16))); The int type specifies the base type, while the attribute specifies the vector size for the variable, measured in bytes. For example, the declaration above causes the compiler to set the mode for the v4si type to be 16 bytes wide and divided into int sized units. For a 32-bit int this means a vector of 4 units of 4 bytes, and the corresponding mode of foo is V4SI. The vector_size attribute is only applicable to integral and floating scalars, although arrays, pointers, and function return values are allowed in conjunction with this construct. Only sizes that are positive power-of-two multiples of the base type size are currently allowed. All the basic integer types can be used as base types, both as signed and as unsigned: char, short, int, long, long long. In addition, float and double can be used to build floating-point vector types. Specifying a combination that is not valid for the current architecture causes GCC to synthesize the instructions using a narrower mode. For example, if you specify a variable of type V4SI and your architecture does not allow for this specific SIMD type, GCC produces code that uses 4 SIs. The types defined in this manner can be used with a subset of normal C operations. Currently, GCC allows using the following operators on these types: +, -, *, /, unary minus, ^, |, &, ~, %. The operations behave like C++ valarrays. Addition is defined as the addition of the corresponding elements of the operands. For example, in the code below, each of the 4 elements in a is added to the corresponding 4 elements in b and the resulting vector is stored in c. typedef int v4si __attribute__ ((vector_size (16))); v4si a, b, c; c = a + b; Subtraction, multiplication, division, and the logical operations operate in a similar manner. Likewise, the result of using the unary minus or complement operators on a vector type is a vector whose elements are the negative or complemented values of the corresponding elements in the operand. It is possible to use shifting operators <<, >> on integer-type vectors. The operation is defined as following: {a0, a1, …, an} >> {b0, b1, …, bn} == {a0 >> b0, a1 >> b1, …, an >> bn}. Vector operands must have the same number of elements. For convenience, it is allowed to use a binary vector operation where one operand is a scalar. In that case the compiler transforms the scalar operand into a vector where each element is the scalar from the operation. The transformation happens only if the scalar could be safely converted to the vector-element type. Consider the following code. typedef int v4si __attribute__ ((vector_size (16))); v4si a, b, c; long l; a = b + 1; /* a = b + {1,1,1,1}; */ a = 2 * b; /* a = {2,2,2,2} * b; */ a = l + a; /* Error, cannot convert long to int. */ Vectors can be subscripted as if the vector were an array with the same number of elements and base type. Out of bound accesses invoke undefined behavior at run time. Warnings for out of bound accesses for vector subscription can be enabled with -Warray-bounds. Vector comparison is supported with standard comparison operators: ==, !=, <, <=, >, >=. Comparison operands can be vector expressions of integer-type or real-type. Comparison between integer-type vectors and real-type vectors are not supported. The result of the comparison is a vector of the same width and number of elements as the comparison operands with a signed integral element type. Vectors are compared element-wise producing 0 when comparison is false and -1 (constant of the appropriate type where all bits are set) otherwise. Consider the following example. typedef int v4si __attribute__ ((vector_size (16))); v4si a = {1,2,3,4}; v4si b = {3,2,1,4}; v4si c; c = a > b; /* The result would be {0, 0,-1, 0} */ c = a == b; /* The result would be {0,-1, 0,-1} */ In C++, the ternary operator ?: is available. a?b:c, where b and c are vectors of the same type and a is an integer vector with the same number of elements of the same size as b and c, computes all three arguments and creates a vector {a[0]?b[0]:c[0], a[1]?b[1]:c[1], …}. Note that unlike in OpenCL, a is thus interpreted as a != 0 and not a < 0. As in the case of binary operations, this syntax is also accepted when one of b or c is a scalar that is then transformed into a vector. If both b and c are scalars and the type of true?b:c has the same size as the element type of a, then b and c are converted to a vector type whose elements have this type and with the same number of elements as a. In C++, the logic operators !, &&, || are available for vectors. !v is equivalent to v == 0, a && b is equivalent to a!=0 & b!=0 and a || b is equivalent to a!=0 | b!=0. For mixed operations between a scalar s and a vector v, s && v is equivalent to s?v!=0:0 (the evaluation is short-circuit) and v && s is equivalent to v!=0 & (s?-1:0). Vector shuffling is available using functions __builtin_shuffle (vec, mask) and __builtin_shuffle (vec0, vec1, mask). Both functions construct a permutation of elements from one or two vectors and return a vector of the same type as the input vector(s). The mask is an integral vector with the same width (W) and element count (N) as the output vector. The elements of the input vectors are numbered in memory ordering of vec0 beginning at 0 and vec1 beginning at N. The elements of mask are considered modulo N in the single-operand case and modulo 2*N in the two-operand case. Consider the following example, typedef int v4si __attribute__ ((vector_size (16))); v4si a = {1,2,3,4}; v4si b = {5,6,7,8}; v4si mask1 = {0,1,1,3}; v4si mask2 = {0,4,2,5}; v4si res; res = __builtin_shuffle (a, mask1); /* res is {1,2,2,4} */ res = __builtin_shuffle (a, b, mask2); /* res is {1,5,3,6} */ Note that __builtin_shuffle is intentionally semantically compatible with the OpenCL shuffle and shuffle2 functions. You can declare variables and use them in function calls and returns, as well as in assignments and some casts. You can specify a vector type as a return type for a function. Vector types can also be used as function arguments. It is possible to cast from one vector type to another, provided they are of the same size (in fact, you can also cast vectors to and from other datatypes of the same size). You cannot operate between vectors of different lengths or different signedness without a cast. Vector conversion is available using the __builtin_convertvector (vec, vectype) function. vec must be an expression with integral or floating vector type and vectype an integral or floating vector type with the same number of elements. The result has vectype type and value of a C cast of every element of vec to the element type of vectype. Consider the following example, typedef int v4si __attribute__ ((vector_size (16))); typedef float v4sf __attribute__ ((vector_size (16))); typedef double v4df __attribute__ ((vector_size (32))); typedef unsigned long long v4di __attribute__ ((vector_size (32))); v4si a = {1,-2,3,-4}; v4sf b = {1.5f,-2.5f,3.f,7.f}; v4di c = {1ULL,5ULL,0ULL,10ULL}; v4sf d = __builtin_convertvector (a, v4sf); /* d is {1.f,-2.f,3.f,-4.f} */ /* Equivalent of: v4sf d = { (float)a[0], (float)a[1], (float)a[2], (float)a[3] }; */ v4df e = __builtin_convertvector (a, v4df); /* e is {1.,-2.,3.,-4.} */ v4df f = __builtin_convertvector (b, v4df); /* f is {1.5,-2.5,3.,7.} */ v4si g = __builtin_convertvector (f, v4si); /* g is {1,-2,3,7} */ v4si h = __builtin_convertvector (c, v4si); /* h is {1,5,0,10} */ Sometimes it is desirable to write code using a mix of generic vector operations (for clarity) and machine-specific vector intrinsics (to access vector instructions that are not exposed via generic built-ins). On x86, intrinsic functions for integer vectors typically use the same vector type __m128i irrespective of how they interpret the vector, making it necessary to cast their arguments and return values from/to other vector types. In C, you can make use of a union type: #include typedef unsigned char u8x16 __attribute__ ((vector_size (16))); typedef unsigned int u32x4 __attribute__ ((vector_size (16))); typedef union { __m128i mm; u8x16 u8; u32x4 u32; } v128; for variables that can be used with both built-in operators and x86 intrinsics: v128 x, y = { 0 }; memcpy (&x, ptr, sizeof x); y.u8 += 0x80; x.mm = _mm_adds_epu8 (x.mm, y.mm); x.u32 &= 0xffffff; /* Instead of a variable, a compound literal may be used to pass the return value of an intrinsic call to a function expecting the union: */ v128 foo (v128); x = foo ((v128) {_mm_adds_epu8 (x.mm, y.mm)}); ────────────────────────────────────────────────────────────────────────────── 6.53 Support for offsetof GCC implements for both C and C++ a syntactic extension to implement the offsetof macro. primary: "__builtin_offsetof" "(" typename "," offsetof_member_designator ")" offsetof_member_designator: identifier | offsetof_member_designator "." identifier | offsetof_member_designator "[" expr "]" This extension is sufficient such that #define offsetof(type, member) __builtin_offsetof (type, member) is a suitable definition of the offsetof macro. In C++, type may be dependent. In either case, member may consist of a single identifier, or a sequence of member accesses and array references. ────────────────────────────────────────────────────────────────────────────── 6.54 Legacy __sync Built-in Functions for Atomic Memory Access The following built-in functions are intended to be compatible with those described in the Intel Itanium Processor-specific Application Binary Interface, section 7.4. As such, they depart from normal GCC practice by not using the '__builtin_' prefix and also by being overloaded so that they work on multiple types. The definition given in the Intel documentation allows only for the use of the types int, long, long long or their unsigned counterparts. GCC allows any scalar type that is 1, 2, 4 or 8 bytes in size other than the C type _Bool or the C++ type bool. Operations on pointer arguments are performed as if the operands were of the uintptr_t type. That is, they are not scaled by the size of the type to which the pointer points. These functions are implemented in terms of the '__atomic' builtins (see __atomic Builtins). They should not be used for new code which should use the '__atomic' builtins instead. Not all operations are supported by all target processors. If a particular operation cannot be implemented on the target processor, a warning is generated and a call to an external function is generated. The external function carries the same name as the built-in version, with an additional suffix '_n' where n is the size of the data type. In most cases, these built-in functions are considered a full barrier. That is, no memory operand is moved across the operation, either forward or backward. Further, instructions are issued as necessary to prevent the processor from speculating loads across the operation and from queuing stores after the operation. All of the routines are described in the Intel documentation to take “an optional list of variables protected by the memory barrier”. It's not clear what is meant by that; it could mean that only the listed variables are protected, or it could mean a list of additional variables to be protected. The list is ignored by GCC which treats it as empty. GCC interprets an empty list as meaning that all globally accessible variables should be protected. type __sync_fetch_and_add (type *ptr, type value, ...) type __sync_fetch_and_sub (type *ptr, type value, ...) type __sync_fetch_and_or (type *ptr, type value, ...) type __sync_fetch_and_and (type *ptr, type value, ...) type __sync_fetch_and_xor (type *ptr, type value, ...) type __sync_fetch_and_nand (type *ptr, type value, ...) These built-in functions perform the operation suggested by the name, and returns the value that had previously been in memory. That is, operations on integer operands have the following semantics. Operations on pointer arguments are performed as if the operands were of the uintptr_t type. That is, they are not scaled by the size of the type to which the pointer points. { tmp = *ptr; *ptr op= value; return tmp; } { tmp = *ptr; *ptr = ~(tmp & value); return tmp; } // nand The object pointed to by the first argument must be of integer or pointer type. It must not be a boolean type. Note: GCC 4.4 and later implement __sync_fetch_and_nand as *ptr = ~(tmp & value) instead of *ptr = ~tmp & value. type __sync_add_and_fetch (type *ptr, type value, ...) type __sync_sub_and_fetch (type *ptr, type value, ...) type __sync_or_and_fetch (type *ptr, type value, ...) type __sync_and_and_fetch (type *ptr, type value, ...) type __sync_xor_and_fetch (type *ptr, type value, ...) type __sync_nand_and_fetch (type *ptr, type value, ...) These built-in functions perform the operation suggested by the name, and return the new value. That is, operations on integer operands have the following semantics. Operations on pointer operands are performed as if the operand's type were uintptr_t. { *ptr op= value; return *ptr; } { *ptr = ~(*ptr & value); return *ptr; } // nand The same constraints on arguments apply as for the corresponding __sync_op_and_fetch built-in functions. Note: GCC 4.4 and later implement __sync_nand_and_fetch as *ptr = ~(*ptr & value) instead of *ptr = ~*ptr & value. bool __sync_bool_compare_and_swap (type *ptr, type oldval, type newval, ...) type __sync_val_compare_and_swap (type *ptr, type oldval, type newval, ...) These built-in functions perform an atomic compare and swap. That is, if the current value of *ptr is oldval, then write newval into *ptr. The “bool” version returns true if the comparison is successful and newval is written. The “val” version returns the contents of *ptr before the operation. __sync_synchronize (...) This built-in function issues a full memory barrier. type __sync_lock_test_and_set (type *ptr, type value, ...) This built-in function, as described by Intel, is not a traditional test-and-set operation, but rather an atomic exchange operation. It writes value into *ptr, and returns the previous contents of *ptr. Many targets have only minimal support for such locks, and do not support a full exchange operation. In this case, a target may support reduced functionality here by which the only valid value to store is the immediate constant 1. The exact value actually stored in *ptr is implementation defined. This built-in function is not a full barrier, but rather an acquire barrier. This means that references after the operation cannot move to (or be speculated to) before the operation, but previous memory stores may not be globally visible yet, and previous memory loads may not yet be satisfied. void __sync_lock_release (type *ptr, ...) This built-in function releases the lock acquired by __sync_lock_test_and_set. Normally this means writing the constant 0 to *ptr. This built-in function is not a full barrier, but rather a release barrier. This means that all previous memory stores are globally visible, and all previous memory loads have been satisfied, but following memory reads are not prevented from being speculated to before the barrier. ────────────────────────────────────────────────────────────────────────────── Next: __atomic Builtins, Previous: Offsetof, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Integer Overflow Builtins Link: prev: __sync Builtins Next: Integer Overflow Builtins, Previous: __sync Builtins, Up: C ────────────────────────────────────────────────────────────────────────────── 6.55 Built-in Functions for Memory Model Aware Atomic Operations The following built-in functions approximately match the requirements for the C++11 memory model. They are all identified by being prefixed with '__atomic' and most are overloaded so that they work with multiple types. These functions are intended to replace the legacy '__sync' builtins. The main difference is that the memory order that is requested is a parameter to the functions. New code should always use the '__atomic' builtins rather than the '__sync' builtins. Note that the '__atomic' builtins assume that programs will conform to the C++11 memory model. In particular, they assume that programs are free of data races. See the C++11 standard for detailed requirements. The '__atomic' builtins can be used with any integral scalar or pointer type that is 1, 2, 4, or 8 bytes in length. 16-byte integral types are also allowed if '__int128' (see __int128) is supported by the architecture. The four non-arithmetic functions (load, store, exchange, and compare_exchange) all have a generic version as well. This generic version works on any data type. It uses the lock-free built-in function if the specific data type size makes that possible; otherwise, an external call is left to be resolved at run time. This external call is the same format with the addition of a 'size_t' parameter inserted as the first parameter indicating the size of the object being pointed to. All objects must be the same size. There are 6 different memory orders that can be specified. These map to the C++11 memory orders with the same names, see the C++11 standard or the GCC wiki on atomic synchronization for detailed definitions. Individual targets may also support additional memory orders for use on specific architectures. Refer to the target documentation for details of these. An atomic operation can both constrain code motion and be mapped to hardware instructions for synchronization between threads (e.g., a fence). To which extent this happens is controlled by the memory orders, which are listed here in approximately ascending order of strength. The description of each memory order is only meant to roughly illustrate the effects and is not a specification; see the C++11 memory model for precise semantics. __ATOMIC_RELAXED Implies no inter-thread ordering constraints. __ATOMIC_CONSUME This is currently implemented using the stronger __ATOMIC_ACQUIRE memory order because of a deficiency in C++11's semantics for memory_order_consume. __ATOMIC_ACQUIRE Creates an inter-thread happens-before constraint from the release (or stronger) semantic store to this acquire load. Can prevent hoisting of code to before the operation. __ATOMIC_RELEASE Creates an inter-thread happens-before constraint to acquire (or stronger) semantic loads that read from this release store. Can prevent sinking of code to after the operation. __ATOMIC_ACQ_REL Combines the effects of both __ATOMIC_ACQUIRE and __ATOMIC_RELEASE. __ATOMIC_SEQ_CST Enforces total ordering with all other __ATOMIC_SEQ_CST operations. Note that in the C++11 memory model, fences (e.g., '__atomic_thread_fence') take effect in combination with other atomic operations on specific memory locations (e.g., atomic loads); operations on specific memory locations do not necessarily affect other operations in the same way. Target architectures are encouraged to provide their own patterns for each of the atomic built-in functions. If no target is provided, the original non-memory model set of '__sync' atomic built-in functions are used, along with any required synchronization fences surrounding it in order to achieve the proper behavior. Execution in this case is subject to the same restrictions as those built-in functions. If there is no pattern or mechanism to provide a lock-free instruction sequence, a call is made to an external routine with the same parameters to be resolved at run time. When implementing patterns for these built-in functions, the memory order parameter can be ignored as long as the pattern implements the most restrictive __ATOMIC_SEQ_CST memory order. Any of the other memory orders execute correctly with this memory order but they may not execute as efficiently as they could with a more appropriate implementation of the relaxed requirements. Note that the C++11 standard allows for the memory order parameter to be determined at run time rather than at compile time. These built-in functions map any run-time value to __ATOMIC_SEQ_CST rather than invoke a runtime library call or inline a switch statement. This is standard compliant, safe, and the simplest approach for now. The memory order parameter is a signed int, but only the lower 16 bits are reserved for the memory order. The remainder of the signed int is reserved for target use and should be 0. Use of the predefined atomic values ensures proper usage. Built-in Function: type __atomic_load_n (type *ptr, int memorder) This built-in function implements an atomic load operation. It returns the contents of *ptr. The valid memory order variants are __ATOMIC_RELAXED, __ATOMIC_SEQ_CST, __ATOMIC_ACQUIRE, and __ATOMIC_CONSUME. Built-in Function: void __atomic_load (type *ptr, type *ret, int memorder) This is the generic version of an atomic load. It returns the contents of *ptr in *ret. Built-in Function: void __atomic_store_n (type *ptr, type val, int memorder) This built-in function implements an atomic store operation. It writes val into *ptr. The valid memory order variants are __ATOMIC_RELAXED, __ATOMIC_SEQ_CST, and __ATOMIC_RELEASE. Built-in Function: void __atomic_store (type *ptr, type *val, int memorder) This is the generic version of an atomic store. It stores the value of *val into *ptr. Built-in Function: type __atomic_exchange_n (type *ptr, type val, int memorder) This built-in function implements an atomic exchange operation. It writes val into *ptr, and returns the previous contents of *ptr. The valid memory order variants are __ATOMIC_RELAXED, __ATOMIC_SEQ_CST, __ATOMIC_ACQUIRE, __ATOMIC_RELEASE, and __ATOMIC_ACQ_REL. Built-in Function: void __atomic_exchange (type *ptr, type *val, type *ret, int memorder) This is the generic version of an atomic exchange. It stores the contents of *val into *ptr. The original value of *ptr is copied into *ret. Built-in Function: bool __atomic_compare_exchange_n (type *ptr, type *expected, type desired, bool weak, int success_memorder, int failure_memorder) This built-in function implements an atomic compare and exchange operation. This compares the contents of *ptr with the contents of *expected. If equal, the operation is a read-modify-write operation that writes desired into *ptr. If they are not equal, the operation is a read and the current contents of *ptr are written into *expected. weak is true for weak compare_exchange, which may fail spuriously, and false for the strong variation, which never fails spuriously. Many targets only offer the strong variation and ignore the parameter. When in doubt, use the strong variation. If desired is written into *ptr then true is returned and memory is affected according to the memory order specified by success_memorder. There are no restrictions on what memory order can be used here. Otherwise, false is returned and memory is affected according to failure_memorder. This memory order cannot be __ATOMIC_RELEASE nor __ATOMIC_ACQ_REL. It also cannot be a stronger order than that specified by success_memorder. Built-in Function: bool __atomic_compare_exchange (type *ptr, type *expected, type *desired, bool weak, int success_memorder, int failure_memorder) This built-in function implements the generic version of __atomic_compare_exchange. The function is virtually identical to __atomic_compare_exchange_n, except the desired value is also a pointer. Built-in Function: type __atomic_add_fetch (type *ptr, type val, int memorder) Built-in Function: type __atomic_sub_fetch (type *ptr, type val, int memorder) Built-in Function: type __atomic_and_fetch (type *ptr, type val, int memorder) Built-in Function: type __atomic_xor_fetch (type *ptr, type val, int memorder) Built-in Function: type __atomic_or_fetch (type *ptr, type val, int memorder) Built-in Function: type __atomic_nand_fetch (type *ptr, type val, int memorder) These built-in functions perform the operation suggested by the name, and return the result of the operation. Operations on pointer arguments are performed as if the operands were of the uintptr_t type. That is, they are not scaled by the size of the type to which the pointer points. { *ptr op= val; return *ptr; } { *ptr = ~(*ptr & val); return *ptr; } // nand The object pointed to by the first argument must be of integer or pointer type. It must not be a boolean type. All memory orders are valid. Built-in Function: type __atomic_fetch_add (type *ptr, type val, int memorder) Built-in Function: type __atomic_fetch_sub (type *ptr, type val, int memorder) Built-in Function: type __atomic_fetch_and (type *ptr, type val, int memorder) Built-in Function: type __atomic_fetch_xor (type *ptr, type val, int memorder) Built-in Function: type __atomic_fetch_or (type *ptr, type val, int memorder) Built-in Function: type __atomic_fetch_nand (type *ptr, type val, int memorder) These built-in functions perform the operation suggested by the name, and return the value that had previously been in *ptr. Operations on pointer arguments are performed as if the operands were of the uintptr_t type. That is, they are not scaled by the size of the type to which the pointer points. { tmp = *ptr; *ptr op= val; return tmp; } { tmp = *ptr; *ptr = ~(*ptr & val); return tmp; } // nand The same constraints on arguments apply as for the corresponding __atomic_op_fetch built-in functions. All memory orders are valid. Built-in Function: bool __atomic_test_and_set (void *ptr, int memorder) This built-in function performs an atomic test-and-set operation on the byte at *ptr. The byte is set to some implementation defined nonzero “set” value and the return value is true if and only if the previous contents were “set”. It should be only used for operands of type bool or char. For other types only part of the value may be set. All memory orders are valid. Built-in Function: void __atomic_clear (bool *ptr, int memorder) This built-in function performs an atomic clear operation on *ptr. After the operation, *ptr contains 0. It should be only used for operands of type bool or char and in conjunction with __atomic_test_and_set. For other types it may only clear partially. If the type is not bool prefer using __atomic_store. The valid memory order variants are __ATOMIC_RELAXED, __ATOMIC_SEQ_CST, and __ATOMIC_RELEASE. Built-in Function: void __atomic_thread_fence (int memorder) This built-in function acts as a synchronization fence between threads based on the specified memory order. All memory orders are valid. Built-in Function: void __atomic_signal_fence (int memorder) This built-in function acts as a synchronization fence between a thread and signal handlers based in the same thread. All memory orders are valid. Built-in Function: bool __atomic_always_lock_free (size_t size, void *ptr) This built-in function returns true if objects of size bytes always generate lock-free atomic instructions for the target architecture. size must resolve to a compile-time constant and the result also resolves to a compile-time constant. ptr is an optional pointer to the object that may be used to determine alignment. A value of 0 indicates typical alignment should be used. The compiler may also ignore this parameter. if (__atomic_always_lock_free (sizeof (long long), 0)) Built-in Function: bool __atomic_is_lock_free (size_t size, void *ptr) This built-in function returns true if objects of size bytes always generate lock-free atomic instructions for the target architecture. If the built-in function is not known to be lock-free, a call is made to a runtime routine named __atomic_is_lock_free. ptr is an optional pointer to the object that may be used to determine alignment. A value of 0 indicates typical alignment should be used. The compiler may also ignore this parameter. ────────────────────────────────────────────────────────────────────────────── 6.56 Built-in Functions to Perform Arithmetic with Overflow Checking The following built-in functions allow performing simple arithmetic operations together with checking whether the operations overflowed. Built-in Function: bool __builtin_add_overflow (type1 a, type2 b, type3 *res) Built-in Function: bool __builtin_sadd_overflow (int a, int b, int *res) Built-in Function: bool __builtin_saddl_overflow (long int a, long int b, long int *res) Built-in Function: bool __builtin_saddll_overflow (long long int a, long long int b, long long int *res) Built-in Function: bool __builtin_uadd_overflow (unsigned int a, unsigned int b, unsigned int *res) Built-in Function: bool __builtin_uaddl_overflow (unsigned long int a, unsigned long int b, unsigned long int *res) Built-in Function: bool __builtin_uaddll_overflow (unsigned long long int a, unsigned long long int b, unsigned long long int *res) These built-in functions promote the first two operands into infinite precision signed type and perform addition on those promoted operands. The result is then cast to the type the third pointer argument points to and stored there. If the stored result is equal to the infinite precision result, the built-in functions return false, otherwise they return true. As the addition is performed in infinite signed precision, these built-in functions have fully defined behavior for all argument values. The first built-in function allows arbitrary integral types for operands and the result type must be pointer to some integral type other than enumerated or boolean type, the rest of the built-in functions have explicit integer types. The compiler will attempt to use hardware instructions to implement these built-in functions where possible, like conditional jump on overflow after addition, conditional jump on carry etc. Built-in Function: bool __builtin_sub_overflow (type1 a, type2 b, type3 *res) Built-in Function: bool __builtin_ssub_overflow (int a, int b, int *res) Built-in Function: bool __builtin_ssubl_overflow (long int a, long int b, long int *res) Built-in Function: bool __builtin_ssubll_overflow (long long int a, long long int b, long long int *res) Built-in Function: bool __builtin_usub_overflow (unsigned int a, unsigned int b, unsigned int *res) Built-in Function: bool __builtin_usubl_overflow (unsigned long int a, unsigned long int b, unsigned long int *res) Built-in Function: bool __builtin_usubll_overflow (unsigned long long int a, unsigned long long int b, unsigned long long int *res) These built-in functions are similar to the add overflow checking built-in functions above, except they perform subtraction, subtract the second argument from the first one, instead of addition. Built-in Function: bool __builtin_mul_overflow (type1 a, type2 b, type3 *res) Built-in Function: bool __builtin_smul_overflow (int a, int b, int *res) Built-in Function: bool __builtin_smull_overflow (long int a, long int b, long int *res) Built-in Function: bool __builtin_smulll_overflow (long long int a, long long int b, long long int *res) Built-in Function: bool __builtin_umul_overflow (unsigned int a, unsigned int b, unsigned int *res) Built-in Function: bool __builtin_umull_overflow (unsigned long int a, unsigned long int b, unsigned long int *res) Built-in Function: bool __builtin_umulll_overflow (unsigned long long int a, unsigned long long int b, unsigned long long int *res) These built-in functions are similar to the add overflow checking built-in functions above, except they perform multiplication, instead of addition. The following built-in functions allow checking if simple arithmetic operation would overflow. Built-in Function: bool __builtin_add_overflow_p (type1 a, type2 b, type3 c) Built-in Function: bool __builtin_sub_overflow_p (type1 a, type2 b, type3 c) Built-in Function: bool __builtin_mul_overflow_p (type1 a, type2 b, type3 c) These built-in functions are similar to __builtin_add_overflow, __builtin_sub_overflow, or __builtin_mul_overflow, except that they don't store the result of the arithmetic operation anywhere and the last argument is not a pointer, but some expression with integral type other than enumerated or boolean type. The built-in functions promote the first two operands into infinite precision signed type and perform addition on those promoted operands. The result is then cast to the type of the third argument. If the cast result is equal to the infinite precision result, the built-in functions return false, otherwise they return true. The value of the third argument is ignored, just the side effects in the third argument are evaluated, and no integral argument promotions are performed on the last argument. If the third argument is a bit-field, the type used for the result cast has the precision and signedness of the given bit-field, rather than precision and signedness of the underlying type. For example, the following macro can be used to portably check, at compile-time, whether or not adding two constant integers will overflow, and perform the addition only when it is known to be safe and not to trigger a -Woverflow warning. #define INT_ADD_OVERFLOW_P(a, b) \ __builtin_add_overflow_p (a, b, (__typeof__ ((a) + (b))) 0) enum { A = INT_MAX, B = 3, C = INT_ADD_OVERFLOW_P (A, B) ? 0 : A + B, D = __builtin_add_overflow_p (1, SCHAR_MAX, (signed char) 0) }; The compiler will attempt to use hardware instructions to implement these built-in functions where possible, like conditional jump on overflow after addition, conditional jump on carry etc. ────────────────────────────────────────────────────────────────────────────── 6.57 x86-Specific Memory Model Extensions for Transactional Memory The x86 architecture supports additional memory ordering flags to mark critical sections for hardware lock elision. These must be specified in addition to an existing memory order to atomic intrinsics. __ATOMIC_HLE_ACQUIRE Start lock elision on a lock variable. Memory order must be __ATOMIC_ACQUIRE or stronger. __ATOMIC_HLE_RELEASE End lock elision on a lock variable. Memory order must be __ATOMIC_RELEASE or stronger. When a lock acquire fails, it is required for good performance to abort the transaction quickly. This can be done with a _mm_pause. #include // For _mm_pause int lockvar; /* Acquire lock with lock elision */ while (__atomic_exchange_n(&lockvar, 1, __ATOMIC_ACQUIRE|__ATOMIC_HLE_ACQUIRE)) _mm_pause(); /* Abort failed transaction */ ... /* Free lock with lock elision */ __atomic_store_n(&lockvar, 0, __ATOMIC_RELEASE|__ATOMIC_HLE_RELEASE); ────────────────────────────────────────────────────────────────────────────── 6.58 Object Size Checking Built-in Functions GCC implements a limited buffer overflow protection mechanism that can prevent some buffer overflow attacks by determining the sizes of objects into which data is about to be written and preventing the writes when the size isn't sufficient. The built-in functions described below yield the best results when used together and when optimization is enabled. For example, to detect object sizes across function boundaries or to follow pointer assignments through non-trivial control flow they rely on various optimization passes enabled with -O2. However, to a limited extent, they can be used without optimization as well. Built-in Function: size_t __builtin_object_size (const void * ptr, int type) is a built-in construct that returns a constant number of bytes from ptr to the end of the object ptr pointer points to (if known at compile time). To determine the sizes of dynamically allocated objects the function relies on the allocation functions called to obtain the storage to be declared with the alloc_size attribute (see Common Function Attributes). __builtin_object_size never evaluates its arguments for side effects. If there are any side effects in them, it returns (size_t) -1 for type 0 or 1 and (size_t) 0 for type 2 or 3. If there are multiple objects ptr can point to and all of them are known at compile time, the returned number is the maximum of remaining byte counts in those objects if type & 2 is 0 and minimum if nonzero. If it is not possible to determine which objects ptr points to at compile time, __builtin_object_size should return (size_t) -1 for type 0 or 1 and (size_t) 0 for type 2 or 3. type is an integer constant from 0 to 3. If the least significant bit is clear, objects are whole variables, if it is set, a closest surrounding subobject is considered the object a pointer points to. The second bit determines if maximum or minimum of remaining bytes is computed. struct V { char buf1[10]; int b; char buf2[10]; } var; char *p = &var.buf1[1], *q = &var.b; /* Here the object p points to is var. */ assert (__builtin_object_size (p, 0) == sizeof (var) - 1); /* The subobject p points to is var.buf1. */ assert (__builtin_object_size (p, 1) == sizeof (var.buf1) - 1); /* The object q points to is var. */ assert (__builtin_object_size (q, 0) == (char *) (&var + 1) - (char *) &var.b); /* The subobject q points to is var.b. */ assert (__builtin_object_size (q, 1) == sizeof (var.b)); There are built-in functions added for many common string operation functions, e.g., for memcpy __builtin___memcpy_chk built-in is provided. This built-in has an additional last argument, which is the number of bytes remaining in the object the dest argument points to or (size_t) -1 if the size is not known. The built-in functions are optimized into the normal string functions like memcpy if the last argument is (size_t) -1 or if it is known at compile time that the destination object will not be overflowed. If the compiler can determine at compile time that the object will always be overflowed, it issues a warning. The intended use can be e.g. #undef memcpy #define bos0(dest) __builtin_object_size (dest, 0) #define memcpy(dest, src, n) \ __builtin___memcpy_chk (dest, src, n, bos0 (dest)) char *volatile p; char buf[10]; /* It is unknown what object p points to, so this is optimized into plain memcpy - no checking is possible. */ memcpy (p, "abcde", n); /* Destination is known and length too. It is known at compile time there will be no overflow. */ memcpy (&buf[5], "abcde", 5); /* Destination is known, but the length is not known at compile time. This will result in __memcpy_chk call that can check for overflow at run time. */ memcpy (&buf[5], "abcde", n); /* Destination is known and it is known at compile time there will be overflow. There will be a warning and __memcpy_chk call that will abort the program at run time. */ memcpy (&buf[6], "abcde", 5); Such built-in functions are provided for memcpy, mempcpy, memmove, memset, strcpy, stpcpy, strncpy, strcat and strncat. There are also checking built-in functions for formatted output functions. int __builtin___sprintf_chk (char *s, int flag, size_t os, const char *fmt, ...); int __builtin___snprintf_chk (char *s, size_t maxlen, int flag, size_t os, const char *fmt, ...); int __builtin___vsprintf_chk (char *s, int flag, size_t os, const char *fmt, va_list ap); int __builtin___vsnprintf_chk (char *s, size_t maxlen, int flag, size_t os, const char *fmt, va_list ap); The added flag argument is passed unchanged to __sprintf_chk etc. functions and can contain implementation specific flags on what additional security measures the checking function might take, such as handling %n differently. The os argument is the object size s points to, like in the other built-in functions. There is a small difference in the behavior though, if os is (size_t) -1, the built-in functions are optimized into the non-checking functions only if flag is 0, otherwise the checking function is called with os argument set to (size_t) -1. In addition to this, there are checking built-in functions __builtin___printf_chk, __builtin___vprintf_chk, __builtin___fprintf_chk and __builtin___vfprintf_chk. These have just one additional argument, flag, right before format string fmt. If the compiler is able to optimize them to fputc etc. functions, it does, otherwise the checking function is called and the flag argument passed to it. ────────────────────────────────────────────────────────────────────────────── Next: Other Builtins, Previous: x86 specific memory model extensions for Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Target Builtins Link: prev: Object Size Checking Next: Target Builtins, Previous: Object Size Checking, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.59 Other Built-in Functions Provided by GCC GCC provides a large number of built-in functions other than the ones mentioned above. Some of these are for internal use in the processing of exceptions or variable-length argument lists and are not documented here because they may change from time to time; we do not recommend general use of these functions. The remaining functions are provided for optimization purposes. With the exception of built-ins that have library equivalents such as the standard C library functions discussed below, or that expand to library calls, GCC built-in functions are always expanded inline and thus do not have corresponding entry points and their address cannot be obtained. Attempting to use them in an expression other than a function call results in a compile-time error. GCC includes built-in versions of many of the functions in the standard C library. These functions come in two forms: one whose names start with the __builtin_ prefix, and the other without. Both forms have the same type (including prototype), the same address (when their address is taken), and the same meaning as the C library functions even if you specify the -fno-builtin option see C Dialect Options). Many of these functions are only optimized in certain cases; if they are not optimized in a particular case, a call to the library function is emitted. Outside strict ISO C mode (-ansi, -std=c90, -std=c99 or -std=c11), the functions _exit, alloca, bcmp, bzero, dcgettext, dgettext, dremf, dreml, drem, exp10f, exp10l, exp10, ffsll, ffsl, ffs, fprintf_unlocked, fputs_unlocked, gammaf, gammal, gamma, gammaf_r, gammal_r, gamma_r, gettext, index, isascii, j0f, j0l, j0, j1f, j1l, j1, jnf, jnl, jn, lgammaf_r, lgammal_r, lgamma_r, mempcpy, pow10f, pow10l, pow10, printf_unlocked, rindex, scalbf, scalbl, scalb, signbit, signbitf, signbitl, signbitd32, signbitd64, signbitd128, significandf, significandl, significand, sincosf, sincosl, sincos, stpcpy, stpncpy, strcasecmp, strdup, strfmon, strncasecmp, strndup, strnlen, toascii, y0f, y0l, y0, y1f, y1l, y1, ynf, ynl and yn may be handled as built-in functions. All these functions have corresponding versions prefixed with __builtin_, which may be used even in strict C90 mode. The ISO C99 functions _Exit, acoshf, acoshl, acosh, asinhf, asinhl, asinh, atanhf, atanhl, atanh, cabsf, cabsl, cabs, cacosf, cacoshf, cacoshl, cacosh, cacosl, cacos, cargf, cargl, carg, casinf, casinhf, casinhl, casinh, casinl, casin, catanf, catanhf, catanhl, catanh, catanl, catan, cbrtf, cbrtl, cbrt, ccosf, ccoshf, ccoshl, ccosh, ccosl, ccos, cexpf, cexpl, cexp, cimagf, cimagl, cimag, clogf, clogl, clog, conjf, conjl, conj, copysignf, copysignl, copysign, cpowf, cpowl, cpow, cprojf, cprojl, cproj, crealf, creall, creal, csinf, csinhf, csinhl, csinh, csinl, csin, csqrtf, csqrtl, csqrt, ctanf, ctanhf, ctanhl, ctanh, ctanl, ctan, erfcf, erfcl, erfc, erff, erfl, erf, exp2f, exp2l, exp2, expm1f, expm1l, expm1, fdimf, fdiml, fdim, fmaf, fmal, fmaxf, fmaxl, fmax, fma, fminf, fminl, fmin, hypotf, hypotl, hypot, ilogbf, ilogbl, ilogb, imaxabs, isblank, iswblank, lgammaf, lgammal, lgamma, llabs, llrintf, llrintl, llrint, llroundf, llroundl, llround, log1pf, log1pl, log1p, log2f, log2l, log2, logbf, logbl, logb, lrintf, lrintl, lrint, lroundf, lroundl, lround, nearbyintf, nearbyintl, nearbyint, nextafterf, nextafterl, nextafter, nexttowardf, nexttowardl, nexttoward, remainderf, remainderl, remainder, remquof, remquol, remquo, rintf, rintl, rint, roundf, roundl, round, scalblnf, scalblnl, scalbln, scalbnf, scalbnl, scalbn, snprintf, tgammaf, tgammal, tgamma, truncf, truncl, trunc, vfscanf, vscanf, vsnprintf and vsscanf are handled as built-in functions except in strict ISO C90 mode (-ansi or -std=c90). There are also built-in versions of the ISO C99 functions acosf, acosl, asinf, asinl, atan2f, atan2l, atanf, atanl, ceilf, ceill, cosf, coshf, coshl, cosl, expf, expl, fabsf, fabsl, floorf, floorl, fmodf, fmodl, frexpf, frexpl, ldexpf, ldexpl, log10f, log10l, logf, logl, modfl, modf, powf, powl, sinf, sinhf, sinhl, sinl, sqrtf, sqrtl, tanf, tanhf, tanhl and tanl that are recognized in any mode since ISO C90 reserves these names for the purpose to which ISO C99 puts them. All these functions have corresponding versions prefixed with __builtin_. There are also built-in functions __builtin_fabsfn, __builtin_fabsfnx, __builtin_copysignfn and __builtin_copysignfnx, corresponding to the TS 18661-3 functions fabsfn, fabsfnx, copysignfn and copysignfnx, for supported types _Floatn and _Floatnx. There are also GNU extension functions clog10, clog10f and clog10l which names are reserved by ISO C99 for future use. All these functions have versions prefixed with __builtin_. The ISO C94 functions iswalnum, iswalpha, iswcntrl, iswdigit, iswgraph, iswlower, iswprint, iswpunct, iswspace, iswupper, iswxdigit, towlower and towupper are handled as built-in functions except in strict ISO C90 mode (-ansi or -std=c90). The ISO C90 functions abort, abs, acos, asin, atan2, atan, calloc, ceil, cosh, cos, exit, exp, fabs, floor, fmod, fprintf, fputs, frexp, fscanf, isalnum, isalpha, iscntrl, isdigit, isgraph, islower, isprint, ispunct, isspace, isupper, isxdigit, tolower, toupper, labs, ldexp, log10, log, malloc, memchr, memcmp, memcpy, memset, modf, pow, printf, putchar, puts, scanf, sinh, sin, snprintf, sprintf, sqrt, sscanf, strcat, strchr, strcmp, strcpy, strcspn, strlen, strncat, strncmp, strncpy, strpbrk, strrchr, strspn, strstr, tanh, tan, vfprintf, vprintf and vsprintf are all recognized as built-in functions unless -fno-builtin is specified (or -fno-builtin-function is specified for an individual function). All of these functions have corresponding versions prefixed with __builtin_. GCC provides built-in versions of the ISO C99 floating-point comparison macros that avoid raising exceptions for unordered operands. They have the same names as the standard macros ( isgreater, isgreaterequal, isless, islessequal, islessgreater, and isunordered) , with __builtin_ prefixed. We intend for a library implementor to be able to simply #define each standard macro to its built-in equivalent. In the same fashion, GCC provides fpclassify, isfinite, isinf_sign, isnormal and signbit built-ins used with __builtin_ prefixed. The isinf and isnan built-in functions appear both with and without the __builtin_ prefix. Built-in Function: void *__builtin_alloca (size_t size) The __builtin_alloca function must be called at block scope. The function allocates an object size bytes large on the stack of the calling function. The object is aligned on the default stack alignment boundary for the target determined by the __BIGGEST_ALIGNMENT__ macro. The __builtin_alloca function returns a pointer to the first byte of the allocated object. The lifetime of the allocated object ends just before the calling function returns to its caller. This is so even when __builtin_alloca is called within a nested block. For example, the following function allocates eight objects of n bytes each on the stack, storing a pointer to each in consecutive elements of the array a. It then passes the array to function g which can safely use the storage pointed to by each of the array elements. void f (unsigned n) { void *a [8]; for (int i = 0; i != 8; ++i) a [i] = __builtin_alloca (n); g (a, n); // safe } Since the __builtin_alloca function doesn't validate its argument it is the responsibility of its caller to make sure the argument doesn't cause it to exceed the stack size limit. The __builtin_alloca function is provided to make it possible to allocate on the stack arrays of bytes with an upper bound that may be computed at run time. Since C99 Variable Length Arrays offer similar functionality under a portable, more convenient, and safer interface they are recommended instead, in both C99 and C++ programs where GCC provides them as an extension. See Variable Length, for details. Built-in Function: void *__builtin_alloca_with_align (size_t size, size_t alignment) The __builtin_alloca_with_align function must be called at block scope. The function allocates an object size bytes large on the stack of the calling function. The allocated object is aligned on the boundary specified by the argument alignment whose unit is given in bits (not bytes). The size argument must be positive and not exceed the stack size limit. The alignment argument must be a constant integer expression that evaluates to a power of 2 greater than or equal to CHAR_BIT and less than some unspecified maximum. Invocations with other values are rejected with an error indicating the valid bounds. The function returns a pointer to the first byte of the allocated object. The lifetime of the allocated object ends at the end of the block in which the function was called. The allocated storage is released no later than just before the calling function returns to its caller, but may be released at the end of the block in which the function was called. For example, in the following function the call to g is unsafe because when overalign is non-zero, the space allocated by __builtin_alloca_with_align may have been released at the end of the if statement in which it was called. void f (unsigned n, bool overalign) { void *p; if (overalign) p = __builtin_alloca_with_align (n, 64 /* bits */); else p = __builtin_alloc (n); g (p, n); // unsafe } Since the __builtin_alloca_with_align function doesn't validate its size argument it is the responsibility of its caller to make sure the argument doesn't cause it to exceed the stack size limit. The __builtin_alloca_with_align function is provided to make it possible to allocate on the stack overaligned arrays of bytes with an upper bound that may be computed at run time. Since C99 Variable Length Arrays offer the same functionality under a portable, more convenient, and safer interface they are recommended instead, in both C99 and C++ programs where GCC provides them as an extension. See Variable Length, for details. Built-in Function: void *__builtin_alloca_with_align_and_max (size_t size, size_t alignment, size_t max_size) Similar to __builtin_alloca_with_align but takes an extra argument specifying an upper bound for size in case its value cannot be computed at compile time, for use by -fstack-usage, -Wstack-usage and -Walloca-larger-than. max_size must be a constant integer expression, it has no effect on code generation and no attempt is made to check its compatibility with size. Built-in Function: bool __builtin_has_attribute (type-or-expression, attribute) The __builtin_has_attribute function evaluates to an integer constant expression equal to true if the symbol or type referenced by the type-or-expression argument has been declared with the attribute referenced by the second argument. For an type-or-expression argument that does not reference a symbol, since attributes do not apply to expressions the built-in consider the type of the argument. Neither argument is evaluated. The type-or-expression argument is subject to the same restrictions as the argument to typeof (see Typeof). The attribute argument is an attribute name optionally followed by a comma-separated list of arguments enclosed in parentheses. Both forms of attribute names—with and without double leading and trailing underscores—are recognized. See Attribute Syntax, for details. When no attribute arguments are specified for an attribute that expects one or more arguments the function returns true if type-or-expression has been declared with the attribute regardless of the attribute argument values. Arguments provided for an attribute that expects some are validated and matched up to the provided number. The function returns true if all provided arguments match. For example, the first call to the function below evaluates to true because x is declared with the aligned attribute but the second call evaluates to false because x is declared aligned (8) and not aligned (4). __attribute__ ((aligned (8))) int x; _Static_assert (__builtin_has_attribute (x, aligned), "aligned"); _Static_assert (!