Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 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.

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 (Ubuntu 4.4.3-4ubuntu5) version 4.4.3. 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.

• G++ and GCC: You can compile C or C++ programs.
• Standards: Language standards supported by GCC.
• Invoking GCC: Command options supported by ‘gcc’.
• C Implementation: How GCC implements the ISO C specification.
• C Extensions: GNU extensions to the C language family.
• C++ Extensions: GNU extensions to the C++ language.
• Objective-C: GNU Objective-C runtime features.
• Compatibility: Binary Compatibility
• Gcov: gcov---a test coverage program.
• Trouble: If you have trouble using GCC.
• Bugs: How, why and where to report bugs.
• Service: How to find suppliers of support for GCC.
• Contributing: How to contribute to testing and developing GCC.
• Funding: How to help assure funding for free software.
• GNU Project: The GNU Project and GNU/Linux.
• Copying: GNU General Public License says how you can copy and share GCC.
• GNU Free Documentation License: How you can copy and share this manual.
• Contributors: People who have contributed to GCC.
• Option Index: Index to command line options.
• Keyword Index: Index of concepts and symbol names.

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++, Java, Fortran, and Ada.

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 Pascal, 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

GCC supports three versions of the C standard, although support for the most recent version is not yet complete.

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. This standard, in both its forms, is commonly known as C89, or occasionally as C90, from the dates of ratification. The ANSI standard, but not the ISO standard, also came with a Rationale document. To select this standard in GCC, use one of the options -ansi, -std=c89 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. GCC has incomplete support for this standard version; see http://gcc.gnu.org/gcc-4.4/c99status.html for details. To select this standard, use -std=c99 or -std=iso9899:1999. (While in development, drafts of this standard version were referred to as C9X.)

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.

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. Use of the -std options listed above will disable 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=gnu89 (for C89 with GNU extensions) or -std=gnu99 (for C99 with GNU extensions). The default, if no C language dialect options are given, is -std=gnu89; this will change to -std=gnu99 in some future release when the C99 support is complete. Some features that are part of the C99 standard are accepted as extensions in C89 mode.

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 <float.h>, <limits.h>, <stdarg.h>, and <stddef.h>; since AMD1, also those in <iso646.h>; and in C99, also those in <stdbool.h> and <stdint.h>. 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 would be a freestanding environment; a program using the facilities of an operating system would normally be in a hosted implementation.

GCC aims towards being usable as a conforming freestanding implementation, or as the compiler for a conforming hosted implementation. By default, it will act 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 will then define __STDC_HOSTED__ to 0 and 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; to use the facilities of a hosted environment, you will 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 will emit 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 ISO C++ standard (1998) and contains experimental support for the upcoming ISO C++ standard (200x).

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 or -std=c++98; 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).

The ISO C++ committee is working on a new ISO C++ standard, dubbed C++0x, that is intended to be published by 2009. C++0x contains several changes to the C++ language, some of which have been implemented in an experimental C++0x mode in GCC. The C++0x mode in GCC tracks the draft working paper for the C++0x standard; the latest working paper is available on the ISO C++ committee's web site at http://www.open-std.org/jtc1/sc22/wg21/. For information regarding the C++0x features available in the experimental C++0x mode, see http://gcc.gnu.org/gcc-4.3/cxx0x_status.html. To select this standard in GCC, use the option -std=c++0x; 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).

By default, GCC provides some extensions to the C++ language; See Options Controlling C++ Dialect. Use of the -std option listed above will disable these extensions. 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++0x (for C++0x with GNU extensions). The default, if no C++ language dialect options are given, is -std=gnu++98.

2.3 Objective-C and Objective-C++ languages

There is no formal written standard for Objective-C or Objective-C++. The most authoritative manual is “Object-Oriented Programming and the Objective-C Language”, available at a number of web sites:

• http://developer.apple.com/documentation/Cocoa/Conceptual/ObjectiveC/ is a recent (and periodically updated) version;
• http://www.toodarkpark.org/computers/objc/ is an older example;
• http://www.gnustep.org and http://gcc.gnu.org/readings.html have additional useful information.

See GNAT Reference Manual, for information on standard conformance and compatibility of the Ada compiler.

See Standards, for details of standards supported by GNU Fortran.

See Compatibility with the Java Platform, for details of compatibility between gcj and the Java Platform.

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.

Other options are passed on to one stage 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.

See Compiling C++ Programs, for a summary of special options for 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 would be -fno-foo. This manual documents only one of these two forms, whichever one is not the default.

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 Objective-C++ Dialect Options: Variations on Objective-C and Objective-C++.
• Language Independent Options: Controlling how diagnostics should be formatted.
• Warning Options: How picky should the compiler be?
• Debugging Options: Symbol tables, measurements, and debugging dumps.
• Optimize Options: How much optimization?
• 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.
• Spec Files: How to pass switches to sub-processes.
• Target Options: Running a cross-compiler, or an old version of GCC.
• Submodel Options: Specifying minor hardware or convention variations, such as 68010 vs 68020.
• Code Gen Options: Specifying conventions for function calls, data layout and register usage.
• Environment Variables: Env vars that affect GCC.
• Precompiled Headers: Compiling a header once, and using it many times.
• Running Protoize: Automatically adding or removing function prototypes.

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  -combine  -pipe  -pass-exit-codes  
          -x language  -v  -###  --help[=class[,...]]  --target-help  
          --version -wrapper@file -fplugin=file -fplugin-arg-name=arg

C Language Options

See Options Controlling C Dialect.

          -ansi  -std=standard  -fgnu89-inline 
          -aux-info filename 
          -fno-asm  -fno-builtin  -fno-builtin-function 
          -fhosted  -ffreestanding -fopenmp -fms-extensions 
          -trigraphs  -no-integrated-cpp  -traditional  -traditional-cpp 
          -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  -fcheck-new 
          -fconserve-space  -ffriend-injection 
          -fno-elide-constructors 
          -fno-enforce-eh-specs 
          -ffor-scope  -fno-for-scope  -fno-gnu-keywords 
          -fno-implicit-templates 
          -fno-implicit-inline-templates 
          -