__builtin_has_attribute (x, aligned (4)), "aligned (4)"); Due to a limitation the __builtin_has_attribute function returns false for the mode attribute even if the type or variable referenced by the type-or-expression argument was declared with one. The function is also not supported with labels, and in C with enumerators. Note that unlike the __has_attribute preprocessor operator which is suitable for use in #if preprocessing directives __builtin_has_attribute is an intrinsic function that is not recognized in such contexts. Built-in Function: type __builtin_speculation_safe_value (type val, type failval) This built-in function can be used to help mitigate against unsafe speculative execution. type may be any integral type or any pointer type. 1. If the CPU is not speculatively executing the code, then val is returned. 2. If the CPU is executing speculatively then either: * The function may cause execution to pause until it is known that the code is no-longer being executed speculatively (in which case val can be returned, as above); or * The function may use target-dependent speculation tracking state to cause failval to be returned when it is known that speculative execution has incorrectly predicted a conditional branch operation. The second argument, failval, is optional and defaults to zero if omitted. GCC defines the preprocessor macro __HAVE_BUILTIN_SPECULATION_SAFE_VALUE for targets that have been updated to support this builtin. The built-in function can be used where a variable appears to be used in a safe way, but the CPU, due to speculative execution may temporarily ignore the bounds checks. Consider, for example, the following function: int array[500]; int f (unsigned untrusted_index) { if (untrusted_index < 500) return array[untrusted_index]; return 0; } If the function is called repeatedly with untrusted_index less than the limit of 500, then a branch predictor will learn that the block of code that returns a value stored in array will be executed. If the function is subsequently called with an out-of-range value it will still try to execute that block of code first until the CPU determines that the prediction was incorrect (the CPU will unwind any incorrect operations at that point). However, depending on how the result of the function is used, it might be possible to leave traces in the cache that can reveal what was stored at the out-of-bounds location. The built-in function can be used to provide some protection against leaking data in this way by changing the code to: int array[500]; int f (unsigned untrusted_index) { if (untrusted_index < 500) return array[__builtin_speculation_safe_value (untrusted_index)]; return 0; } The built-in function will either cause execution to stall until the conditional branch has been fully resolved, or it may permit speculative execution to continue, but using 0 instead of untrusted_value if that exceeds the limit. If accessing any memory location is potentially unsafe when speculative execution is incorrect, then the code can be rewritten as int array[500]; int f (unsigned untrusted_index) { if (untrusted_index < 500) return *__builtin_speculation_safe_value (&array[untrusted_index], NULL); return 0; } which will cause a NULL pointer to be used for the unsafe case. Built-in Function: int __builtin_types_compatible_p (type1, type2) You can use the built-in function __builtin_types_compatible_p to determine whether two types are the same. This built-in function returns 1 if the unqualified versions of the types type1 and type2 (which are types, not expressions) are compatible, 0 otherwise. The result of this built-in function can be used in integer constant expressions. This built-in function ignores top level qualifiers (e.g., const, volatile). For example, int is equivalent to const int. The type int[] and int[5] are compatible. On the other hand, int and char * are not compatible, even if the size of their types, on the particular architecture are the same. Also, the amount of pointer indirection is taken into account when determining similarity. Consequently, short * is not similar to short **. Furthermore, two types that are typedefed are considered compatible if their underlying types are compatible. An enum type is not considered to be compatible with another enum type even if both are compatible with the same integer type; this is what the C standard specifies. For example, enum {foo, bar} is not similar to enum {hot, dog}. You typically use this function in code whose execution varies depending on the arguments' types. For example: #define foo(x) \ ({ \ typeof (x) tmp = (x); \ if (__builtin_types_compatible_p (typeof (x), long double)) \ tmp = foo_long_double (tmp); \ else if (__builtin_types_compatible_p (typeof (x), double)) \ tmp = foo_double (tmp); \ else if (__builtin_types_compatible_p (typeof (x), float)) \ tmp = foo_float (tmp); \ else \ abort (); \ tmp; \ }) Note: This construct is only available for C. Built-in Function: type __builtin_call_with_static_chain (call_exp, pointer_exp) The call_exp expression must be a function call, and the pointer_exp expression must be a pointer. The pointer_exp is passed to the function call in the target's static chain location. The result of builtin is the result of the function call. Note: This builtin is only available for C. This builtin can be used to call Go closures from C. Built-in Function: type __builtin_choose_expr (const_exp, exp1, exp2) You can use the built-in function __builtin_choose_expr to evaluate code depending on the value of a constant expression. This built-in function returns exp1 if const_exp, which is an integer constant expression, is nonzero. Otherwise it returns exp2. This built-in function is analogous to the '? :' operator in C, except that the expression returned has its type unaltered by promotion rules. Also, the built-in function does not evaluate the expression that is not chosen. For example, if const_exp evaluates to true, exp2 is not evaluated even if it has side effects. This built-in function can return an lvalue if the chosen argument is an lvalue. If exp1 is returned, the return type is the same as exp1's type. Similarly, if exp2 is returned, its return type is the same as exp2. Example: #define foo(x) \ __builtin_choose_expr ( \ __builtin_types_compatible_p (typeof (x), double), \ foo_double (x), \ __builtin_choose_expr ( \ __builtin_types_compatible_p (typeof (x), float), \ foo_float (x), \ /* The void expression results in a compile-time error \ when assigning the result to something. */ \ (void)0)) Note: This construct is only available for C. Furthermore, the unused expression (exp1 or exp2 depending on the value of const_exp) may still generate syntax errors. This may change in future revisions. Built-in Function: type __builtin_tgmath (functions, arguments) The built-in function __builtin_tgmath, available only for C and Objective-C, calls a function determined according to the rules of macros. It is intended to be used in implementations of that header, so that expansions of macros from that header only expand each of their arguments once, to avoid problems when calls to such macros are nested inside the arguments of other calls to such macros; in addition, it results in better diagnostics for invalid calls to macros than implementations using other GNU C language features. For example, the pow type-generic macro might be defined as: #define pow(a, b) __builtin_tgmath (powf, pow, powl, \ cpowf, cpow, cpowl, a, b) The arguments to __builtin_tgmath are at least two pointers to functions, followed by the arguments to the type-generic macro (which will be passed as arguments to the selected function). All the pointers to functions must be pointers to prototyped functions, none of which may have variable arguments, and all of which must have the same number of parameters; the number of parameters of the first function determines how many arguments to __builtin_tgmath are interpreted as function pointers, and how many as the arguments to the called function. The types of the specified functions must all be different, but related to each other in the same way as a set of functions that may be selected between by a macro in . This means that the functions are parameterized by a floating-point type t, different for each such function. The function return types may all be the same type, or they may be t for each function, or they may be the real type corresponding to t for each function (if some of the types t are complex). Likewise, for each parameter position, the type of the parameter in that position may always be the same type, or may be t for each function (this case must apply for at least one parameter position), or may be the real type corresponding to t for each function. The standard rules for macros are used to find a common type u from the types of the arguments for parameters whose types vary between the functions; complex integer types (a GNU extension) are treated like _Complex double for this purpose (or _Complex _Float64 if all the function return types are the same _Floatn or _Floatnx type). If the function return types vary, or are all the same integer type, the function called is the one for which t is u, and it is an error if there is no such function. If the function return types are all the same floating-point type, the type-generic macro is taken to be one of those from TS 18661 that rounds the result to a narrower type; if there is a function for which t is u, it is called, and otherwise the first function, if any, for which t has at least the range and precision of u is called, and it is an error if there is no such function. Built-in Function: type __builtin_complex (real, imag) The built-in function __builtin_complex is provided for use in implementing the ISO C11 macros CMPLXF, CMPLX and CMPLXL. real and imag must have the same type, a real binary floating-point type, and the result has the corresponding complex type with real and imaginary parts real and imag. Unlike 'real + I * imag', this works even when infinities, NaNs and negative zeros are involved. Built-in Function: int __builtin_constant_p (exp) You can use the built-in function __builtin_constant_p to determine if a value is known to be constant at compile time and hence that GCC can perform constant-folding on expressions involving that value. The argument of the function is the value to test. The function returns the integer 1 if the argument is known to be a compile-time constant and 0 if it is not known to be a compile-time constant. A return of 0 does not indicate that the value is not a constant, but merely that GCC cannot prove it is a constant with the specified value of the -O option. You typically use this function in an embedded application where memory is a critical resource. If you have some complex calculation, you may want it to be folded if it involves constants, but need to call a function if it does not. For example: #define Scale_Value(X) \ (__builtin_constant_p (X) \ ? ((X) * SCALE + OFFSET) : Scale (X)) You may use this built-in function in either a macro or an inline function. However, if you use it in an inlined function and pass an argument of the function as the argument to the built-in, GCC never returns 1 when you call the inline function with a string constant or compound literal (see Compound Literals) and does not return 1 when you pass a constant numeric value to the inline function unless you specify the -O option. You may also use __builtin_constant_p in initializers for static data. For instance, you can write static const int table[] = { __builtin_constant_p (EXPRESSION) ? (EXPRESSION) : -1, /* … */ }; This is an acceptable initializer even if EXPRESSION is not a constant expression, including the case where __builtin_constant_p returns 1 because EXPRESSION can be folded to a constant but EXPRESSION contains operands that are not otherwise permitted in a static initializer (for example, 0 && foo ()). GCC must be more conservative about evaluating the built-in in this case, because it has no opportunity to perform optimization. Built-in Function: bool __builtin_is_constant_evaluated (void) The __builtin_is_constant_evaluated function is available only in C++. The built-in is intended to be used by implementations of the std::is_constant_evaluated C++ function. Programs should make use of the latter function rather than invoking the built-in directly. The main use case of the built-in is to determine whether a constexpr function is being called in a constexpr context. A call to the function evaluates to a core constant expression with the value true if and only if it occurs within the evaluation of an expression or conversion that is manifestly constant-evaluated as defined in the C++ standard. Manifestly constant-evaluated contexts include constant-expressions, the conditions of constexpr if statements, constraint-expressions, and initializers of variables usable in constant expressions. For more details refer to the latest revision of the C++ standard. Built-in Function: long __builtin_expect (long exp, long c) You may use __builtin_expect to provide the compiler with branch prediction information. In general, you should prefer to use actual profile feedback for this (-fprofile-arcs), as programmers are notoriously bad at predicting how their programs actually perform. However, there are applications in which this data is hard to collect. The return value is the value of exp, which should be an integral expression. The semantics of the built-in are that it is expected that exp == c. For example: if (__builtin_expect (x, 0)) foo (); indicates that we do not expect to call foo, since we expect x to be zero. Since you are limited to integral expressions for exp, you should use constructions such as if (__builtin_expect (ptr != NULL, 1)) foo (*ptr); when testing pointer or floating-point values. For the purposes of branch prediction optimizations, the probability that a __builtin_expect expression is true is controlled by GCC's builtin-expect-probability parameter, which defaults to 90%. You can also use __builtin_expect_with_probability to explicitly assign a probability value to individual expressions. Built-in Function: long __builtin_expect_with_probability (long exp, long c, double probability) This function has the same semantics as __builtin_expect, but the caller provides the expected probability that exp == c. The last argument, probability, is a floating-point value in the range 0.0 to 1.0, inclusive. The probability argument must be constant floating-point expression. Built-in Function: void __builtin_trap (void) This function causes the program to exit abnormally. GCC implements this function by using a target-dependent mechanism (such as intentionally executing an illegal instruction) or by calling abort. The mechanism used may vary from release to release so you should not rely on any particular implementation. Built-in Function: void __builtin_unreachable (void) If control flow reaches the point of the __builtin_unreachable, the program is undefined. It is useful in situations where the compiler cannot deduce the unreachability of the code. One such case is immediately following an asm statement that either never terminates, or one that transfers control elsewhere and never returns. In this example, without the __builtin_unreachable, GCC issues a warning that control reaches the end of a non-void function. It also generates code to return after the asm. int f (int c, int v) { if (c) { return v; } else { asm("jmp error_handler"); __builtin_unreachable (); } } Because the asm statement unconditionally transfers control out of the function, control never reaches the end of the function body. The __builtin_unreachable is in fact unreachable and communicates this fact to the compiler. Another use for __builtin_unreachable is following a call a function that never returns but that is not declared __attribute__((noreturn)), as in this example: void function_that_never_returns (void); int g (int c) { if (c) { return 1; } else { function_that_never_returns (); __builtin_unreachable (); } } Built-in Function: void * __builtin_assume_aligned (const void *exp, size_t align, ...) This function returns its first argument, and allows the compiler to assume that the returned pointer is at least align bytes aligned. This built-in can have either two or three arguments, if it has three, the third argument should have integer type, and if it is nonzero means misalignment offset. For example: void *x = __builtin_assume_aligned (arg, 16); means that the compiler can assume x, set to arg, is at least 16-byte aligned, while: void *x = __builtin_assume_aligned (arg, 32, 8); means that the compiler can assume for x, set to arg, that (char *) x - 8 is 32-byte aligned. Built-in Function: int __builtin_LINE () This function is the equivalent of the preprocessor __LINE__ macro and returns a constant integer expression that evaluates to the line number of the invocation of the built-in. When used as a C++ default argument for a function F, it returns the line number of the call to F. Built-in Function: const char * __builtin_FUNCTION () This function is the equivalent of the __FUNCTION__ symbol and returns an address constant pointing to the name of the function from which the built-in was invoked, or the empty string if the invocation is not at function scope. When used as a C++ default argument for a function F, it returns the name of F's caller or the empty string if the call was not made at function scope. Built-in Function: const char * __builtin_FILE () This function is the equivalent of the preprocessor __FILE__ macro and returns an address constant pointing to the file name containing the invocation of the built-in, or the empty string if the invocation is not at function scope. When used as a C++ default argument for a function F, it returns the file name of the call to F or the empty string if the call was not made at function scope. For example, in the following, each call to function foo will print a line similar to "file.c:123: foo: message" with the name of the file and the line number of the printf call, the name of the function foo, followed by the word message. const char* function (const char *func = __builtin_FUNCTION ()) { return func; } void foo (void) { printf ("%s:%i: %s: message\n", file (), line (), function ()); } Built-in Function: void __builtin___clear_cache (void *begin, void *end) This function is used to flush the processor's instruction cache for the region of memory between begin inclusive and end exclusive. Some targets require that the instruction cache be flushed, after modifying memory containing code, in order to obtain deterministic behavior. If the target does not require instruction cache flushes, __builtin___clear_cache has no effect. Otherwise either instructions are emitted in-line to clear the instruction cache or a call to the __clear_cache function in libgcc is made. Built-in Function: void __builtin_prefetch (const void *addr, ...) This function is used to minimize cache-miss latency by moving data into a cache before it is accessed. You can insert calls to __builtin_prefetch into code for which you know addresses of data in memory that is likely to be accessed soon. If the target supports them, data prefetch instructions are generated. If the prefetch is done early enough before the access then the data will be in the cache by the time it is accessed. The value of addr is the address of the memory to prefetch. There are two optional arguments, rw and locality. The value of rw is a compile-time constant one or zero; one means that the prefetch is preparing for a write to the memory address and zero, the default, means that the prefetch is preparing for a read. The value locality must be a compile-time constant integer between zero and three. A value of zero means that the data has no temporal locality, so it need not be left in the cache after the access. A value of three means that the data has a high degree of temporal locality and should be left in all levels of cache possible. Values of one and two mean, respectively, a low or moderate degree of temporal locality. The default is three. for (i = 0; i < n; i++) { a[i] = a[i] + b[i]; __builtin_prefetch (&a[i+j], 1, 1); __builtin_prefetch (&b[i+j], 0, 1); /* … */ } Data prefetch does not generate faults if addr is invalid, but the address expression itself must be valid. For example, a prefetch of p->next does not fault if p->next is not a valid address, but evaluation faults if p is not a valid address. If the target does not support data prefetch, the address expression is evaluated if it includes side effects but no other code is generated and GCC does not issue a warning. Built-in Function: size_t __builtin_object_size (const void * ptr, int type) Returns the size of an object pointed to by ptr. See Object Size Checking, for a detailed description of the function. Built-in Function: double __builtin_huge_val (void) Returns a positive infinity, if supported by the floating-point format, else DBL_MAX. This function is suitable for implementing the ISO C macro HUGE_VAL. Built-in Function: float __builtin_huge_valf (void) Similar to __builtin_huge_val, except the return type is float. Built-in Function: long double __builtin_huge_vall (void) Similar to __builtin_huge_val, except the return type is long double. Built-in Function: _Floatn __builtin_huge_valfn (void) Similar to __builtin_huge_val, except the return type is _Floatn. Built-in Function: _Floatnx __builtin_huge_valfnx (void) Similar to __builtin_huge_val, except the return type is _Floatnx. Built-in Function: int __builtin_fpclassify (int, int, int, int, int, ...) This built-in implements the C99 fpclassify functionality. The first five int arguments should be the target library's notion of the possible FP classes and are used for return values. They must be constant values and they must appear in this order: FP_NAN, FP_INFINITE, FP_NORMAL, FP_SUBNORMAL and FP_ZERO. The ellipsis is for exactly one floating-point value to classify. GCC treats the last argument as type-generic, which means it does not do default promotion from float to double. Built-in Function: double __builtin_inf (void) Similar to __builtin_huge_val, except a warning is generated if the target floating-point format does not support infinities. Built-in Function: _Decimal32 __builtin_infd32 (void) Similar to __builtin_inf, except the return type is _Decimal32. Built-in Function: _Decimal64 __builtin_infd64 (void) Similar to __builtin_inf, except the return type is _Decimal64. Built-in Function: _Decimal128 __builtin_infd128 (void) Similar to __builtin_inf, except the return type is _Decimal128. Built-in Function: float __builtin_inff (void) Similar to __builtin_inf, except the return type is float. This function is suitable for implementing the ISO C99 macro INFINITY. Built-in Function: long double __builtin_infl (void) Similar to __builtin_inf, except the return type is long double. Built-in Function: _Floatn __builtin_inffn (void) Similar to __builtin_inf, except the return type is _Floatn. Built-in Function: _Floatn __builtin_inffnx (void) Similar to __builtin_inf, except the return type is _Floatnx. Built-in Function: int __builtin_isinf_sign (...) Similar to isinf, except the return value is -1 for an argument of -Inf and 1 for an argument of +Inf. Note while the parameter list is an ellipsis, this function only accepts exactly one floating-point argument. GCC treats this parameter as type-generic, which means it does not do default promotion from float to double. Built-in Function: double __builtin_nan (const char *str) This is an implementation of the ISO C99 function nan. Since ISO C99 defines this function in terms of strtod, which we do not implement, a description of the parsing is in order. The string is parsed as by strtol; that is, the base is recognized by leading '0' or '0x' prefixes. The number parsed is placed in the significand such that the least significant bit of the number is at the least significant bit of the significand. The number is truncated to fit the significand field provided. The significand is forced to be a quiet NaN. This function, if given a string literal all of which would have been consumed by strtol, is evaluated early enough that it is considered a compile-time constant. Built-in Function: _Decimal32 __builtin_nand32 (const char *str) Similar to __builtin_nan, except the return type is _Decimal32. Built-in Function: _Decimal64 __builtin_nand64 (const char *str) Similar to __builtin_nan, except the return type is _Decimal64. Built-in Function: _Decimal128 __builtin_nand128 (const char *str) Similar to __builtin_nan, except the return type is _Decimal128. Built-in Function: float __builtin_nanf (const char *str) Similar to __builtin_nan, except the return type is float. Built-in Function: long double __builtin_nanl (const char *str) Similar to __builtin_nan, except the return type is long double. Built-in Function: _Floatn __builtin_nanfn (const char *str) Similar to __builtin_nan, except the return type is _Floatn. Built-in Function: _Floatnx __builtin_nanfnx (const char *str) Similar to __builtin_nan, except the return type is _Floatnx. Built-in Function: double __builtin_nans (const char *str) Similar to __builtin_nan, except the significand is forced to be a signaling NaN. The nans function is proposed by WG14 N965. Built-in Function: float __builtin_nansf (const char *str) Similar to __builtin_nans, except the return type is float. Built-in Function: long double __builtin_nansl (const char *str) Similar to __builtin_nans, except the return type is long double. Built-in Function: _Floatn __builtin_nansfn (const char *str) Similar to __builtin_nans, except the return type is _Floatn. Built-in Function: _Floatnx __builtin_nansfnx (const char *str) Similar to __builtin_nans, except the return type is _Floatnx. Built-in Function: int __builtin_ffs (int x) Returns one plus the index of the least significant 1-bit of x, or if x is zero, returns zero. Built-in Function: int __builtin_clz (unsigned int x) Returns the number of leading 0-bits in x, starting at the most significant bit position. If x is 0, the result is undefined. Built-in Function: int __builtin_ctz (unsigned int x) Returns the number of trailing 0-bits in x, starting at the least significant bit position. If x is 0, the result is undefined. Built-in Function: int __builtin_clrsb (int x) Returns the number of leading redundant sign bits in x, i.e. the number of bits following the most significant bit that are identical to it. There are no special cases for 0 or other values. Built-in Function: int __builtin_popcount (unsigned int x) Returns the number of 1-bits in x. Built-in Function: int __builtin_parity (unsigned int x) Returns the parity of x, i.e. the number of 1-bits in x modulo 2. Built-in Function: int __builtin_ffsl (long) Similar to __builtin_ffs, except the argument type is long. Built-in Function: int __builtin_clzl (unsigned long) Similar to __builtin_clz, except the argument type is unsigned long. Built-in Function: int __builtin_ctzl (unsigned long) Similar to __builtin_ctz, except the argument type is unsigned long. Built-in Function: int __builtin_clrsbl (long) Similar to __builtin_clrsb, except the argument type is long. Built-in Function: int __builtin_popcountl (unsigned long) Similar to __builtin_popcount, except the argument type is unsigned long. Built-in Function: int __builtin_parityl (unsigned long) Similar to __builtin_parity, except the argument type is unsigned long. Built-in Function: int __builtin_ffsll (long long) Similar to __builtin_ffs, except the argument type is long long. Built-in Function: int __builtin_clzll (unsigned long long) Similar to __builtin_clz, except the argument type is unsigned long long. Built-in Function: int __builtin_ctzll (unsigned long long) Similar to __builtin_ctz, except the argument type is unsigned long long. Built-in Function: int __builtin_clrsbll (long long) Similar to __builtin_clrsb, except the argument type is long long. Built-in Function: int __builtin_popcountll (unsigned long long) Similar to __builtin_popcount, except the argument type is unsigned long long. Built-in Function: int __builtin_parityll (unsigned long long) Similar to __builtin_parity, except the argument type is unsigned long long. Built-in Function: double __builtin_powi (double, int) Returns the first argument raised to the power of the second. Unlike the pow function no guarantees about precision and rounding are made. Built-in Function: float __builtin_powif (float, int) Similar to __builtin_powi, except the argument and return types are float. Built-in Function: long double __builtin_powil (long double, int) Similar to __builtin_powi, except the argument and return types are long double. Built-in Function: uint16_t __builtin_bswap16 (uint16_t x) Returns x with the order of the bytes reversed; for example, 0xaabb becomes 0xbbaa. Byte here always means exactly 8 bits. Built-in Function: uint32_t __builtin_bswap32 (uint32_t x) Similar to __builtin_bswap16, except the argument and return types are 32 bit. Built-in Function: uint64_t __builtin_bswap64 (uint64_t x) Similar to __builtin_bswap32, except the argument and return types are 64 bit. Built-in Function: Pmode __builtin_extend_pointer (void * x) On targets where the user visible pointer size is smaller than the size of an actual hardware address this function returns the extended user pointer. Targets where this is true included ILP32 mode on x86_64 or Aarch64. This function is mainly useful when writing inline assembly code. Built-in Function: int __builtin_goacc_parlevel_id (int x) Returns the openacc gang, worker or vector id depending on whether x is 0, 1 or 2. Built-in Function: int __builtin_goacc_parlevel_size (int x) Returns the openacc gang, worker or vector size depending on whether x is 0, 1 or 2. ────────────────────────────────────────────────────────────────────────────── 6.60 Built-in Functions Specific to Particular Target Machines On some target machines, GCC supports many built-in functions specific to those machines. Generally these generate calls to specific machine instructions, but allow the compiler to schedule those calls. • AArch64 Built-in Functions: • Alpha Built-in Functions: • Altera Nios II Built-in Functions: • ARC Built-in Functions: • ARC SIMD Built-in Functions: • ARM iWMMXt Built-in Functions: • ARM C Language Extensions (ACLE): • ARM Floating Point Status and Control Intrinsics: • ARM ARMv8-M Security Extensions: • AVR Built-in Functions: • Blackfin Built-in Functions: • FR-V Built-in Functions: • MIPS DSP Built-in Functions: • MIPS Paired-Single Support: • MIPS Loongson Built-in Functions: • MIPS SIMD Architecture (MSA) Support: • Other MIPS Built-in Functions: • MSP430 Built-in Functions: • NDS32 Built-in Functions: • picoChip Built-in Functions: • Basic PowerPC Built-in Functions: • PowerPC AltiVec/VSX Built-in Functions: • PowerPC Hardware Transactional Memory Built-in Functions: • PowerPC Atomic Memory Operation Functions: • RX Built-in Functions: • S/390 System z Built-in Functions: • SH Built-in Functions: • SPARC VIS Built-in Functions: • SPU Built-in Functions: • TI C6X Built-in Functions: • TILE-Gx Built-in Functions: • TILEPro Built-in Functions: • x86 Built-in Functions: • x86 transactional memory intrinsics: • x86 control-flow protection intrinsics: ────────────────────────────────────────────────────────────────────────────── Next: Target Format Checks, Previous: Other Builtins, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: FR-V Built-in Functions Link: next: Directly-mapped Integer Functions Link: prev: FR-V Built-in Functions Next: Directly-mapped Integer Functions, Up: FR-V Built-in Functions ────────────────────────────────────────────────────────────────────────────── 6.60.12.1 Argument Types The arguments to the built-in functions can be divided into three groups: register numbers, compile-time constants and run-time values. In order to make this classification clear at a glance, the arguments and return values are given the following pseudo types: Pseudo type Real C type Constant? Description uh unsigned short No an unsigned halfword uw1 unsigned int No an unsigned word sw1 int No a signed word uw2 unsigned long long No an unsigned doubleword sw2 long long No a signed doubleword const int Yes an integer constant acc int Yes an ACC register number iacc int Yes an IACC register number These pseudo types are not defined by GCC, they are simply a notational convenience used in this manual. Arguments of type uh, uw1, sw1, uw2 and sw2 are evaluated at run time. They correspond to register operands in the underlying FR-V instructions. const arguments represent immediate operands in the underlying FR-V instructions. They must be compile-time constants. acc arguments are evaluated at compile time and specify the number of an accumulator register. For example, an acc argument of 2 selects the ACC2 register. iacc arguments are similar to acc arguments but specify the number of an IACC register. See see Other Built-in Functions for more details. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: FR-V Built-in Functions Link: next: Other Built-in Functions Link: prev: Directly-mapped Media Functions Next: Other Built-in Functions, Previous: Directly-mapped Media Functions, ────────────────────────────────────────────────────────────────────────────── 6.60.12.4 Raw Read/Write Functions This sections describes built-in functions related to read and write instructions to access memory. These functions generate membar instructions to flush the I/O load and stores where appropriate, as described in Fujitsu's manual described above. unsigned char __builtin_read8 (void *data) unsigned short __builtin_read16 (void *data) unsigned long __builtin_read32 (void *data) unsigned long long __builtin_read64 (void *data) void __builtin_write8 (void *data, unsigned char datum) void __builtin_write16 (void *data, unsigned short datum) void __builtin_write32 (void *data, unsigned long datum) void __builtin_write64 (void *data, unsigned long long datum) Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: FR-V Built-in Functions Link: next: MIPS DSP Built-in Functions Link: prev: Raw read/write Functions Previous: Raw read/write Functions, Up: FR-V Built-in Functions ────────────────────────────────────────────────────────────────────────────── 6.60.12.5 Other Built-in Functions This section describes built-in functions that are not named after a specific FR-V instruction. sw2 __IACCreadll (iacc reg) Return the full 64-bit value of IACC0. The reg argument is reserved for future expansion and must be 0. sw1 __IACCreadl (iacc reg) Return the value of IACC0H if reg is 0 and IACC0L if reg is 1. Other values of reg are rejected as invalid. void __IACCsetll (iacc reg, sw2 x) Set the full 64-bit value of IACC0 to x. The reg argument is reserved for future expansion and must be 0. void __IACCsetl (iacc reg, sw1 x) Set IACC0H to x if reg is 0 and IACC0L to x if reg is 1. Other values of reg are rejected as invalid. void __data_prefetch0 (const void *x) Use the dcpl instruction to load the contents of address x into the data cache. void __data_prefetch (const void *x) Use the nldub instruction to load the contents of address x into the data cache. The instruction is issued in slot I1. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: MIPS Loongson Built-in Functions Link: next: Paired-Single Built-in Functions Link: prev: MIPS Loongson Built-in Functions Next: Paired-Single Built-in Functions, Up: MIPS Loongson Built-in ────────────────────────────────────────────────────────────────────────────── 6.60.15.1 Paired-Single Arithmetic The table below lists the v2sf operations for which hardware support exists. a, b and c are v2sf values and x is an integral value. C code MIPS instruction a + b add.ps a - b sub.ps -a neg.ps a * b mul.ps a * b + c madd.ps a * b - c msub.ps -(a * b + c) nmadd.ps -(a * b - c) nmsub.ps x ? a : b movn.ps/movz.ps Note that the multiply-accumulate instructions can be disabled using the command-line option -mno-fused-madd. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: MIPS Loongson Built-in Functions Link: next: MIPS-3D Built-in Functions Link: prev: Paired-Single Arithmetic Next: MIPS-3D Built-in Functions, Previous: Paired-Single Arithmetic, Up: ────────────────────────────────────────────────────────────────────────────── 6.60.15.2 Paired-Single Built-in Functions The following paired-single functions map directly to a particular MIPS instruction. Please refer to the architecture specification for details on what each instruction does. v2sf __builtin_mips_pll_ps (v2sf, v2sf) Pair lower lower (pll.ps). v2sf __builtin_mips_pul_ps (v2sf, v2sf) Pair upper lower (pul.ps). v2sf __builtin_mips_plu_ps (v2sf, v2sf) Pair lower upper (plu.ps). v2sf __builtin_mips_puu_ps (v2sf, v2sf) Pair upper upper (puu.ps). v2sf __builtin_mips_cvt_ps_s (float, float) Convert pair to paired single (cvt.ps.s). float __builtin_mips_cvt_s_pl (v2sf) Convert pair lower to single (cvt.s.pl). float __builtin_mips_cvt_s_pu (v2sf) Convert pair upper to single (cvt.s.pu). v2sf __builtin_mips_abs_ps (v2sf) Absolute value (abs.ps). v2sf __builtin_mips_alnv_ps (v2sf, v2sf, int) Align variable (alnv.ps). Note: The value of the third parameter must be 0 or 4 modulo 8, otherwise the result is unpredictable. Please read the instruction description for details. The following multi-instruction functions are also available. In each case, cond can be any of the 16 floating-point conditions: f, un, eq, ueq, olt, ult, ole, ule, sf, ngle, seq, ngl, lt, nge, le or ngt. v2sf __builtin_mips_movt_c_cond_ps (v2sf a, v2sf b, v2sf c, v2sf d) v2sf __builtin_mips_movf_c_cond_ps (v2sf a, v2sf b, v2sf c, v2sf d) Conditional move based on floating-point comparison (c.cond.ps, movt.ps/movf.ps). The movt functions return the value x computed by: c.cond.ps cc,a,b mov.ps x,c movt.ps x,d,cc The movf functions are similar but use movf.ps instead of movt.ps. int __builtin_mips_upper_c_cond_ps (v2sf a, v2sf b) int __builtin_mips_lower_c_cond_ps (v2sf a, v2sf b) Comparison of two paired-single values (c.cond.ps, bc1t/bc1f). These functions compare a and b using c.cond.ps and return either the upper or lower half of the result. For example: v2sf a, b; if (__builtin_mips_upper_c_eq_ps (a, b)) upper_halves_are_equal (); else upper_halves_are_unequal (); if (__builtin_mips_lower_c_eq_ps (a, b)) lower_halves_are_equal (); else lower_halves_are_unequal (); ────────────────────────────────────────────────────────────────────────────── Next: MIPS-3D Built-in Functions, Previous: Paired-Single Arithmetic, Up: Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Target Builtins Link: next: MSP430 Built-in Functions Link: prev: MIPS SIMD Architecture Built-in Functions Next: MSP430 Built-in Functions, Previous: MIPS SIMD Architecture (MSA) ────────────────────────────────────────────────────────────────────────────── 6.60.17 Other MIPS Built-in Functions GCC provides other MIPS-specific built-in functions: void __builtin_mips_cache (int op, const volatile void *addr) Insert a 'cache' instruction with operands op and addr. GCC defines the preprocessor macro ___GCC_HAVE_BUILTIN_MIPS_CACHE when this function is available. unsigned int __builtin_mips_get_fcsr (void) void __builtin_mips_set_fcsr (unsigned int value) Get and set the contents of the floating-point control and status register (FPU control register 31). These functions are only available in hard-float code but can be called in both MIPS16 and non-MIPS16 contexts. __builtin_mips_set_fcsr can be used to change any bit of the register except the condition codes, which GCC assumes are preserved. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Target Builtins Link: next: x86 transactional memory intrinsics Link: prev: TILEPro Built-in Functions Next: x86 transactional memory intrinsics, Previous: TILEPro Built-in ────────────────────────────────────────────────────────────────────────────── 6.60.33 x86 Built-in Functions These built-in functions are available for the x86-32 and x86-64 family of computers, depending on the command-line switches used. If you specify command-line switches such as -msse, the compiler could use the extended instruction sets even if the built-ins are not used explicitly in the program. For this reason, applications that perform run-time CPU detection must compile separate files for each supported architecture, using the appropriate flags. In particular, the file containing the CPU detection code should be compiled without these options. The following machine modes are available for use with MMX built-in functions (see Vector Extensions): V2SI for a vector of two 32-bit integers, V4HI for a vector of four 16-bit integers, and V8QI for a vector of eight 8-bit integers. Some of the built-in functions operate on MMX registers as a whole 64-bit entity, these use V1DI as their mode. If 3DNow! extensions are enabled, V2SF is used as a mode for a vector of two 32-bit floating-point values. If SSE extensions are enabled, V4SF is used for a vector of four 32-bit floating-point values. Some instructions use a vector of four 32-bit integers, these use V4SI. Finally, some instructions operate on an entire vector register, interpreting it as a 128-bit integer, these use mode TI. The x86-32 and x86-64 family of processors use additional built-in functions for efficient use of TF (__float128) 128-bit floating point and TC 128-bit complex floating-point values. The following floating-point built-in functions are always available. All of them implement the function that is part of the name. __float128 __builtin_fabsq (__float128) __float128 __builtin_copysignq (__float128, __float128) The following built-in functions are always available. __float128 __builtin_infq (void) Similar to __builtin_inf, except the return type is __float128. __float128 __builtin_huge_valq (void) Similar to __builtin_huge_val, except the return type is __float128. __float128 __builtin_nanq (void) Similar to __builtin_nan, except the return type is __float128. __float128 __builtin_nansq (void) Similar to __builtin_nans, except the return type is __float128. The following built-in function is always available. void __builtin_ia32_pause (void) Generates the pause machine instruction with a compiler memory barrier. The following built-in functions are always available and can be used to check the target platform type. Built-in Function: void __builtin_cpu_init (void) This function runs the CPU detection code to check the type of CPU and the features supported. This built-in function needs to be invoked along with the built-in functions to check CPU type and features, __builtin_cpu_is and __builtin_cpu_supports, only when used in a function that is executed before any constructors are called. The CPU detection code is automatically executed in a very high priority constructor. For example, this function has to be used in ifunc resolvers that check for CPU type using the built-in functions __builtin_cpu_is and __builtin_cpu_supports, or in constructors on targets that don't support constructor priority. static void (*resolve_memcpy (void)) (void) { // ifunc resolvers fire before constructors, explicitly call the init // function. __builtin_cpu_init (); if (__builtin_cpu_supports ("ssse3")) return ssse3_memcpy; // super fast memcpy with ssse3 instructions. else return default_memcpy; } void *memcpy (void *, const void *, size_t) __attribute__ ((ifunc ("resolve_memcpy"))); Built-in Function: int __builtin_cpu_is (const char *cpuname) This function returns a positive integer if the run-time CPU is of type cpuname and returns 0 otherwise. The following CPU names can be detected: 'amd' AMD CPU. 'intel' Intel CPU. 'atom' Intel Atom CPU. 'slm' Intel Silvermont CPU. 'core2' Intel Core 2 CPU. 'corei7' Intel Core i7 CPU. 'nehalem' Intel Core i7 Nehalem CPU. 'westmere' Intel Core i7 Westmere CPU. 'sandybridge' Intel Core i7 Sandy Bridge CPU. 'ivybridge' Intel Core i7 Ivy Bridge CPU. 'haswell' Intel Core i7 Haswell CPU. 'broadwell' Intel Core i7 Broadwell CPU. 'skylake' Intel Core i7 Skylake CPU. 'skylake-avx512' Intel Core i7 Skylake AVX512 CPU. 'cannonlake' Intel Core i7 Cannon Lake CPU. 'icelake-client' Intel Core i7 Ice Lake Client CPU. 'icelake-server' Intel Core i7 Ice Lake Server CPU. 'cascadelake' Intel Core i7 Cascadelake CPU. 'bonnell' Intel Atom Bonnell CPU. 'silvermont' Intel Atom Silvermont CPU. 'goldmont' Intel Atom Goldmont CPU. 'goldmont-plus' Intel Atom Goldmont Plus CPU. 'tremont' Intel Atom Tremont CPU. 'knl' Intel Knights Landing CPU. 'knm' Intel Knights Mill CPU. 'amdfam10h' AMD Family 10h CPU. 'barcelona' AMD Family 10h Barcelona CPU. 'shanghai' AMD Family 10h Shanghai CPU. 'istanbul' AMD Family 10h Istanbul CPU. 'btver1' AMD Family 14h CPU. 'amdfam15h' AMD Family 15h CPU. 'bdver1' AMD Family 15h Bulldozer version 1. 'bdver2' AMD Family 15h Bulldozer version 2. 'bdver3' AMD Family 15h Bulldozer version 3. 'bdver4' AMD Family 15h Bulldozer version 4. 'btver2' AMD Family 16h CPU. 'amdfam17h' AMD Family 17h CPU. 'znver1' AMD Family 17h Zen version 1. 'znver2' AMD Family 17h Zen version 2. Here is an example: if (__builtin_cpu_is ("corei7")) { do_corei7 (); // Core i7 specific implementation. } else { do_generic (); // Generic implementation. } Built-in Function: int __builtin_cpu_supports (const char *feature) This function returns a positive integer if the run-time CPU supports feature and returns 0 otherwise. The following features can be detected: 'cmov' CMOV instruction. 'mmx' MMX instructions. 'popcnt' POPCNT instruction. 'sse' SSE instructions. 'sse2' SSE2 instructions. 'sse3' SSE3 instructions. 'ssse3' SSSE3 instructions. 'sse4.1' SSE4.1 instructions. 'sse4.2' SSE4.2 instructions. 'avx' AVX instructions. 'avx2' AVX2 instructions. 'sse4a' SSE4A instructions. 'fma4' FMA4 instructions. 'xop' XOP instructions. 'fma' FMA instructions. 'avx512f' AVX512F instructions. 'bmi' BMI instructions. 'bmi2' BMI2 instructions. 'aes' AES instructions. 'pclmul' PCLMUL instructions. 'avx512vl' AVX512VL instructions. 'avx512bw' AVX512BW instructions. 'avx512dq' AVX512DQ instructions. 'avx512cd' AVX512CD instructions. 'avx512er' AVX512ER instructions. 'avx512pf' AVX512PF instructions. 'avx512vbmi' AVX512VBMI instructions. 'avx512ifma' AVX512IFMA instructions. 'avx5124vnniw' AVX5124VNNIW instructions. 'avx5124fmaps' AVX5124FMAPS instructions. 'avx512vpopcntdq' AVX512VPOPCNTDQ instructions. 'avx512vbmi2' AVX512VBMI2 instructions. 'gfni' GFNI instructions. 'vpclmulqdq' VPCLMULQDQ instructions. 'avx512vnni' AVX512VNNI instructions. 'avx512bitalg' AVX512BITALG instructions. Here is an example: if (__builtin_cpu_supports ("popcnt")) { asm("popcnt %1,%0" : "=r"(count) : "rm"(n) : "cc"); } else { count = generic_countbits (n); //generic implementation. } The following built-in functions are made available by -mmmx. All of them generate the machine instruction that is part of the name. v8qi __builtin_ia32_paddb (v8qi, v8qi) v4hi __builtin_ia32_paddw (v4hi, v4hi) v2si __builtin_ia32_paddd (v2si, v2si) v8qi __builtin_ia32_psubb (v8qi, v8qi) v4hi __builtin_ia32_psubw (v4hi, v4hi) v2si __builtin_ia32_psubd (v2si, v2si) v8qi __builtin_ia32_paddsb (v8qi, v8qi) v4hi __builtin_ia32_paddsw (v4hi, v4hi) v8qi __builtin_ia32_psubsb (v8qi, v8qi) v4hi __builtin_ia32_psubsw (v4hi, v4hi) v8qi __builtin_ia32_paddusb (v8qi, v8qi) v4hi __builtin_ia32_paddusw (v4hi, v4hi) v8qi __builtin_ia32_psubusb (v8qi, v8qi) v4hi __builtin_ia32_psubusw (v4hi, v4hi) v4hi __builtin_ia32_pmullw (v4hi, v4hi) v4hi __builtin_ia32_pmulhw (v4hi, v4hi) di __builtin_ia32_pand (di, di) di __builtin_ia32_pandn (di,di) di __builtin_ia32_por (di, di) di __builtin_ia32_pxor (di, di) v8qi __builtin_ia32_pcmpeqb (v8qi, v8qi) v4hi __builtin_ia32_pcmpeqw (v4hi, v4hi) v2si __builtin_ia32_pcmpeqd (v2si, v2si) v8qi __builtin_ia32_pcmpgtb (v8qi, v8qi) v4hi __builtin_ia32_pcmpgtw (v4hi, v4hi) v2si __builtin_ia32_pcmpgtd (v2si, v2si) v8qi __builtin_ia32_punpckhbw (v8qi, v8qi) v4hi __builtin_ia32_punpckhwd (v4hi, v4hi) v2si __builtin_ia32_punpckhdq (v2si, v2si) v8qi __builtin_ia32_punpcklbw (v8qi, v8qi) v4hi __builtin_ia32_punpcklwd (v4hi, v4hi) v2si __builtin_ia32_punpckldq (v2si, v2si) v8qi __builtin_ia32_packsswb (v4hi, v4hi) v4hi __builtin_ia32_packssdw (v2si, v2si) v8qi __builtin_ia32_packuswb (v4hi, v4hi) v4hi __builtin_ia32_psllw (v4hi, v4hi) v2si __builtin_ia32_pslld (v2si, v2si) v1di __builtin_ia32_psllq (v1di, v1di) v4hi __builtin_ia32_psrlw (v4hi, v4hi) v2si __builtin_ia32_psrld (v2si, v2si) v1di __builtin_ia32_psrlq (v1di, v1di) v4hi __builtin_ia32_psraw (v4hi, v4hi) v2si __builtin_ia32_psrad (v2si, v2si) v4hi __builtin_ia32_psllwi (v4hi, int) v2si __builtin_ia32_pslldi (v2si, int) v1di __builtin_ia32_psllqi (v1di, int) v4hi __builtin_ia32_psrlwi (v4hi, int) v2si __builtin_ia32_psrldi (v2si, int) v1di __builtin_ia32_psrlqi (v1di, int) v4hi __builtin_ia32_psrawi (v4hi, int) v2si __builtin_ia32_psradi (v2si, int) The following built-in functions are made available either with -msse, or with -m3dnowa. All of them generate the machine instruction that is part of the name. v4hi __builtin_ia32_pmulhuw (v4hi, v4hi) v8qi __builtin_ia32_pavgb (v8qi, v8qi) v4hi __builtin_ia32_pavgw (v4hi, v4hi) v1di __builtin_ia32_psadbw (v8qi, v8qi) v8qi __builtin_ia32_pmaxub (v8qi, v8qi) v4hi __builtin_ia32_pmaxsw (v4hi, v4hi) v8qi __builtin_ia32_pminub (v8qi, v8qi) v4hi __builtin_ia32_pminsw (v4hi, v4hi) int __builtin_ia32_pmovmskb (v8qi) void __builtin_ia32_maskmovq (v8qi, v8qi, char *) void __builtin_ia32_movntq (di *, di) void __builtin_ia32_sfence (void) The following built-in functions are available when -msse is used. All of them generate the machine instruction that is part of the name. int __builtin_ia32_comieq (v4sf, v4sf) int __builtin_ia32_comineq (v4sf, v4sf) int __builtin_ia32_comilt (v4sf, v4sf) int __builtin_ia32_comile (v4sf, v4sf) int __builtin_ia32_comigt (v4sf, v4sf) int __builtin_ia32_comige (v4sf, v4sf) int __builtin_ia32_ucomieq (v4sf, v4sf) int __builtin_ia32_ucomineq (v4sf, v4sf) int __builtin_ia32_ucomilt (v4sf, v4sf) int __builtin_ia32_ucomile (v4sf, v4sf) int __builtin_ia32_ucomigt (v4sf, v4sf) int __builtin_ia32_ucomige (v4sf, v4sf) v4sf __builtin_ia32_addps (v4sf, v4sf) v4sf __builtin_ia32_subps (v4sf, v4sf) v4sf __builtin_ia32_mulps (v4sf, v4sf) v4sf __builtin_ia32_divps (v4sf, v4sf) v4sf __builtin_ia32_addss (v4sf, v4sf) v4sf __builtin_ia32_subss (v4sf, v4sf) v4sf __builtin_ia32_mulss (v4sf, v4sf) v4sf __builtin_ia32_divss (v4sf, v4sf) v4sf __builtin_ia32_cmpeqps (v4sf, v4sf) v4sf __builtin_ia32_cmpltps (v4sf, v4sf) v4sf __builtin_ia32_cmpleps (v4sf, v4sf) v4sf __builtin_ia32_cmpgtps (v4sf, v4sf) v4sf __builtin_ia32_cmpgeps (v4sf, v4sf) v4sf __builtin_ia32_cmpunordps (v4sf, v4sf) v4sf __builtin_ia32_cmpneqps (v4sf, v4sf) v4sf __builtin_ia32_cmpnltps (v4sf, v4sf) v4sf __builtin_ia32_cmpnleps (v4sf, v4sf) v4sf __builtin_ia32_cmpngtps (v4sf, v4sf) v4sf __builtin_ia32_cmpngeps (v4sf, v4sf) v4sf __builtin_ia32_cmpordps (v4sf, v4sf) v4sf __builtin_ia32_cmpeqss (v4sf, v4sf) v4sf __builtin_ia32_cmpltss (v4sf, v4sf) v4sf __builtin_ia32_cmpless (v4sf, v4sf) v4sf __builtin_ia32_cmpunordss (v4sf, v4sf) v4sf __builtin_ia32_cmpneqss (v4sf, v4sf) v4sf __builtin_ia32_cmpnltss (v4sf, v4sf) v4sf __builtin_ia32_cmpnless (v4sf, v4sf) v4sf __builtin_ia32_cmpordss (v4sf, v4sf) v4sf __builtin_ia32_maxps (v4sf, v4sf) v4sf __builtin_ia32_maxss (v4sf, v4sf) v4sf __builtin_ia32_minps (v4sf, v4sf) v4sf __builtin_ia32_minss (v4sf, v4sf) v4sf __builtin_ia32_andps (v4sf, v4sf) v4sf __builtin_ia32_andnps (v4sf, v4sf) v4sf __builtin_ia32_orps (v4sf, v4sf) v4sf __builtin_ia32_xorps (v4sf, v4sf) v4sf __builtin_ia32_movss (v4sf, v4sf) v4sf __builtin_ia32_movhlps (v4sf, v4sf) v4sf __builtin_ia32_movlhps (v4sf, v4sf) v4sf __builtin_ia32_unpckhps (v4sf, v4sf) v4sf __builtin_ia32_unpcklps (v4sf, v4sf) v4sf __builtin_ia32_cvtpi2ps (v4sf, v2si) v4sf __builtin_ia32_cvtsi2ss (v4sf, int) v2si __builtin_ia32_cvtps2pi (v4sf) int __builtin_ia32_cvtss2si (v4sf) v2si __builtin_ia32_cvttps2pi (v4sf) int __builtin_ia32_cvttss2si (v4sf) v4sf __builtin_ia32_rcpps (v4sf) v4sf __builtin_ia32_rsqrtps (v4sf) v4sf __builtin_ia32_sqrtps (v4sf) v4sf __builtin_ia32_rcpss (v4sf) v4sf __builtin_ia32_rsqrtss (v4sf) v4sf __builtin_ia32_sqrtss (v4sf) v4sf __builtin_ia32_shufps (v4sf, v4sf, int) void __builtin_ia32_movntps (float *, v4sf) int __builtin_ia32_movmskps (v4sf) The following built-in functions are available when -msse is used. v4sf __builtin_ia32_loadups (float *) Generates the movups machine instruction as a load from memory. void __builtin_ia32_storeups (float *, v4sf) Generates the movups machine instruction as a store to memory. v4sf __builtin_ia32_loadss (float *) Generates the movss machine instruction as a load from memory. v4sf __builtin_ia32_loadhps (v4sf, const v2sf *) Generates the movhps machine instruction as a load from memory. v4sf __builtin_ia32_loadlps (v4sf, const v2sf *) Generates the movlps machine instruction as a load from memory void __builtin_ia32_storehps (v2sf *, v4sf) Generates the movhps machine instruction as a store to memory. void __builtin_ia32_storelps (v2sf *, v4sf) Generates the movlps machine instruction as a store to memory. The following built-in functions are available when -msse2 is used. All of them generate the machine instruction that is part of the name. int __builtin_ia32_comisdeq (v2df, v2df) int __builtin_ia32_comisdlt (v2df, v2df) int __builtin_ia32_comisdle (v2df, v2df) int __builtin_ia32_comisdgt (v2df, v2df) int __builtin_ia32_comisdge (v2df, v2df) int __builtin_ia32_comisdneq (v2df, v2df) int __builtin_ia32_ucomisdeq (v2df, v2df) int __builtin_ia32_ucomisdlt (v2df, v2df) int __builtin_ia32_ucomisdle (v2df, v2df) int __builtin_ia32_ucomisdgt (v2df, v2df) int __builtin_ia32_ucomisdge (v2df, v2df) int __builtin_ia32_ucomisdneq (v2df, v2df) v2df __builtin_ia32_cmpeqpd (v2df, v2df) v2df __builtin_ia32_cmpltpd (v2df, v2df) v2df __builtin_ia32_cmplepd (v2df, v2df) v2df __builtin_ia32_cmpgtpd (v2df, v2df) v2df __builtin_ia32_cmpgepd (v2df, v2df) v2df __builtin_ia32_cmpunordpd (v2df, v2df) v2df __builtin_ia32_cmpneqpd (v2df, v2df) v2df __builtin_ia32_cmpnltpd (v2df, v2df) v2df __builtin_ia32_cmpnlepd (v2df, v2df) v2df __builtin_ia32_cmpngtpd (v2df, v2df) v2df __builtin_ia32_cmpngepd (v2df, v2df) v2df __builtin_ia32_cmpordpd (v2df, v2df) v2df __builtin_ia32_cmpeqsd (v2df, v2df) v2df __builtin_ia32_cmpltsd (v2df, v2df) v2df __builtin_ia32_cmplesd (v2df, v2df) v2df __builtin_ia32_cmpunordsd (v2df, v2df) v2df __builtin_ia32_cmpneqsd (v2df, v2df) v2df __builtin_ia32_cmpnltsd (v2df, v2df) v2df __builtin_ia32_cmpnlesd (v2df, v2df) v2df __builtin_ia32_cmpordsd (v2df, v2df) v2di __builtin_ia32_paddq (v2di, v2di) v2di __builtin_ia32_psubq (v2di, v2di) v2df __builtin_ia32_addpd (v2df, v2df) v2df __builtin_ia32_subpd (v2df, v2df) v2df __builtin_ia32_mulpd (v2df, v2df) v2df __builtin_ia32_divpd (v2df, v2df) v2df __builtin_ia32_addsd (v2df, v2df) v2df __builtin_ia32_subsd (v2df, v2df) v2df __builtin_ia32_mulsd (v2df, v2df) v2df __builtin_ia32_divsd (v2df, v2df) v2df __builtin_ia32_minpd (v2df, v2df) v2df __builtin_ia32_maxpd (v2df, v2df) v2df __builtin_ia32_minsd (v2df, v2df) v2df __builtin_ia32_maxsd (v2df, v2df) v2df __builtin_ia32_andpd (v2df, v2df) v2df __builtin_ia32_andnpd (v2df, v2df) v2df __builtin_ia32_orpd (v2df, v2df) v2df __builtin_ia32_xorpd (v2df, v2df) v2df __builtin_ia32_movsd (v2df, v2df) v2df __builtin_ia32_unpckhpd (v2df, v2df) v2df __builtin_ia32_unpcklpd (v2df, v2df) v16qi __builtin_ia32_paddb128 (v16qi, v16qi) v8hi __builtin_ia32_paddw128 (v8hi, v8hi) v4si __builtin_ia32_paddd128 (v4si, v4si) v2di __builtin_ia32_paddq128 (v2di, v2di) v16qi __builtin_ia32_psubb128 (v16qi, v16qi) v8hi __builtin_ia32_psubw128 (v8hi, v8hi) v4si __builtin_ia32_psubd128 (v4si, v4si) v2di __builtin_ia32_psubq128 (v2di, v2di) v8hi __builtin_ia32_pmullw128 (v8hi, v8hi) v8hi __builtin_ia32_pmulhw128 (v8hi, v8hi) v2di __builtin_ia32_pand128 (v2di, v2di) v2di __builtin_ia32_pandn128 (v2di, v2di) v2di __builtin_ia32_por128 (v2di, v2di) v2di __builtin_ia32_pxor128 (v2di, v2di) v16qi __builtin_ia32_pavgb128 (v16qi, v16qi) v8hi __builtin_ia32_pavgw128 (v8hi, v8hi) v16qi __builtin_ia32_pcmpeqb128 (v16qi, v16qi) v8hi __builtin_ia32_pcmpeqw128 (v8hi, v8hi) v4si __builtin_ia32_pcmpeqd128 (v4si, v4si) v16qi __builtin_ia32_pcmpgtb128 (v16qi, v16qi) v8hi __builtin_ia32_pcmpgtw128 (v8hi, v8hi) v4si __builtin_ia32_pcmpgtd128 (v4si, v4si) v16qi __builtin_ia32_pmaxub128 (v16qi, v16qi) v8hi __builtin_ia32_pmaxsw128 (v8hi, v8hi) v16qi __builtin_ia32_pminub128 (v16qi, v16qi) v8hi __builtin_ia32_pminsw128 (v8hi, v8hi) v16qi __builtin_ia32_punpckhbw128 (v16qi, v16qi) v8hi __builtin_ia32_punpckhwd128 (v8hi, v8hi) v4si __builtin_ia32_punpckhdq128 (v4si, v4si) v2di __builtin_ia32_punpckhqdq128 (v2di, v2di) v16qi __builtin_ia32_punpcklbw128 (v16qi, v16qi) v8hi __builtin_ia32_punpcklwd128 (v8hi, v8hi) v4si __builtin_ia32_punpckldq128 (v4si, v4si) v2di __builtin_ia32_punpcklqdq128 (v2di, v2di) v16qi __builtin_ia32_packsswb128 (v8hi, v8hi) v8hi __builtin_ia32_packssdw128 (v4si, v4si) v16qi __builtin_ia32_packuswb128 (v8hi, v8hi) v8hi __builtin_ia32_pmulhuw128 (v8hi, v8hi) void __builtin_ia32_maskmovdqu (v16qi, v16qi) v2df __builtin_ia32_loadupd (double *) void __builtin_ia32_storeupd (double *, v2df) v2df __builtin_ia32_loadhpd (v2df, double const *) v2df __builtin_ia32_loadlpd (v2df, double const *) int __builtin_ia32_movmskpd (v2df) int __builtin_ia32_pmovmskb128 (v16qi) void __builtin_ia32_movnti (int *, int) void __builtin_ia32_movnti64 (long long int *, long long int) void __builtin_ia32_movntpd (double *, v2df) void __builtin_ia32_movntdq (v2df *, v2df) v4si __builtin_ia32_pshufd (v4si, int) v8hi __builtin_ia32_pshuflw (v8hi, int) v8hi __builtin_ia32_pshufhw (v8hi, int) v2di __builtin_ia32_psadbw128 (v16qi, v16qi) v2df __builtin_ia32_sqrtpd (v2df) v2df __builtin_ia32_sqrtsd (v2df) v2df __builtin_ia32_shufpd (v2df, v2df, int) v2df __builtin_ia32_cvtdq2pd (v4si) v4sf __builtin_ia32_cvtdq2ps (v4si) v4si __builtin_ia32_cvtpd2dq (v2df) v2si __builtin_ia32_cvtpd2pi (v2df) v4sf __builtin_ia32_cvtpd2ps (v2df) v4si __builtin_ia32_cvttpd2dq (v2df) v2si __builtin_ia32_cvttpd2pi (v2df) v2df __builtin_ia32_cvtpi2pd (v2si) int __builtin_ia32_cvtsd2si (v2df) int __builtin_ia32_cvttsd2si (v2df) long long __builtin_ia32_cvtsd2si64 (v2df) long long __builtin_ia32_cvttsd2si64 (v2df) v4si __builtin_ia32_cvtps2dq (v4sf) v2df __builtin_ia32_cvtps2pd (v4sf) v4si __builtin_ia32_cvttps2dq (v4sf) v2df __builtin_ia32_cvtsi2sd (v2df, int) v2df __builtin_ia32_cvtsi642sd (v2df, long long) v4sf __builtin_ia32_cvtsd2ss (v4sf, v2df) v2df __builtin_ia32_cvtss2sd (v2df, v4sf) void __builtin_ia32_clflush (const void *) void __builtin_ia32_lfence (void) void __builtin_ia32_mfence (void) v16qi __builtin_ia32_loaddqu (const char *) void __builtin_ia32_storedqu (char *, v16qi) v1di __builtin_ia32_pmuludq (v2si, v2si) v2di __builtin_ia32_pmuludq128 (v4si, v4si) v8hi __builtin_ia32_psllw128 (v8hi, v8hi) v4si __builtin_ia32_pslld128 (v4si, v4si) v2di __builtin_ia32_psllq128 (v2di, v2di) v8hi __builtin_ia32_psrlw128 (v8hi, v8hi) v4si __builtin_ia32_psrld128 (v4si, v4si) v2di __builtin_ia32_psrlq128 (v2di, v2di) v8hi __builtin_ia32_psraw128 (v8hi, v8hi) v4si __builtin_ia32_psrad128 (v4si, v4si) v2di __builtin_ia32_pslldqi128 (v2di, int) v8hi __builtin_ia32_psllwi128 (v8hi, int) v4si __builtin_ia32_pslldi128 (v4si, int) v2di __builtin_ia32_psllqi128 (v2di, int) v2di __builtin_ia32_psrldqi128 (v2di, int) v8hi __builtin_ia32_psrlwi128 (v8hi, int) v4si __builtin_ia32_psrldi128 (v4si, int) v2di __builtin_ia32_psrlqi128 (v2di, int) v8hi __builtin_ia32_psrawi128 (v8hi, int) v4si __builtin_ia32_psradi128 (v4si, int) v4si __builtin_ia32_pmaddwd128 (v8hi, v8hi) v2di __builtin_ia32_movq128 (v2di) The following built-in functions are available when -msse3 is used. All of them generate the machine instruction that is part of the name. v2df __builtin_ia32_addsubpd (v2df, v2df) v4sf __builtin_ia32_addsubps (v4sf, v4sf) v2df __builtin_ia32_haddpd (v2df, v2df) v4sf __builtin_ia32_haddps (v4sf, v4sf) v2df __builtin_ia32_hsubpd (v2df, v2df) v4sf __builtin_ia32_hsubps (v4sf, v4sf) v16qi __builtin_ia32_lddqu (char const *) void __builtin_ia32_monitor (void *, unsigned int, unsigned int) v4sf __builtin_ia32_movshdup (v4sf) v4sf __builtin_ia32_movsldup (v4sf) void __builtin_ia32_mwait (unsigned int, unsigned int) The following built-in functions are available when -mssse3 is used. All of them generate the machine instruction that is part of the name. v2si __builtin_ia32_phaddd (v2si, v2si) v4hi __builtin_ia32_phaddw (v4hi, v4hi) v4hi __builtin_ia32_phaddsw (v4hi, v4hi) v2si __builtin_ia32_phsubd (v2si, v2si) v4hi __builtin_ia32_phsubw (v4hi, v4hi) v4hi __builtin_ia32_phsubsw (v4hi, v4hi) v4hi __builtin_ia32_pmaddubsw (v8qi, v8qi) v4hi __builtin_ia32_pmulhrsw (v4hi, v4hi) v8qi __builtin_ia32_pshufb (v8qi, v8qi) v8qi __builtin_ia32_psignb (v8qi, v8qi) v2si __builtin_ia32_psignd (v2si, v2si) v4hi __builtin_ia32_psignw (v4hi, v4hi) v1di __builtin_ia32_palignr (v1di, v1di, int) v8qi __builtin_ia32_pabsb (v8qi) v2si __builtin_ia32_pabsd (v2si) v4hi __builtin_ia32_pabsw (v4hi) The following built-in functions are available when -mssse3 is used. All of them generate the machine instruction that is part of the name. v4si __builtin_ia32_phaddd128 (v4si, v4si) v8hi __builtin_ia32_phaddw128 (v8hi, v8hi) v8hi __builtin_ia32_phaddsw128 (v8hi, v8hi) v4si __builtin_ia32_phsubd128 (v4si, v4si) v8hi __builtin_ia32_phsubw128 (v8hi, v8hi) v8hi __builtin_ia32_phsubsw128 (v8hi, v8hi) v8hi __builtin_ia32_pmaddubsw128 (v16qi, v16qi) v8hi __builtin_ia32_pmulhrsw128 (v8hi, v8hi) v16qi __builtin_ia32_pshufb128 (v16qi, v16qi) v16qi __builtin_ia32_psignb128 (v16qi, v16qi) v4si __builtin_ia32_psignd128 (v4si, v4si) v8hi __builtin_ia32_psignw128 (v8hi, v8hi) v2di __builtin_ia32_palignr128 (v2di, v2di, int) v16qi __builtin_ia32_pabsb128 (v16qi) v4si __builtin_ia32_pabsd128 (v4si) v8hi __builtin_ia32_pabsw128 (v8hi) The following built-in functions are available when -msse4.1 is used. All of them generate the machine instruction that is part of the name. v2df __builtin_ia32_blendpd (v2df, v2df, const int) v4sf __builtin_ia32_blendps (v4sf, v4sf, const int) v2df __builtin_ia32_blendvpd (v2df, v2df, v2df) v4sf __builtin_ia32_blendvps (v4sf, v4sf, v4sf) v2df __builtin_ia32_dppd (v2df, v2df, const int) v4sf __builtin_ia32_dpps (v4sf, v4sf, const int) v4sf __builtin_ia32_insertps128 (v4sf, v4sf, const int) v2di __builtin_ia32_movntdqa (v2di *); v16qi __builtin_ia32_mpsadbw128 (v16qi, v16qi, const int) v8hi __builtin_ia32_packusdw128 (v4si, v4si) v16qi __builtin_ia32_pblendvb128 (v16qi, v16qi, v16qi) v8hi __builtin_ia32_pblendw128 (v8hi, v8hi, const int) v2di __builtin_ia32_pcmpeqq (v2di, v2di) v8hi __builtin_ia32_phminposuw128 (v8hi) v16qi __builtin_ia32_pmaxsb128 (v16qi, v16qi) v4si __builtin_ia32_pmaxsd128 (v4si, v4si) v4si __builtin_ia32_pmaxud128 (v4si, v4si) v8hi __builtin_ia32_pmaxuw128 (v8hi, v8hi) v16qi __builtin_ia32_pminsb128 (v16qi, v16qi) v4si __builtin_ia32_pminsd128 (v4si, v4si) v4si __builtin_ia32_pminud128 (v4si, v4si) v8hi __builtin_ia32_pminuw128 (v8hi, v8hi) v4si __builtin_ia32_pmovsxbd128 (v16qi) v2di __builtin_ia32_pmovsxbq128 (v16qi) v8hi __builtin_ia32_pmovsxbw128 (v16qi) v2di __builtin_ia32_pmovsxdq128 (v4si) v4si __builtin_ia32_pmovsxwd128 (v8hi) v2di __builtin_ia32_pmovsxwq128 (v8hi) v4si __builtin_ia32_pmovzxbd128 (v16qi) v2di __builtin_ia32_pmovzxbq128 (v16qi) v8hi __builtin_ia32_pmovzxbw128 (v16qi) v2di __builtin_ia32_pmovzxdq128 (v4si) v4si __builtin_ia32_pmovzxwd128 (v8hi) v2di __builtin_ia32_pmovzxwq128 (v8hi) v2di __builtin_ia32_pmuldq128 (v4si, v4si) v4si __builtin_ia32_pmulld128 (v4si, v4si) int __builtin_ia32_ptestc128 (v2di, v2di) int __builtin_ia32_ptestnzc128 (v2di, v2di) int __builtin_ia32_ptestz128 (v2di, v2di) v2df __builtin_ia32_roundpd (v2df, const int) v4sf __builtin_ia32_roundps (v4sf, const int) v2df __builtin_ia32_roundsd (v2df, v2df, const int) v4sf __builtin_ia32_roundss (v4sf, v4sf, const int) The following built-in functions are available when -msse4.1 is used. v4sf __builtin_ia32_vec_set_v4sf (v4sf, float, const int) Generates the insertps machine instruction. int __builtin_ia32_vec_ext_v16qi (v16qi, const int) Generates the pextrb machine instruction. v16qi __builtin_ia32_vec_set_v16qi (v16qi, int, const int) Generates the pinsrb machine instruction. v4si __builtin_ia32_vec_set_v4si (v4si, int, const int) Generates the pinsrd machine instruction. v2di __builtin_ia32_vec_set_v2di (v2di, long long, const int) Generates the pinsrq machine instruction in 64bit mode. The following built-in functions are changed to generate new SSE4.1 instructions when -msse4.1 is used. float __builtin_ia32_vec_ext_v4sf (v4sf, const int) Generates the extractps machine instruction. int __builtin_ia32_vec_ext_v4si (v4si, const int) Generates the pextrd machine instruction. long long __builtin_ia32_vec_ext_v2di (v2di, const int) Generates the pextrq machine instruction in 64bit mode. The following built-in functions are available when -msse4.2 is used. All of them generate the machine instruction that is part of the name. v16qi __builtin_ia32_pcmpestrm128 (v16qi, int, v16qi, int, const int) int __builtin_ia32_pcmpestri128 (v16qi, int, v16qi, int, const int) int __builtin_ia32_pcmpestria128 (v16qi, int, v16qi, int, const int) int __builtin_ia32_pcmpestric128 (v16qi, int, v16qi, int, const int) int __builtin_ia32_pcmpestrio128 (v16qi, int, v16qi, int, const int) int __builtin_ia32_pcmpestris128 (v16qi, int, v16qi, int, const int) int __builtin_ia32_pcmpestriz128 (v16qi, int, v16qi, int, const int) v16qi __builtin_ia32_pcmpistrm128 (v16qi, v16qi, const int) int __builtin_ia32_pcmpistri128 (v16qi, v16qi, const int) int __builtin_ia32_pcmpistria128 (v16qi, v16qi, const int) int __builtin_ia32_pcmpistric128 (v16qi, v16qi, const int) int __builtin_ia32_pcmpistrio128 (v16qi, v16qi, const int) int __builtin_ia32_pcmpistris128 (v16qi, v16qi, const int) int __builtin_ia32_pcmpistriz128 (v16qi, v16qi, const int) v2di __builtin_ia32_pcmpgtq (v2di, v2di) The following built-in functions are available when -msse4.2 is used. unsigned int __builtin_ia32_crc32qi (unsigned int, unsigned char) Generates the crc32b machine instruction. unsigned int __builtin_ia32_crc32hi (unsigned int, unsigned short) Generates the crc32w machine instruction. unsigned int __builtin_ia32_crc32si (unsigned int, unsigned int) Generates the crc32l machine instruction. unsigned long long __builtin_ia32_crc32di (unsigned long long, unsigned long long) Generates the crc32q machine instruction. The following built-in functions are changed to generate new SSE4.2 instructions when -msse4.2 is used. int __builtin_popcount (unsigned int) Generates the popcntl machine instruction. int __builtin_popcountl (unsigned long) Generates the popcntl or popcntq machine instruction, depending on the size of unsigned long. int __builtin_popcountll (unsigned long long) Generates the popcntq machine instruction. The following built-in functions are available when -mavx is used. All of them generate the machine instruction that is part of the name. v4df __builtin_ia32_addpd256 (v4df,v4df) v8sf __builtin_ia32_addps256 (v8sf,v8sf) v4df __builtin_ia32_addsubpd256 (v4df,v4df) v8sf __builtin_ia32_addsubps256 (v8sf,v8sf) v4df __builtin_ia32_andnpd256 (v4df,v4df) v8sf __builtin_ia32_andnps256 (v8sf,v8sf) v4df __builtin_ia32_andpd256 (v4df,v4df) v8sf __builtin_ia32_andps256 (v8sf,v8sf) v4df __builtin_ia32_blendpd256 (v4df,v4df,int) v8sf __builtin_ia32_blendps256 (v8sf,v8sf,int) v4df __builtin_ia32_blendvpd256 (v4df,v4df,v4df) v8sf __builtin_ia32_blendvps256 (v8sf,v8sf,v8sf) v2df __builtin_ia32_cmppd (v2df,v2df,int) v4df __builtin_ia32_cmppd256 (v4df,v4df,int) v4sf __builtin_ia32_cmpps (v4sf,v4sf,int) v8sf __builtin_ia32_cmpps256 (v8sf,v8sf,int) v2df __builtin_ia32_cmpsd (v2df,v2df,int) v4sf __builtin_ia32_cmpss (v4sf,v4sf,int) v4df __builtin_ia32_cvtdq2pd256 (v4si) v8sf __builtin_ia32_cvtdq2ps256 (v8si) v4si __builtin_ia32_cvtpd2dq256 (v4df) v4sf __builtin_ia32_cvtpd2ps256 (v4df) v8si __builtin_ia32_cvtps2dq256 (v8sf) v4df __builtin_ia32_cvtps2pd256 (v4sf) v4si __builtin_ia32_cvttpd2dq256 (v4df) v8si __builtin_ia32_cvttps2dq256 (v8sf) v4df __builtin_ia32_divpd256 (v4df,v4df) v8sf __builtin_ia32_divps256 (v8sf,v8sf) v8sf __builtin_ia32_dpps256 (v8sf,v8sf,int) v4df __builtin_ia32_haddpd256 (v4df,v4df) v8sf __builtin_ia32_haddps256 (v8sf,v8sf) v4df __builtin_ia32_hsubpd256 (v4df,v4df) v8sf __builtin_ia32_hsubps256 (v8sf,v8sf) v32qi __builtin_ia32_lddqu256 (pcchar) v32qi __builtin_ia32_loaddqu256 (pcchar) v4df __builtin_ia32_loadupd256 (pcdouble) v8sf __builtin_ia32_loadups256 (pcfloat) v2df __builtin_ia32_maskloadpd (pcv2df,v2df) v4df __builtin_ia32_maskloadpd256 (pcv4df,v4df) v4sf __builtin_ia32_maskloadps (pcv4sf,v4sf) v8sf __builtin_ia32_maskloadps256 (pcv8sf,v8sf) void __builtin_ia32_maskstorepd (pv2df,v2df,v2df) void __builtin_ia32_maskstorepd256 (pv4df,v4df,v4df) void __builtin_ia32_maskstoreps (pv4sf,v4sf,v4sf) void __builtin_ia32_maskstoreps256 (pv8sf,v8sf,v8sf) v4df __builtin_ia32_maxpd256 (v4df,v4df) v8sf __builtin_ia32_maxps256 (v8sf,v8sf) v4df __builtin_ia32_minpd256 (v4df,v4df) v8sf __builtin_ia32_minps256 (v8sf,v8sf) v4df __builtin_ia32_movddup256 (v4df) int __builtin_ia32_movmskpd256 (v4df) int __builtin_ia32_movmskps256 (v8sf) v8sf __builtin_ia32_movshdup256 (v8sf) v8sf __builtin_ia32_movsldup256 (v8sf) v4df __builtin_ia32_mulpd256 (v4df,v4df) v8sf __builtin_ia32_mulps256 (v8sf,v8sf) v4df __builtin_ia32_orpd256 (v4df,v4df) v8sf __builtin_ia32_orps256 (v8sf,v8sf) v2df __builtin_ia32_pd_pd256 (v4df) v4df __builtin_ia32_pd256_pd (v2df) v4sf __builtin_ia32_ps_ps256 (v8sf) v8sf __builtin_ia32_ps256_ps (v4sf) int __builtin_ia32_ptestc256 (v4di,v4di,ptest) int __builtin_ia32_ptestnzc256 (v4di,v4di,ptest) int __builtin_ia32_ptestz256 (v4di,v4di,ptest) v8sf __builtin_ia32_rcpps256 (v8sf) v4df __builtin_ia32_roundpd256 (v4df,int) v8sf __builtin_ia32_roundps256 (v8sf,int) v8sf __builtin_ia32_rsqrtps_nr256 (v8sf) v8sf __builtin_ia32_rsqrtps256 (v8sf) v4df __builtin_ia32_shufpd256 (v4df,v4df,int) v8sf __builtin_ia32_shufps256 (v8sf,v8sf,int) v4si __builtin_ia32_si_si256 (v8si) v8si __builtin_ia32_si256_si (v4si) v4df __builtin_ia32_sqrtpd256 (v4df) v8sf __builtin_ia32_sqrtps_nr256 (v8sf) v8sf __builtin_ia32_sqrtps256 (v8sf) void __builtin_ia32_storedqu256 (pchar,v32qi) void __builtin_ia32_storeupd256 (pdouble,v4df) void __builtin_ia32_storeups256 (pfloat,v8sf) v4df __builtin_ia32_subpd256 (v4df,v4df) v8sf __builtin_ia32_subps256 (v8sf,v8sf) v4df __builtin_ia32_unpckhpd256 (v4df,v4df) v8sf __builtin_ia32_unpckhps256 (v8sf,v8sf) v4df __builtin_ia32_unpcklpd256 (v4df,v4df) v8sf __builtin_ia32_unpcklps256 (v8sf,v8sf) v4df __builtin_ia32_vbroadcastf128_pd256 (pcv2df) v8sf __builtin_ia32_vbroadcastf128_ps256 (pcv4sf) v4df __builtin_ia32_vbroadcastsd256 (pcdouble) v4sf __builtin_ia32_vbroadcastss (pcfloat) v8sf __builtin_ia32_vbroadcastss256 (pcfloat) v2df __builtin_ia32_vextractf128_pd256 (v4df,int) v4sf __builtin_ia32_vextractf128_ps256 (v8sf,int) v4si __builtin_ia32_vextractf128_si256 (v8si,int) v4df __builtin_ia32_vinsertf128_pd256 (v4df,v2df,int) v8sf __builtin_ia32_vinsertf128_ps256 (v8sf,v4sf,int) v8si __builtin_ia32_vinsertf128_si256 (v8si,v4si,int) v4df __builtin_ia32_vperm2f128_pd256 (v4df,v4df,int) v8sf __builtin_ia32_vperm2f128_ps256 (v8sf,v8sf,int) v8si __builtin_ia32_vperm2f128_si256 (v8si,v8si,int) v2df __builtin_ia32_vpermil2pd (v2df,v2df,v2di,int) v4df __builtin_ia32_vpermil2pd256 (v4df,v4df,v4di,int) v4sf __builtin_ia32_vpermil2ps (v4sf,v4sf,v4si,int) v8sf __builtin_ia32_vpermil2ps256 (v8sf,v8sf,v8si,int) v2df __builtin_ia32_vpermilpd (v2df,int) v4df __builtin_ia32_vpermilpd256 (v4df,int) v4sf __builtin_ia32_vpermilps (v4sf,int) v8sf __builtin_ia32_vpermilps256 (v8sf,int) v2df __builtin_ia32_vpermilvarpd (v2df,v2di) v4df __builtin_ia32_vpermilvarpd256 (v4df,v4di) v4sf __builtin_ia32_vpermilvarps (v4sf,v4si) v8sf __builtin_ia32_vpermilvarps256 (v8sf,v8si) int __builtin_ia32_vtestcpd (v2df,v2df,ptest) int __builtin_ia32_vtestcpd256 (v4df,v4df,ptest) int __builtin_ia32_vtestcps (v4sf,v4sf,ptest) int __builtin_ia32_vtestcps256 (v8sf,v8sf,ptest) int __builtin_ia32_vtestnzcpd (v2df,v2df,ptest) int __builtin_ia32_vtestnzcpd256 (v4df,v4df,ptest) int __builtin_ia32_vtestnzcps (v4sf,v4sf,ptest) int __builtin_ia32_vtestnzcps256 (v8sf,v8sf,ptest) int __builtin_ia32_vtestzpd (v2df,v2df,ptest) int __builtin_ia32_vtestzpd256 (v4df,v4df,ptest) int __builtin_ia32_vtestzps (v4sf,v4sf,ptest) int __builtin_ia32_vtestzps256 (v8sf,v8sf,ptest) void __builtin_ia32_vzeroall (void) void __builtin_ia32_vzeroupper (void) v4df __builtin_ia32_xorpd256 (v4df,v4df) v8sf __builtin_ia32_xorps256 (v8sf,v8sf) The following built-in functions are available when -mavx2 is used. All of them generate the machine instruction that is part of the name. v32qi __builtin_ia32_mpsadbw256 (v32qi,v32qi,int) v32qi __builtin_ia32_pabsb256 (v32qi) v16hi __builtin_ia32_pabsw256 (v16hi) v8si __builtin_ia32_pabsd256 (v8si) v16hi __builtin_ia32_packssdw256 (v8si,v8si) v32qi __builtin_ia32_packsswb256 (v16hi,v16hi) v16hi __builtin_ia32_packusdw256 (v8si,v8si) v32qi __builtin_ia32_packuswb256 (v16hi,v16hi) v32qi __builtin_ia32_paddb256 (v32qi,v32qi) v16hi __builtin_ia32_paddw256 (v16hi,v16hi) v8si __builtin_ia32_paddd256 (v8si,v8si) v4di __builtin_ia32_paddq256 (v4di,v4di) v32qi __builtin_ia32_paddsb256 (v32qi,v32qi) v16hi __builtin_ia32_paddsw256 (v16hi,v16hi) v32qi __builtin_ia32_paddusb256 (v32qi,v32qi) v16hi __builtin_ia32_paddusw256 (v16hi,v16hi) v4di __builtin_ia32_palignr256 (v4di,v4di,int) v4di __builtin_ia32_andsi256 (v4di,v4di) v4di __builtin_ia32_andnotsi256 (v4di,v4di) v32qi __builtin_ia32_pavgb256 (v32qi,v32qi) v16hi __builtin_ia32_pavgw256 (v16hi,v16hi) v32qi __builtin_ia32_pblendvb256 (v32qi,v32qi,v32qi) v16hi __builtin_ia32_pblendw256 (v16hi,v16hi,int) v32qi __builtin_ia32_pcmpeqb256 (v32qi,v32qi) v16hi __builtin_ia32_pcmpeqw256 (v16hi,v16hi) v8si __builtin_ia32_pcmpeqd256 (c8si,v8si) v4di __builtin_ia32_pcmpeqq256 (v4di,v4di) v32qi __builtin_ia32_pcmpgtb256 (v32qi,v32qi) v16hi __builtin_ia32_pcmpgtw256 (16hi,v16hi) v8si __builtin_ia32_pcmpgtd256 (v8si,v8si) v4di __builtin_ia32_pcmpgtq256 (v4di,v4di) v16hi __builtin_ia32_phaddw256 (v16hi,v16hi) v8si __builtin_ia32_phaddd256 (v8si,v8si) v16hi __builtin_ia32_phaddsw256 (v16hi,v16hi) v16hi __builtin_ia32_phsubw256 (v16hi,v16hi) v8si __builtin_ia32_phsubd256 (v8si,v8si) v16hi __builtin_ia32_phsubsw256 (v16hi,v16hi) v32qi __builtin_ia32_pmaddubsw256 (v32qi,v32qi) v16hi __builtin_ia32_pmaddwd256 (v16hi,v16hi) v32qi __builtin_ia32_pmaxsb256 (v32qi,v32qi) v16hi __builtin_ia32_pmaxsw256 (v16hi,v16hi) v8si __builtin_ia32_pmaxsd256 (v8si,v8si) v32qi __builtin_ia32_pmaxub256 (v32qi,v32qi) v16hi __builtin_ia32_pmaxuw256 (v16hi,v16hi) v8si __builtin_ia32_pmaxud256 (v8si,v8si) v32qi __builtin_ia32_pminsb256 (v32qi,v32qi) v16hi __builtin_ia32_pminsw256 (v16hi,v16hi) v8si __builtin_ia32_pminsd256 (v8si,v8si) v32qi __builtin_ia32_pminub256 (v32qi,v32qi) v16hi __builtin_ia32_pminuw256 (v16hi,v16hi) v8si __builtin_ia32_pminud256 (v8si,v8si) int __builtin_ia32_pmovmskb256 (v32qi) v16hi __builtin_ia32_pmovsxbw256 (v16qi) v8si __builtin_ia32_pmovsxbd256 (v16qi) v4di __builtin_ia32_pmovsxbq256 (v16qi) v8si __builtin_ia32_pmovsxwd256 (v8hi) v4di __builtin_ia32_pmovsxwq256 (v8hi) v4di __builtin_ia32_pmovsxdq256 (v4si) v16hi __builtin_ia32_pmovzxbw256 (v16qi) v8si __builtin_ia32_pmovzxbd256 (v16qi) v4di __builtin_ia32_pmovzxbq256 (v16qi) v8si __builtin_ia32_pmovzxwd256 (v8hi) v4di __builtin_ia32_pmovzxwq256 (v8hi) v4di __builtin_ia32_pmovzxdq256 (v4si) v4di __builtin_ia32_pmuldq256 (v8si,v8si) v16hi __builtin_ia32_pmulhrsw256 (v16hi, v16hi) v16hi __builtin_ia32_pmulhuw256 (v16hi,v16hi) v16hi __builtin_ia32_pmulhw256 (v16hi,v16hi) v16hi __builtin_ia32_pmullw256 (v16hi,v16hi) v8si __builtin_ia32_pmulld256 (v8si,v8si) v4di __builtin_ia32_pmuludq256 (v8si,v8si) v4di __builtin_ia32_por256 (v4di,v4di) v16hi __builtin_ia32_psadbw256 (v32qi,v32qi) v32qi __builtin_ia32_pshufb256 (v32qi,v32qi) v8si __builtin_ia32_pshufd256 (v8si,int) v16hi __builtin_ia32_pshufhw256 (v16hi,int) v16hi __builtin_ia32_pshuflw256 (v16hi,int) v32qi __builtin_ia32_psignb256 (v32qi,v32qi) v16hi __builtin_ia32_psignw256 (v16hi,v16hi) v8si __builtin_ia32_psignd256 (v8si,v8si) v4di __builtin_ia32_pslldqi256 (v4di,int) v16hi __builtin_ia32_psllwi256 (16hi,int) v16hi __builtin_ia32_psllw256(v16hi,v8hi) v8si __builtin_ia32_pslldi256 (v8si,int) v8si __builtin_ia32_pslld256(v8si,v4si) v4di __builtin_ia32_psllqi256 (v4di,int) v4di __builtin_ia32_psllq256(v4di,v2di) v16hi __builtin_ia32_psrawi256 (v16hi,int) v16hi __builtin_ia32_psraw256 (v16hi,v8hi) v8si __builtin_ia32_psradi256 (v8si,int) v8si __builtin_ia32_psrad256 (v8si,v4si) v4di __builtin_ia32_psrldqi256 (v4di, int) v16hi __builtin_ia32_psrlwi256 (v16hi,int) v16hi __builtin_ia32_psrlw256 (v16hi,v8hi) v8si __builtin_ia32_psrldi256 (v8si,int) v8si __builtin_ia32_psrld256 (v8si,v4si) v4di __builtin_ia32_psrlqi256 (v4di,int) v4di __builtin_ia32_psrlq256(v4di,v2di) v32qi __builtin_ia32_psubb256 (v32qi,v32qi) v32hi __builtin_ia32_psubw256 (v16hi,v16hi) v8si __builtin_ia32_psubd256 (v8si,v8si) v4di __builtin_ia32_psubq256 (v4di,v4di) v32qi __builtin_ia32_psubsb256 (v32qi,v32qi) v16hi __builtin_ia32_psubsw256 (v16hi,v16hi) v32qi __builtin_ia32_psubusb256 (v32qi,v32qi) v16hi __builtin_ia32_psubusw256 (v16hi,v16hi) v32qi __builtin_ia32_punpckhbw256 (v32qi,v32qi) v16hi __builtin_ia32_punpckhwd256 (v16hi,v16hi) v8si __builtin_ia32_punpckhdq256 (v8si,v8si) v4di __builtin_ia32_punpckhqdq256 (v4di,v4di) v32qi __builtin_ia32_punpcklbw256 (v32qi,v32qi) v16hi __builtin_ia32_punpcklwd256 (v16hi,v16hi) v8si __builtin_ia32_punpckldq256 (v8si,v8si) v4di __builtin_ia32_punpcklqdq256 (v4di,v4di) v4di __builtin_ia32_pxor256 (v4di,v4di) v4di __builtin_ia32_movntdqa256 (pv4di) v4sf __builtin_ia32_vbroadcastss_ps (v4sf) v8sf __builtin_ia32_vbroadcastss_ps256 (v4sf) v4df __builtin_ia32_vbroadcastsd_pd256 (v2df) v4di __builtin_ia32_vbroadcastsi256 (v2di) v4si __builtin_ia32_pblendd128 (v4si,v4si) v8si __builtin_ia32_pblendd256 (v8si,v8si) v32qi __builtin_ia32_pbroadcastb256 (v16qi) v16hi __builtin_ia32_pbroadcastw256 (v8hi) v8si __builtin_ia32_pbroadcastd256 (v4si) v4di __builtin_ia32_pbroadcastq256 (v2di) v16qi __builtin_ia32_pbroadcastb128 (v16qi) v8hi __builtin_ia32_pbroadcastw128 (v8hi) v4si __builtin_ia32_pbroadcastd128 (v4si) v2di __builtin_ia32_pbroadcastq128 (v2di) v8si __builtin_ia32_permvarsi256 (v8si,v8si) v4df __builtin_ia32_permdf256 (v4df,int) v8sf __builtin_ia32_permvarsf256 (v8sf,v8sf) v4di __builtin_ia32_permdi256 (v4di,int) v4di __builtin_ia32_permti256 (v4di,v4di,int) v4di __builtin_ia32_extract128i256 (v4di,int) v4di __builtin_ia32_insert128i256 (v4di,v2di,int) v8si __builtin_ia32_maskloadd256 (pcv8si,v8si) v4di __builtin_ia32_maskloadq256 (pcv4di,v4di) v4si __builtin_ia32_maskloadd (pcv4si,v4si) v2di __builtin_ia32_maskloadq (pcv2di,v2di) void __builtin_ia32_maskstored256 (pv8si,v8si,v8si) void __builtin_ia32_maskstoreq256 (pv4di,v4di,v4di) void __builtin_ia32_maskstored (pv4si,v4si,v4si) void __builtin_ia32_maskstoreq (pv2di,v2di,v2di) v8si __builtin_ia32_psllv8si (v8si,v8si) v4si __builtin_ia32_psllv4si (v4si,v4si) v4di __builtin_ia32_psllv4di (v4di,v4di) v2di __builtin_ia32_psllv2di (v2di,v2di) v8si __builtin_ia32_psrav8si (v8si,v8si) v4si __builtin_ia32_psrav4si (v4si,v4si) v8si __builtin_ia32_psrlv8si (v8si,v8si) v4si __builtin_ia32_psrlv4si (v4si,v4si) v4di __builtin_ia32_psrlv4di (v4di,v4di) v2di __builtin_ia32_psrlv2di (v2di,v2di) v2df __builtin_ia32_gathersiv2df (v2df, pcdouble,v4si,v2df,int) v4df __builtin_ia32_gathersiv4df (v4df, pcdouble,v4si,v4df,int) v2df __builtin_ia32_gatherdiv2df (v2df, pcdouble,v2di,v2df,int) v4df __builtin_ia32_gatherdiv4df (v4df, pcdouble,v4di,v4df,int) v4sf __builtin_ia32_gathersiv4sf (v4sf, pcfloat,v4si,v4sf,int) v8sf __builtin_ia32_gathersiv8sf (v8sf, pcfloat,v8si,v8sf,int) v4sf __builtin_ia32_gatherdiv4sf (v4sf, pcfloat,v2di,v4sf,int) v4sf __builtin_ia32_gatherdiv4sf256 (v4sf, pcfloat,v4di,v4sf,int) v2di __builtin_ia32_gathersiv2di (v2di, pcint64,v4si,v2di,int) v4di __builtin_ia32_gathersiv4di (v4di, pcint64,v4si,v4di,int) v2di __builtin_ia32_gatherdiv2di (v2di, pcint64,v2di,v2di,int) v4di __builtin_ia32_gatherdiv4di (v4di, pcint64,v4di,v4di,int) v4si __builtin_ia32_gathersiv4si (v4si, pcint,v4si,v4si,int) v8si __builtin_ia32_gathersiv8si (v8si, pcint,v8si,v8si,int) v4si __builtin_ia32_gatherdiv4si (v4si, pcint,v2di,v4si,int) v4si __builtin_ia32_gatherdiv4si256 (v4si, pcint,v4di,v4si,int) The following built-in functions are available when -maes is used. All of them generate the machine instruction that is part of the name. v2di __builtin_ia32_aesenc128 (v2di, v2di) v2di __builtin_ia32_aesenclast128 (v2di, v2di) v2di __builtin_ia32_aesdec128 (v2di, v2di) v2di __builtin_ia32_aesdeclast128 (v2di, v2di) v2di __builtin_ia32_aeskeygenassist128 (v2di, const int) v2di __builtin_ia32_aesimc128 (v2di) The following built-in function is available when -mpclmul is used. v2di __builtin_ia32_pclmulqdq128 (v2di, v2di, const int) Generates the pclmulqdq machine instruction. The following built-in function is available when -mfsgsbase is used. All of them generate the machine instruction that is part of the name. unsigned int __builtin_ia32_rdfsbase32 (void) unsigned long long __builtin_ia32_rdfsbase64 (void) unsigned int __builtin_ia32_rdgsbase32 (void) unsigned long long __builtin_ia32_rdgsbase64 (void) void _writefsbase_u32 (unsigned int) void _writefsbase_u64 (unsigned long long) void _writegsbase_u32 (unsigned int) void _writegsbase_u64 (unsigned long long) The following built-in function is available when -mrdrnd is used. All of them generate the machine instruction that is part of the name. unsigned int __builtin_ia32_rdrand16_step (unsigned short *) unsigned int __builtin_ia32_rdrand32_step (unsigned int *) unsigned int __builtin_ia32_rdrand64_step (unsigned long long *) The following built-in function is available when -mptwrite is used. All of them generate the machine instruction that is part of the name. void __builtin_ia32_ptwrite32 (unsigned) void __builtin_ia32_ptwrite64 (unsigned long long) The following built-in functions are available when -msse4a is used. All of them generate the machine instruction that is part of the name. void __builtin_ia32_movntsd (double *, v2df) void __builtin_ia32_movntss (float *, v4sf) v2di __builtin_ia32_extrq (v2di, v16qi) v2di __builtin_ia32_extrqi (v2di, const unsigned int, const unsigned int) v2di __builtin_ia32_insertq (v2di, v2di) v2di __builtin_ia32_insertqi (v2di, v2di, const unsigned int, const unsigned int) The following built-in functions are available when -mxop is used. v2df __builtin_ia32_vfrczpd (v2df) v4sf __builtin_ia32_vfrczps (v4sf) v2df __builtin_ia32_vfrczsd (v2df) v4sf __builtin_ia32_vfrczss (v4sf) v4df __builtin_ia32_vfrczpd256 (v4df) v8sf __builtin_ia32_vfrczps256 (v8sf) v2di __builtin_ia32_vpcmov (v2di, v2di, v2di) v2di __builtin_ia32_vpcmov_v2di (v2di, v2di, v2di) v4si __builtin_ia32_vpcmov_v4si (v4si, v4si, v4si) v8hi __builtin_ia32_vpcmov_v8hi (v8hi, v8hi, v8hi) v16qi __builtin_ia32_vpcmov_v16qi (v16qi, v16qi, v16qi) v2df __builtin_ia32_vpcmov_v2df (v2df, v2df, v2df) v4sf __builtin_ia32_vpcmov_v4sf (v4sf, v4sf, v4sf) v4di __builtin_ia32_vpcmov_v4di256 (v4di, v4di, v4di) v8si __builtin_ia32_vpcmov_v8si256 (v8si, v8si, v8si) v16hi __builtin_ia32_vpcmov_v16hi256 (v16hi, v16hi, v16hi) v32qi __builtin_ia32_vpcmov_v32qi256 (v32qi, v32qi, v32qi) v4df __builtin_ia32_vpcmov_v4df256 (v4df, v4df, v4df) v8sf __builtin_ia32_vpcmov_v8sf256 (v8sf, v8sf, v8sf) v16qi __builtin_ia32_vpcomeqb (v16qi, v16qi) v8hi __builtin_ia32_vpcomeqw (v8hi, v8hi) v4si __builtin_ia32_vpcomeqd (v4si, v4si) v2di __builtin_ia32_vpcomeqq (v2di, v2di) v16qi __builtin_ia32_vpcomequb (v16qi, v16qi) v4si __builtin_ia32_vpcomequd (v4si, v4si) v2di __builtin_ia32_vpcomequq (v2di, v2di) v8hi __builtin_ia32_vpcomequw (v8hi, v8hi) v8hi __builtin_ia32_vpcomeqw (v8hi, v8hi) v16qi __builtin_ia32_vpcomfalseb (v16qi, v16qi) v4si __builtin_ia32_vpcomfalsed (v4si, v4si) v2di __builtin_ia32_vpcomfalseq (v2di, v2di) v16qi __builtin_ia32_vpcomfalseub (v16qi, v16qi) v4si __builtin_ia32_vpcomfalseud (v4si, v4si) v2di __builtin_ia32_vpcomfalseuq (v2di, v2di) v8hi __builtin_ia32_vpcomfalseuw (v8hi, v8hi) v8hi __builtin_ia32_vpcomfalsew (v8hi, v8hi) v16qi __builtin_ia32_vpcomgeb (v16qi, v16qi) v4si __builtin_ia32_vpcomged (v4si, v4si) v2di __builtin_ia32_vpcomgeq (v2di, v2di) v16qi __builtin_ia32_vpcomgeub (v16qi, v16qi) v4si __builtin_ia32_vpcomgeud (v4si, v4si) v2di __builtin_ia32_vpcomgeuq (v2di, v2di) v8hi __builtin_ia32_vpcomgeuw (v8hi, v8hi) v8hi __builtin_ia32_vpcomgew (v8hi, v8hi) v16qi __builtin_ia32_vpcomgtb (v16qi, v16qi) v4si __builtin_ia32_vpcomgtd (v4si, v4si) v2di __builtin_ia32_vpcomgtq (v2di, v2di) v16qi __builtin_ia32_vpcomgtub (v16qi, v16qi) v4si __builtin_ia32_vpcomgtud (v4si, v4si) v2di __builtin_ia32_vpcomgtuq (v2di, v2di) v8hi __builtin_ia32_vpcomgtuw (v8hi, v8hi) v8hi __builtin_ia32_vpcomgtw (v8hi, v8hi) v16qi __builtin_ia32_vpcomleb (v16qi, v16qi) v4si __builtin_ia32_vpcomled (v4si, v4si) v2di __builtin_ia32_vpcomleq (v2di, v2di) v16qi __builtin_ia32_vpcomleub (v16qi, v16qi) v4si __builtin_ia32_vpcomleud (v4si, v4si) v2di __builtin_ia32_vpcomleuq (v2di, v2di) v8hi __builtin_ia32_vpcomleuw (v8hi, v8hi) v8hi __builtin_ia32_vpcomlew (v8hi, v8hi) v16qi __builtin_ia32_vpcomltb (v16qi, v16qi) v4si __builtin_ia32_vpcomltd (v4si, v4si) v2di __builtin_ia32_vpcomltq (v2di, v2di) v16qi __builtin_ia32_vpcomltub (v16qi, v16qi) v4si __builtin_ia32_vpcomltud (v4si, v4si) v2di __builtin_ia32_vpcomltuq (v2di, v2di) v8hi __builtin_ia32_vpcomltuw (v8hi, v8hi) v8hi __builtin_ia32_vpcomltw (v8hi, v8hi) v16qi __builtin_ia32_vpcomneb (v16qi, v16qi) v4si __builtin_ia32_vpcomned (v4si, v4si) v2di __builtin_ia32_vpcomneq (v2di, v2di) v16qi __builtin_ia32_vpcomneub (v16qi, v16qi) v4si __builtin_ia32_vpcomneud (v4si, v4si) v2di __builtin_ia32_vpcomneuq (v2di, v2di) v8hi __builtin_ia32_vpcomneuw (v8hi, v8hi) v8hi __builtin_ia32_vpcomnew (v8hi, v8hi) v16qi __builtin_ia32_vpcomtrueb (v16qi, v16qi) v4si __builtin_ia32_vpcomtrued (v4si, v4si) v2di __builtin_ia32_vpcomtrueq (v2di, v2di) v16qi __builtin_ia32_vpcomtrueub (v16qi, v16qi) v4si __builtin_ia32_vpcomtrueud (v4si, v4si) v2di __builtin_ia32_vpcomtrueuq (v2di, v2di) v8hi __builtin_ia32_vpcomtrueuw (v8hi, v8hi) v8hi __builtin_ia32_vpcomtruew (v8hi, v8hi) v4si __builtin_ia32_vphaddbd (v16qi) v2di __builtin_ia32_vphaddbq (v16qi) v8hi __builtin_ia32_vphaddbw (v16qi) v2di __builtin_ia32_vphadddq (v4si) v4si __builtin_ia32_vphaddubd (v16qi) v2di __builtin_ia32_vphaddubq (v16qi) v8hi __builtin_ia32_vphaddubw (v16qi) v2di __builtin_ia32_vphaddudq (v4si) v4si __builtin_ia32_vphadduwd (v8hi) v2di __builtin_ia32_vphadduwq (v8hi) v4si __builtin_ia32_vphaddwd (v8hi) v2di __builtin_ia32_vphaddwq (v8hi) v8hi __builtin_ia32_vphsubbw (v16qi) v2di __builtin_ia32_vphsubdq (v4si) v4si __builtin_ia32_vphsubwd (v8hi) v4si __builtin_ia32_vpmacsdd (v4si, v4si, v4si) v2di __builtin_ia32_vpmacsdqh (v4si, v4si, v2di) v2di __builtin_ia32_vpmacsdql (v4si, v4si, v2di) v4si __builtin_ia32_vpmacssdd (v4si, v4si, v4si) v2di __builtin_ia32_vpmacssdqh (v4si, v4si, v2di) v2di __builtin_ia32_vpmacssdql (v4si, v4si, v2di) v4si __builtin_ia32_vpmacsswd (v8hi, v8hi, v4si) v8hi __builtin_ia32_vpmacssww (v8hi, v8hi, v8hi) v4si __builtin_ia32_vpmacswd (v8hi, v8hi, v4si) v8hi __builtin_ia32_vpmacsww (v8hi, v8hi, v8hi) v4si __builtin_ia32_vpmadcsswd (v8hi, v8hi, v4si) v4si __builtin_ia32_vpmadcswd (v8hi, v8hi, v4si) v16qi __builtin_ia32_vpperm (v16qi, v16qi, v16qi) v16qi __builtin_ia32_vprotb (v16qi, v16qi) v4si __builtin_ia32_vprotd (v4si, v4si) v2di __builtin_ia32_vprotq (v2di, v2di) v8hi __builtin_ia32_vprotw (v8hi, v8hi) v16qi __builtin_ia32_vpshab (v16qi, v16qi) v4si __builtin_ia32_vpshad (v4si, v4si) v2di __builtin_ia32_vpshaq (v2di, v2di) v8hi __builtin_ia32_vpshaw (v8hi, v8hi) v16qi __builtin_ia32_vpshlb (v16qi, v16qi) v4si __builtin_ia32_vpshld (v4si, v4si) v2di __builtin_ia32_vpshlq (v2di, v2di) v8hi __builtin_ia32_vpshlw (v8hi, v8hi) The following built-in functions are available when -mfma4 is used. All of them generate the machine instruction that is part of the name. v2df __builtin_ia32_vfmaddpd (v2df, v2df, v2df) v4sf __builtin_ia32_vfmaddps (v4sf, v4sf, v4sf) v2df __builtin_ia32_vfmaddsd (v2df, v2df, v2df) v4sf __builtin_ia32_vfmaddss (v4sf, v4sf, v4sf) v2df __builtin_ia32_vfmsubpd (v2df, v2df, v2df) v4sf __builtin_ia32_vfmsubps (v4sf, v4sf, v4sf) v2df __builtin_ia32_vfmsubsd (v2df, v2df, v2df) v4sf __builtin_ia32_vfmsubss (v4sf, v4sf, v4sf) v2df __builtin_ia32_vfnmaddpd (v2df, v2df, v2df) v4sf __builtin_ia32_vfnmaddps (v4sf, v4sf, v4sf) v2df __builtin_ia32_vfnmaddsd (v2df, v2df, v2df) v4sf __builtin_ia32_vfnmaddss (v4sf, v4sf, v4sf) v2df __builtin_ia32_vfnmsubpd (v2df, v2df, v2df) v4sf __builtin_ia32_vfnmsubps (v4sf, v4sf, v4sf) v2df __builtin_ia32_vfnmsubsd (v2df, v2df, v2df) v4sf __builtin_ia32_vfnmsubss (v4sf, v4sf, v4sf) v2df __builtin_ia32_vfmaddsubpd (v2df, v2df, v2df) v4sf __builtin_ia32_vfmaddsubps (v4sf, v4sf, v4sf) v2df __builtin_ia32_vfmsubaddpd (v2df, v2df, v2df) v4sf __builtin_ia32_vfmsubaddps (v4sf, v4sf, v4sf) v4df __builtin_ia32_vfmaddpd256 (v4df, v4df, v4df) v8sf __builtin_ia32_vfmaddps256 (v8sf, v8sf, v8sf) v4df __builtin_ia32_vfmsubpd256 (v4df, v4df, v4df) v8sf __builtin_ia32_vfmsubps256 (v8sf, v8sf, v8sf) v4df __builtin_ia32_vfnmaddpd256 (v4df, v4df, v4df) v8sf __builtin_ia32_vfnmaddps256 (v8sf, v8sf, v8sf) v4df __builtin_ia32_vfnmsubpd256 (v4df, v4df, v4df) v8sf __builtin_ia32_vfnmsubps256 (v8sf, v8sf, v8sf) v4df __builtin_ia32_vfmaddsubpd256 (v4df, v4df, v4df) v8sf __builtin_ia32_vfmaddsubps256 (v8sf, v8sf, v8sf) v4df __builtin_ia32_vfmsubaddpd256 (v4df, v4df, v4df) v8sf __builtin_ia32_vfmsubaddps256 (v8sf, v8sf, v8sf) The following built-in functions are available when -mlwp is used. void __builtin_ia32_llwpcb16 (void *); void __builtin_ia32_llwpcb32 (void *); void __builtin_ia32_llwpcb64 (void *); void * __builtin_ia32_llwpcb16 (void); void * __builtin_ia32_llwpcb32 (void); void * __builtin_ia32_llwpcb64 (void); void __builtin_ia32_lwpval16 (unsigned short, unsigned int, unsigned short) void __builtin_ia32_lwpval32 (unsigned int, unsigned int, unsigned int) void __builtin_ia32_lwpval64 (unsigned __int64, unsigned int, unsigned int) unsigned char __builtin_ia32_lwpins16 (unsigned short, unsigned int, unsigned short) unsigned char __builtin_ia32_lwpins32 (unsigned int, unsigned int, unsigned int) unsigned char __builtin_ia32_lwpins64 (unsigned __int64, unsigned int, unsigned int) The following built-in functions are available when -mbmi is used. All of them generate the machine instruction that is part of the name. unsigned int __builtin_ia32_bextr_u32(unsigned int, unsigned int); unsigned long long __builtin_ia32_bextr_u64 (unsigned long long, unsigned long long); The following built-in functions are available when -mbmi2 is used. All of them generate the machine instruction that is part of the name. unsigned int _bzhi_u32 (unsigned int, unsigned int) unsigned int _pdep_u32 (unsigned int, unsigned int) unsigned int _pext_u32 (unsigned int, unsigned int) unsigned long long _bzhi_u64 (unsigned long long, unsigned long long) unsigned long long _pdep_u64 (unsigned long long, unsigned long long) unsigned long long _pext_u64 (unsigned long long, unsigned long long) The following built-in functions are available when -mlzcnt is used. All of them generate the machine instruction that is part of the name. unsigned short __builtin_ia32_lzcnt_u16(unsigned short); unsigned int __builtin_ia32_lzcnt_u32(unsigned int); unsigned long long __builtin_ia32_lzcnt_u64 (unsigned long long); The following built-in functions are available when -mfxsr is used. All of them generate the machine instruction that is part of the name. void __builtin_ia32_fxsave (void *) void __builtin_ia32_fxrstor (void *) void __builtin_ia32_fxsave64 (void *) void __builtin_ia32_fxrstor64 (void *) The following built-in functions are available when -mxsave is used. All of them generate the machine instruction that is part of the name. void __builtin_ia32_xsave (void *, long long) void __builtin_ia32_xrstor (void *, long long) void __builtin_ia32_xsave64 (void *, long long) void __builtin_ia32_xrstor64 (void *, long long) The following built-in functions are available when -mxsaveopt is used. All of them generate the machine instruction that is part of the name. void __builtin_ia32_xsaveopt (void *, long long) void __builtin_ia32_xsaveopt64 (void *, long long) The following built-in functions are available when -mtbm is used. Both of them generate the immediate form of the bextr machine instruction. unsigned int __builtin_ia32_bextri_u32 (unsigned int, const unsigned int); unsigned long long __builtin_ia32_bextri_u64 (unsigned long long, const unsigned long long); The following built-in functions are available when -m3dnow is used. All of them generate the machine instruction that is part of the name. void __builtin_ia32_femms (void) v8qi __builtin_ia32_pavgusb (v8qi, v8qi) v2si __builtin_ia32_pf2id (v2sf) v2sf __builtin_ia32_pfacc (v2sf, v2sf) v2sf __builtin_ia32_pfadd (v2sf, v2sf) v2si __builtin_ia32_pfcmpeq (v2sf, v2sf) v2si __builtin_ia32_pfcmpge (v2sf, v2sf) v2si __builtin_ia32_pfcmpgt (v2sf, v2sf) v2sf __builtin_ia32_pfmax (v2sf, v2sf) v2sf __builtin_ia32_pfmin (v2sf, v2sf) v2sf __builtin_ia32_pfmul (v2sf, v2sf) v2sf __builtin_ia32_pfrcp (v2sf) v2sf __builtin_ia32_pfrcpit1 (v2sf, v2sf) v2sf __builtin_ia32_pfrcpit2 (v2sf, v2sf) v2sf __builtin_ia32_pfrsqrt (v2sf) v2sf __builtin_ia32_pfsub (v2sf, v2sf) v2sf __builtin_ia32_pfsubr (v2sf, v2sf) v2sf __builtin_ia32_pi2fd (v2si) v4hi __builtin_ia32_pmulhrw (v4hi, v4hi) The following built-in functions are available when -m3dnowa is used. All of them generate the machine instruction that is part of the name. v2si __builtin_ia32_pf2iw (v2sf) v2sf __builtin_ia32_pfnacc (v2sf, v2sf) v2sf __builtin_ia32_pfpnacc (v2sf, v2sf) v2sf __builtin_ia32_pi2fw (v2si) v2sf __builtin_ia32_pswapdsf (v2sf) v2si __builtin_ia32_pswapdsi (v2si) The following built-in functions are available when -mrtm is used They are used for restricted transactional memory. These are the internal low level functions. Normally the functions in x86 transactional memory intrinsics should be used instead. int __builtin_ia32_xbegin () void __builtin_ia32_xend () void __builtin_ia32_xabort (status) int __builtin_ia32_xtest () The following built-in functions are available when -mmwaitx is used. All of them generate the machine instruction that is part of the name. void __builtin_ia32_monitorx (void *, unsigned int, unsigned int) void __builtin_ia32_mwaitx (unsigned int, unsigned int, unsigned int) The following built-in functions are available when -mclzero is used. All of them generate the machine instruction that is part of the name. void __builtin_i32_clzero (void *) The following built-in functions are available when -mpku is used. They generate reads and writes to PKRU. void __builtin_ia32_wrpkru (unsigned int) unsigned int __builtin_ia32_rdpkru () The following built-in functions are available when -mcet or -mshstk option is used. They support shadow stack machine instructions from Intel Control-flow Enforcement Technology (CET). Each built-in function generates the machine instruction that is part of the function's name. These are the internal low-level functions. Normally the functions in x86 control-flow protection intrinsics should be used instead. unsigned int __builtin_ia32_rdsspd (void) unsigned long long __builtin_ia32_rdsspq (void) void __builtin_ia32_incsspd (unsigned int) void __builtin_ia32_incsspq (unsigned long long) void __builtin_ia32_saveprevssp(void); void __builtin_ia32_rstorssp(void *); void __builtin_ia32_wrssd(unsigned int, void *); void __builtin_ia32_wrssq(unsigned long long, void *); void __builtin_ia32_wrussd(unsigned int, void *); void __builtin_ia32_wrussq(unsigned long long, void *); void __builtin_ia32_setssbsy(void); void __builtin_ia32_clrssbsy(void *); ────────────────────────────────────────────────────────────────────────────── 6.60.34 x86 Transactional Memory Intrinsics These hardware transactional memory intrinsics for x86 allow you to use memory transactions with RTM (Restricted Transactional Memory). This support is enabled with the -mrtm option. For using HLE (Hardware Lock Elision) see x86 specific memory model extensions for transactional memory instead. A memory transaction commits all changes to memory in an atomic way, as visible to other threads. If the transaction fails it is rolled back and all side effects discarded. Generally there is no guarantee that a memory transaction ever succeeds and suitable fallback code always needs to be supplied. RTM Function: unsigned _xbegin () Start a RTM (Restricted Transactional Memory) transaction. Returns _XBEGIN_STARTED when the transaction started successfully (note this is not 0, so the constant has to be explicitly tested). If the transaction aborts, all side effects are undone and an abort code encoded as a bit mask is returned. The following macros are defined: _XABORT_EXPLICIT Transaction was explicitly aborted with _xabort. The parameter passed to _xabort is available with _XABORT_CODE(status). _XABORT_RETRY Transaction retry is possible. _XABORT_CONFLICT Transaction abort due to a memory conflict with another thread. _XABORT_CAPACITY Transaction abort due to the transaction using too much memory. _XABORT_DEBUG Transaction abort due to a debug trap. _XABORT_NESTED Transaction abort in an inner nested transaction. There is no guarantee any transaction ever succeeds, so there always needs to be a valid fallback path. RTM Function: void _xend () Commit the current transaction. When no transaction is active this faults. All memory side effects of the transaction become visible to other threads in an atomic manner. RTM Function: int _xtest () Return a nonzero value if a transaction is currently active, otherwise 0. RTM Function: void _xabort (status) Abort the current transaction. When no transaction is active this is a no-op. The status is an 8-bit constant; its value is encoded in the return value from _xbegin. Here is an example showing handling for _XABORT_RETRY and a fallback path for other failures: #include int n_tries, max_tries; unsigned status = _XABORT_EXPLICIT; ... for (n_tries = 0; n_tries < max_tries; n_tries++) { status = _xbegin (); if (status == _XBEGIN_STARTED || !(status & _XABORT_RETRY)) break; } if (status == _XBEGIN_STARTED) { ... transaction code... _xend (); } else { ... non-transactional fallback path... } Note that, in most cases, the transactional and non-transactional code must synchronize together to ensure consistency. ────────────────────────────────────────────────────────────────────────────── Next: x86 control-flow protection intrinsics, Previous: x86 Built-in Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Target Builtins Link: next: Target Format Checks Link: prev: x86 transactional memory intrinsics Previous: x86 transactional memory intrinsics, Up: Target Builtins ────────────────────────────────────────────────────────────────────────────── 6.60.35 x86 Control-Flow Protection Intrinsics CET Function: ret_type _get_ssp (void) Get the current value of shadow stack pointer if shadow stack support from Intel CET is enabled in the hardware or 0 otherwise. The ret_type is unsigned long long for 64-bit targets and unsigned int for 32-bit targets. CET Function: void _inc_ssp (unsigned int) Increment the current shadow stack pointer by the size specified by the function argument. The argument is masked to a byte value for security reasons, so to increment by more than 255 bytes you must call the function multiple times. The shadow stack unwind code looks like: #include /* Unwind the shadow stack for EH. */ #define _Unwind_Frames_Extra(x) \ do \ { \ _Unwind_Word ssp = _get_ssp (); \ if (ssp != 0) \ { \ _Unwind_Word tmp = (x); \ while (tmp > 255) \ { \ _inc_ssp (tmp); \ tmp -= 255; \ } \ _inc_ssp (tmp); \ } \ } \ while (0) This code runs unconditionally on all 64-bit processors. For 32-bit processors the code runs on those that support multi-byte NOP instructions. ────────────────────────────────────────────────────────────────────────────── 6.61 Format Checks Specific to Particular Target Machines For some target machines, GCC supports additional options to the format attribute (see Declaring Attributes of Functions). • Solaris Format Checks: • Darwin Format Checks: Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Target Format Checks Link: next: Darwin Format Checks Link: prev: Target Format Checks ────────────────────────────────────────────────────────────────────────────── 6.61.1 Solaris Format Checks Solaris targets support the cmn_err (or __cmn_err__) format check. cmn_err accepts a subset of the standard printf conversions, and the two-argument %b conversion for displaying bit-fields. See the Solaris man page for cmn_err for more information. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: AArch64 Pragmas Link: prev: Darwin Format Checks Next: Unnamed Fields, Previous: Target Format Checks, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.62 Pragmas Accepted by GCC GCC supports several types of pragmas, primarily in order to compile code originally written for other compilers. Note that in general we do not recommend the use of pragmas; See Function Attributes, for further explanation. The GNU C preprocessor recognizes several pragmas in addition to the compiler pragmas documented here. Refer to the CPP manual for more information. • AArch64 Pragmas: • ARM Pragmas: • M32C Pragmas: • MeP Pragmas: • RS/6000 and PowerPC Pragmas: • S/390 Pragmas: • Darwin Pragmas: • Solaris Pragmas: • Symbol-Renaming Pragmas: • Structure-Layout Pragmas: • Weak Pragmas: • Diagnostic Pragmas: • Visibility Pragmas: • Push/Pop Macro Pragmas: • Function Specific Option Pragmas: • Loop-Specific Pragmas: Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Pragmas Link: next: Symbol-Renaming Pragmas Link: prev: Darwin Pragmas Next: Symbol-Renaming Pragmas, Previous: Darwin Pragmas, Up: Pragmas ────────────────────────────────────────────────────────────────────────────── 6.62.8 Solaris Pragmas The Solaris target supports #pragma redefine_extname (see Symbol-Renaming Pragmas). It also supports additional #pragma directives for compatibility with the system compiler. align alignment (variable [, variable]...) Increase the minimum alignment of each variable to alignment. This is the same as GCC's aligned attribute see Variable Attributes). Macro expansion occurs on the arguments to this pragma when compiling C and Objective-C. It does not currently occur when compiling C++, but this is a bug which may be fixed in a future release. fini (function [, function]...) This pragma causes each listed function to be called after main, or during shared module unloading, by adding a call to the .fini section. init (function [, function]...) This pragma causes each listed function to be called during initialization (before main) or during shared module loading, by adding a call to the .init section. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Pragmas Link: next: Structure-Layout Pragmas Link: prev: Solaris Pragmas Next: Structure-Layout Pragmas, Previous: Solaris Pragmas, Up: Pragmas ────────────────────────────────────────────────────────────────────────────── 6.62.9 Symbol-Renaming Pragmas GCC supports a #pragma directive that changes the name used in assembly for a given declaration. While this pragma is supported on all platforms, it is intended primarily to provide compatibility with the Solaris system headers. This effect can also be achieved using the asm labels extension (see Asm Labels). redefine_extname oldname newname This pragma gives the C function oldname the assembly symbol newname. The preprocessor macro __PRAGMA_REDEFINE_EXTNAME is defined if this pragma is available (currently on all platforms). This pragma and the asm labels extension interact in a complicated manner. Here are some corner cases you may want to be aware of: 1. This pragma silently applies only to declarations with external linkage. The asm label feature does not have this restriction. 2. In C++, this pragma silently applies only to declarations with “C” linkage. Again, asm labels do not have this restriction. 3. If either of the ways of changing the assembly name of a declaration are applied to a declaration whose assembly name has already been determined (either by a previous use of one of these features, or because the compiler needed the assembly name in order to generate code), and the new name is different, a warning issues and the name does not change. 4. The oldname used by #pragma redefine_extname is always the C-language name. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Pragmas Link: next: Weak Pragmas Link: prev: Symbol-Renaming Pragmas Next: Weak Pragmas, Previous: Symbol-Renaming Pragmas, Up: Pragmas ────────────────────────────────────────────────────────────────────────────── 6.62.10 Structure-Layout Pragmas For compatibility with Microsoft Windows compilers, GCC supports a set of #pragma directives that change the maximum alignment of members of structures (other than zero-width bit-fields), unions, and classes subsequently defined. The n value below always is required to be a small power of two and specifies the new alignment in bytes. 1. #pragma pack(n) simply sets the new alignment. 2. #pragma pack() sets the alignment to the one that was in effect when compilation started (see also command-line option -fpack-struct[=n] see Code Gen Options). 3. #pragma pack(push[,n]) pushes the current alignment setting on an internal stack and then optionally sets the new alignment. 4. #pragma pack(pop) restores the alignment setting to the one saved at the top of the internal stack (and removes that stack entry). Note that #pragma pack([n]) does not influence this internal stack; thus it is possible to have #pragma pack(push) followed by multiple #pragma pack(n) instances and finalized by a single #pragma pack(pop). Some targets, e.g. x86 and PowerPC, support the #pragma ms_struct directive which lays out structures and unions subsequently defined as the documented __attribute__ ((ms_struct)). 1. #pragma ms_struct on turns on the Microsoft layout. 2. #pragma ms_struct off turns off the Microsoft layout. 3. #pragma ms_struct reset goes back to the default layout. Most targets also support the #pragma scalar_storage_order directive which lays out structures and unions subsequently defined as the documented __attribute__ ((scalar_storage_order)). 1. #pragma scalar_storage_order big-endian sets the storage order of the scalar fields to big-endian. 2. #pragma scalar_storage_order little-endian sets the storage order of the scalar fields to little-endian. 3. #pragma scalar_storage_order default goes back to the endianness that was in effect when compilation started (see also command-line option -fsso-struct=endianness see C Dialect Options). ────────────────────────────────────────────────────────────────────────────── 6.62.11 Weak Pragmas For compatibility with SVR4, GCC supports a set of #pragma directives for declaring symbols to be weak, and defining weak aliases. #pragma weak symbol This pragma declares symbol to be weak, as if the declaration had the attribute of the same name. The pragma may appear before or after the declaration of symbol. It is not an error for symbol to never be defined at all. #pragma weak symbol1 = symbol2 This pragma declares symbol1 to be a weak alias of symbol2. It is an error if symbol2 is not defined in the current translation unit. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Pragmas Link: next: Visibility Pragmas Link: prev: Weak Pragmas Next: Visibility Pragmas, Previous: Weak Pragmas, Up: Pragmas ────────────────────────────────────────────────────────────────────────────── 6.62.12 Diagnostic Pragmas GCC allows the user to selectively enable or disable certain types of diagnostics, and change the kind of the diagnostic. For example, a project's policy might require that all sources compile with -Werror but certain files might have exceptions allowing specific types of warnings. Or, a project might selectively enable diagnostics and treat them as errors depending on which preprocessor macros are defined. #pragma GCC diagnostic kind option Modifies the disposition of a diagnostic. Note that not all diagnostics are modifiable; at the moment only warnings (normally controlled by '-W…') can be controlled, and not all of them. Use -fdiagnostics-show-option to determine which diagnostics are controllable and which option controls them. kind is 'error' to treat this diagnostic as an error, 'warning' to treat it like a warning (even if -Werror is in effect), or 'ignored' if the diagnostic is to be ignored. option is a double quoted string that matches the command-line option. #pragma GCC diagnostic warning "-Wformat" #pragma GCC diagnostic error "-Wformat" #pragma GCC diagnostic ignored "-Wformat" Note that these pragmas override any command-line options. GCC keeps track of the location of each pragma, and issues diagnostics according to the state as of that point in the source file. Thus, pragmas occurring after a line do not affect diagnostics caused by that line. #pragma GCC diagnostic push #pragma GCC diagnostic pop Causes GCC to remember the state of the diagnostics as of each push, and restore to that point at each pop. If a pop has no matching push, the command-line options are restored. #pragma GCC diagnostic error "-Wuninitialized" foo(a); /* error is given for this one */ #pragma GCC diagnostic push #pragma GCC diagnostic ignored "-Wuninitialized" foo(b); /* no diagnostic for this one */ #pragma GCC diagnostic pop foo(c); /* error is given for this one */ #pragma GCC diagnostic pop foo(d); /* depends on command-line options */ GCC also offers a simple mechanism for printing messages during compilation. #pragma message string Prints string as a compiler message on compilation. The message is informational only, and is neither a compilation warning nor an error. Newlines can be included in the string by using the '\n' escape sequence. #pragma message "Compiling " __FILE__ "..." string may be parenthesized, and is printed with location information. For example, #define DO_PRAGMA(x) _Pragma (#x) #define TODO(x) DO_PRAGMA(message ("TODO - " #x)) TODO(Remember to fix this) prints '/tmp/file.c:4: note: #pragma message: TODO - Remember to fix this'. #pragma GCC error message Generates an error message. This pragma is considered to indicate an error in the compilation, and it will be treated as such. Newlines can be included in the string by using the '\n' escape sequence. They will be displayed as newlines even if the -fmessage-length option is set to zero. The error is only generated if the pragma is present in the code after pre-processing has been completed. It does not matter however if the code containing the pragma is unreachable: #if 0 #pragma GCC error "this error is not seen" #endif void foo (void) { return; #pragma GCC error "this error is seen" } #pragma GCC warning message This is just like 'pragma GCC error' except that a warning message is issued instead of an error message. Unless -Werror is in effect, in which case this pragma will generate an error as well. ────────────────────────────────────────────────────────────────────────────── 6.62.13 Visibility Pragmas #pragma GCC visibility push(visibility) #pragma GCC visibility pop This pragma allows the user to set the visibility for multiple declarations without having to give each a visibility attribute (see Function Attributes). In C++, '#pragma GCC visibility' affects only namespace-scope declarations. Class members and template specializations are not affected; if you want to override the visibility for a particular member or instantiation, you must use an attribute. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Pragmas Link: next: Function Specific Option Pragmas Link: prev: Visibility Pragmas Next: Function Specific Option Pragmas, Previous: Visibility Pragmas, Up: ────────────────────────────────────────────────────────────────────────────── 6.62.14 Push/Pop Macro Pragmas For compatibility with Microsoft Windows compilers, GCC supports '#pragma push_macro("macro_name")' and '#pragma pop_macro("macro_name")'. #pragma push_macro("macro_name") This pragma saves the value of the macro named as macro_name to the top of the stack for this macro. #pragma pop_macro("macro_name") This pragma sets the value of the macro named as macro_name to the value on top of the stack for this macro. If the stack for macro_name is empty, the value of the macro remains unchanged. For example: #define X 1 #pragma push_macro("X") #undef X #define X -1 #pragma pop_macro("X") int x [X]; In this example, the definition of X as 1 is saved by #pragma push_macro and restored by #pragma pop_macro. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Pragmas Link: next: Loop-Specific Pragmas Link: prev: Push/Pop Macro Pragmas Next: Loop-Specific Pragmas, Previous: Push/Pop Macro Pragmas, Up: Pragmas ────────────────────────────────────────────────────────────────────────────── 6.62.15 Function Specific Option Pragmas #pragma GCC target (string, …) This pragma allows you to set target-specific options for functions defined later in the source file. One or more strings can be specified. Each function that is defined after this point is treated as if it had been declared with one target(string) attribute for each string argument. The parentheses around the strings in the pragma are optional. See Function Attributes, for more information about the target attribute and the attribute syntax. The #pragma GCC target pragma is presently implemented for x86, ARM, AArch64, PowerPC, S/390, and Nios II targets only. #pragma GCC optimize (string, …) This pragma allows you to set global optimization options for functions defined later in the source file. One or more strings can be specified. Each function that is defined after this point is treated as if it had been declared with one optimize(string) attribute for each string argument. The parentheses around the strings in the pragma are optional. See Function Attributes, for more information about the optimize attribute and the attribute syntax. #pragma GCC push_options #pragma GCC pop_options These pragmas maintain a stack of the current target and optimization options. It is intended for include files where you temporarily want to switch to using a different '#pragma GCC target' or '#pragma GCC optimize' and then to pop back to the previous options. #pragma GCC reset_options This pragma clears the current #pragma GCC target and #pragma GCC optimize to use the default switches as specified on the command line. ────────────────────────────────────────────────────────────────────────────── Next: Loop-Specific Pragmas, Previous: Push/Pop Macro Pragmas, Up: Pragmas Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Pragmas Link: next: Unnamed Fields Link: prev: Function Specific Option Pragmas Previous: Function Specific Option Pragmas, Up: Pragmas ────────────────────────────────────────────────────────────────────────────── 6.62.16 Loop-Specific Pragmas #pragma GCC ivdep With this pragma, the programmer asserts that there are no loop-carried dependencies which would prevent consecutive iterations of the following loop from executing concurrently with SIMD (single instruction multiple data) instructions. For example, the compiler can only unconditionally vectorize the following loop with the pragma: void foo (int n, int *a, int *b, int *c) { int i, j; #pragma GCC ivdep for (i = 0; i < n; ++i) a[i] = b[i] + c[i]; } In this example, using the restrict qualifier had the same effect. In the following example, that would not be possible. Assume k < -m or k >= m. Only with the pragma, the compiler knows that it can unconditionally vectorize the following loop: void ignore_vec_dep (int *a, int k, int c, int m) { #pragma GCC ivdep for (int i = 0; i < m; i++) a[i] = a[i + k] * c; } #pragma GCC unroll n You can use this pragma to control how many times a loop should be unrolled. It must be placed immediately before a for, while or do loop or a #pragma GCC ivdep, and applies only to the loop that follows. n is an integer constant expression specifying the unrolling factor. The values of 0 and 1 block any unrolling of the loop. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: Thread-Local Link: prev: Loop-Specific Pragmas Next: Thread-Local, Previous: Pragmas, Up: C Extensions ────────────────────────────────────────────────────────────────────────────── 6.63 Unnamed Structure and Union Fields As permitted by ISO C11 and for compatibility with other compilers, GCC allows you to define a structure or union that contains, as fields, structures and unions without names. For example: struct { int a; union { int b; float c; }; int d; } foo; In this example, you are able to access members of the unnamed union with code like 'foo.b'. Note that only unnamed structs and unions are allowed, you may not have, for example, an unnamed int. You must never create such structures that cause ambiguous field definitions. For example, in this structure: struct { int a; struct { int a; }; } foo; it is ambiguous which a is being referred to with 'foo.a'. The compiler gives errors for such constructs. Unless -fms-extensions is used, the unnamed field must be a structure or union definition without a tag (for example, 'struct { int a; };'). If -fms-extensions is used, the field may also be a definition with a tag such as 'struct foo { int a; };', a reference to a previously defined structure or union such as 'struct foo;', or a reference to a typedef name for a previously defined structure or union type. The option -fplan9-extensions enables -fms-extensions as well as two other extensions. First, a pointer to a structure is automatically converted to a pointer to an anonymous field for assignments and function calls. For example: struct s1 { int a; }; struct s2 { struct s1; }; extern void f1 (struct s1 *); void f2 (struct s2 *p) { f1 (p); } In the call to f1 inside f2, the pointer p is converted into a pointer to the anonymous field. Second, when the type of an anonymous field is a typedef for a struct or union, code may refer to the field using the name of the typedef. typedef struct { int a; } s1; struct s2 { s1; }; s1 f1 (struct s2 *p) { return p->s1; } These usages are only permitted when they are not ambiguous. ────────────────────────────────────────────────────────────────────────────── 6.64 Thread-Local Storage Thread-local storage (TLS) is a mechanism by which variables are allocated such that there is one instance of the variable per extant thread. The runtime model GCC uses to implement this originates in the IA-64 processor-specific ABI, but has since been migrated to other processors as well. It requires significant support from the linker (ld), dynamic linker (ld.so), and system libraries (libc.so and libpthread.so), so it is not available everywhere. At the user level, the extension is visible with a new storage class keyword: __thread. For example: __thread int i; extern __thread struct state s; static __thread char *p; The __thread specifier may be used alone, with the extern or static specifiers, but with no other storage class specifier. When used with extern or static, __thread must appear immediately after the other storage class specifier. The __thread specifier may be applied to any global, file-scoped static, function-scoped static, or static data member of a class. It may not be applied to block-scoped automatic or non-static data member. When the address-of operator is applied to a thread-local variable, it is evaluated at run time and returns the address of the current thread's instance of that variable. An address so obtained may be used by any thread. When a thread terminates, any pointers to thread-local variables in that thread become invalid. No static initialization may refer to the address of a thread-local variable. In C++, if an initializer is present for a thread-local variable, it must be a constant-expression, as defined in 5.19.2 of the ANSI/ISO C++ standard. See ELF Handling For Thread-Local Storage for a detailed explanation of the four thread-local storage addressing models, and how the runtime is expected to function. • C99 Thread-Local Edits: • C++98 Thread-Local Edits: ────────────────────────────────────────────────────────────────────────────── Next: Binary constants, Previous: Unnamed Fields, Up: C Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Thread-Local Link: next: C++98 Thread-Local Edits Link: prev: Thread-Local ────────────────────────────────────────────────────────────────────────────── 6.64.1 ISO/IEC 9899:1999 Edits for Thread-Local Storage The following are a set of changes to ISO/IEC 9899:1999 (aka C99) that document the exact semantics of the language extension. * 5.1.2 Execution environments Add new text after paragraph 1 Within either execution environment, a thread is a flow of control within a program. It is implementation defined whether or not there may be more than one thread associated with a program. It is implementation defined how threads beyond the first are created, the name and type of the function called at thread startup, and how threads may be terminated. However, objects with thread storage duration shall be initialized before thread startup. * 6.2.4 Storage durations of objects Add new text before paragraph 3 An object whose identifier is declared with the storage-class specifier __thread has thread storage duration. Its lifetime is the entire execution of the thread, and its stored value is initialized only once, prior to thread startup. * 6.4.1 Keywords Add __thread. * 6.7.1 Storage-class specifiers Add __thread to the list of storage class specifiers in paragraph 1. Change paragraph 2 to With the exception of __thread, at most one storage-class specifier may be given […]. The __thread specifier may be used alone, or immediately following extern or static. Add new text after paragraph 6 The declaration of an identifier for a variable that has block scope that specifies __thread shall also specify either extern or static. The __thread specifier shall be used only with variables. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Thread-Local Link: next: Binary constants Link: prev: C99 Thread-Local Edits ────────────────────────────────────────────────────────────────────────────── 6.64.2 ISO/IEC 14882:1998 Edits for Thread-Local Storage The following are a set of changes to ISO/IEC 14882:1998 (aka C++98) that document the exact semantics of the language extension. * [intro.execution] New text after paragraph 4 A thread is a flow of control within the abstract machine. It is implementation defined whether or not there may be more than one thread. New text after paragraph 7 It is unspecified whether additional action must be taken to ensure when and whether side effects are visible to other threads. * [lex.key] Add __thread. * [basic.start.main] Add after paragraph 5 The thread that begins execution at the main function is called the main thread. It is implementation defined how functions beginning threads other than the main thread are designated or typed. A function so designated, as well as the main function, is called a thread startup function. It is implementation defined what happens if a thread startup function returns. It is implementation defined what happens to other threads when any thread calls exit. * [basic.start.init] Add after paragraph 4 The storage for an object of thread storage duration shall be statically initialized before the first statement of the thread startup function. An object of thread storage duration shall not require dynamic initialization. * [basic.start.term] Add after paragraph 3 The type of an object with thread storage duration shall not have a non-trivial destructor, nor shall it be an array type whose elements (directly or indirectly) have non-trivial destructors. * [basic.stc] Add “thread storage duration” to the list in paragraph 1. Change paragraph 2 Thread, static, and automatic storage durations are associated with objects introduced by declarations […]. Add __thread to the list of specifiers in paragraph 3. * [basic.stc.thread] New section before [basic.stc.static] The keyword __thread applied to a non-local object gives the object thread storage duration. A local variable or class data member declared both static and __thread gives the variable or member thread storage duration. * [basic.stc.static] Change paragraph 1 All objects that have neither thread storage duration, dynamic storage duration nor are local […]. * [dcl.stc] Add __thread to the list in paragraph 1. Change paragraph 1 With the exception of __thread, at most one storage-class-specifier shall appear in a given decl-specifier-seq. The __thread specifier may be used alone, or immediately following the extern or static specifiers. […] Add after paragraph 5 The __thread specifier can be applied only to the names of objects and to anonymous unions. * [class.mem] Add after paragraph 6 Non-static members shall not be __thread. ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C Extensions Link: next: C++ Extensions Link: prev: C++98 Thread-Local Edits ────────────────────────────────────────────────────────────────────────────── 6.65 Binary Constants using the '0b' Prefix Integer constants can be written as binary constants, consisting of a sequence of '0' and '1' digits, prefixed by '0b' or '0B'. This is particularly useful in environments that operate a lot on the bit level (like microcontrollers). The following statements are identical: i = 42; i = 0x2a; i = 052; i = 0b101010; The type of these constants follows the same rules as for octal or hexadecimal integer constants, so suffixes like 'L' or 'UL' can be applied. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Top Link: next: C++ Volatiles Link: prev: Binary constants ────────────────────────────────────────────────────────────────────────────── 7 Extensions to the C++ Language The GNU compiler provides these extensions to the C++ language (and you can also use most of the C language extensions in your C++ programs). If you want to write code that checks whether these features are available, you can test for the GNU compiler the same way as for C programs: check for a predefined macro __GNUC__. You can also use __GNUG__ to test specifically for GNU C++ (see Predefined Macros in The GNU C Preprocessor). • C++ Volatiles: What constitutes an access to a volatile object. • Restricted Pointers: C99 restricted pointers and references. • Vague Linkage: Where G++ puts inlines, vtables and such. • C++ Interface: You can use a single C++ header file for both declarations and definitions. • Template Instantiation: Methods for ensuring that exactly one copy of each needed template instantiation is emitted. • Bound member functions: You can extract a function pointer to the method denoted by a '->*' or '.*' expression. • C++ Attributes: Variable, function, and type attributes for C++ only. • Function Multiversioning: Declaring multiple function versions. • Type Traits: Compiler support for type traits. • C++ Concepts: Improved support for generic programming. • Deprecated Features: Things will disappear from G++. • Backwards Compatibility: Compatibilities with earlier definitions of C++. ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C++ Extensions Link: next: Restricted Pointers Link: prev: C++ Extensions ────────────────────────────────────────────────────────────────────────────── 7.1 When is a Volatile C++ Object Accessed? The C++ standard differs from the C standard in its treatment of volatile objects. It fails to specify what constitutes a volatile access, except to say that C++ should behave in a similar manner to C with respect to volatiles, where possible. However, the different lvalueness of expressions between C and C++ complicate the behavior. G++ behaves the same as GCC for volatile access, See Volatiles, for a description of GCC's behavior. The C and C++ language specifications differ when an object is accessed in a void context: volatile int *src = somevalue; *src; The C++ standard specifies that such expressions do not undergo lvalue to rvalue conversion, and that the type of the dereferenced object may be incomplete. The C++ standard does not specify explicitly that it is lvalue to rvalue conversion that is responsible for causing an access. There is reason to believe that it is, because otherwise certain simple expressions become undefined. However, because it would surprise most programmers, G++ treats dereferencing a pointer to volatile object of complete type as GCC would do for an equivalent type in C. When the object has incomplete type, G++ issues a warning; if you wish to force an error, you must force a conversion to rvalue with, for instance, a static cast. When using a reference to volatile, G++ does not treat equivalent expressions as accesses to volatiles, but instead issues a warning that no volatile is accessed. The rationale for this is that otherwise it becomes difficult to determine where volatile access occur, and not possible to ignore the return value from functions returning volatile references. Again, if you wish to force a read, cast the reference to an rvalue. G++ implements the same behavior as GCC does when assigning to a volatile object—there is no reread of the assigned-to object, the assigned rvalue is reused. Note that in C++ assignment expressions are lvalues, and if used as an lvalue, the volatile object is referred to. For instance, vref refers to vobj, as expected, in the following example: volatile int vobj; volatile int &vref = vobj = something; ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C++ Extensions Link: next: Vague Linkage Link: prev: C++ Volatiles Next: Vague Linkage, Previous: C++ Volatiles, Up: C++ Extensions ────────────────────────────────────────────────────────────────────────────── 7.2 Restricting Pointer Aliasing As with the C front end, G++ understands the C99 feature of restricted pointers, specified with the __restrict__, or __restrict type qualifier. Because you cannot compile C++ by specifying the -std=c99 language flag, restrict is not a keyword in C++. In addition to allowing restricted pointers, you can specify restricted references, which indicate that the reference is not aliased in the local context. void fn (int *__restrict__ rptr, int &__restrict__ rref) { /* … */ } In the body of fn, rptr points to an unaliased integer and rref refers to a (different) unaliased integer. You may also specify whether a member function's this pointer is unaliased by using __restrict__ as a member function qualifier. void T::fn () __restrict__ { /* … */ } Within the body of T::fn, this has the effective definition T *__restrict__ const this. Notice that the interpretation of a __restrict__ member function qualifier is different to that of const or volatile qualifier, in that it is applied to the pointer rather than the object. This is consistent with other compilers that implement restricted pointers. As with all outermost parameter qualifiers, __restrict__ is ignored in function definition matching. This means you only need to specify __restrict__ in a function definition, rather than in a function prototype as well. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C++ Extensions Link: next: C++ Interface Link: prev: Restricted Pointers Next: C++ Interface, Previous: Restricted Pointers, Up: C++ Extensions ────────────────────────────────────────────────────────────────────────────── 7.3 Vague Linkage There are several constructs in C++ that require space in the object file but are not clearly tied to a single translation unit. We say that these constructs have “vague linkage”. Typically such constructs are emitted wherever they are needed, though sometimes we can be more clever. Inline Functions Inline functions are typically defined in a header file which can be included in many different compilations. Hopefully they can usually be inlined, but sometimes an out-of-line copy is necessary, if the address of the function is taken or if inlining fails. In general, we emit an out-of-line copy in all translation units where one is needed. As an exception, we only emit inline virtual functions with the vtable, since it always requires a copy. Local static variables and string constants used in an inline function are also considered to have vague linkage, since they must be shared between all inlined and out-of-line instances of the function. VTables C++ virtual functions are implemented in most compilers using a lookup table, known as a vtable. The vtable contains pointers to the virtual functions provided by a class, and each object of the class contains a pointer to its vtable (or vtables, in some multiple-inheritance situations). If the class declares any non-inline, non-pure virtual functions, the first one is chosen as the “key method” for the class, and the vtable is only emitted in the translation unit where the key method is defined. Note: If the chosen key method is later defined as inline, the vtable is still emitted in every translation unit that defines it. Make sure that any inline virtuals are declared inline in the class body, even if they are not defined there. type_info objects C++ requires information about types to be written out in order to implement 'dynamic_cast', 'typeid' and exception handling. For polymorphic classes (classes with virtual functions), the 'type_info' object is written out along with the vtable so that 'dynamic_cast' can determine the dynamic type of a class object at run time. For all other types, we write out the 'type_info' object when it is used: when applying 'typeid' to an expression, throwing an object, or referring to a type in a catch clause or exception specification. Template Instantiations Most everything in this section also applies to template instantiations, but there are other options as well. See Where's the Template?. When used with GNU ld version 2.8 or later on an ELF system such as GNU/Linux or Solaris 2, or on Microsoft Windows, duplicate copies of these constructs will be discarded at link time. This is known as COMDAT support. On targets that don't support COMDAT, but do support weak symbols, GCC uses them. This way one copy overrides all the others, but the unused copies still take up space in the executable. For targets that do not support either COMDAT or weak symbols, most entities with vague linkage are emitted as local symbols to avoid duplicate definition errors from the linker. This does not happen for local statics in inlines, however, as having multiple copies almost certainly breaks things. See Declarations and Definitions in One Header, for another way to control placement of these constructs. ────────────────────────────────────────────────────────────────────────────── Next: C++ Interface, Previous: Restricted Pointers, Up: C++ Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C++ Extensions Link: next: Template Instantiation Link: prev: Vague Linkage Next: Template Instantiation, Previous: Vague Linkage, Up: C++ Extensions ────────────────────────────────────────────────────────────────────────────── 7.4 C++ Interface and Implementation Pragmas #pragma interface and #pragma implementation provide the user with a way of explicitly directing the compiler to emit entities with vague linkage (and debugging information) in a particular translation unit. Note: These #pragmas have been superceded as of GCC 2.7.2 by COMDAT support and the “key method” heuristic mentioned in Vague Linkage. Using them can actually cause your program to grow due to unnecessary out-of-line copies of inline functions. #pragma interface #pragma interface "subdir/objects.h" Use this directive in header files that define object classes, to save space in most of the object files that use those classes. Normally, local copies of certain information (backup copies of inline member functions, debugging information, and the internal tables that implement virtual functions) must be kept in each object file that includes class definitions. You can use this pragma to avoid such duplication. When a header file containing '#pragma interface' is included in a compilation, this auxiliary information is not generated (unless the main input source file itself uses '#pragma implementation'). Instead, the object files contain references to be resolved at link time. The second form of this directive is useful for the case where you have multiple headers with the same name in different directories. If you use this form, you must specify the same string to '#pragma implementation'. #pragma implementation #pragma implementation "objects.h" Use this pragma in a main input file, when you want full output from included header files to be generated (and made globally visible). The included header file, in turn, should use '#pragma interface'. Backup copies of inline member functions, debugging information, and the internal tables used to implement virtual functions are all generated in implementation files. If you use '#pragma implementation' with no argument, it applies to an include file with the same basename^4 as your source file. For example, in allclass.cc, giving just '#pragma implementation' by itself is equivalent to '#pragma implementation "allclass.h"'. Use the string argument if you want a single implementation file to include code from multiple header files. (You must also use '#include' to include the header file; '#pragma implementation' only specifies how to use the file—it doesn't actually include it.) There is no way to split up the contents of a single header file into multiple implementation files. '#pragma implementation' and '#pragma interface' also have an effect on function inlining. If you define a class in a header file marked with '#pragma interface', the effect on an inline function defined in that class is similar to an explicit extern declaration—the compiler emits no code at all to define an independent version of the function. Its definition is used only for inlining with its callers. Conversely, when you include the same header file in a main source file that declares it as '#pragma implementation', the compiler emits code for the function itself; this defines a version of the function that can be found via pointers (or by callers compiled without inlining). If all calls to the function can be inlined, you can avoid emitting the function by compiling with -fno-implement-inlines. If any calls are not inlined, you will get linker errors. Footnotes (4) A file's basename is the name stripped of all leading path information and of trailing suffixes, such as '.h' or '.C' or '.cc'. ────────────────────────────────────────────────────────────────────────────── Next: Template Instantiation, Previous: Vague Linkage, Up: C++ Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C++ Extensions Link: next: Bound member functions Link: prev: C++ Interface Next: Bound member functions, Previous: C++ Interface, Up: C++ Extensions ────────────────────────────────────────────────────────────────────────────── 7.5 Where's the Template? C++ templates were the first language feature to require more intelligence from the environment than was traditionally found on a UNIX system. Somehow the compiler and linker have to make sure that each template instance occurs exactly once in the executable if it is needed, and not at all otherwise. There are two basic approaches to this problem, which are referred to as the Borland model and the Cfront model. Borland model Borland C++ solved the template instantiation problem by adding the code equivalent of common blocks to their linker; the compiler emits template instances in each translation unit that uses them, and the linker collapses them together. The advantage of this model is that the linker only has to consider the object files themselves; there is no external complexity to worry about. The disadvantage is that compilation time is increased because the template code is being compiled repeatedly. Code written for this model tends to include definitions of all templates in the header file, since they must be seen to be instantiated. Cfront model The AT&T C++ translator, Cfront, solved the template instantiation problem by creating the notion of a template repository, an automatically maintained place where template instances are stored. A more modern version of the repository works as follows: As individual object files are built, the compiler places any template definitions and instantiations encountered in the repository. At link time, the link wrapper adds in the objects in the repository and compiles any needed instances that were not previously emitted. The advantages of this model are more optimal compilation speed and the ability to use the system linker; to implement the Borland model a compiler vendor also needs to replace the linker. The disadvantages are vastly increased complexity, and thus potential for error; for some code this can be just as transparent, but in practice it can been very difficult to build multiple programs in one directory and one program in multiple directories. Code written for this model tends to separate definitions of non-inline member templates into a separate file, which should be compiled separately. G++ implements the Borland model on targets where the linker supports it, including ELF targets (such as GNU/Linux), Mac OS X and Microsoft Windows. Otherwise G++ implements neither automatic model. You have the following options for dealing with template instantiations: 1. Do nothing. Code written for the Borland model works fine, but each translation unit contains instances of each of the templates it uses. The duplicate instances will be discarded by the linker, but in a large program, this can lead to an unacceptable amount of code duplication in object files or shared libraries. Duplicate instances of a template can be avoided by defining an explicit instantiation in one object file, and preventing the compiler from doing implicit instantiations in any other object files by using an explicit instantiation declaration, using the extern template syntax: extern template int max (int, int); This syntax is defined in the C++ 2011 standard, but has been supported by G++ and other compilers since well before 2011. Explicit instantiations can be used for the largest or most frequently duplicated instances, without having to know exactly which other instances are used in the rest of the program. You can scatter the explicit instantiations throughout your program, perhaps putting them in the translation units where the instances are used or the translation units that define the templates themselves; you can put all of the explicit instantiations you need into one big file; or you can create small files like #include "Foo.h" #include "Foo.cc" template class Foo; template ostream& operator << (ostream&, const Foo&); for each of the instances you need, and create a template instantiation library from those. This is the simplest option, but also offers flexibility and fine-grained control when necessary. It is also the most portable alternative and programs using this approach will work with most modern compilers. 2. Compile your template-using code with -frepo. The compiler generates files with the extension '.rpo' listing all of the template instantiations used in the corresponding object files that could be instantiated there; the link wrapper, 'collect2', then updates the '.rpo' files to tell the compiler where to place those instantiations and rebuild any affected object files. The link-time overhead is negligible after the first pass, as the compiler continues to place the instantiations in the same files. This can be a suitable option for application code written for the Borland model, as it usually just works. Code written for the Cfront model needs to be modified so that the template definitions are available at one or more points of instantiation; usually this is as simple as adding #include to the end of each template header. For library code, if you want the library to provide all of the template instantiations it needs, just try to link all of its object files together; the link will fail, but cause the instantiations to be generated as a side effect. Be warned, however, that this may cause conflicts if multiple libraries try to provide the same instantiations. For greater control, use explicit instantiation as described in the next option. 3. Compile your code with -fno-implicit-templates to disable the implicit generation of template instances, and explicitly instantiate all the ones you use. This approach requires more knowledge of exactly which instances you need than do the others, but it's less mysterious and allows greater control if you want to ensure that only the intended instances are used. If you are using Cfront-model code, you can probably get away with not using -fno-implicit-templates when compiling files that don't '#include' the member template definitions. If you use one big file to do the instantiations, you may want to compile it without -fno-implicit-templates so you get all of the instances required by your explicit instantiations (but not by any other files) without having to specify them as well. In addition to forward declaration of explicit instantiations (with extern), G++ has extended the template instantiation syntax to support instantiation of the compiler support data for a template class (i.e. the vtable) without instantiating any of its members (with inline), and instantiation of only the static data members of a template class, without the support data or member functions (with static): inline template class Foo; static template class Foo; ────────────────────────────────────────────────────────────────────────────── Next: Bound member functions, Previous: C++ Interface, Up: C++ Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C++ Extensions Link: next: C++ Attributes Link: prev: Template Instantiation Next: C++ Attributes, Previous: Template Instantiation, Up: C++ Extensions ────────────────────────────────────────────────────────────────────────────── 7.6 Extracting the Function Pointer from a Bound Pointer to Member Function In C++, pointer to member functions (PMFs) are implemented using a wide pointer of sorts to handle all the possible call mechanisms; the PMF needs to store information about how to adjust the 'this' pointer, and if the function pointed to is virtual, where to find the vtable, and where in the vtable to look for the member function. If you are using PMFs in an inner loop, you should really reconsider that decision. If that is not an option, you can extract the pointer to the function that would be called for a given object/PMF pair and call it directly inside the inner loop, to save a bit of time. Note that you still pay the penalty for the call through a function pointer; on most modern architectures, such a call defeats the branch prediction features of the CPU. This is also true of normal virtual function calls. The syntax for this extension is extern A a; extern int (A::*fp)(); typedef int (*fptr)(A *); fptr p = (fptr)(a.*fp); For PMF constants (i.e. expressions of the form '&Klasse::Member'), no object is needed to obtain the address of the function. They can be converted to function pointers directly: fptr p1 = (fptr)(&A::foo); You must specify -Wno-pmf-conversions to use this extension. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C++ Extensions Link: next: Function Multiversioning Link: prev: Bound member functions Next: Function Multiversioning, Previous: Bound member functions, Up: C++ ────────────────────────────────────────────────────────────────────────────── 7.7 C++-Specific Variable, Function, and Type Attributes Some attributes only make sense for C++ programs. abi_tag ("tag", ...) The abi_tag attribute can be applied to a function, variable, or class declaration. It modifies the mangled name of the entity to incorporate the tag name, in order to distinguish the function or class from an earlier version with a different ABI; perhaps the class has changed size, or the function has a different return type that is not encoded in the mangled name. The attribute can also be applied to an inline namespace, but does not affect the mangled name of the namespace; in this case it is only used for -Wabi-tag warnings and automatic tagging of functions and variables. Tagging inline namespaces is generally preferable to tagging individual declarations, but the latter is sometimes necessary, such as when only certain members of a class need to be tagged. The argument can be a list of strings of arbitrary length. The strings are sorted on output, so the order of the list is unimportant. A redeclaration of an entity must not add new ABI tags, since doing so would change the mangled name. The ABI tags apply to a name, so all instantiations and specializations of a template have the same tags. The attribute will be ignored if applied to an explicit specialization or instantiation. The -Wabi-tag flag enables a warning about a class which does not have all the ABI tags used by its subobjects and virtual functions; for users with code that needs to coexist with an earlier ABI, using this option can help to find all affected types that need to be tagged. When a type involving an ABI tag is used as the type of a variable or return type of a function where that tag is not already present in the signature of the function, the tag is automatically applied to the variable or function. -Wabi-tag also warns about this situation; this warning can be avoided by explicitly tagging the variable or function or moving it into a tagged inline namespace. init_priority (priority) In Standard C++, objects defined at namespace scope are guaranteed to be initialized in an order in strict accordance with that of their definitions in a given translation unit. No guarantee is made for initializations across translation units. However, GNU C++ allows users to control the order of initialization of objects defined at namespace scope with the init_priority attribute by specifying a relative priority, a constant integral expression currently bounded between 101 and 65535 inclusive. Lower numbers indicate a higher priority. In the following example, A would normally be created before B, but the init_priority attribute reverses that order: Some_Class A __attribute__ ((init_priority (2000))); Some_Class B __attribute__ ((init_priority (543))); Note that the particular values of priority do not matter; only their relative ordering. warn_unused For C++ types with non-trivial constructors and/or destructors it is impossible for the compiler to determine whether a variable of this type is truly unused if it is not referenced. This type attribute informs the compiler that variables of this type should be warned about if they appear to be unused, just like variables of fundamental types. This attribute is appropriate for types which just represent a value, such as std::string; it is not appropriate for types which control a resource, such as std::lock_guard. This attribute is also accepted in C, but it is unnecessary because C does not have constructors or destructors. ────────────────────────────────────────────────────────────────────────────── Next: Function Multiversioning, Previous: Bound member functions, Up: C++ Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C++ Extensions Link: next: Type Traits Link: prev: C++ Attributes Next: Type Traits, Previous: C++ Attributes, Up: C++ Extensions ────────────────────────────────────────────────────────────────────────────── 7.8 Function Multiversioning With the GNU C++ front end, for x86 targets, you may specify multiple versions of a function, where each function is specialized for a specific target feature. At runtime, the appropriate version of the function is automatically executed depending on the characteristics of the execution platform. Here is an example. __attribute__ ((target ("default"))) int foo () { // The default version of foo. return 0; } __attribute__ ((target ("sse4.2"))) int foo () { // foo version for SSE4.2 return 1; } __attribute__ ((target ("arch=atom"))) int foo () { // foo version for the Intel ATOM processor return 2; } __attribute__ ((target ("arch=amdfam10"))) int foo () { // foo version for the AMD Family 0x10 processors. return 3; } int main () { int (*p)() = &foo; assert ((*p) () == foo ()); return 0; } In the above example, four versions of function foo are created. The first version of foo with the target attribute "default" is the default version. This version gets executed when no other target specific version qualifies for execution on a particular platform. A new version of foo is created by using the same function signature but with a different target string. Function foo is called or a pointer to it is taken just like a regular function. GCC takes care of doing the dispatching to call the right version at runtime. Refer to the GCC wiki on Function Multiversioning for more details. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C++ Extensions Link: next: C++ Concepts Link: prev: Function Multiversioning Next: C++ Concepts, Previous: Function Multiversioning, Up: C++ Extensions ────────────────────────────────────────────────────────────────────────────── 7.9 Type Traits The C++ front end implements syntactic extensions that allow compile-time determination of various characteristics of a type (or of a pair of types). __has_nothrow_assign (type) If type is const-qualified or is a reference type then the trait is false. Otherwise if __has_trivial_assign (type) is true then the trait is true, else if type is a cv-qualified class or union type with copy assignment operators that are known not to throw an exception then the trait is true, else it is false. Requires: type shall be a complete type, (possibly cv-qualified) void, or an array of unknown bound. __has_nothrow_copy (type) If __has_trivial_copy (type) is true then the trait is true, else if type is a cv-qualified class or union type with copy constructors that are known not to throw an exception then the trait is true, else it is false. Requires: type shall be a complete type, (possibly cv-qualified) void, or an array of unknown bound. __has_nothrow_constructor (type) If __has_trivial_constructor (type) is true then the trait is true, else if type is a cv class or union type (or array thereof) with a default constructor that is known not to throw an exception then the trait is true, else it is false. Requires: type shall be a complete type, (possibly cv-qualified) void, or an array of unknown bound. __has_trivial_assign (type) If type is const- qualified or is a reference type then the trait is false. Otherwise if __is_pod (type) is true then the trait is true, else if type is a cv-qualified class or union type with a trivial copy assignment ([class.copy]) then the trait is true, else it is false. Requires: type shall be a complete type, (possibly cv-qualified) void, or an array of unknown bound. __has_trivial_copy (type) If __is_pod (type) is true or type is a reference type then the trait is true, else if type is a cv class or union type with a trivial copy constructor ([class.copy]) then the trait is true, else it is false. Requires: type shall be a complete type, (possibly cv-qualified) void, or an array of unknown bound. __has_trivial_constructor (type) If __is_pod (type) is true then the trait is true, else if type is a cv-qualified class or union type (or array thereof) with a trivial default constructor ([class.ctor]) then the trait is true, else it is false. Requires: type shall be a complete type, (possibly cv-qualified) void, or an array of unknown bound. __has_trivial_destructor (type) If __is_pod (type) is true or type is a reference type then the trait is true, else if type is a cv class or union type (or array thereof) with a trivial destructor ([class.dtor]) then the trait is true, else it is false. Requires: type shall be a complete type, (possibly cv-qualified) void, or an array of unknown bound. __has_virtual_destructor (type) If type is a class type with a virtual destructor ([class.dtor]) then the trait is true, else it is false. Requires: type shall be a complete type, (possibly cv-qualified) void, or an array of unknown bound. __is_abstract (type) If type is an abstract class ([class.abstract]) then the trait is true, else it is false. Requires: type shall be a complete type, (possibly cv-qualified) void, or an array of unknown bound. __is_base_of (base_type, derived_type) If base_type is a base class of derived_type ([class.derived]) then the trait is true, otherwise it is false. Top-level cv-qualifications of base_type and derived_type are ignored. For the purposes of this trait, a class type is considered is own base. Requires: if __is_class (base_type) and __is_class (derived_type) are true and base_type and derived_type are not the same type (disregarding cv-qualifiers), derived_type shall be a complete type. A diagnostic is produced if this requirement is not met. __is_class (type) If type is a cv-qualified class type, and not a union type ([basic.compound]) the trait is true, else it is false. __is_empty (type) If __is_class (type) is false then the trait is false. Otherwise type is considered empty if and only if: type has no non-static data members, or all non-static data members, if any, are bit-fields of length 0, and type has no virtual members, and type has no virtual base classes, and type has no base classes base_type for which __is_empty (base_type) is false. Requires: type shall be a complete type, (possibly cv-qualified) void, or an array of unknown bound. __is_enum (type) If type is a cv enumeration type ([basic.compound]) the trait is true, else it is false. __is_literal_type (type) If type is a literal type ([basic.types]) the trait is true, else it is false. Requires: type shall be a complete type, (possibly cv-qualified) void, or an array of unknown bound. __is_pod (type) If type is a cv POD type ([basic.types]) then the trait is true, else it is false. Requires: type shall be a complete type, (possibly cv-qualified) void, or an array of unknown bound. __is_polymorphic (type) If type is a polymorphic class ([class.virtual]) then the trait is true, else it is false. Requires: type shall be a complete type, (possibly cv-qualified) void, or an array of unknown bound. __is_standard_layout (type) If type is a standard-layout type ([basic.types]) the trait is true, else it is false. Requires: type shall be a complete type, (possibly cv-qualified) void, or an array of unknown bound. __is_trivial (type) If type is a trivial type ([basic.types]) the trait is true, else it is false. Requires: type shall be a complete type, (possibly cv-qualified) void, or an array of unknown bound. __is_union (type) If type is a cv union type ([basic.compound]) the trait is true, else it is false. __underlying_type (type) The underlying type of type. Requires: type shall be an enumeration type ([dcl.enum]). __integer_pack (length) When used as the pattern of a pack expansion within a template definition, expands to a template argument pack containing integers from 0 to length-1. This is provided for efficient implementation of std::make_integer_sequence. ────────────────────────────────────────────────────────────────────────────── Next: C++ Concepts, Previous: Function Multiversioning, Up: C++ Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C++ Extensions Link: next: Deprecated Features Link: prev: Type Traits Next: Deprecated Features, Previous: Type Traits, Up: C++ Extensions ────────────────────────────────────────────────────────────────────────────── 7.10 C++ Concepts C++ concepts provide much-improved support for generic programming. In particular, they allow the specification of constraints on template arguments. The constraints are used to extend the usual overloading and partial specialization capabilities of the language, allowing generic data structures and algorithms to be “refined” based on their properties rather than their type names. The following keywords are reserved for concepts. assumes States an expression as an assumption, and if possible, verifies that the assumption is valid. For example, assume(n > 0). axiom Introduces an axiom definition. Axioms introduce requirements on values. forall Introduces a universally quantified object in an axiom. For example, forall (int n) n + 0 == n). concept Introduces a concept definition. Concepts are sets of syntactic and semantic requirements on types and their values. requires Introduces constraints on template arguments or requirements for a member function of a class template. The front end also exposes a number of internal mechanism that can be used to simplify the writing of type traits. Note that some of these traits are likely to be removed in the future. __is_same (type1, type2) A binary type trait: true whenever the type arguments are the same. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C++ Extensions Link: next: Backwards Compatibility Link: prev: C++ Concepts Next: Backwards Compatibility, Previous: C++ Concepts, Up: C++ Extensions ────────────────────────────────────────────────────────────────────────────── 7.11 Deprecated Features In the past, the GNU C++ compiler was extended to experiment with new features, at a time when the C++ language was still evolving. Now that the C++ standard is complete, some of those features are superseded by superior alternatives. Using the old features might cause a warning in some cases that the feature will be dropped in the future. In other cases, the feature might be gone already. G++ allows a virtual function returning 'void *' to be overridden by one returning a different pointer type. This extension to the covariant return type rules is now deprecated and will be removed from a future version. The use of default arguments in function pointers, function typedefs and other places where they are not permitted by the standard is deprecated and will be removed from a future version of G++. G++ allows floating-point literals to appear in integral constant expressions, e.g. ' enum E { e = int(2.2 * 3.7) } ' This extension is deprecated and will be removed from a future version. G++ allows static data members of const floating-point type to be declared with an initializer in a class definition. The standard only allows initializers for static members of const integral types and const enumeration types so this extension has been deprecated and will be removed from a future version. G++ allows attributes to follow a parenthesized direct initializer, e.g. ' int f (0) __attribute__ ((something)); ' This extension has been ignored since G++ 3.3 and is deprecated. G++ allows anonymous structs and unions to have members that are not public non-static data members (i.e. fields). These extensions are deprecated. ────────────────────────────────────────────────────────────────────────────── Next: Backwards Compatibility, Previous: C++ Concepts, Up: C++ Extensions Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C++ Extensions Link: next: Objective-C Link: prev: Deprecated Features ────────────────────────────────────────────────────────────────────────────── 7.12 Backwards Compatibility Now that there is a definitive ISO standard C++, G++ has a specification to adhere to. The C++ language evolved over time, and features that used to be acceptable in previous drafts of the standard, such as the ARM [Annotated C++ Reference Manual], are no longer accepted. In order to allow compilation of C++ written to such drafts, G++ contains some backwards compatibilities. All such backwards compatibility features are liable to disappear in future versions of G++. They should be considered deprecated. See Deprecated Features. Implicit C language Old C system header files did not contain an extern "C" {…} scope to set the language. On such systems, all system header files are implicitly scoped inside a C language scope. Such headers must correctly prototype function argument types, there is no leeway for () to indicate an unspecified set of arguments. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Objective-C Link: next: Modern GNU Objective-C runtime API Link: prev: Objective-C ────────────────────────────────────────────────────────────────────────────── 8.1 GNU Objective-C Runtime API This section is specific for the GNU Objective-C runtime. If you are using a different runtime, you can skip it. The GNU Objective-C runtime provides an API that allows you to interact with the Objective-C runtime system, querying the live runtime structures and even manipulating them. This allows you for example to inspect and navigate classes, methods and protocols; to define new classes or new methods, and even to modify existing classes or protocols. If you are using a “Foundation” library such as GNUstep-Base, this library will provide you with a rich set of functionality to do most of the inspection tasks, and you probably will only need direct access to the GNU Objective-C runtime API to define new classes or methods. • Modern GNU Objective-C runtime API: • Traditional GNU Objective-C runtime API: Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: GNU Objective-C runtime API Link: next: Executing code before main Link: prev: Modern GNU Objective-C runtime API Previous: Modern GNU Objective-C runtime API, Up: GNU Objective-C runtime ────────────────────────────────────────────────────────────────────────────── 8.1.2 Traditional GNU Objective-C Runtime API The GNU Objective-C runtime used to provide a different API, which we call the “traditional” GNU Objective-C runtime API. Functions belonging to this API are easy to recognize because they use a different naming convention, such as class_get_super_class() (traditional API) instead of class_getSuperclass() (modern API). Software using this API includes the file objc/objc-api.h where it is declared. Starting with GCC 4.7.0, the traditional GNU runtime API is no longer available. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Objective-C Link: next: What you can and what you cannot do in +load Link: prev: Traditional GNU Objective-C runtime API Next: Type encoding, Previous: GNU Objective-C runtime API, Up: ────────────────────────────────────────────────────────────────────────────── 8.2 +load: Executing Code before main This section is specific for the GNU Objective-C runtime. If you are using a different runtime, you can skip it. The GNU Objective-C runtime provides a way that allows you to execute code before the execution of the program enters the main function. The code is executed on a per-class and a per-category basis, through a special class method +load. This facility is very useful if you want to initialize global variables which can be accessed by the program directly, without sending a message to the class first. The usual way to initialize global variables, in the +initialize method, might not be useful because +initialize is only called when the first message is sent to a class object, which in some cases could be too late. Suppose for example you have a FileStream class that declares Stdin, Stdout and Stderr as global variables, like below: FileStream *Stdin = nil; FileStream *Stdout = nil; FileStream *Stderr = nil; @implementation FileStream + (void)initialize { Stdin = [[FileStream new] initWithFd:0]; Stdout = [[FileStream new] initWithFd:1]; Stderr = [[FileStream new] initWithFd:2]; } /* Other methods here */ @end In this example, the initialization of Stdin, Stdout and Stderr in +initialize occurs too late. The programmer can send a message to one of these objects before the variables are actually initialized, thus sending messages to the nil object. The +initialize method which actually initializes the global variables is not invoked until the first message is sent to the class object. The solution would require these variables to be initialized just before entering main. The correct solution of the above problem is to use the +load method instead of +initialize: @implementation FileStream + (void)load { Stdin = [[FileStream new] initWithFd:0]; Stdout = [[FileStream new] initWithFd:1]; Stderr = [[FileStream new] initWithFd:2]; } /* Other methods here */ @end The +load is a method that is not overridden by categories. If a class and a category of it both implement +load, both methods are invoked. This allows some additional initializations to be performed in a category. This mechanism is not intended to be a replacement for +initialize. You should be aware of its limitations when you decide to use it instead of +initialize. • What you can and what you cannot do in +load: ────────────────────────────────────────────────────────────────────────────── Next: Type encoding, Previous: GNU Objective-C runtime API, Up: Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Executing code before main Link: next: Type encoding Link: prev: Executing code before main ────────────────────────────────────────────────────────────────────────────── 8.2.1 What You Can and Cannot Do in +load +load is to be used only as a last resort. Because it is executed very early, most of the Objective-C runtime machinery will not be ready when +load is executed; hence +load works best for executing C code that is independent on the Objective-C runtime. The +load implementation in the GNU runtime guarantees you the following things: * you can write whatever C code you like; * you can allocate and send messages to objects whose class is implemented in the same file; * the +load implementation of all super classes of a class are executed before the +load of that class is executed; * the +load implementation of a class is executed before the +load implementation of any category. In particular, the following things, even if they can work in a particular case, are not guaranteed: * allocation of or sending messages to arbitrary objects; * allocation of or sending messages to objects whose classes have a category implemented in the same file; * sending messages to Objective-C constant strings (@"this is a constant string"); You should make no assumptions about receiving +load in sibling classes when you write +load of a class. The order in which sibling classes receive +load is not guaranteed. The order in which +load and +initialize are called could be problematic if this matters. If you don't allocate objects inside +load, it is guaranteed that +load is called before +initialize. If you create an object inside +load the +initialize method of object's class is invoked even if +load was not invoked. Note if you explicitly call +load on a class, +initialize will be called first. To avoid possible problems try to implement only one of these methods. The +load method is also invoked when a bundle is dynamically loaded into your running program. This happens automatically without any intervening operation from you. When you write bundles and you need to write +load you can safely create and send messages to objects whose classes already exist in the running program. The same restrictions as above apply to classes defined in bundle. ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Objective-C Link: next: Legacy type encoding Link: prev: What you can and what you cannot do in +load Next: Garbage Collection, Previous: Executing code before main, Up: ────────────────────────────────────────────────────────────────────────────── 8.3 Type Encoding This is an advanced section. Type encodings are used extensively by the compiler and by the runtime, but you generally do not need to know about them to use Objective-C. The Objective-C compiler generates type encodings for all the types. These type encodings are used at runtime to find out information about selectors and methods and about objects and classes. The types are encoded in the following way: _Bool B char c unsigned char C short s unsigned short S int i unsigned int I long l unsigned long L long long q unsigned long long Q float f double d long double D void v id @ Class # SEL : char* * an enum is encoded exactly as the integer type that the enum compiler uses for it, which depends on the enumeration values. Often the compiler users unsigned int, which is then encoded as I. unknown type ? Complex types j followed by the inner type. For example _Complex double is encoded as "jd". b followed by the starting position of the bit-field, bit-fields the type of the bit-field and the size of the bit-field (the bit-fields encoding was changed from the NeXT's compiler encoding, see below) The encoding of bit-fields has changed to allow bit-fields to be properly handled by the runtime functions that compute sizes and alignments of types that contain bit-fields. The previous encoding contained only the size of the bit-field. Using only this information it is not possible to reliably compute the size occupied by the bit-field. This is very important in the presence of the Boehm's garbage collector because the objects are allocated using the typed memory facility available in this collector. The typed memory allocation requires information about where the pointers are located inside the object. The position in the bit-field is the position, counting in bits, of the bit closest to the beginning of the structure. The non-atomic types are encoded as follows: pointers '^' followed by the pointed type. arrays '[' followed by the number of elements in the array followed by the type of the elements followed by ']' '{' followed by the name of the structure (or '?' if the structures structure is unnamed), the '=' sign, the type of the members and by '}' '(' followed by the name of the structure (or '?' if the union unions is unnamed), the '=' sign, the type of the members followed by ')' '![' followed by the vector_size (the number of bytes composing vectors the vector) followed by a comma, followed by the alignment (in bytes) of the vector, followed by the type of the elements followed by ']' Here are some types and their encodings, as they are generated by the compiler on an i386 machine: Objective-C type Compiler encoding int a[10]; [10i] struct { int i; float f[3]; int a:3; {?=i[3f]b128i3b131i2c} int b:2; char c; } int a __attribute__ ((vector_size (16))); ![16,16i] (alignment depends on the machine) In addition to the types the compiler also encodes the type specifiers. The table below describes the encoding of the current Objective-C type specifiers: Specifier Encoding const r in n inout N out o bycopy O byref R oneway V The type specifiers are encoded just before the type. Unlike types however, the type specifiers are only encoded when they appear in method argument types. Note how const interacts with pointers: Objective-C type Compiler encoding const int ri const int* ^ri int *const r^i const int* is a pointer to a const int, and so is encoded as ^ri. int* const, instead, is a const pointer to an int, and so is encoded as r^i. Finally, there is a complication when encoding const char * versus char * const. Because char * is encoded as * and not as ^c, there is no way to express the fact that r applies to the pointer or to the pointee. Hence, it is assumed as a convention that r* means const char * (since it is what is most often meant), and there is no way to encode char *const. char *const would simply be encoded as *, and the const is lost. • Legacy type encoding: • @encode: • Method signatures: ────────────────────────────────────────────────────────────────────────────── Next: Garbage Collection, Previous: Executing code before main, Up: Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Type encoding Link: next: @encode Link: prev: Type encoding ────────────────────────────────────────────────────────────────────────────── 8.3.1 Legacy Type Encoding Unfortunately, historically GCC used to have a number of bugs in its encoding code. The NeXT runtime expects GCC to emit type encodings in this historical format (compatible with GCC-3.3), so when using the NeXT runtime, GCC will introduce on purpose a number of incorrect encodings: * the read-only qualifier of the pointee gets emitted before the '^'. The read-only qualifier of the pointer itself gets ignored, unless it is a typedef. Also, the 'r' is only emitted for the outermost type. * 32-bit longs are encoded as 'l' or 'L', but not always. For typedefs, the compiler uses 'i' or 'I' instead if encoding a struct field or a pointer. * enums are always encoded as 'i' (int) even if they are actually unsigned or long. In addition to that, the NeXT runtime uses a different encoding for bitfields. It encodes them as b followed by the size, without a bit offset or the underlying field type. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Type encoding Link: next: Method signatures Link: prev: Legacy type encoding Next: Method signatures, Previous: Legacy type encoding, Up: Type encoding ────────────────────────────────────────────────────────────────────────────── 8.3.2 @encode GNU Objective-C supports the @encode syntax that allows you to create a type encoding from a C/Objective-C type. For example, @encode(int) is compiled by the compiler into "i". @encode does not support type qualifiers other than const. For example, @encode(const char*) is valid and is compiled into "r*", while @encode(bycopy char *) is invalid and will cause a compilation error. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Type encoding Link: next: Garbage Collection Link: prev: @encode ────────────────────────────────────────────────────────────────────────────── 8.3.3 Method Signatures This section documents the encoding of method types, which is rarely needed to use Objective-C. You should skip it at a first reading; the runtime provides functions that will work on methods and can walk through the list of parameters and interpret them for you. These functions are part of the public “API” and are the preferred way to interact with method signatures from user code. But if you need to debug a problem with method signatures and need to know how they are implemented (i.e., the “ABI”), read on. Methods have their “signature” encoded and made available to the runtime. The “signature” encodes all the information required to dynamically build invocations of the method at runtime: return type and arguments. The “signature” is a null-terminated string, composed of the following: * The return type, including type qualifiers. For example, a method returning int would have i here. * The total size (in bytes) required to pass all the parameters. This includes the two hidden parameters (the object self and the method selector _cmd). * Each argument, with the type encoding, followed by the offset (in bytes) of the argument in the list of parameters. For example, a method with no arguments and returning int would have the signature i8@0:4 if the size of a pointer is 4. The signature is interpreted as follows: the i is the return type (an int), the 8 is the total size of the parameters in bytes (two pointers each of size 4), the @0 is the first parameter (an object at byte offset 0) and :4 is the second parameter (a SEL at byte offset 4). You can easily find more examples by running the “strings” program on an Objective-C object file compiled by GCC. You'll see a lot of strings that look very much like i8@0:4. They are signatures of Objective-C methods. ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Objective-C Link: next: Constant string objects Link: prev: Method signatures Next: Constant string objects, Previous: Type encoding, Up: Objective-C ────────────────────────────────────────────────────────────────────────────── 8.4 Garbage Collection This section is specific for the GNU Objective-C runtime. If you are using a different runtime, you can skip it. Support for garbage collection with the GNU runtime has been added by using a powerful conservative garbage collector, known as the Boehm-Demers-Weiser conservative garbage collector. To enable the support for it you have to configure the compiler using an additional argument, --enable-objc-gc. This will build the boehm-gc library, and build an additional runtime library which has several enhancements to support the garbage collector. The new library has a new name, libobjc_gc.a to not conflict with the non-garbage-collected library. When the garbage collector is used, the objects are allocated using the so-called typed memory allocation mechanism available in the Boehm-Demers-Weiser collector. This mode requires precise information on where pointers are located inside objects. This information is computed once per class, immediately after the class has been initialized. There is a new runtime function class_ivar_set_gcinvisible() which can be used to declare a so-called weak pointer reference. Such a pointer is basically hidden for the garbage collector; this can be useful in certain situations, especially when you want to keep track of the allocated objects, yet allow them to be collected. This kind of pointers can only be members of objects, you cannot declare a global pointer as a weak reference. Every type which is a pointer type can be declared a weak pointer, including id, Class and SEL. Here is an example of how to use this feature. Suppose you want to implement a class whose instances hold a weak pointer reference; the following class does this: @interface WeakPointer : Object { const void* weakPointer; } - initWithPointer:(const void*)p; - (const void*)weakPointer; @end @implementation WeakPointer + (void)initialize { if (self == objc_lookUpClass ("WeakPointer")) class_ivar_set_gcinvisible (self, "weakPointer", YES); } - initWithPointer:(const void*)p { weakPointer = p; return self; } - (const void*)weakPointer { return weakPointer; } @end Weak pointers are supported through a new type character specifier represented by the '!' character. The class_ivar_set_gcinvisible() function adds or removes this specifier to the string type description of the instance variable named as argument. ────────────────────────────────────────────────────────────────────────────── Next: Constant string objects, Previous: Type encoding, Up: Objective-C Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Objective-C Link: next: compatibility_alias Link: prev: Garbage Collection Next: compatibility_alias, Previous: Garbage Collection, Up: Objective-C ────────────────────────────────────────────────────────────────────────────── 8.5 Constant String Objects GNU Objective-C provides constant string objects that are generated directly by the compiler. You declare a constant string object by prefixing a C constant string with the character '@': id myString = @"this is a constant string object"; The constant string objects are by default instances of the NXConstantString class which is provided by the GNU Objective-C runtime. To get the definition of this class you must include the objc/NXConstStr.h header file. User defined libraries may want to implement their own constant string class. To be able to support them, the GNU Objective-C compiler provides a new command line options -fconstant-string-class=class-name. The provided class should adhere to a strict structure, the same as NXConstantString's structure: @interface MyConstantStringClass { Class isa; char *c_string; unsigned int len; } @end NXConstantString inherits from Object; user class libraries may choose to inherit the customized constant string class from a different class than Object. There is no requirement in the methods the constant string class has to implement, but the final ivar layout of the class must be the compatible with the given structure. When the compiler creates the statically allocated constant string object, the c_string field will be filled by the compiler with the string; the length field will be filled by the compiler with the string length; the isa pointer will be filled with NULL by the compiler, and it will later be fixed up automatically at runtime by the GNU Objective-C runtime library to point to the class which was set by the -fconstant-string-class option when the object file is loaded (if you wonder how it works behind the scenes, the name of the class to use, and the list of static objects to fixup, are stored by the compiler in the object file in a place where the GNU runtime library will find them at runtime). As a result, when a file is compiled with the -fconstant-string-class option, all the constant string objects will be instances of the class specified as argument to this option. It is possible to have multiple compilation units referring to different constant string classes, neither the compiler nor the linker impose any restrictions in doing this. ────────────────────────────────────────────────────────────────────────────── Next: compatibility_alias, Previous: Garbage Collection, Up: Objective-C Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Objective-C Link: next: Exceptions Link: prev: Constant string objects Next: Exceptions, Previous: Constant string objects, Up: Objective-C ────────────────────────────────────────────────────────────────────────────── 8.6 compatibility_alias The keyword @compatibility_alias allows you to define a class name as equivalent to another class name. For example: @compatibility_alias WOApplication GSWApplication; tells the compiler that each time it encounters WOApplication as a class name, it should replace it with GSWApplication (that is, WOApplication is just an alias for GSWApplication). There are some constraints on how this can be used— * WOApplication (the alias) must not be an existing class; * GSWApplication (the real class) must be an existing class. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Objective-C Link: next: Synchronization Link: prev: compatibility_alias Next: Synchronization, Previous: compatibility_alias, Up: Objective-C ────────────────────────────────────────────────────────────────────────────── 8.7 Exceptions GNU Objective-C provides exception support built into the language, as in the following example: @try { … @throw expr; … } @catch (AnObjCClass *exc) { … @throw expr; … @throw; … } @catch (AnotherClass *exc) { … } @catch (id allOthers) { … } @finally { … @throw expr; … } The @throw statement may appear anywhere in an Objective-C or Objective-C++ program; when used inside of a @catch block, the @throw may appear without an argument (as shown above), in which case the object caught by the @catch will be rethrown. Note that only (pointers to) Objective-C objects may be thrown and caught using this scheme. When an object is thrown, it will be caught by the nearest @catch clause capable of handling objects of that type, analogously to how catch blocks work in C++ and Java. A @catch(id …) clause (as shown above) may also be provided to catch any and all Objective-C exceptions not caught by previous @catch clauses (if any). The @finally clause, if present, will be executed upon exit from the immediately preceding @try … @catch section. This will happen regardless of whether any exceptions are thrown, caught or rethrown inside the @try … @catch section, analogously to the behavior of the finally clause in Java. There are several caveats to using the new exception mechanism: * The -fobjc-exceptions command line option must be used when compiling Objective-C files that use exceptions. * With the GNU runtime, exceptions are always implemented as “native” exceptions and it is recommended that the -fexceptions and -shared-libgcc options are used when linking. * With the NeXT runtime, although currently designed to be binary compatible with NS_HANDLER-style idioms provided by the NSException class, the new exceptions can only be used on Mac OS X 10.3 (Panther) and later systems, due to additional functionality needed in the NeXT Objective-C runtime. * As mentioned above, the new exceptions do not support handling types other than Objective-C objects. Furthermore, when used from Objective-C++, the Objective-C exception model does not interoperate with C++ exceptions at this time. This means you cannot @throw an exception from Objective-C and catch it in C++, or vice versa (i.e., throw … @catch). ────────────────────────────────────────────────────────────────────────────── Next: Synchronization, Previous: compatibility_alias, Up: Objective-C Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Objective-C Link: next: Fast enumeration Link: prev: Exceptions Next: Fast enumeration, Previous: Exceptions, Up: Objective-C ────────────────────────────────────────────────────────────────────────────── 8.8 Synchronization GNU Objective-C provides support for synchronized blocks: @synchronized (ObjCClass *guard) { … } Upon entering the @synchronized block, a thread of execution shall first check whether a lock has been placed on the corresponding guard object by another thread. If it has, the current thread shall wait until the other thread relinquishes its lock. Once guard becomes available, the current thread will place its own lock on it, execute the code contained in the @synchronized block, and finally relinquish the lock (thereby making guard available to other threads). Unlike Java, Objective-C does not allow for entire methods to be marked @synchronized. Note that throwing exceptions out of @synchronized blocks is allowed, and will cause the guarding object to be unlocked properly. Because of the interactions between synchronization and exception handling, you can only use @synchronized when compiling with exceptions enabled, that is with the command line option -fobjc-exceptions. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Objective-C Link: next: Using fast enumeration Link: prev: Synchronization Next: Messaging with the GNU Objective-C runtime, Previous: ────────────────────────────────────────────────────────────────────────────── 8.9 Fast Enumeration • Using fast enumeration: • c99-like fast enumeration syntax: • Fast enumeration details: • Fast enumeration protocol: Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Fast enumeration Link: next: c99-like fast enumeration syntax Link: prev: Fast enumeration Next: c99-like fast enumeration syntax, Up: Fast enumeration ────────────────────────────────────────────────────────────────────────────── 8.9.1 Using Fast Enumeration GNU Objective-C provides support for the fast enumeration syntax: id array = …; id object; for (object in array) { /* Do something with 'object' */ } array needs to be an Objective-C object (usually a collection object, for example an array, a dictionary or a set) which implements the “Fast Enumeration Protocol” (see below). If you are using a Foundation library such as GNUstep Base or Apple Cocoa Foundation, all collection objects in the library implement this protocol and can be used in this way. The code above would iterate over all objects in array. For each of them, it assigns it to object, then executes the Do something with 'object' statements. Here is a fully worked-out example using a Foundation library (which provides the implementation of NSArray, NSString and NSLog): NSArray *array = [NSArray arrayWithObjects: @"1", @"2", @"3", nil]; NSString *object; for (object in array) NSLog (@"Iterating over %@", object); Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Fast enumeration Link: next: Fast enumeration details Link: prev: Using fast enumeration Next: Fast enumeration details, Previous: Using fast enumeration, Up: Fast ────────────────────────────────────────────────────────────────────────────── 8.9.2 C99-Like Fast Enumeration Syntax A c99-like declaration syntax is also allowed: id array = …; for (id object in array) { /* Do something with 'object' */ } this is completely equivalent to: id array = …; { id object; for (object in array) { /* Do something with 'object' */ } } but can save some typing. Note that the option -std=c99 is not required to allow this syntax in Objective-C. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Fast enumeration Link: next: Fast enumeration protocol Link: prev: c99-like fast enumeration syntax Next: Fast enumeration protocol, Previous: c99-like fast enumeration ────────────────────────────────────────────────────────────────────────────── 8.9.3 Fast Enumeration Details Here is a more technical description with the gory details. Consider the code for (object expression in collection expression) { statements } here is what happens when you run it: * collection expression is evaluated exactly once and the result is used as the collection object to iterate over. This means it is safe to write code such as for (object in [NSDictionary keyEnumerator]) …. * the iteration is implemented by the compiler by repeatedly getting batches of objects from the collection object using the fast enumeration protocol (see below), then iterating over all objects in the batch. This is faster than a normal enumeration where objects are retrieved one by one (hence the name “fast enumeration”). * if there are no objects in the collection, then object expression is set to nil and the loop immediately terminates. * if there are objects in the collection, then for each object in the collection (in the order they are returned) object expression is set to the object, then statements are executed. * statements can contain break and continue commands, which will abort the iteration or skip to the next loop iteration as expected. * when the iteration ends because there are no more objects to iterate over, object expression is set to nil. This allows you to determine whether the iteration finished because a break command was used (in which case object expression will remain set to the last object that was iterated over) or because it iterated over all the objects (in which case object expression will be set to nil). * statements must not make any changes to the collection object; if they do, it is a hard error and the fast enumeration terminates by invoking objc_enumerationMutation, a runtime function that normally aborts the program but which can be customized by Foundation libraries via objc_set_mutation_handler to do something different, such as raising an exception. ────────────────────────────────────────────────────────────────────────────── Next: Fast enumeration protocol, Previous: c99-like fast enumeration Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Fast enumeration Link: next: Messaging with the GNU Objective-C runtime Link: prev: Fast enumeration details Previous: Fast enumeration details, Up: Fast enumeration ────────────────────────────────────────────────────────────────────────────── 8.9.4 Fast Enumeration Protocol If you want your own collection object to be usable with fast enumeration, you need to have it implement the method - (unsigned long) countByEnumeratingWithState: (NSFastEnumerationState *)state objects: (id *)objects count: (unsigned long)len; where NSFastEnumerationState must be defined in your code as follows: typedef struct { unsigned long state; id *itemsPtr; unsigned long *mutationsPtr; unsigned long extra[5]; } NSFastEnumerationState; If no NSFastEnumerationState is defined in your code, the compiler will automatically replace NSFastEnumerationState * with struct __objcFastEnumerationState *, where that type is silently defined by the compiler in an identical way. This can be confusing and we recommend that you define NSFastEnumerationState (as shown above) instead. The method is called repeatedly during a fast enumeration to retrieve batches of objects. Each invocation of the method should retrieve the next batch of objects. The return value of the method is the number of objects in the current batch; this should not exceed len, which is the maximum size of a batch as requested by the caller. The batch itself is returned in the itemsPtr field of the NSFastEnumerationState struct. To help with returning the objects, the objects array is a C array preallocated by the caller (on the stack) of size len. In many cases you can put the objects you want to return in that objects array, then do itemsPtr = objects. But you don't have to; if your collection already has the objects to return in some form of C array, it could return them from there instead. The state and extra fields of the NSFastEnumerationState structure allows your collection object to keep track of the state of the enumeration. In a simple array implementation, state may keep track of the index of the last object that was returned, and extra may be unused. The mutationsPtr field of the NSFastEnumerationState is used to keep track of mutations. It should point to a number; before working on each object, the fast enumeration loop will check that this number has not changed. If it has, a mutation has happened and the fast enumeration will abort. So, mutationsPtr could be set to point to some sort of version number of your collection, which is increased by one every time there is a change (for example when an object is added or removed). Or, if you are content with less strict mutation checks, it could point to the number of objects in your collection or some other value that can be checked to perform an approximate check that the collection has not been mutated. Finally, note how we declared the len argument and the return value to be of type unsigned long. They could also be declared to be of type unsigned int and everything would still work. ────────────────────────────────────────────────────────────────────────────── Previous: Fast enumeration details, Up: Fast enumeration Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Messaging with the GNU Objective-C runtime Link: next: Forwarding hook Link: prev: Messaging with the GNU Objective-C runtime Next: Forwarding hook, Up: Messaging with the GNU Objective-C runtime ────────────────────────────────────────────────────────────────────────────── 8.10.1 Dynamically Registering Methods If objc_msg_lookup() does not find a suitable method implementation, because the receiver does not implement the required method, it tries to see if the class can dynamically register the method. To do so, the runtime checks if the class of the receiver implements the method + (BOOL) resolveInstanceMethod: (SEL)selector; in the case of an instance method, or + (BOOL) resolveClassMethod: (SEL)selector; in the case of a class method. If the class implements it, the runtime invokes it, passing as argument the selector of the original method, and if it returns YES, the runtime tries the lookup again, which could now succeed if a matching method was added dynamically by +resolveInstanceMethod: or +resolveClassMethod:. This allows classes to dynamically register methods (by adding them to the class using class_addMethod) when they are first called. To do so, a class should implement +resolveInstanceMethod: (or, depending on the case, +resolveClassMethod:) and have it recognize the selectors of methods that can be registered dynamically at runtime, register them, and return YES. It should return NO for methods that it does not dynamically registered at runtime. If +resolveInstanceMethod: (or +resolveClassMethod:) is not implemented or returns NO, the runtime then tries the forwarding hook. Support for +resolveInstanceMethod: and resolveClassMethod: was added to the GNU Objective-C runtime in GCC version 4.6. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Messaging with the GNU Objective-C runtime Link: next: Compatibility Link: prev: Dynamically registering methods Previous: Dynamically registering methods, Up: Messaging with the GNU ────────────────────────────────────────────────────────────────────────────── 8.10.2 Forwarding Hook The GNU Objective-C runtime provides a hook, called __objc_msg_forward2, which is called by objc_msg_lookup() when it cannot find a method implementation in the runtime tables and after calling +resolveInstanceMethod: and +resolveClassMethod: has been attempted and did not succeed in dynamically registering the method. To configure the hook, you set the global variable __objc_msg_forward2 to a function with the same argument and return types of objc_msg_lookup(). When objc_msg_lookup() cannot find a method implementation, it invokes the hook function you provided to get a method implementation to return. So, in practice __objc_msg_forward2 allows you to extend objc_msg_lookup() by adding some custom code that is called to do a further lookup when no standard method implementation can be found using the normal lookup. This hook is generally reserved for “Foundation” libraries such as GNUstep Base, which use it to implement their high-level method forwarding API, typically based around the forwardInvocation: method. So, unless you are implementing your own “Foundation” library, you should not set this hook. In a typical forwarding implementation, the __objc_msg_forward2 hook function determines the argument and return type of the method that is being looked up, and then creates a function that takes these arguments and has that return type, and returns it to the caller. Creating this function is non-trivial and is typically performed using a dedicated library such as libffi. The forwarding method implementation thus created is returned by objc_msg_lookup() and is executed as if it was a normal method implementation. When the forwarding method implementation is called, it is usually expected to pack all arguments into some sort of object (typically, an NSInvocation in a “Foundation” library), and hand it over to the programmer (forwardInvocation:) who is then allowed to manipulate the method invocation using a high-level API provided by the “Foundation” library. For example, the programmer may want to examine the method invocation arguments and name and potentially change them before forwarding the method invocation to one or more local objects (performInvocation:) or even to remote objects (by using Distributed Objects or some other mechanism). When all this completes, the return value is passed back and must be returned correctly to the original caller. Note that the GNU Objective-C runtime currently provides no support for method forwarding or method invocations other than the __objc_msg_forward2 hook. If the forwarding hook does not exist or returns NULL, the runtime currently attempts forwarding using an older, deprecated API, and if that fails, it aborts the program. In future versions of the GNU Objective-C runtime, the runtime will immediately abort. ────────────────────────────────────────────────────────────────────────────── Previous: Dynamically registering methods, Up: Messaging with the GNU Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Top Link: next: Gcov Link: prev: Forwarding hook ────────────────────────────────────────────────────────────────────────────── 9 Binary Compatibility Binary compatibility encompasses several related concepts: application binary interface (ABI) The set of runtime conventions followed by all of the tools that deal with binary representations of a program, including compilers, assemblers, linkers, and language runtime support. Some ABIs are formal with a written specification, possibly designed by multiple interested parties. Others are simply the way things are actually done by a particular set of tools. ABI conformance A compiler conforms to an ABI if it generates code that follows all of the specifications enumerated by that ABI. A library conforms to an ABI if it is implemented according to that ABI. An application conforms to an ABI if it is built using tools that conform to that ABI and does not contain source code that specifically changes behavior specified by the ABI. calling conventions Calling conventions are a subset of an ABI that specify of how arguments are passed and function results are returned. interoperability Different sets of tools are interoperable if they generate files that can be used in the same program. The set of tools includes compilers, assemblers, linkers, libraries, header files, startup files, and debuggers. Binaries produced by different sets of tools are not interoperable unless they implement the same ABI. This applies to different versions of the same tools as well as tools from different vendors. intercallability Whether a function in a binary built by one set of tools can call a function in a binary built by a different set of tools is a subset of interoperability. implementation-defined features Language standards include lists of implementation-defined features whose behavior can vary from one implementation to another. Some of these features are normally covered by a platform's ABI and others are not. The features that are not covered by an ABI generally affect how a program behaves, but not intercallability. compatibility Conformance to the same ABI and the same behavior of implementation-defined features are both relevant for compatibility. The application binary interface implemented by a C or C++ compiler affects code generation and runtime support for: * size and alignment of data types * layout of structured types * calling conventions * register usage conventions * interfaces for runtime arithmetic support * object file formats In addition, the application binary interface implemented by a C++ compiler affects code generation and runtime support for: * name mangling * exception handling * invoking constructors and destructors * layout, alignment, and padding of classes * layout and alignment of virtual tables Some GCC compilation options cause the compiler to generate code that does not conform to the platform's default ABI. Other options cause different program behavior for implementation-defined features that are not covered by an ABI. These options are provided for consistency with other compilers that do not follow the platform's default ABI or the usual behavior of implementation-defined features for the platform. Be very careful about using such options. Most platforms have a well-defined ABI that covers C code, but ABIs that cover C++ functionality are not yet common. Starting with GCC 3.2, GCC binary conventions for C++ are based on a written, vendor-neutral C++ ABI that was designed to be specific to 64-bit Itanium but also includes generic specifications that apply to any platform. This C++ ABI is also implemented by other compiler vendors on some platforms, notably GNU/Linux and BSD systems. We have tried hard to provide a stable ABI that will be compatible with future GCC releases, but it is possible that we will encounter problems that make this difficult. Such problems could include different interpretations of the C++ ABI by different vendors, bugs in the ABI, or bugs in the implementation of the ABI in different compilers. GCC's -Wabi switch warns when G++ generates code that is probably not compatible with the C++ ABI. The C++ library used with a C++ compiler includes the Standard C++ Library, with functionality defined in the C++ Standard, plus language runtime support. The runtime support is included in a C++ ABI, but there is no formal ABI for the Standard C++ Library. Two implementations of that library are interoperable if one follows the de-facto ABI of the other and if they are both built with the same compiler, or with compilers that conform to the same ABI for C++ compiler and runtime support. When G++ and another C++ compiler conform to the same C++ ABI, but the implementations of the Standard C++ Library that they normally use do not follow the same ABI for the Standard C++ Library, object files built with those compilers can be used in the same program only if they use the same C++ library. This requires specifying the location of the C++ library header files when invoking the compiler whose usual library is not being used. The location of GCC's C++ header files depends on how the GCC build was configured, but can be seen by using the G++ -v option. With default configuration options for G++ 3.3 the compile line for a different C++ compiler needs to include -Igcc_install_directory/include/c++/3.3 Similarly, compiling code with G++ that must use a C++ library other than the GNU C++ library requires specifying the location of the header files for that other library. The most straightforward way to link a program to use a particular C++ library is to use a C++ driver that specifies that C++ library by default. The g++ driver, for example, tells the linker where to find GCC's C++ library (libstdc++) plus the other libraries and startup files it needs, in the proper order. If a program must use a different C++ library and it's not possible to do the final link using a C++ driver that uses that library by default, it is necessary to tell g++ the location and name of that library. It might also be necessary to specify different startup files and other runtime support libraries, and to suppress the use of GCC's support libraries with one or more of the options -nostdlib, -nostartfiles, and -nodefaultlibs. ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Top Link: next: Gcov Intro Link: prev: Compatibility ────────────────────────────────────────────────────────────────────────────── 10 gcov—a Test Coverage Program gcov is a tool you can use in conjunction with GCC to test code coverage in your programs. • Gcov Intro: Introduction to gcov. • Invoking Gcov: How to use gcov. • Gcov and Optimization: Using gcov with GCC optimization. • Gcov Data Files: The files used by gcov. • Cross-profiling: Data file relocation. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Gcov Link: next: Invoking Gcov Link: prev: Gcov ────────────────────────────────────────────────────────────────────────────── 10.1 Introduction to gcov gcov is a test coverage program. Use it in concert with GCC to analyze your programs to help create more efficient, faster running code and to discover untested parts of your program. You can use gcov as a profiling tool to help discover where your optimization efforts will best affect your code. You can also use gcov along with the other profiling tool, gprof, to assess which parts of your code use the greatest amount of computing time. Profiling tools help you analyze your code's performance. Using a profiler such as gcov or gprof, you can find out some basic performance statistics, such as: * how often each line of code executes * what lines of code are actually executed * how much computing time each section of code uses Once you know these things about how your code works when compiled, you can look at each module to see which modules should be optimized. gcov helps you determine where to work on optimization. Software developers also use coverage testing in concert with testsuites, to make sure software is actually good enough for a release. Testsuites can verify that a program works as expected; a coverage program tests to see how much of the program is exercised by the testsuite. Developers can then determine what kinds of test cases need to be added to the testsuites to create both better testing and a better final product. You should compile your code without optimization if you plan to use gcov because the optimization, by combining some lines of code into one function, may not give you as much information as you need to look for 'hot spots' where the code is using a great deal of computer time. Likewise, because gcov accumulates statistics by line (at the lowest resolution), it works best with a programming style that places only one statement on each line. If you use complicated macros that expand to loops or to other control structures, the statistics are less helpful—they only report on the line where the macro call appears. If your complex macros behave like functions, you can replace them with inline functions to solve this problem. gcov creates a logfile called sourcefile.gcov which indicates how many times each line of a source file sourcefile.c has executed. You can use these logfiles along with gprof to aid in fine-tuning the performance of your programs. gprof gives timing information you can use along with the information you get from gcov. gcov works only on code compiled with GCC. It is not compatible with any other profiling or test coverage mechanism. ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Gcov Link: next: Gcov and Optimization Link: prev: Gcov Intro Next: Gcov and Optimization, Previous: Gcov Intro, Up: Gcov ────────────────────────────────────────────────────────────────────────────── 10.2 Invoking gcov gcov [options] files gcov accepts the following options: -a --all-blocks Write individual execution counts for every basic block. Normally gcov outputs execution counts only for the main blocks of a line. With this option you can determine if blocks within a single line are not being executed. -b --branch-probabilities Write branch frequencies to the output file, and write branch summary info to the standard output. This option allows you to see how often each branch in your program was taken. Unconditional branches will not be shown, unless the -u option is given. -c --branch-counts Write branch frequencies as the number of branches taken, rather than the percentage of branches taken. -d --display-progress Display the progress on the standard output. -f --function-summaries Output summaries for each function in addition to the file level summary. -h --help Display help about using gcov (on the standard output), and exit without doing any further processing. -i --json-format Output gcov file in an easy-to-parse JSON intermediate format which does not require source code for generation. The JSON file is compressed with gzip compression algorithm and the files have .gcov.json.gz extension. Structure of the JSON is following: { "current_working_directory": current_working_directory, "data_file": data_file, "format_version": format_version, "gcc_version": gcc_version "files": [file] } Fields of the root element have following semantics: * current_working_directory: working directory where a compilation unit was compiled * data_file: name of the data file (GCDA) * format_version: semantic version of the format * gcc_version: version of the GCC compiler Each file has the following form: { "file": file_name, "functions": [function], "lines": [line] } Fields of the file element have following semantics: * file_name: name of the source file Each function has the following form: { "blocks": blocks, "blocks_executed": blocks_executed, "demangled_name": "demangled_name, "end_column": end_column, "end_line": end_line, "execution_count": execution_count, "name": name, "start_column": start_column "start_line": start_line } Fields of the function element have following semantics: * blocks: number of blocks that are in the function * blocks_executed: number of executed blocks of the function * demangled_name: demangled name of the function * end_column: column in the source file where the function ends * end_line: line in the source file where the function ends * execution_count: number of executions of the function * name: name of the function * start_column: column in the source file where the function begins * start_line: line in the source file where the function begins Note that line numbers and column numbers number from 1. In the current implementation, start_line and start_column do not include any template parameters and the leading return type but that this is likely to be fixed in the future. Each line has the following form: { "branches": [branch], "count": count, "line_number": line_number, "unexecuted_block": unexecuted_block "function_name": function_name, } Branches are present only with -b option. Fields of the line element have following semantics: * count: number of executions of the line * line_number: line number * unexecuted_block: flag whether the line contains an unexecuted block (not all statements on the line are executed) * function_name: a name of a function this line belongs to (for a line with an inlined statements can be not set) Each branch has the following form: { "count": count, "fallthrough": fallthrough, "throw": throw } Fields of the branch element have following semantics: * count: number of executions of the branch * fallthrough: true when the branch is a fall through branch * throw: true when the branch is an exceptional branch -j --human-readable Write counts in human readable format (like 24.6k). -k --use-colors Use colors for lines of code that have zero coverage. We use red color for non-exceptional lines and cyan for exceptional. Same colors are used for basic blocks with -a option. -l --long-file-names Create long file names for included source files. For example, if the header file x.h contains code, and was included in the file a.c, then running gcov on the file a.c will produce an output file called a.c##x.h.gcov instead of x.h.gcov. This can be useful if x.h is included in multiple source files and you want to see the individual contributions. If you use the '-p' option, both the including and included file names will be complete path names. -m --demangled-names Display demangled function names in output. The default is to show mangled function names. -n --no-output Do not create the gcov output file. -o directory|file --object-directory directory --object-file file Specify either the directory containing the gcov data files, or the object path name. The .gcno, and .gcda data files are searched for using this option. If a directory is specified, the data files are in that directory and named after the input file name, without its extension. If a file is specified here, the data files are named after that file, without its extension. -p --preserve-paths Preserve complete path information in the names of generated .gcov files. Without this option, just the filename component is used. With this option, all directories are used, with '/' characters translated to '#' characters, . directory components removed and unremoveable .. components renamed to '^'. This is useful if sourcefiles are in several different directories. -q --use-hotness-colors Emit perf-like colored output for hot lines. Legend of the color scale is printed at the very beginning of the output file. -r --relative-only Only output information about source files with a relative pathname (after source prefix elision). Absolute paths are usually system header files and coverage of any inline functions therein is normally uninteresting. -s directory --source-prefix directory A prefix for source file names to remove when generating the output coverage files. This option is useful when building in a separate directory, and the pathname to the source directory is not wanted when determining the output file names. Note that this prefix detection is applied before determining whether the source file is absolute. -t --stdout Output to standard output instead of output files. -u --unconditional-branches When branch probabilities are given, include those of unconditional branches. Unconditional branches are normally not interesting. -v --version Display the gcov version number (on the standard output), and exit without doing any further processing. -w --verbose Print verbose informations related to basic blocks and arcs. -x --hash-filenames When using –preserve-paths, gcov uses the full pathname of the source files to create an output filename. This can lead to long filenames that can overflow filesystem limits. This option creates names of the form source-file##md5.gcov, where the source-file component is the final filename part and the md5 component is calculated from the full mangled name that would have been used otherwise. The option is an alternative to the –preserve-paths on systems which have a filesystem limit. gcov should be run with the current directory the same as that when you invoked the compiler. Otherwise it will not be able to locate the source files. gcov produces files called mangledname.gcov in the current directory. These contain the coverage information of the source file they correspond to. One .gcov file is produced for each source (or header) file containing code, which was compiled to produce the data files. The mangledname part of the output file name is usually simply the source file name, but can be something more complicated if the '-l' or '-p' options are given. Refer to those options for details. If you invoke gcov with multiple input files, the contributions from each input file are summed. Typically you would invoke it with the same list of files as the final link of your executable. The .gcov files contain the ':' separated fields along with program source code. The format is execution_count:line_number:source line text Additional block information may succeed each line, when requested by command line option. The execution_count is '-' for lines containing no code. Unexecuted lines are marked '#####' or '=====', depending on whether they are reachable by non-exceptional paths or only exceptional paths such as C++ exception handlers, respectively. Given the '-a' option, unexecuted blocks are marked '$$$$$' or '%%%%%', depending on whether a basic block is reachable via non-exceptional or exceptional paths. Executed basic blocks having a statement with zero execution_count end with '*' character and are colored with magenta color with the -k option. This functionality is not supported in Ada. Note that GCC can completely remove the bodies of functions that are not needed – for instance if they are inlined everywhere. Such functions are marked with '-', which can be confusing. Use the -fkeep-inline-functions and -fkeep-static-functions options to retain these functions and allow gcov to properly show their execution_count. Some lines of information at the start have line_number of zero. These preamble lines are of the form -:0:tag:value The ordering and number of these preamble lines will be augmented as gcov development progresses — do not rely on them remaining unchanged. Use tag to locate a particular preamble line. The additional block information is of the form tag information The information is human readable, but designed to be simple enough for machine parsing too. When printing percentages, 0% and 100% are only printed when the values are exactly 0% and 100% respectively. Other values which would conventionally be rounded to 0% or 100% are instead printed as the nearest non-boundary value. When using gcov, you must first compile your program with a special GCC option '--coverage'. This tells the compiler to generate additional information needed by gcov (basically a flow graph of the program) and also includes additional code in the object files for generating the extra profiling information needed by gcov. These additional files are placed in the directory where the object file is located. Running the program will cause profile output to be generated. For each source file compiled with -fprofile-arcs, an accompanying .gcda file will be placed in the object file directory. Running gcov with your program's source file names as arguments will now produce a listing of the code along with frequency of execution for each line. For example, if your program is called tmp.cpp, this is what you see when you use the basic gcov facility: $ g++ --coverage tmp.cpp $ a.out $ gcov tmp.cpp -m File 'tmp.cpp' Lines executed:92.86% of 14 Creating 'tmp.cpp.gcov' The file tmp.cpp.gcov contains output from gcov. Here is a sample: -: 0:Source:tmp.cpp -: 0:Working directory:/home/gcc/testcase -: 0:Graph:tmp.gcno -: 0:Data:tmp.gcda -: 0:Runs:1 -: 0:Programs:1 -: 1:#include -: 2: -: 3:template -: 4:class Foo -: 5:{ -: 6: public: 1*: 7: Foo(): b (1000) {} ------------------ Foo::Foo(): #####: 7: Foo(): b (1000) {} ------------------ Foo::Foo(): 1: 7: Foo(): b (1000) {} ------------------ 2*: 8: void inc () { b++; } ------------------ Foo::inc(): #####: 8: void inc () { b++; } ------------------ Foo::inc(): 2: 8: void inc () { b++; } ------------------ -: 9: -: 10: private: -: 11: int b; -: 12:}; -: 13: -: 14:template class Foo; -: 15:template class Foo; -: 16: -: 17:int 1: 18:main (void) -: 19:{ -: 20: int i, total; 1: 21: Foo counter; -: 22: 1: 23: counter.inc(); 1: 24: counter.inc(); 1: 25: total = 0; -: 26: 11: 27: for (i = 0; i < 10; i++) 10: 28: total += i; -: 29: 1*: 30: int v = total > 100 ? 1 : 2; -: 31: 1: 32: if (total != 45) #####: 33: printf ("Failure\n"); -: 34: else 1: 35: printf ("Success\n"); 1: 36: return 0; -: 37:} Note that line 7 is shown in the report multiple times. First occurrence presents total number of execution of the line and the next two belong to instances of class Foo constructors. As you can also see, line 30 contains some unexecuted basic blocks and thus execution count has asterisk symbol. When you use the -a option, you will get individual block counts, and the output looks like this: -: 0:Source:tmp.cpp -: 0:Working directory:/home/gcc/testcase -: 0:Graph:tmp.gcno -: 0:Data:tmp.gcda -: 0:Runs:1 -: 0:Programs:1 -: 1:#include -: 2: -: 3:template -: 4:class Foo -: 5:{ -: 6: public: 1*: 7: Foo(): b (1000) {} ------------------ Foo::Foo(): #####: 7: Foo(): b (1000) {} ------------------ Foo::Foo(): 1: 7: Foo(): b (1000) {} ------------------ 2*: 8: void inc () { b++; } ------------------ Foo::inc(): #####: 8: void inc () { b++; } ------------------ Foo::inc(): 2: 8: void inc () { b++; } ------------------ -: 9: -: 10: private: -: 11: int b; -: 12:}; -: 13: -: 14:template class Foo; -: 15:template class Foo; -: 16: -: 17:int 1: 18:main (void) -: 19:{ -: 20: int i, total; 1: 21: Foo counter; 1: 21-block 0 -: 22: 1: 23: counter.inc(); 1: 23-block 0 1: 24: counter.inc(); 1: 24-block 0 1: 25: total = 0; -: 26: 11: 27: for (i = 0; i < 10; i++) 1: 27-block 0 11: 27-block 1 10: 28: total += i; 10: 28-block 0 -: 29: 1*: 30: int v = total > 100 ? 1 : 2; 1: 30-block 0 %%%%%: 30-block 1 1: 30-block 2 -: 31: 1: 32: if (total != 45) 1: 32-block 0 #####: 33: printf ("Failure\n"); %%%%%: 33-block 0 -: 34: else 1: 35: printf ("Success\n"); 1: 35-block 0 1: 36: return 0; 1: 36-block 0 -: 37:} In this mode, each basic block is only shown on one line – the last line of the block. A multi-line block will only contribute to the execution count of that last line, and other lines will not be shown to contain code, unless previous blocks end on those lines. The total execution count of a line is shown and subsequent lines show the execution counts for individual blocks that end on that line. After each block, the branch and call counts of the block will be shown, if the -b option is given. Because of the way GCC instruments calls, a call count can be shown after a line with no individual blocks. As you can see, line 33 contains a basic block that was not executed. When you use the -b option, your output looks like this: -: 0:Source:tmp.cpp -: 0:Working directory:/home/gcc/testcase -: 0:Graph:tmp.gcno -: 0:Data:tmp.gcda -: 0:Runs:1 -: 0:Programs:1 -: 1:#include -: 2: -: 3:template -: 4:class Foo -: 5:{ -: 6: public: 1*: 7: Foo(): b (1000) {} ------------------ Foo::Foo(): function Foo::Foo() called 0 returned 0% blocks executed 0% #####: 7: Foo(): b (1000) {} ------------------ Foo::Foo(): function Foo::Foo() called 1 returned 100% blocks executed 100% 1: 7: Foo(): b (1000) {} ------------------ 2*: 8: void inc () { b++; } ------------------ Foo::inc(): function Foo::inc() called 0 returned 0% blocks executed 0% #####: 8: void inc () { b++; } ------------------ Foo::inc(): function Foo::inc() called 2 returned 100% blocks executed 100% 2: 8: void inc () { b++; } ------------------ -: 9: -: 10: private: -: 11: int b; -: 12:}; -: 13: -: 14:template class Foo; -: 15:template class Foo; -: 16: -: 17:int function main called 1 returned 100% blocks executed 81% 1: 18:main (void) -: 19:{ -: 20: int i, total; 1: 21: Foo counter; call 0 returned 100% branch 1 taken 100% (fallthrough) branch 2 taken 0% (throw) -: 22: 1: 23: counter.inc(); call 0 returned 100% branch 1 taken 100% (fallthrough) branch 2 taken 0% (throw) 1: 24: counter.inc(); call 0 returned 100% branch 1 taken 100% (fallthrough) branch 2 taken 0% (throw) 1: 25: total = 0; -: 26: 11: 27: for (i = 0; i < 10; i++) branch 0 taken 91% (fallthrough) branch 1 taken 9% 10: 28: total += i; -: 29: 1*: 30: int v = total > 100 ? 1 : 2; branch 0 taken 0% (fallthrough) branch 1 taken 100% -: 31: 1: 32: if (total != 45) branch 0 taken 0% (fallthrough) branch 1 taken 100% #####: 33: printf ("Failure\n"); call 0 never executed branch 1 never executed branch 2 never executed -: 34: else 1: 35: printf ("Success\n"); call 0 returned 100% branch 1 taken 100% (fallthrough) branch 2 taken 0% (throw) 1: 36: return 0; -: 37:} For each function, a line is printed showing how many times the function is called, how many times it returns and what percentage of the function's blocks were executed. For each basic block, a line is printed after the last line of the basic block describing the branch or call that ends the basic block. There can be multiple branches and calls listed for a single source line if there are multiple basic blocks that end on that line. In this case, the branches and calls are each given a number. There is no simple way to map these branches and calls back to source constructs. In general, though, the lowest numbered branch or call will correspond to the leftmost construct on the source line. For a branch, if it was executed at least once, then a percentage indicating the number of times the branch was taken divided by the number of times the branch was executed will be printed. Otherwise, the message “never executed” is printed. For a call, if it was executed at least once, then a percentage indicating the number of times the call returned divided by the number of times the call was executed will be printed. This will usually be 100%, but may be less for functions that call exit or longjmp, and thus may not return every time they are called. The execution counts are cumulative. If the example program were executed again without removing the .gcda file, the count for the number of times each line in the source was executed would be added to the results of the previous run(s). This is potentially useful in several ways. For example, it could be used to accumulate data over a number of program runs as part of a test verification suite, or to provide more accurate long-term information over a large number of program runs. The data in the .gcda files is saved immediately before the program exits. For each source file compiled with -fprofile-arcs, the profiling code first attempts to read in an existing .gcda file; if the file doesn't match the executable (differing number of basic block counts) it will ignore the contents of the file. It then adds in the new execution counts and finally writes the data to the file. ────────────────────────────────────────────────────────────────────────────── Next: Gcov and Optimization, Previous: Gcov Intro, Up: Gcov Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Gcov Link: next: Gcov Data Files Link: prev: Invoking Gcov Next: Gcov Data Files, Previous: Invoking Gcov, Up: Gcov ────────────────────────────────────────────────────────────────────────────── 10.3 Using gcov with GCC Optimization If you plan to use gcov to help optimize your code, you must first compile your program with a special GCC option '--coverage'. Aside from that, you can use any other GCC options; but if you want to prove that every single line in your program was executed, you should not compile with optimization at the same time. On some machines the optimizer can eliminate some simple code lines by combining them with other lines. For example, code like this: if (a != b) c = 1; else c = 0; can be compiled into one instruction on some machines. In this case, there is no way for gcov to calculate separate execution counts for each line because there isn't separate code for each line. Hence the gcov output looks like this if you compiled the program with optimization: 100: 12:if (a != b) 100: 13: c = 1; 100: 14:else 100: 15: c = 0; The output shows that this block of code, combined by optimization, executed 100 times. In one sense this result is correct, because there was only one instruction representing all four of these lines. However, the output does not indicate how many times the result was 0 and how many times the result was 1. Inlineable functions can create unexpected line counts. Line counts are shown for the source code of the inlineable function, but what is shown depends on where the function is inlined, or if it is not inlined at all. If the function is not inlined, the compiler must emit an out of line copy of the function, in any object file that needs it. If fileA.o and fileB.o both contain out of line bodies of a particular inlineable function, they will also both contain coverage counts for that function. When fileA.o and fileB.o are linked together, the linker will, on many systems, select one of those out of line bodies for all calls to that function, and remove or ignore the other. Unfortunately, it will not remove the coverage counters for the unused function body. Hence when instrumented, all but one use of that function will show zero counts. If the function is inlined in several places, the block structure in each location might not be the same. For instance, a condition might now be calculable at compile time in some instances. Because the coverage of all the uses of the inline function will be shown for the same source lines, the line counts themselves might seem inconsistent. Long-running applications can use the __gcov_reset and __gcov_dump facilities to restrict profile collection to the program region of interest. Calling __gcov_reset(void) will clear all profile counters to zero, and calling __gcov_dump(void) will cause the profile information collected at that point to be dumped to .gcda output files. Instrumented applications use a static destructor with priority 99 to invoke the __gcov_dump function. Thus __gcov_dump is executed after all user defined static destructors, as well as handlers registered with atexit. If an executable loads a dynamic shared object via dlopen functionality, -Wl,--dynamic-list-data is needed to dump all profile data. Profiling run-time library reports various errors related to profile manipulation and profile saving. Errors are printed into standard error output or 'GCOV_ERROR_FILE' file, if environment variable is used. In order to terminate immediately after an errors occurs set 'GCOV_EXIT_AT_ERROR' environment variable. That can help users to find profile clashing which leads to a misleading profile. ────────────────────────────────────────────────────────────────────────────── Next: Gcov Data Files, Previous: Invoking Gcov, Up: Gcov Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Gcov Link: next: Cross-profiling Link: prev: Gcov and Optimization Next: Cross-profiling, Previous: Gcov and Optimization, Up: Gcov ────────────────────────────────────────────────────────────────────────────── 10.4 Brief Description of gcov Data Files gcov uses two files for profiling. The names of these files are derived from the original object file by substituting the file suffix with either .gcno, or .gcda. The files contain coverage and profile data stored in a platform-independent format. The .gcno files are placed in the same directory as the object file. By default, the .gcda files are also stored in the same directory as the object file, but the GCC -fprofile-dir option may be used to store the .gcda files in a separate directory. The .gcno notes file is generated when the source file is compiled with the GCC -ftest-coverage option. It contains information to reconstruct the basic block graphs and assign source line numbers to blocks. The .gcda count data file is generated when a program containing object files built with the GCC -fprofile-arcs option is executed. A separate .gcda file is created for each object file compiled with this option. It contains arc transition counts, value profile counts, and some summary information. It is not recommended to access the coverage files directly. Consumers should use the intermediate format that is provided by gcov tool via --json-format option. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Top Link: next: Gcov-tool Intro Link: prev: Cross-profiling ────────────────────────────────────────────────────────────────────────────── 11 gcov-tool—an Offline Gcda Profile Processing Tool gcov-tool is a tool you can use in conjunction with GCC to manipulate or process gcda profile files offline. • Gcov-tool Intro: Introduction to gcov-tool. • Invoking Gcov-tool: How to use gcov-tool. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Gcov-tool Link: next: Invoking Gcov-tool Link: prev: Gcov-tool ────────────────────────────────────────────────────────────────────────────── 11.1 Introduction to gcov-tool gcov-tool is an offline tool to process gcc's gcda profile files. Current gcov-tool supports the following functionalities: * merge two sets of profiles with weights. * read one set of profile and rewrite profile contents. One can scale or normalize the count values. Examples of the use cases for this tool are: * Collect the profiles for different set of inputs, and use this tool to merge them. One can specify the weight to factor in the relative importance of each input. * Rewrite the profile after removing a subset of the gcda files, while maintaining the consistency of the summary and the histogram. * It can also be used to debug or libgcov code as the tools shares the majority code as the runtime library. Note that for the merging operation, this profile generated offline may contain slight different values from the online merged profile. Here are a list of typical differences: * histogram difference: This offline tool recomputes the histogram after merging the counters. The resulting histogram, therefore, is precise. The online merging does not have this capability – the histogram is merged from two histograms and the result is an approximation. * summary checksum difference: Summary checksum uses a CRC32 operation. The value depends on the link list order of gcov-info objects. This order is different in gcov-tool from that in the online merge. It's expected to have different summary checksums. It does not really matter as the compiler does not use this checksum anywhere. * value profile counter values difference: Some counter values for value profile are runtime dependent, like heap addresses. It's normal to see some difference in these kind of counters. ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Gcov-tool Link: next: Gcov-dump Link: prev: Gcov-tool Intro ────────────────────────────────────────────────────────────────────────────── 11.2 Invoking gcov-tool gcov-tool [global-options] SUB_COMMAND [sub_command-options] profile_dir gcov-tool accepts the following options: -h --help Display help about using gcov-tool (on the standard output), and exit without doing any further processing. -v --version Display the gcov-tool version number (on the standard output), and exit without doing any further processing. merge Merge two profile directories. -o directory --output directory Set the output profile directory. Default output directory name is merged_profile. -v --verbose Set the verbose mode. -w w1,w2 --weight w1,w2 Set the merge weights of the directory1 and directory2, respectively. The default weights are 1 for both. rewrite Read the specified profile directory and rewrite to a new directory. -n long_long_value --normalize Normalize the profile. The specified value is the max counter value in the new profile. -o directory --output directory Set the output profile directory. Default output name is rewrite_profile. -s float_or_simple-frac_value --scale float_or_simple-frac_value Scale the profile counters. The specified value can be in floating point value, or simple fraction value form, such 1, 2, 2/3, and 5/3. -v --verbose Set the verbose mode. overlap Compute the overlap score between the two specified profile directories. The overlap score is computed based on the arc profiles. It is defined as the sum of min (p1_counter[i] / p1_sum_all, p2_counter[i] / p2_sum_all), for all arc counter i, where p1_counter[i] and p2_counter[i] are two matched counters and p1_sum_all and p2_sum_all are the sum of counter values in profile 1 and profile 2, respectively. -f --function Print function level overlap score. -F --fullname Print full gcda filename. -h --hotonly Only print info for hot objects/functions. -o --object Print object level overlap score. -t float --hot_threshold Set the threshold for hot counter value. -v --verbose Set the verbose mode. ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Top Link: next: Gcov-dump Intro Link: prev: Invoking Gcov-tool ────────────────────────────────────────────────────────────────────────────── 12 gcov-dump—an Offline Gcda and Gcno Profile Dump Tool • Gcov-dump Intro: Introduction to gcov-dump. • Invoking Gcov-dump: How to use gcov-dump. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Gcov-dump Link: next: Invoking Gcov-dump Link: prev: Gcov-dump ────────────────────────────────────────────────────────────────────────────── 12.1 Introduction to gcov-dump gcov-dump is a tool you can use in conjunction with GCC to dump content of gcda and gcno profile files offline. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Gcov-dump Link: next: Trouble Link: prev: Gcov-dump Intro ────────────────────────────────────────────────────────────────────────────── 12.2 Invoking gcov-dump Usage: gcov-dump [OPTION] ... gcovfiles gcov-dump accepts the following options: -h --help Display help about using gcov-dump (on the standard output), and exit without doing any further processing. -l --long Dump content of records. -p --positions Dump positions of records. -v --version Display the gcov-dump version number (on the standard output), and exit without doing any further processing. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Top Link: next: Actual Bugs Link: prev: Invoking Gcov-dump ────────────────────────────────────────────────────────────────────────────── 13 Known Causes of Trouble with GCC This section describes known problems that affect users of GCC. Most of these are not GCC bugs per se—if they were, we would fix them. But the result for a user may be like the result of a bug. Some of these problems are due to bugs in other software, some are missing features that are too much work to add, and some are places where people's opinions differ as to what is best. • Actual Bugs: Bugs we will fix later. • Interoperation: Problems using GCC with other compilers, and with certain linkers, assemblers and debuggers. • Incompatibilities: GCC is incompatible with traditional C. • Fixed Headers: GCC uses corrected versions of system header files. This is necessary, but doesn't always work smoothly. • Standard Libraries: GCC uses the system C library, which might not be compliant with the ISO C standard. • Disappointments: Regrettable things we cannot change, but not quite bugs. • C++ Misunderstandings: Common misunderstandings with GNU C++. • Non-bugs: Things we think are right, but some others disagree. • Warnings and Errors: Which problems in your code get warnings, and which get errors. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Trouble Link: next: Incompatibilities Link: prev: Actual Bugs Next: Incompatibilities, Previous: Actual Bugs, Up: Trouble ────────────────────────────────────────────────────────────────────────────── 13.2 Interoperation This section lists various difficulties encountered in using GCC together with other compilers or with the assemblers, linkers, libraries and debuggers on certain systems. * On many platforms, GCC supports a different ABI for C++ than do other compilers, so the object files compiled by GCC cannot be used with object files generated by another C++ compiler. An area where the difference is most apparent is name mangling. The use of different name mangling is intentional, to protect you from more subtle problems. Compilers differ as to many internal details of C++ implementation, including: how class instances are laid out, how multiple inheritance is implemented, and how virtual function calls are handled. If the name encoding were made the same, your programs would link against libraries provided from other compilers—but the programs would then crash when run. Incompatible libraries are then detected at link time, rather than at run time. * On some BSD systems, including some versions of Ultrix, use of profiling causes static variable destructors (currently used only in C++) not to be run. * On a SPARC, GCC aligns all values of type double on an 8-byte boundary, and it expects every double to be so aligned. The Sun compiler usually gives double values 8-byte alignment, with one exception: function arguments of type double may not be aligned. As a result, if a function compiled with Sun CC takes the address of an argument of type double and passes this pointer of type double * to a function compiled with GCC, dereferencing the pointer may cause a fatal signal. One way to solve this problem is to compile your entire program with GCC. Another solution is to modify the function that is compiled with Sun CC to copy the argument into a local variable; local variables are always properly aligned. A third solution is to modify the function that uses the pointer to dereference it via the following function access_double instead of directly with '*': inline double access_double (double *unaligned_ptr) { union d2i { double d; int i[2]; }; union d2i *p = (union d2i *) unaligned_ptr; union d2i u; u.i[0] = p->i[0]; u.i[1] = p->i[1]; return u.d; } Storing into the pointer can be done likewise with the same union. * On Solaris, the malloc function in the libmalloc.a library may allocate memory that is only 4 byte aligned. Since GCC on the SPARC assumes that doubles are 8 byte aligned, this may result in a fatal signal if doubles are stored in memory allocated by the libmalloc.a library. The solution is to not use the libmalloc.a library. Use instead malloc and related functions from libc.a; they do not have this problem. * On the HP PA machine, ADB sometimes fails to work on functions compiled with GCC. Specifically, it fails to work on functions that use alloca or variable-size arrays. This is because GCC doesn't generate HP-UX unwind descriptors for such functions. It may even be impossible to generate them. * Debugging (-g) is not supported on the HP PA machine, unless you use the preliminary GNU tools. * Taking the address of a label may generate errors from the HP-UX PA assembler. GAS for the PA does not have this problem. * Using floating point parameters for indirect calls to static functions will not work when using the HP assembler. There simply is no way for GCC to specify what registers hold arguments for static functions when using the HP assembler. GAS for the PA does not have this problem. * In extremely rare cases involving some very large functions you may receive errors from the HP linker complaining about an out of bounds unconditional branch offset. This used to occur more often in previous versions of GCC, but is now exceptionally rare. If you should run into it, you can work around by making your function smaller. * GCC compiled code sometimes emits warnings from the HP-UX assembler of the form: (warning) Use of GR3 when frame >= 8192 may cause conflict. These warnings are harmless and can be safely ignored. * In extremely rare cases involving some very large functions you may receive errors from the AIX Assembler complaining about a displacement that is too large. If you should run into it, you can work around by making your function smaller. * The libstdc++.a library in GCC relies on the SVR4 dynamic linker semantics which merges global symbols between libraries and applications, especially necessary for C++ streams functionality. This is not the default behavior of AIX shared libraries and dynamic linking. libstdc++.a is built on AIX with “runtime-linking” enabled so that symbol merging can occur. To utilize this feature, the application linked with libstdc++.a must include the -Wl,-brtl flag on the link line. G++ cannot impose this because this option may interfere with the semantics of the user program and users may not always use 'g++' to link his or her application. Applications are not required to use the -Wl,-brtl flag on the link line—the rest of the libstdc++.a library which is not dependent on the symbol merging semantics will continue to function correctly. * An application can interpose its own definition of functions for functions invoked by libstdc++.a with “runtime-linking” enabled on AIX. To accomplish this the application must be linked with “runtime-linking” option and the functions explicitly must be exported by the application (-Wl,-brtl,-bE:exportfile). * AIX on the RS/6000 provides support (NLS) for environments outside of the United States. Compilers and assemblers use NLS to support locale-specific representations of various objects including floating-point numbers ('.' vs ',' for separating decimal fractions). There have been problems reported where the library linked with GCC does not produce the same floating-point formats that the assembler accepts. If you have this problem, set the LANG environment variable to 'C' or 'En_US'. * Even if you specify -fdollars-in-identifiers, you cannot successfully use '$' in identifiers on the RS/6000 due to a restriction in the IBM assembler. GAS supports these identifiers. ────────────────────────────────────────────────────────────────────────────── Next: Incompatibilities, Previous: Actual Bugs, Up: Trouble Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Trouble Link: next: Fixed Headers Link: prev: Interoperation Next: Fixed Headers, Previous: Interoperation, Up: Trouble ────────────────────────────────────────────────────────────────────────────── 13.3 Incompatibilities of GCC There are several noteworthy incompatibilities between GNU C and K&R (non-ISO) versions of C. * GCC normally makes string constants read-only. If several identical-looking string constants are used, GCC stores only one copy of the string. One consequence is that you cannot call mktemp with a string constant argument. The function mktemp always alters the string its argument points to. Another consequence is that sscanf does not work on some very old systems when passed a string constant as its format control string or input. This is because sscanf incorrectly tries to write into the string constant. Likewise fscanf and scanf. The solution to these problems is to change the program to use char-array variables with initialization strings for these purposes instead of string constants. * -2147483648 is positive. This is because 2147483648 cannot fit in the type int, so (following the ISO C rules) its data type is unsigned long int. Negating this value yields 2147483648 again. * GCC does not substitute macro arguments when they appear inside of string constants. For example, the following macro in GCC #define foo(a) "a" will produce output "a" regardless of what the argument a is. * When you use setjmp and longjmp, the only automatic variables guaranteed to remain valid are those declared volatile. This is a consequence of automatic register allocation. Consider this function: jmp_buf j; foo () { int a, b; a = fun1 (); if (setjmp (j)) return a; a = fun2 (); /* longjmp (j) may occur in fun3. */ return a + fun3 (); } Here a may or may not be restored to its first value when the longjmp occurs. If a is allocated in a register, then its first value is restored; otherwise, it keeps the last value stored in it. If you use the -W option with the -O option, you will get a warning when GCC thinks such a problem might be possible. * Programs that use preprocessing directives in the middle of macro arguments do not work with GCC. For example, a program like this will not work: foobar ( #define luser hack) ISO C does not permit such a construct. * K&R compilers allow comments to cross over an inclusion boundary (i.e. started in an include file and ended in the including file). * Declarations of external variables and functions within a block apply only to the block containing the declaration. In other words, they have the same scope as any other declaration in the same place. In some other C compilers, an extern declaration affects all the rest of the file even if it happens within a block. * In traditional C, you can combine long, etc., with a typedef name, as shown here: typedef int foo; typedef long foo bar; In ISO C, this is not allowed: long and other type modifiers require an explicit int. * PCC allows typedef names to be used as function parameters. * Traditional C allows the following erroneous pair of declarations to appear together in a given scope: typedef int foo; typedef foo foo; * GCC treats all characters of identifiers as significant. According to K&R-1 (2.2), “No more than the first eight characters are significant, although more may be used.”. Also according to K&R-1 (2.2), “An identifier is a sequence of letters and digits; the first character must be a letter. The underscore _ counts as a letter.”, but GCC also allows dollar signs in identifiers. * PCC allows whitespace in the middle of compound assignment operators such as '+='. GCC, following the ISO standard, does not allow this. * GCC complains about unterminated character constants inside of preprocessing conditionals that fail. Some programs have English comments enclosed in conditionals that are guaranteed to fail; if these comments contain apostrophes, GCC will probably report an error. For example, this code would produce an error: #if 0 You can't expect this to work. #endif The best solution to such a problem is to put the text into an actual C comment delimited by '/*…*/'. * Many user programs contain the declaration 'long time ();'. In the past, the system header files on many systems did not actually declare time, so it did not matter what type your program declared it to return. But in systems with ISO C headers, time is declared to return time_t, and if that is not the same as long, then 'long time ();' is erroneous. The solution is to change your program to use appropriate system headers ( on systems with ISO C headers) and not to declare time if the system header files declare it, or failing that to use time_t as the return type of time. * When compiling functions that return float, PCC converts it to a double. GCC actually returns a float. If you are concerned with PCC compatibility, you should declare your functions to return double; you might as well say what you mean. * When compiling functions that return structures or unions, GCC output code normally uses a method different from that used on most versions of Unix. As a result, code compiled with GCC cannot call a structure-returning function compiled with PCC, and vice versa. The method used by GCC is as follows: a structure or union which is 1, 2, 4 or 8 bytes long is returned like a scalar. A structure or union with any other size is stored into an address supplied by the caller (usually in a special, fixed register, but on some machines it is passed on the stack). The target hook TARGET_STRUCT_VALUE_RTX tells GCC where to pass this address. By contrast, PCC on most target machines returns structures and unions of any size by copying the data into an area of static storage, and then returning the address of that storage as if it were a pointer value. The caller must copy the data from that memory area to the place where the value is wanted. GCC does not use this method because it is slower and nonreentrant. On some newer machines, PCC uses a reentrant convention for all structure and union returning. GCC on most of these machines uses a compatible convention when returning structures and unions in memory, but still returns small structures and unions in registers. You can tell GCC to use a compatible convention for all structure and union returning with the option -fpcc-struct-return. * GCC complains about program fragments such as '0x74ae-0x4000' which appear to be two hexadecimal constants separated by the minus operator. Actually, this string is a single preprocessing token. Each such token must correspond to one token in C. Since this does not, GCC prints an error message. Although it may appear obvious that what is meant is an operator and two values, the ISO C standard specifically requires that this be treated as erroneous. A preprocessing token is a preprocessing number if it begins with a digit and is followed by letters, underscores, digits, periods and 'e+', 'e-', 'E+', 'E-', 'p+', 'p-', 'P+', or 'P-' character sequences. (In strict C90 mode, the sequences 'p+', 'p-', 'P+' and 'P-' cannot appear in preprocessing numbers.) To make the above program fragment valid, place whitespace in front of the minus sign. This whitespace will end the preprocessing number. ────────────────────────────────────────────────────────────────────────────── Next: Fixed Headers, Previous: Interoperation, Up: Trouble Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Trouble Link: next: Standard Libraries Link: prev: Incompatibilities Next: Standard Libraries, Previous: Incompatibilities, Up: Trouble ────────────────────────────────────────────────────────────────────────────── 13.4 Fixed Header Files GCC needs to install corrected versions of some system header files. This is because most target systems have some header files that won't work with GCC unless they are changed. Some have bugs, some are incompatible with ISO C, and some depend on special features of other compilers. Installing GCC automatically creates and installs the fixed header files, by running a program called fixincludes. Normally, you don't need to pay attention to this. But there are cases where it doesn't do the right thing automatically. * If you update the system's header files, such as by installing a new system version, the fixed header files of GCC are not automatically updated. They can be updated using the mkheaders script installed in libexecdir/gcc/target/version/install-tools/. * On some systems, header file directories contain machine-specific symbolic links in certain places. This makes it possible to share most of the header files among hosts running the same version of the system on different machine models. The programs that fix the header files do not understand this special way of using symbolic links; therefore, the directory of fixed header files is good only for the machine model used to build it. It is possible to make separate sets of fixed header files for the different machine models, and arrange a structure of symbolic links so as to use the proper set, but you'll have to do this by hand. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Trouble Link: next: Disappointments Link: prev: Fixed Headers Next: Disappointments, Previous: Fixed Headers, Up: Trouble ────────────────────────────────────────────────────────────────────────────── 13.5 Standard Libraries GCC by itself attempts to be a conforming freestanding implementation. See Language Standards Supported by GCC, for details of what this means. Beyond the library facilities required of such an implementation, the rest of the C library is supplied by the vendor of the operating system. If that C library doesn't conform to the C standards, then your programs might get warnings (especially when using -Wall) that you don't expect. For example, the sprintf function on SunOS 4.1.3 returns char * while the C standard says that sprintf returns an int. The fixincludes program could make the prototype for this function match the Standard, but that would be wrong, since the function will still return char *. If you need a Standard compliant library, then you need to find one, as GCC does not provide one. The GNU C library (called glibc) provides ISO C, POSIX, BSD, SystemV and X/Open compatibility for GNU/Linux and HURD-based GNU systems; no recent version of it supports other systems, though some very old versions did. Version 2.2 of the GNU C library includes nearly complete C99 support. You could also ask your operating system vendor if newer libraries are available. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Trouble Link: next: C++ Misunderstandings Link: prev: Standard Libraries Next: C++ Misunderstandings, Previous: Standard Libraries, Up: Trouble ────────────────────────────────────────────────────────────────────────────── 13.6 Disappointments and Misunderstandings These problems are perhaps regrettable, but we don't know any practical way around them. * Certain local variables aren't recognized by debuggers when you compile with optimization. This occurs because sometimes GCC optimizes the variable out of existence. There is no way to tell the debugger how to compute the value such a variable “would have had”, and it is not clear that would be desirable anyway. So GCC simply does not mention the eliminated variable when it writes debugging information. You have to expect a certain amount of disagreement between the executable and your source code, when you use optimization. * Users often think it is a bug when GCC reports an error for code like this: int foo (struct mumble *); struct mumble { … }; int foo (struct mumble *x) { … } This code really is erroneous, because the scope of struct mumble in the prototype is limited to the argument list containing it. It does not refer to the struct mumble defined with file scope immediately below—they are two unrelated types with similar names in different scopes. But in the definition of foo, the file-scope type is used because that is available to be inherited. Thus, the definition and the prototype do not match, and you get an error. This behavior may seem silly, but it's what the ISO standard specifies. It is easy enough for you to make your code work by moving the definition of struct mumble above the prototype. It's not worth being incompatible with ISO C just to avoid an error for the example shown above. * Accesses to bit-fields even in volatile objects works by accessing larger objects, such as a byte or a word. You cannot rely on what size of object is accessed in order to read or write the bit-field; it may even vary for a given bit-field according to the precise usage. If you care about controlling the amount of memory that is accessed, use volatile but do not use bit-fields. * GCC comes with shell scripts to fix certain known problems in system header files. They install corrected copies of various header files in a special directory where only GCC will normally look for them. The scripts adapt to various systems by searching all the system header files for the problem cases that we know about. If new system header files are installed, nothing automatically arranges to update the corrected header files. They can be updated using the mkheaders script installed in libexecdir/gcc/target/version/install-tools/. * On 68000 and x86 systems, for instance, you can get paradoxical results if you test the precise values of floating point numbers. For example, you can find that a floating point value which is not a NaN is not equal to itself. This results from the fact that the floating point registers hold a few more bits of precision than fit in a double in memory. Compiled code moves values between memory and floating point registers at its convenience, and moving them into memory truncates them. You can partially avoid this problem by using the -ffloat-store option (see Optimize Options). * On AIX and other platforms without weak symbol support, templates need to be instantiated explicitly and symbols for static members of templates will not be generated. * On AIX, GCC scans object files and library archives for static constructors and destructors when linking an application before the linker prunes unreferenced symbols. This is necessary to prevent the AIX linker from mistakenly assuming that static constructor or destructor are unused and removing them before the scanning can occur. All static constructors and destructors found will be referenced even though the modules in which they occur may not be used by the program. This may lead to both increased executable size and unexpected symbol references. ────────────────────────────────────────────────────────────────────────────── Next: C++ Misunderstandings, Previous: Standard Libraries, Up: Trouble Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Trouble Link: next: Static Definitions Link: prev: Disappointments ────────────────────────────────────────────────────────────────────────────── 13.7 Common Misunderstandings with GNU C++ C++ is a complex language and an evolving one, and its standard definition (the ISO C++ standard) was only recently completed. As a result, your C++ compiler may occasionally surprise you, even when its behavior is correct. This section discusses some areas that frequently give rise to questions of this sort. • Static Definitions: Static member declarations are not definitions • Name lookup: Name lookup, templates, and accessing members of base classes • Temporaries: Temporaries may vanish before you expect • Copy Assignment: Copy Assignment operators copy virtual bases twice Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C++ Misunderstandings Link: next: Name lookup Link: prev: C++ Misunderstandings ────────────────────────────────────────────────────────────────────────────── 13.7.1 Declare and Define Static Members When a class has static data members, it is not enough to declare the static member; you must also define it. For example: class Foo { … void method(); static int bar; }; This declaration only establishes that the class Foo has an int named Foo::bar, and a member function named Foo::method. But you still need to define both method and bar elsewhere. According to the ISO standard, you must supply an initializer in one (and only one) source file, such as: int Foo::bar = 0; Other C++ compilers may not correctly implement the standard behavior. As a result, when you switch to g++ from one of these compilers, you may discover that a program that appeared to work correctly in fact does not conform to the standard: g++ reports as undefined symbols any static data members that lack definitions. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C++ Misunderstandings Link: next: Temporaries Link: prev: Static Definitions Next: Temporaries, Previous: Static Definitions, Up: C++ Misunderstandings ────────────────────────────────────────────────────────────────────────────── 13.7.2 Name Lookup, Templates, and Accessing Members of Base Classes The C++ standard prescribes that all names that are not dependent on template parameters are bound to their present definitions when parsing a template function or class.^5 Only names that are dependent are looked up at the point of instantiation. For example, consider void foo(double); struct A { template void f () { foo (1); // 1 int i = N; // 2 T t; t.bar(); // 3 foo (t); // 4 } static const int N; }; Here, the names foo and N appear in a context that does not depend on the type of T. The compiler will thus require that they are defined in the context of use in the template, not only before the point of instantiation, and will here use ::foo(double) and A::N, respectively. In particular, it will convert the integer value to a double when passing it to ::foo(double). Conversely, bar and the call to foo in the fourth marked line are used in contexts that do depend on the type of T, so they are only looked up at the point of instantiation, and you can provide declarations for them after declaring the template, but before instantiating it. In particular, if you instantiate A::f, the last line will call an overloaded ::foo(int) if one was provided, even if after the declaration of struct A. This distinction between lookup of dependent and non-dependent names is called two-stage (or dependent) name lookup. G++ implements it since version 3.4. Two-stage name lookup sometimes leads to situations with behavior different from non-template codes. The most common is probably this: template struct Base { int i; }; template struct Derived : public Base { int get_i() { return i; } }; In get_i(), i is not used in a dependent context, so the compiler will look for a name declared at the enclosing namespace scope (which is the global scope here). It will not look into the base class, since that is dependent and you may declare specializations of Base even after declaring Derived, so the compiler cannot really know what i would refer to. If there is no global variable i, then you will get an error message. In order to make it clear that you want the member of the base class, you need to defer lookup until instantiation time, at which the base class is known. For this, you need to access i in a dependent context, by either using this->i (remember that this is of type Derived*, so is obviously dependent), or using Base::i. Alternatively, Base::i might be brought into scope by a using-declaration. Another, similar example involves calling member functions of a base class: template struct Base { int f(); }; template struct Derived : Base { int g() { return f(); }; }; Again, the call to f() is not dependent on template arguments (there are no arguments that depend on the type T, and it is also not otherwise specified that the call should be in a dependent context). Thus a global declaration of such a function must be available, since the one in the base class is not visible until instantiation time. The compiler will consequently produce the following error message: x.cc: In member function `int Derived::g()': x.cc:6: error: there are no arguments to `f' that depend on a template parameter, so a declaration of `f' must be available x.cc:6: error: (if you use `-fpermissive', G++ will accept your code, but allowing the use of an undeclared name is deprecated) To make the code valid either use this->f(), or Base::f(). Using the -fpermissive flag will also let the compiler accept the code, by marking all function calls for which no declaration is visible at the time of definition of the template for later lookup at instantiation time, as if it were a dependent call. We do not recommend using -fpermissive to work around invalid code, and it will also only catch cases where functions in base classes are called, not where variables in base classes are used (as in the example above). Note that some compilers (including G++ versions prior to 3.4) get these examples wrong and accept above code without an error. Those compilers do not implement two-stage name lookup correctly. Footnotes (5) The C++ standard just uses the term “dependent” for names that depend on the type or value of template parameters. This shorter term will also be used in the rest of this section. ────────────────────────────────────────────────────────────────────────────── Next: Temporaries, Previous: Static Definitions, Up: C++ Misunderstandings Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C++ Misunderstandings Link: next: Copy Assignment Link: prev: Name lookup Next: Copy Assignment, Previous: Name lookup, Up: C++ Misunderstandings ────────────────────────────────────────────────────────────────────────────── 13.7.3 Temporaries May Vanish Before You Expect It is dangerous to use pointers or references to portions of a temporary object. The compiler may very well delete the object before you expect it to, leaving a pointer to garbage. The most common place where this problem crops up is in classes like string classes, especially ones that define a conversion function to type char * or const char *—which is one reason why the standard string class requires you to call the c_str member function. However, any class that returns a pointer to some internal structure is potentially subject to this problem. For example, a program may use a function strfunc that returns string objects, and another function charfunc that operates on pointers to char: string strfunc (); void charfunc (const char *); void f () { const char *p = strfunc().c_str(); … charfunc (p); … charfunc (p); } In this situation, it may seem reasonable to save a pointer to the C string returned by the c_str member function and use that rather than call c_str repeatedly. However, the temporary string created by the call to strfunc is destroyed after p is initialized, at which point p is left pointing to freed memory. Code like this may run successfully under some other compilers, particularly obsolete cfront-based compilers that delete temporaries along with normal local variables. However, the GNU C++ behavior is standard-conforming, so if your program depends on late destruction of temporaries it is not portable. The safe way to write such code is to give the temporary a name, which forces it to remain until the end of the scope of the name. For example: const string& tmp = strfunc (); charfunc (tmp.c_str ()); ────────────────────────────────────────────────────────────────────────────── Next: Copy Assignment, Previous: Name lookup, Up: C++ Misunderstandings Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: C++ Misunderstandings Link: next: Non-bugs Link: prev: Temporaries ────────────────────────────────────────────────────────────────────────────── 13.7.4 Implicit Copy-Assignment for Virtual Bases When a base class is virtual, only one subobject of the base class belongs to each full object. Also, the constructors and destructors are invoked only once, and called from the most-derived class. However, such objects behave unspecified when being assigned. For example: struct Base{ char *name; Base(char *n) : name(strdup(n)){} Base& operator= (const Base& other){ free (name); name = strdup (other.name); } }; struct A:virtual Base{ int val; A():Base("A"){} }; struct B:virtual Base{ int bval; B():Base("B"){} }; struct Derived:public A, public B{ Derived():Base("Derived"){} }; void func(Derived &d1, Derived &d2) { d1 = d2; } The C++ standard specifies that 'Base::Base' is only called once when constructing or copy-constructing a Derived object. It is unspecified whether 'Base::operator=' is called more than once when the implicit copy-assignment for Derived objects is invoked (as it is inside 'func' in the example). G++ implements the “intuitive” algorithm for copy-assignment: assign all direct bases, then assign all members. In that algorithm, the virtual base subobject can be encountered more than once. In the example, copying proceeds in the following order: 'val', 'name' (via strdup), 'bval', and 'name' again. If application code relies on copy-assignment, a user-defined copy-assignment operator removes any uncertainties. With such an operator, the application can define whether and how the virtual base subobject is assigned. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Trouble Link: next: Warnings and Errors Link: prev: Copy Assignment Next: Warnings and Errors, Previous: C++ Misunderstandings, Up: Trouble ────────────────────────────────────────────────────────────────────────────── 13.8 Certain Changes We Don't Want to Make This section lists changes that people frequently request, but which we do not make because we think GCC is better without them. * Checking the number and type of arguments to a function which has an old-fashioned definition and no prototype. Such a feature would work only occasionally—only for calls that appear in the same file as the called function, following the definition. The only way to check all calls reliably is to add a prototype for the function. But adding a prototype eliminates the motivation for this feature. So the feature is not worthwhile. * Warning about using an expression whose type is signed as a shift count. Shift count operands are probably signed more often than unsigned. Warning about this would cause far more annoyance than good. * Warning about assigning a signed value to an unsigned variable. Such assignments must be very common; warning about them would cause more annoyance than good. * Warning when a non-void function value is ignored. C contains many standard functions that return a value that most programs choose to ignore. One obvious example is printf. Warning about this practice only leads the defensive programmer to clutter programs with dozens of casts to void. Such casts are required so frequently that they become visual noise. Writing those casts becomes so automatic that they no longer convey useful information about the intentions of the programmer. For functions where the return value should never be ignored, use the warn_unused_result function attribute (see Function Attributes). * Making -fshort-enums the default. This would cause storage layout to be incompatible with most other C compilers. And it doesn't seem very important, given that you can get the same result in other ways. The case where it matters most is when the enumeration-valued object is inside a structure, and in that case you can specify a field width explicitly. * Making bit-fields unsigned by default on particular machines where “the ABI standard” says to do so. The ISO C standard leaves it up to the implementation whether a bit-field declared plain int is signed or not. This in effect creates two alternative dialects of C. The GNU C compiler supports both dialects; you can specify the signed dialect with -fsigned-bitfields and the unsigned dialect with -funsigned-bitfields. However, this leaves open the question of which dialect to use by default. Currently, the preferred dialect makes plain bit-fields signed, because this is simplest. Since int is the same as signed int in every other context, it is cleanest for them to be the same in bit-fields as well. Some computer manufacturers have published Application Binary Interface standards which specify that plain bit-fields should be unsigned. It is a mistake, however, to say anything about this issue in an ABI. This is because the handling of plain bit-fields distinguishes two dialects of C. Both dialects are meaningful on every type of machine. Whether a particular object file was compiled using signed bit-fields or unsigned is of no concern to other object files, even if they access the same bit-fields in the same data structures. A given program is written in one or the other of these two dialects. The program stands a chance to work on most any machine if it is compiled with the proper dialect. It is unlikely to work at all if compiled with the wrong dialect. Many users appreciate the GNU C compiler because it provides an environment that is uniform across machines. These users would be inconvenienced if the compiler treated plain bit-fields differently on certain machines. Occasionally users write programs intended only for a particular machine type. On these occasions, the users would benefit if the GNU C compiler were to support by default the same dialect as the other compilers on that machine. But such applications are rare. And users writing a program to run on more than one type of machine cannot possibly benefit from this kind of compatibility. This is why GCC does and will treat plain bit-fields in the same fashion on all types of machines (by default). There are some arguments for making bit-fields unsigned by default on all machines. If, for example, this becomes a universal de facto standard, it would make sense for GCC to go along with it. This is something to be considered in the future. (Of course, users strongly concerned about portability should indicate explicitly in each bit-field whether it is signed or not. In this way, they write programs which have the same meaning in both C dialects.) * Undefining __STDC__ when -ansi is not used. Currently, GCC defines __STDC__ unconditionally. This provides good results in practice. Programmers normally use conditionals on __STDC__ to ask whether it is safe to use certain features of ISO C, such as function prototypes or ISO token concatenation. Since plain gcc supports all the features of ISO C, the correct answer to these questions is “yes”. Some users try to use __STDC__ to check for the availability of certain library facilities. This is actually incorrect usage in an ISO C program, because the ISO C standard says that a conforming freestanding implementation should define __STDC__ even though it does not have the library facilities. 'gcc -ansi -pedantic' is a conforming freestanding implementation, and it is therefore required to define __STDC__, even though it does not come with an ISO C library. Sometimes people say that defining __STDC__ in a compiler that does not completely conform to the ISO C standard somehow violates the standard. This is illogical. The standard is a standard for compilers that claim to support ISO C, such as 'gcc -ansi'—not for other compilers such as plain gcc. Whatever the ISO C standard says is relevant to the design of plain gcc without -ansi only for pragmatic reasons, not as a requirement. GCC normally defines __STDC__ to be 1, and in addition defines __STRICT_ANSI__ if you specify the -ansi option, or a -std option for strict conformance to some version of ISO C. On some hosts, system include files use a different convention, where __STDC__ is normally 0, but is 1 if the user specifies strict conformance to the C Standard. GCC follows the host convention when processing system include files, but when processing user files it follows the usual GNU C convention. * Undefining __STDC__ in C++. Programs written to compile with C++-to-C translators get the value of __STDC__ that goes with the C compiler that is subsequently used. These programs must test __STDC__ to determine what kind of C preprocessor that compiler uses: whether they should concatenate tokens in the ISO C fashion or in the traditional fashion. These programs work properly with GNU C++ if __STDC__ is defined. They would not work otherwise. In addition, many header files are written to provide prototypes in ISO C but not in traditional C. Many of these header files can work without change in C++ provided __STDC__ is defined. If __STDC__ is not defined, they will all fail, and will all need to be changed to test explicitly for C++ as well. * Deleting “empty” loops. Historically, GCC has not deleted “empty” loops under the assumption that the most likely reason you would put one in a program is to have a delay, so deleting them will not make real programs run any faster. However, the rationale here is that optimization of a nonempty loop cannot produce an empty one. This held for carefully written C compiled with less powerful optimizers but is not always the case for carefully written C++ or with more powerful optimizers. Thus GCC will remove operations from loops whenever it can determine those operations are not externally visible (apart from the time taken to execute them, of course). In case the loop can be proved to be finite, GCC will also remove the loop itself. Be aware of this when performing timing tests, for instance the following loop can be completely removed, provided some_expression can provably not change any global state. { int sum = 0; int ix; for (ix = 0; ix != 10000; ix++) sum += some_expression; } Even though sum is accumulated in the loop, no use is made of that summation, so the accumulation can be removed. * Making side effects happen in the same order as in some other compiler. It is never safe to depend on the order of evaluation of side effects. For example, a function call like this may very well behave differently from one compiler to another: void func (int, int); int i = 2; func (i++, i++); There is no guarantee (in either the C or the C++ standard language definitions) that the increments will be evaluated in any particular order. Either increment might happen first. func might get the arguments '2, 3', or it might get '3, 2', or even '2, 2'. * Making certain warnings into errors by default. Some ISO C testsuites report failure when the compiler does not produce an error message for a certain program. ISO C requires a “diagnostic” message for certain kinds of invalid programs, but a warning is defined by GCC to count as a diagnostic. If GCC produces a warning but not an error, that is correct ISO C support. If testsuites call this “failure”, they should be run with the GCC option -pedantic-errors, which will turn these warnings into errors. ────────────────────────────────────────────────────────────────────────────── Next: Warnings and Errors, Previous: C++ Misunderstandings, Up: Trouble Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Trouble Link: next: Bugs Link: prev: Non-bugs ────────────────────────────────────────────────────────────────────────────── 13.9 Warning Messages and Error Messages The GNU compiler can produce two kinds of diagnostics: errors and warnings. Each kind has a different purpose: * Errors report problems that make it impossible to compile your program. GCC reports errors with the source file name and line number where the problem is apparent. * Warnings report other unusual conditions in your code that may indicate a problem, although compilation can (and does) proceed. Warning messages also report the source file name and line number, but include the text 'warning:' to distinguish them from error messages. Warnings may indicate danger points where you should check to make sure that your program really does what you intend; or the use of obsolete features; or the use of nonstandard features of GNU C or C++. Many warnings are issued only if you ask for them, with one of the -W options (for instance, -Wall requests a variety of useful warnings). GCC always tries to compile your program if possible; it never gratuitously rejects a program whose meaning is clear merely because (for instance) it fails to conform to a standard. In some cases, however, the C and C++ standards specify that certain extensions are forbidden, and a diagnostic must be issued by a conforming compiler. The -pedantic option tells GCC to issue warnings in such cases; -pedantic-errors says to make them errors instead. This does not mean that all non-ISO constructs get warnings or errors. See Options to Request or Suppress Warnings, for more detail on these and related command-line options. ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Top Link: next: Bug Criteria Link: prev: Warnings and Errors ────────────────────────────────────────────────────────────────────────────── 14 Reporting Bugs Your bug reports play an essential role in making GCC reliable. When you encounter a problem, the first thing to do is to see if it is already known. See Trouble. If it isn't known, then you should report the problem. • Criteria: Have you really found a bug? • Reporting: How to report a bug effectively. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Bugs Link: next: Bug Reporting Link: prev: Bugs ────────────────────────────────────────────────────────────────────────────── 14.1 Have You Found a Bug? If you are not sure whether you have found a bug, here are some guidelines: * If the compiler gets a fatal signal, for any input whatever, that is a compiler bug. Reliable compilers never crash. * If the compiler produces invalid assembly code, for any input whatever (except an asm statement), that is a compiler bug, unless the compiler reports errors (not just warnings) which would ordinarily prevent the assembler from being run. * If the compiler produces valid assembly code that does not correctly execute the input source code, that is a compiler bug. However, you must double-check to make sure, because you may have a program whose behavior is undefined, which happened by chance to give the desired results with another C or C++ compiler. For example, in many nonoptimizing compilers, you can write 'x;' at the end of a function instead of 'return x;', with the same results. But the value of the function is undefined if return is omitted; it is not a bug when GCC produces different results. Problems often result from expressions with two increment operators, as in f (*p++, *p++). Your previous compiler might have interpreted that expression the way you intended; GCC might interpret it another way. Neither compiler is wrong. The bug is in your code. After you have localized the error to a single source line, it should be easy to check for these things. If your program is correct and well defined, you have found a compiler bug. * If the compiler produces an error message for valid input, that is a compiler bug. * If the compiler does not produce an error message for invalid input, that is a compiler bug. However, you should note that your idea of “invalid input” might be someone else's idea of “an extension” or “support for traditional practice”. * If you are an experienced user of one of the languages GCC supports, your suggestions for improvement of GCC are welcome in any case. ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Bugs Link: next: Service Link: prev: Bug Criteria ────────────────────────────────────────────────────────────────────────────── 14.2 How and Where to Report Bugs Bugs should be reported to the bug database at http://gcc.gnu.org/bugs/. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Top Link: next: Contributing Link: prev: Bug Reporting ────────────────────────────────────────────────────────────────────────────── 15 How To Get Help with GCC If you need help installing, using or changing GCC, there are two ways to find it: * Send a message to a suitable network mailing list. First try gcc-help@gcc.gnu.org (for help installing or using GCC), and if that brings no response, try gcc@gcc.gnu.org. For help changing GCC, ask gcc@gcc.gnu.org. If you think you have found a bug in GCC, please report it following the instructions at see Bug Reporting. * Look in the service directory for someone who might help you for a fee. The service directory is found at https://www.fsf.org/resources/service. For further information, see http://gcc.gnu.org/faq.html#support. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Top Link: next: Funding Link: prev: Service ────────────────────────────────────────────────────────────────────────────── 16 Contributing to GCC Development If you would like to help pretest GCC releases to assure they work well, current development sources are available via Git (see http://gcc.gnu.org/git.html). Source and binary snapshots are also available for FTP; see http://gcc.gnu.org/snapshots.html. If you would like to work on improvements to GCC, please read the advice at these URLs: http://gcc.gnu.org/contribute.html http://gcc.gnu.org/contributewhy.html for information on how to make useful contributions and avoid duplication of effort. Suggested projects are listed at http://gcc.gnu.org/projects/. Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Top Link: next: GNU Project Link: prev: Contributing ────────────────────────────────────────────────────────────────────────────── Funding Free Software If you want to have more free software a few years from now, it makes sense for you to help encourage people to contribute funds for its development. The most effective approach known is to encourage commercial redistributors to donate. Users of free software systems can boost the pace of development by encouraging for-a-fee distributors to donate part of their selling price to free software developers—the Free Software Foundation, and others. The way to convince distributors to do this is to demand it and expect it from them. So when you compare distributors, judge them partly by how much they give to free software development. Show distributors they must compete to be the one who gives the most. To make this approach work, you must insist on numbers that you can compare, such as, “We will donate ten dollars to the Frobnitz project for each disk sold.” Don't be satisfied with a vague promise, such as “A portion of the profits are donated,” since it doesn't give a basis for comparison. Even a precise fraction “of the profits from this disk” is not very meaningful, since creative accounting and unrelated business decisions can greatly alter what fraction of the sales price counts as profit. If the price you pay is $50, ten percent of the profit is probably less than a dollar; it might be a few cents, or nothing at all. Some redistributors do development work themselves. This is useful too; but to keep everyone honest, you need to inquire how much they do, and what kind. Some kinds of development make much more long-term difference than others. For example, maintaining a separate version of a program contributes very little; maintaining the standard version of a program for the whole community contributes much. Easy new ports contribute little, since someone else would surely do them; difficult ports such as adding a new CPU to the GNU Compiler Collection contribute more; major new features or packages contribute the most. By establishing the idea that supporting further development is “the proper thing to do” when distributing free software for a fee, we can assure a steady flow of resources into making more free software. Copyright © 1994 Free Software Foundation, Inc. Verbatim copying and redistribution of this section is permitted without royalty; alteration is not permitted. ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Top Link: next: Copying Link: prev: Funding ────────────────────────────────────────────────────────────────────────────── The GNU Project and GNU/Linux The GNU Project was launched in 1984 to develop a complete Unix-like operating system which is free software: the GNU system. (GNU is a recursive acronym for “GNU's Not Unix”; it is pronounced “guh-NEW”.) Variants of the GNU operating system, which use the kernel Linux, are now widely used; though these systems are often referred to as “Linux”, they are more accurately called GNU/Linux systems. For more information, see: http://www.gnu.org/ http://www.gnu.org/gnu/linux-and-gnu.html Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Top Link: next: GNU Free Documentation License Link: prev: GNU Project Next: GNU Free Documentation License, Previous: GNU Project, Up: Top ────────────────────────────────────────────────────────────────────────────── GNU General Public License Version 3, 29 June 2007 Copyright © 2007 Free Software Foundation, Inc. http://fsf.org/ Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. Preamble The GNU General Public License is a free, copyleft license for software and other kinds of works. The licenses for most software and other practical works are designed to take away your freedom to share and change the works. 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If the disclaimer of warranty and limitation of liability provided above cannot be given local legal effect according to their terms, reviewing courts shall apply local law that most closely approximates an absolute waiver of all civil liability in connection with the Program, unless a warranty or assumption of liability accompanies a copy of the Program in return for a fee. END OF TERMS AND CONDITIONS How to Apply These Terms to Your New Programs If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms. To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively state the exclusion of warranty; and each file should have at least the “copyright” line and a pointer to where the full notice is found. one line to give the program's name and a brief idea of what it does. Copyright (C) year name of author This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/. Also add information on how to contact you by electronic and paper mail. If the program does terminal interaction, make it output a short notice like this when it starts in an interactive mode: program Copyright (C) year name of author This program comes with ABSOLUTELY NO WARRANTY; for details type 'show w'. This is free software, and you are welcome to redistribute it under certain conditions; type 'show c' for details. The hypothetical commands 'show w' and 'show c' should show the appropriate parts of the General Public License. Of course, your program's commands might be different; for a GUI interface, you would use an “about box”. You should also get your employer (if you work as a programmer) or school, if any, to sign a “copyright disclaimer” for the program, if necessary. For more information on this, and how to apply and follow the GNU GPL, see http://www.gnu.org/licenses/. The GNU General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Lesser General Public License instead of this License. But first, please read http://www.gnu.org/philosophy/why-not-lgpl.html. ────────────────────────────────────────────────────────────────────────────── Next: GNU Free Documentation License, Previous: GNU Project, Up: Top Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Top Link: next: Contributors Link: prev: Copying ────────────────────────────────────────────────────────────────────────────── GNU Free Documentation License Version 1.3, 3 November 2008 Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc. http://fsf.org/ Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. * PREAMBLE The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others. This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software. We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference. * APPLICABILITY AND DEFINITIONS This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law. A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language. 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The “Title Page” means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, “Title Page” means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text. The “publisher” means any person or entity that distributes copies of the Document to the public. A section “Entitled XYZ” means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To “Preserve the Title” of such a section when you modify the Document means that it remains a section “Entitled XYZ” according to this definition. The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License. * VERBATIM COPYING You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3. 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To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles. You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard. You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one. The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version. * COMBINING DOCUMENTS You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers. 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You must delete all sections Entitled “Endorsements.” * COLLECTIONS OF DOCUMENTS You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects. You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document. * AGGREGATION WITH INDEPENDENT WORKS A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation's users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document. If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document's Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate. * TRANSLATION Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail. If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title. * TERMINATION You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License. However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation. Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice. Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it. * FUTURE REVISIONS OF THIS LICENSE The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/. Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy's public statement of acceptance of a version permanently authorizes you to choose that version for the Document. * RELICENSING “Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the site means any set of copyrightable works thus published on the MMC site. “CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization. “Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document. An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008. The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing. ADDENDUM: How to use this License for your documents To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page: Copyright (C) year your name. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''. If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this: with the Invariant Sections being list their titles, with the Front-Cover Texts being list, and with the Back-Cover Texts being list. If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation. If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software. ────────────────────────────────────────────────────────────────────────────── Link: start: Top Link: index: Option Index Link: contents: Table of Contents Link: up: Top Link: next: Option Index Link: prev: GNU Free Documentation License Next: Option Index, Previous: GNU Free Documentation License, Up: Top ────────────────────────────────────────────────────────────────────────────── Contributors to GCC The GCC project would like to thank its many contributors. Without them the project would not have been nearly as successful as it has been. Any omissions in this list are accidental. Feel free to contact law@redhat.com or gerald@pfeifer.com if you have been left out or some of your contributions are not listed. Please keep this list in alphabetical order. * Analog Devices helped implement the support for complex data types and iterators. * John David Anglin for threading-related fixes and improvements to libstdc++-v3, and the HP-UX port. * James van Artsdalen wrote the code that makes efficient use of the Intel 80387 register stack. * Abramo and Roberto Bagnara for the SysV68 Motorola 3300 Delta Series port. * Alasdair Baird for various bug fixes. * Giovanni Bajo for analyzing lots of complicated C++ problem reports. * Peter Barada for his work to improve code generation for new ColdFire cores. * Gerald Baumgartner added the signature extension to the C++ front end. * Godmar Back for his Java improvements and encouragement. * Scott Bambrough for help porting the Java compiler. * Wolfgang Bangerth for processing tons of bug reports. * Jon Beniston for his Microsoft Windows port of Java and port to Lattice Mico32. * Daniel Berlin for better DWARF 2 support, faster/better optimizations, improved alias analysis, plus migrating GCC to Bugzilla. * Geoff Berry for his Java object serialization work and various patches. * David Binderman tests weekly snapshots of GCC trunk against Fedora Rawhide for several architectures. * Laurynas Biveinis for memory management work and DJGPP port fixes. * Uros Bizjak for the implementation of x87 math built-in functions and for various middle end and i386 back end improvements and bug fixes. * Eric Blake for helping to make GCJ and libgcj conform to the specifications. * Janne Blomqvist for contributions to GNU Fortran. * Hans-J. Boehm for his garbage collector, IA-64 libffi port, and other Java work. * Segher Boessenkool for helping maintain the PowerPC port and the instruction combiner plus various contributions to the middle end. * Neil Booth for work on cpplib, lang hooks, debug hooks and other miscellaneous clean-ups. * Steven Bosscher for integrating the GNU Fortran front end into GCC and for contributing to the tree-ssa branch. * Eric Botcazou for fixing middle- and backend bugs left and right. * Per Bothner for his direction via the steering committee and various improvements to the infrastructure for supporting new languages. Chill front end implementation. Initial implementations of cpplib, fix-header, config.guess, libio, and past C++ library (libg++) maintainer. Dreaming up, designing and implementing much of GCJ. * Devon Bowen helped port GCC to the Tahoe. * Don Bowman for mips-vxworks contributions. * James Bowman for the FT32 port. * Dave Brolley for work on cpplib and Chill. * Paul Brook for work on the ARM architecture and maintaining GNU Fortran. * Robert Brown implemented the support for Encore 32000 systems. * Christian Bruel for improvements to local store elimination. * Herman A.J. ten Brugge for various fixes. * Joerg Brunsmann for Java compiler hacking and help with the GCJ FAQ. * Joe Buck for his direction via the steering committee from its creation to 2013. * Iain Buclaw for the D frontend. * Craig Burley for leadership of the G77 Fortran effort. * Tobias Burnus for contributions to GNU Fortran. * Stephan Buys for contributing Doxygen notes for libstdc++. * Paolo Carlini for libstdc++ work: lots of efficiency improvements to the C++ strings, streambufs and formatted I/O, hard detective work on the frustrating localization issues, and keeping up with the problem reports. * John Carr for his alias work, SPARC hacking, infrastructure improvements, previous contributions to the steering committee, loop optimizations, etc. * Stephane Carrez for 68HC11 and 68HC12 ports. * Steve Chamberlain for support for the Renesas SH and H8 processors and the PicoJava processor, and for GCJ config fixes. * Glenn Chambers for help with the GCJ FAQ. * John-Marc Chandonia for various libgcj patches. * Denis Chertykov for contributing and maintaining the AVR port, the first GCC port for an 8-bit architecture. * Kito Cheng for his work on the RISC-V port, including bringing up the test suite and maintenance. * Scott Christley for his Objective-C contributions. * Eric Christopher for his Java porting help and clean-ups. * Branko Cibej for more warning contributions. * The GNU Classpath project for all of their merged runtime code. * Nick Clifton for arm, mcore, fr30, v850, m32r, msp430 rx work, --help, and other random hacking. * Michael Cook for libstdc++ cleanup patches to reduce warnings. * R. Kelley Cook for making GCC buildable from a read-only directory as well as other miscellaneous build process and documentation clean-ups. * Ralf Corsepius for SH testing and minor bug fixing. * François-Xavier Coudert for contributions to GNU Fortran. * Stan Cox for care and feeding of the x86 port and lots of behind the scenes hacking. * Alex Crain provided changes for the 3b1. * Ian Dall for major improvements to the NS32k port. * Paul Dale for his work to add uClinux platform support to the m68k backend. * Palmer Dabbelt for his work maintaining the RISC-V port. * Dario Dariol contributed the four varieties of sample programs that print a copy of their source. * Russell Davidson for fstream and stringstream fixes in libstdc++. * Bud Davis for work on the G77 and GNU Fortran compilers. * Mo DeJong for GCJ and libgcj bug fixes. * Jerry DeLisle for contributions to GNU Fortran. * DJ Delorie for the DJGPP port, build and libiberty maintenance, various bug fixes, and the M32C, MeP, MSP430, and RL78 ports. * Arnaud Desitter for helping to debug GNU Fortran. * Gabriel Dos Reis for contributions to G++, contributions and maintenance of GCC diagnostics infrastructure, libstdc++-v3, including valarray<>, complex<>, maintaining the numerics library (including that pesky :-) and keeping up-to-date anything to do with numbers. * Ulrich Drepper for his work on glibc, testing of GCC using glibc, ISO C99 support, CFG dumping support, etc., plus support of the C++ runtime libraries including for all kinds of C interface issues, contributing and maintaining complex<>, sanity checking and disbursement, configuration architecture, libio maintenance, and early math work. * François Dumont for his work on libstdc++-v3, especially maintaining and improving debug-mode and associative and unordered containers. * Zdenek Dvorak for a new loop unroller and various fixes. * Michael Eager for his work on the Xilinx MicroBlaze port. * Richard Earnshaw for his ongoing work with the ARM. * David Edelsohn for his direction via the steering committee, ongoing work with the RS6000/PowerPC port, help cleaning up Haifa loop changes, doing the entire AIX port of libstdc++ with his bare hands, and for ensuring GCC properly keeps working on AIX. * Kevin Ediger for the floating point formatting of num_put::do_put in libstdc++. * Phil Edwards for libstdc++ work including configuration hackery, documentation maintainer, chief breaker of the web pages, the occasional iostream bug fix, and work on shared library symbol versioning. * Paul Eggert for random hacking all over GCC. * Mark Elbrecht for various DJGPP improvements, and for libstdc++ configuration support for locales and fstream-related fixes. * Vadim Egorov for libstdc++ fixes in strings, streambufs, and iostreams. * Christian Ehrhardt for dealing with bug reports. * Ben Elliston for his work to move the Objective-C runtime into its own subdirectory and for his work on autoconf. * Revital Eres for work on the PowerPC 750CL port. * Marc Espie for OpenBSD support. * Doug Evans for much of the global optimization framework, arc, m32r, and SPARC work. * Christopher Faylor for his work on the Cygwin port and for caring and feeding the gcc.gnu.org box and saving its users tons of spam. * Fred Fish for BeOS support and Ada fixes. * Ivan Fontes Garcia for the Portuguese translation of the GCJ FAQ. * Peter Gerwinski for various bug fixes and the Pascal front end. * Kaveh R. Ghazi for his direction via the steering committee, amazing work to make '-W -Wall -W* -Werror' useful, and testing GCC on a plethora of platforms. Kaveh extends his gratitude to the CAIP Center at Rutgers University for providing him with computing resources to work on Free Software from the late 1980s to 2010. * John Gilmore for a donation to the FSF earmarked improving GNU Java. * Judy Goldberg for c++ contributions. * Torbjorn Granlund for various fixes and the c-torture testsuite, multiply- and divide-by-constant optimization, improved long long support, improved leaf function register allocation, and his direction via the steering committee. * Jonny Grant for improvements to collect2's --help documentation. * Anthony Green for his -Os contributions, the moxie port, and Java front end work. * Stu Grossman for gdb hacking, allowing GCJ developers to debug Java code. * Michael K. Gschwind contributed the port to the PDP-11. * Richard Biener for his ongoing middle-end contributions and bug fixes and for release management. * Ron Guilmette implemented the protoize and unprotoize tools, the support for DWARF 1 symbolic debugging information, and much of the support for System V Release 4. He has also worked heavily on the Intel 386 and 860 support. * Sumanth Gundapaneni for contributing the CR16 port. * Mostafa Hagog for Swing Modulo Scheduling (SMS) and post reload GCSE. * Bruno Haible for improvements in the runtime overhead for EH, new warnings and assorted bug fixes. * Andrew Haley for his amazing Java compiler and library efforts. * Chris Hanson assisted in making GCC work on HP-UX for the 9000 series 300. * Michael Hayes for various thankless work he's done trying to get the c30/c40 ports functional. Lots of loop and unroll improvements and fixes. * Dara Hazeghi for wading through myriads of target-specific bug reports. * Kate Hedstrom for staking the G77 folks with an initial testsuite. * Richard Henderson for his ongoing SPARC, alpha, ia32, and ia64 work, loop opts, and generally fixing lots of old problems we've ignored for years, flow rewrite and lots of further stuff, including reviewing tons of patches. * Aldy Hernandez for working on the PowerPC port, SIMD support, and various fixes. * Nobuyuki Hikichi of Software Research Associates, Tokyo, contributed the support for the Sony NEWS machine. * Kazu Hirata for caring and feeding the Renesas H8/300 port and various fixes. * Katherine Holcomb for work on GNU Fortran. * Manfred Hollstein for his ongoing work to keep the m88k alive, lots of testing and bug fixing, particularly of GCC configury code. * Steve Holmgren for MachTen patches. * Mat Hostetter for work on the TILE-Gx and TILEPro ports. * Jan Hubicka for his x86 port improvements. * Falk Hueffner for working on C and optimization bug reports. * Bernardo Innocenti for his m68k work, including merging of ColdFire improvements and uClinux support. * Christian Iseli for various bug fixes. * Kamil Iskra for general m68k hacking. * Lee Iverson for random fixes and MIPS testing. * Balaji V. Iyer for Cilk+ development and merging. * Andreas Jaeger for testing and benchmarking of GCC and various bug fixes. * Martin Jambor for his work on inter-procedural optimizations, the switch conversion pass, and scalar replacement of aggregates. * Jakub Jelinek for his SPARC work and sibling call optimizations as well as lots of bug fixes and test cases, and for improving the Java build system. * Janis Johnson for ia64 testing and fixes, her quality improvement sidetracks, and web page maintenance. * Kean Johnston for SCO OpenServer support and various fixes. * Tim Josling for the sample language treelang based originally on Richard Kenner's “toy” language. * Nicolai Josuttis for additional libstdc++ documentation. * Klaus Kaempf for his ongoing work to make alpha-vms a viable target. * Steven G. Kargl for work on GNU Fortran. * David Kashtan of SRI adapted GCC to VMS. * Ryszard Kabatek for many, many libstdc++ bug fixes and optimizations of strings, especially member functions, and for auto_ptr fixes. * Geoffrey Keating for his ongoing work to make the PPC work for GNU/Linux and his automatic regression tester. * Brendan Kehoe for his ongoing work with G++ and for a lot of early work in just about every part of libstdc++. * Oliver M. Kellogg of Deutsche Aerospace contributed the port to the MIL-STD-1750A. * Richard Kenner of the New York University Ultracomputer Research Laboratory wrote the machine descriptions for the AMD 29000, the DEC Alpha, the IBM RT PC, and the IBM RS/6000 as well as the support for instruction attributes. He also made changes to better support RISC processors including changes to common subexpression elimination, strength reduction, function calling sequence handling, and condition code support, in addition to generalizing the code for frame pointer elimination and delay slot scheduling. Richard Kenner was also the head maintainer of GCC for several years. * Mumit Khan for various contributions to the Cygwin and Mingw32 ports and maintaining binary releases for Microsoft Windows hosts, and for massive libstdc++ porting work to Cygwin/Mingw32. * Robin Kirkham for cpu32 support. * Mark Klein for PA improvements. * Thomas Koenig for various bug fixes. * Bruce Korb for the new and improved fixincludes code. * Benjamin Kosnik for his G++ work and for leading the libstdc++-v3 effort. * Maxim Kuvyrkov for contributions to the instruction scheduler, the Android and m68k/Coldfire ports, and optimizations. * Charles LaBrec contributed the support for the Integrated Solutions 68020 system. * Asher Langton and Mike Kumbera for contributing Cray pointer support to GNU Fortran, and for other GNU Fortran improvements. * Jeff Law for his direction via the steering committee, coordinating the entire egcs project and GCC 2.95, rolling out snapshots and releases, handling merges from GCC2, reviewing tons of patches that might have fallen through the cracks else, and random but extensive hacking. * Walter Lee for work on the TILE-Gx and TILEPro ports. * Marc Lehmann for his direction via the steering committee and helping with analysis and improvements of x86 performance. * Victor Leikehman for work on GNU Fortran. * Ted Lemon wrote parts of the RTL reader and printer. * Kriang Lerdsuwanakij for C++ improvements including template as template parameter support, and many C++ fixes. * Warren Levy for tremendous work on libgcj (Java Runtime Library) and random work on the Java front end. * Alain Lichnewsky ported GCC to the MIPS CPU. * Oskar Liljeblad for hacking on AWT and his many Java bug reports and patches. * Robert Lipe for OpenServer support, new testsuites, testing, etc. * Chen Liqin for various S+core related fixes/improvement, and for maintaining the S+core port. * Martin Liska for his work on identical code folding, the sanitizers, HSA, general bug fixing and for running automated regression testing of GCC and reporting numerous bugs. * Weiwen Liu for testing and various bug fixes. * Manuel López-Ibáñez for improving -Wconversion and many other diagnostics fixes and improvements. * Dave Love for his ongoing work with the Fortran front end and runtime libraries. * Martin von Löwis for internal consistency checking infrastructure, various C++ improvements including namespace support, and tons of assistance with libstdc++/compiler merges. * H.J. Lu for his previous contributions to the steering committee, many x86 bug reports, prototype patches, and keeping the GNU/Linux ports working. * Greg McGary for random fixes and (someday) bounded pointers. * Andrew MacLeod for his ongoing work in building a real EH system, various code generation improvements, work on the global optimizer, etc. * Vladimir Makarov for hacking some ugly i960 problems, PowerPC hacking improvements to compile-time performance, overall knowledge and direction in the area of instruction scheduling, design and implementation of the automaton based instruction scheduler and design and implementation of the integrated and local register allocators. * David Malcolm for his work on improving GCC diagnostics, JIT, self-tests and unit testing. * Bob Manson for his behind the scenes work on dejagnu. * John Marino for contributing the DragonFly BSD port. * Philip Martin for lots of libstdc++ string and vector iterator fixes and improvements, and string clean up and testsuites. * Michael Matz for his work on dominance tree discovery, the x86-64 port, link-time optimization framework and general optimization improvements. * All of the Mauve project contributors for Java test code. * Bryce McKinlay for numerous GCJ and libgcj fixes and improvements. * Adam Megacz for his work on the Microsoft Windows port of GCJ. * Michael Meissner for LRS framework, ia32, m32r, v850, m88k, MIPS, powerpc, haifa, ECOFF debug support, and other assorted hacking. * Jason Merrill for his direction via the steering committee and leading the G++ effort. * Martin Michlmayr for testing GCC on several architectures using the entire Debian archive. * David Miller for his direction via the steering committee, lots of SPARC work, improvements in jump.c and interfacing with the Linux kernel developers. * Gary Miller ported GCC to Charles River Data Systems machines. * Alfred Minarik for libstdc++ string and ios bug fixes, and turning the entire libstdc++ testsuite namespace-compatible. * Mark Mitchell for his direction via the steering committee, mountains of C++ work, load/store hoisting out of loops, alias analysis improvements, ISO C restrict support, and serving as release manager from 2000 to 2011. * Alan Modra for various GNU/Linux bits and testing. * Toon Moene for his direction via the steering committee, Fortran maintenance, and his ongoing work to make us make Fortran run fast. * Jason Molenda for major help in the care and feeding of all the services on the gcc.gnu.org (formerly egcs.cygnus.com) machine—mail, web services, ftp services, etc etc. Doing all this work on scrap paper and the backs of envelopes would have been… difficult. * Catherine Moore for fixing various ugly problems we have sent her way, including the haifa bug which was killing the Alpha & PowerPC Linux kernels. * Mike Moreton for his various Java patches. * David Mosberger-Tang for various Alpha improvements, and for the initial IA-64 port. * Stephen Moshier contributed the floating point emulator that assists in cross-compilation and permits support for floating point numbers wider than 64 bits and for ISO C99 support. * Bill Moyer for his behind the scenes work on various issues. * Philippe De Muyter for his work on the m68k port. * Joseph S. Myers for his work on the PDP-11 port, format checking and ISO C99 support, and continuous emphasis on (and contributions to) documentation. * Nathan Myers for his work on libstdc++-v3: architecture and authorship through the first three snapshots, including implementation of locale infrastructure, string, shadow C headers, and the initial project documentation (DESIGN, CHECKLIST, and so forth). Later, more work on MT-safe string and shadow headers. * Felix Natter for documentation on porting libstdc++. * Nathanael Nerode for cleaning up the configuration/build process. * NeXT, Inc. donated the front end that supports the Objective-C language. * Hans-Peter Nilsson for the CRIS and MMIX ports, improvements to the search engine setup, various documentation fixes and other small fixes. * Geoff Noer for his work on getting cygwin native builds working. * Vegard Nossum for running automated regression testing of GCC and reporting numerous bugs. * Diego Novillo for his work on Tree SSA, OpenMP, SPEC performance tracking web pages, GIMPLE tuples, and assorted fixes. * David O'Brien for the FreeBSD/alpha, FreeBSD/AMD x86-64, FreeBSD/ARM, FreeBSD/PowerPC, and FreeBSD/SPARC64 ports and related infrastructure improvements. * Alexandre Oliva for various build infrastructure improvements, scripts and amazing testing work, including keeping libtool issues sane and happy. * Stefan Olsson for work on mt_alloc. * Melissa O'Neill for various NeXT fixes. * Rainer Orth for random MIPS work, including improvements to GCC's o32 ABI support, improvements to dejagnu's MIPS support, Java configuration clean-ups and porting work, and maintaining the IRIX, Solaris 2, and Tru64 UNIX ports. * Steven Pemberton for his contribution of enquire which allowed GCC to determine various properties of the floating point unit and generate float.h in older versions of GCC. * Hartmut Penner for work on the s390 port. * Paul Petersen wrote the machine description for the Alliant FX/8. * Alexandre Petit-Bianco for implementing much of the Java compiler and continued Java maintainership. * Matthias Pfaller for major improvements to the NS32k port. * Gerald Pfeifer for his direction via the steering committee, pointing out lots of problems we need to solve, maintenance of the web pages, and taking care of documentation maintenance in general. * Marek Polacek for his work on the C front end, the sanitizers and general bug fixing. * Andrew Pinski for processing bug reports by the dozen. * Ovidiu Predescu for his work on the Objective-C front end and runtime libraries. * Jerry Quinn for major performance improvements in C++ formatted I/O. * Ken Raeburn for various improvements to checker, MIPS ports and various cleanups in the compiler. * Rolf W. Rasmussen for hacking on AWT. * David Reese of Sun Microsystems contributed to the Solaris on PowerPC port. * John Regehr for running automated regression testing of GCC and reporting numerous bugs. * Volker Reichelt for running automated regression testing of GCC and reporting numerous bugs and for keeping up with the problem reports. * Joern Rennecke for maintaining the sh port, loop, regmove & reload hacking and developing and maintaining the Epiphany port. * Loren J. Rittle for improvements to libstdc++-v3 including the FreeBSD port, threading fixes, thread-related configury changes, critical threading documentation, and solutions to really tricky I/O problems, as well as keeping GCC properly working on FreeBSD and continuous testing. * Craig Rodrigues for processing tons of bug reports. * Ola Rönnerup for work on mt_alloc. * Gavin Romig-Koch for lots of behind the scenes MIPS work. * David Ronis inspired and encouraged Craig to rewrite the G77 documentation in texinfo format by contributing a first pass at a translation of the old g77-0.5.16/f/DOC file. * Ken Rose for fixes to GCC's delay slot filling code. * Ira Rosen for her contributions to the auto-vectorizer. * Paul Rubin wrote most of the preprocessor. * Pétur Runólfsson for major performance improvements in C++ formatted I/O and large file support in C++ filebuf. * Chip Salzenberg for libstdc++ patches and improvements to locales, traits, Makefiles, libio, libtool hackery, and “long long” support. * Juha Sarlin for improvements to the H8 code generator. * Greg Satz assisted in making GCC work on HP-UX for the 9000 series 300. * Roger Sayle for improvements to constant folding and GCC's RTL optimizers as well as for fixing numerous bugs. * Bradley Schatz for his work on the GCJ FAQ. * Peter Schauer wrote the code to allow debugging to work on the Alpha. * William Schelter did most of the work on the Intel 80386 support. * Tobias Schlüter for work on GNU Fortran. * Bernd Schmidt for various code generation improvements and major work in the reload pass, serving as release manager for GCC 2.95.3, and work on the Blackfin and C6X ports. * Peter Schmid for constant testing of libstdc++—especially application testing, going above and beyond what was requested for the release criteria—and libstdc++ header file tweaks. * Jason Schroeder for jcf-dump patches. * Andreas Schwab for his work on the m68k port. * Lars Segerlund for work on GNU Fortran. * Dodji Seketeli for numerous C++ bug fixes and debug info improvements. * Tim Shen for major work on . * Joel Sherrill for his direction via the steering committee, RTEMS contributions and RTEMS testing. * Nathan Sidwell for many C++ fixes/improvements. * Jeffrey Siegal for helping RMS with the original design of GCC, some code which handles the parse tree and RTL data structures, constant folding and help with the original VAX & m68k ports. * Kenny Simpson for prompting libstdc++ fixes due to defect reports from the LWG (thereby keeping GCC in line with updates from the ISO). * Franz Sirl for his ongoing work with making the PPC port stable for GNU/Linux. * Andrey Slepuhin for assorted AIX hacking. * Trevor Smigiel for contributing the SPU port. * Christopher Smith did the port for Convex machines. * Danny Smith for his major efforts on the Mingw (and Cygwin) ports. Retired from GCC maintainership August 2010, having mentored two new maintainers into the role. * Randy Smith finished the Sun FPA support. * Ed Smith-Rowland for his continuous work on libstdc++-v3, special functions, , and various improvements to C++11 features. * Scott Snyder for queue, iterator, istream, and string fixes and libstdc++ testsuite entries. Also for providing the patch to G77 to add rudimentary support for INTEGER*1, INTEGER*2, and LOGICAL*1. * Zdenek Sojka for running automated regression testing of GCC and reporting numerous bugs. * Arseny Solokha for running automated regression testing of GCC and reporting numerous bugs. * Jayant Sonar for contributing the CR16 port. * Brad Spencer for contributions to the GLIBCPP_FORCE_NEW technique. * Richard Stallman, for writing the original GCC and launching the GNU project. * Jan Stein of the Chalmers Computer Society provided support for Genix, as well as part of the 32000 machine description. * Gerhard Steinmetz for running automated regression testing of GCC and reporting numerous bugs. * Nigel Stephens for various mips16 related fixes/improvements. * Jonathan Stone wrote the machine description for the Pyramid computer. * Graham Stott for various infrastructure improvements. * John Stracke for his Java HTTP protocol fixes. * Mike Stump for his Elxsi port, G++ contributions over the years and more recently his vxworks contributions * Jeff Sturm for Java porting help, bug fixes, and encouragement. * Zhendong Su for running automated regression testing of GCC and reporting numerous bugs. * Chengnian Sun for running automated regression testing of GCC and reporting numerous bugs. * Shigeya Suzuki for this fixes for the bsdi platforms. * Ian Lance Taylor for the Go frontend, the initial mips16 and mips64 support, general configury hacking, fixincludes, etc. * Holger Teutsch provided the support for the Clipper CPU. * Gary Thomas for his ongoing work to make the PPC work for GNU/Linux. * Paul Thomas for contributions to GNU Fortran. * Philipp Thomas for random bug fixes throughout the compiler * Jason Thorpe for thread support in libstdc++ on NetBSD. * Kresten Krab Thorup wrote the run time support for the Objective-C language and the fantastic Java bytecode interpreter. * Michael Tiemann for random bug fixes, the first instruction scheduler, initial C++ support, function integration, NS32k, SPARC and M88k machine description work, delay slot scheduling. * Andreas Tobler for his work porting libgcj to Darwin. * Teemu Torma for thread safe exception handling support. * Leonard Tower wrote parts of the parser, RTL generator, and RTL definitions, and of the VAX machine description. * Daniel Towner and Hariharan Sandanagobalane contributed and maintain the picoChip port. * Tom Tromey for internationalization support and for his many Java contributions and libgcj maintainership. * Lassi Tuura for improvements to config.guess to determine HP processor types. * Petter Urkedal for libstdc++ CXXFLAGS, math, and algorithms fixes. * Andy Vaught for the design and initial implementation of the GNU Fortran front end. * Brent Verner for work with the libstdc++ cshadow files and their associated configure steps. * Todd Vierling for contributions for NetBSD ports. * Andrew Waterman for contributing the RISC-V port, as well as maintaining it. * Jonathan Wakely for contributing libstdc++ Doxygen notes and XHTML guidance and maintaining libstdc++. * Dean Wakerley for converting the install documentation from HTML to texinfo in time for GCC 3.0. * Krister Walfridsson for random bug fixes. * Feng Wang for contributions to GNU Fortran. * Stephen M. Webb for time and effort on making libstdc++ shadow files work with the tricky Solaris 8+ headers, and for pushing the build-time header tree. Also, for starting and driving the effort. * John Wehle for various improvements for the x86 code generator, related infrastructure improvements to help x86 code generation, value range propagation and other work, WE32k port. * Ulrich Weigand for work on the s390 port. * Janus Weil for contributions to GNU Fortran. * Zack Weinberg for major work on cpplib and various other bug fixes. * Matt Welsh for help with Linux Threads support in GCJ. * Urban Widmark for help fixing java.io. * Mark Wielaard for new Java library code and his work integrating with Classpath. * Dale Wiles helped port GCC to the Tahoe. * Bob Wilson from Tensilica, Inc. for the Xtensa port. * Jim Wilson for his direction via the steering committee, tackling hard problems in various places that nobody else wanted to work on, strength reduction and other loop optimizations. * Paul Woegerer and Tal Agmon for the CRX port. * Carlo Wood for various fixes. * Tom Wood for work on the m88k port. * Chung-Ju Wu for his work on the Andes NDS32 port. * Canqun Yang for work on GNU Fortran. * Masanobu Yuhara of Fujitsu Laboratories implemented the machine description for the Tron architecture (specifically, the Gmicro). * Kevin Zachmann helped port GCC to the Tahoe. * Ayal Zaks for Swing Modulo Scheduling (SMS). * Qirun Zhang for running automated regression testing of GCC and reporting numerous bugs. * Xiaoqiang Zhang for work on GNU Fortran. * Gilles Zunino for help porting Java to Irix. The following people are recognized for their contributions to GNAT, the Ada front end of GCC: * Bernard Banner * Romain Berrendonner * Geert Bosch * Emmanuel Briot * Joel Brobecker * Ben Brosgol * Vincent Celier * Arnaud Charlet * Chien Chieng * Cyrille Comar * Cyrille Crozes * Robert Dewar * Gary Dismukes * Robert Duff * Ed Falis * Ramon Fernandez * Sam Figueroa * Vasiliy Fofanov * Michael Friess * Franco Gasperoni * Ted Giering * Matthew Gingell * Laurent Guerby * Jerome Guitton * Olivier Hainque * Jerome Hugues * Hristian Kirtchev * Jerome Lambourg * Bruno Leclerc * Albert Lee * Sean McNeil * Javier Miranda * Laurent Nana * Pascal Obry * Dong-Ik Oh * Laurent Pautet * Brett Porter * Thomas Quinot * Nicolas Roche * Pat Rogers * Jose Ruiz * Douglas Rupp * Sergey Rybin * Gail Schenker * Ed Schonberg * Nicolas Setton * Samuel Tardieu The following people are recognized for their contributions of new features, bug reports, testing and integration of classpath/libgcj for GCC version 4.1: * Lillian Angel for JTree implementation and lots Free Swing additions and bug fixes. * Wolfgang Baer for GapContent bug fixes. * Anthony Balkissoon for JList, Free Swing 1.5 updates and mouse event fixes, lots of Free Swing work including JTable editing. * Stuart Ballard for RMI constant fixes. * Goffredo Baroncelli for HTTPURLConnection fixes. * Gary Benson for MessageFormat fixes. * Daniel Bonniot for Serialization fixes. * Chris Burdess for lots of gnu.xml and http protocol fixes, StAX and DOM xml:id support. * Ka-Hing Cheung for TreePath and TreeSelection fixes. * Archie Cobbs for build fixes, VM interface updates, URLClassLoader updates. * Kelley Cook for build fixes. * Martin Cordova for Suggestions for better SocketTimeoutException. * David Daney for BitSet bug fixes, HttpURLConnection rewrite and improvements. * Thomas Fitzsimmons for lots of upgrades to the gtk+ AWT and Cairo 2D support. Lots of imageio framework additions, lots of AWT and Free Swing bug fixes. * Jeroen Frijters for ClassLoader and nio cleanups, serialization fixes, better Proxy support, bug fixes and IKVM integration. * Santiago Gala for AccessControlContext fixes. * Nicolas Geoffray for VMClassLoader and AccessController improvements. * David Gilbert for basic and metal icon and plaf support and lots of documenting, Lots of Free Swing and metal theme additions. MetalIconFactory implementation. * Anthony Green for MIDI framework, ALSA and DSSI providers. * Andrew Haley for Serialization and URLClassLoader fixes, gcj build speedups. * Kim Ho for JFileChooser implementation. * Andrew John Hughes for Locale and net fixes, URI RFC2986 updates, Serialization fixes, Properties XML support and generic branch work, VMIntegration guide update. * Bastiaan Huisman for TimeZone bug fixing. * Andreas Jaeger for mprec updates. * Paul Jenner for better -Werror support. * Ito Kazumitsu for NetworkInterface implementation and updates. * Roman Kennke for BoxLayout, GrayFilter and SplitPane, plus bug fixes all over. Lots of Free Swing work including styled text. * Simon Kitching for String cleanups and optimization suggestions. * Michael Koch for configuration fixes, Locale updates, bug and build fixes. * Guilhem Lavaux for configuration, thread and channel fixes and Kaffe integration. JCL native Pointer updates. Logger bug fixes. * David Lichteblau for JCL support library global/local reference cleanups. * Aaron Luchko for JDWP updates and documentation fixes. * Ziga Mahkovec for Graphics2D upgraded to Cairo 0.5 and new regex features. * Sven de Marothy for BMP imageio support, CSS and TextLayout fixes. GtkImage rewrite, 2D, awt, free swing and date/time fixes and implementing the Qt4 peers. * Casey Marshall for crypto algorithm fixes, FileChannel lock, SystemLogger and FileHandler rotate implementations, NIO FileChannel.map support, security and policy updates. * Bryce McKinlay for RMI work. * Audrius Meskauskas for lots of Free Corba, RMI and HTML work plus testing and documenting. * Kalle Olavi Niemitalo for build fixes. * Rainer Orth for build fixes. * Andrew Overholt for File locking fixes. * Ingo Proetel for Image, Logger and URLClassLoader updates. * Olga Rodimina for MenuSelectionManager implementation. * Jan Roehrich for BasicTreeUI and JTree fixes. * Julian Scheid for documentation updates and gjdoc support. * Christian Schlichtherle for zip fixes and cleanups. * Robert Schuster for documentation updates and beans fixes, TreeNode enumerations and ActionCommand and various fixes, XML and URL, AWT and Free Swing bug fixes. * Keith Seitz for lots of JDWP work. * Christian Thalinger for 64-bit cleanups, Configuration and VM interface fixes and CACAO integration, fdlibm updates. * Gael Thomas for VMClassLoader boot packages support suggestions. * Andreas Tobler for Darwin and Solaris testing and fixing, Qt4 support for Darwin/OS X, Graphics2D support, gtk+ updates. * Dalibor Topic for better DEBUG support, build cleanups and Kaffe integration. Qt4 build infrastructure, SHA1PRNG and GdkPixbugDecoder updates. * Tom Tromey for Eclipse integration, generics work, lots of bug fixes and gcj integration including coordinating The Big Merge. * Mark Wielaard for bug fixes, packaging and release management, Clipboard implementation, system call interrupts and network timeouts and GdkPixpufDecoder fixes. In addition to the above, all of which also contributed time and energy in testing GCC, we would like to thank the following for their contributions to testing: * Michael Abd-El-Malek * Thomas Arend * Bonzo Armstrong * Steven Ashe * Chris Baldwin * David Billinghurst * Jim Blandy * Stephane Bortzmeyer * Horst von Brand * Frank Braun * Rodney Brown * Sidney Cadot * Bradford Castalia * Robert Clark * Jonathan Corbet * Ralph Doncaster * Richard Emberson * Levente Farkas * Graham Fawcett * Mark Fernyhough * Robert A. French * Jörgen Freyh * Mark K. Gardner * Charles-Antoine Gauthier * Yung Shing Gene * David Gilbert * Simon Gornall * Fred Gray * John Griffin * Patrik Hagglund * Phil Hargett * Amancio Hasty * Takafumi Hayashi * Bryan W. Headley * Kevin B. Hendricks * Joep Jansen * Christian Joensson * Michel Kern * David Kidd * Tobias Kuipers * Anand Krishnaswamy * A. O. V. Le Blanc * llewelly * Damon Love * Brad Lucier * Matthias Klose * Martin Knoblauch * Rick Lutowski * Jesse Macnish * Stefan Morrell * Anon A. Mous * Matthias Mueller * Pekka Nikander * Rick Niles * Jon Olson * Magnus Persson * Chris Pollard * Richard Polton * Derk Reefman * David Rees * Paul Reilly * Tom Reilly * Torsten Rueger * Danny Sadinoff * Marc Schifer * Erik Schnetter * Wayne K. Schroll * David Schuler * Vin Shelton * Tim Souder * Adam Sulmicki * Bill Thorson * George Talbot * Pedro A. M. Vazquez * Gregory Warnes * Ian Watson * David E. Young * And many others And finally we'd like to thank everyone who uses the compiler, provides feedback and generally reminds us why we're doing this work in the first place. ────────────────────────────────────────────────────────────────────────────── Next: Option Index, Previous: GNU Free Documentation License, Up: Top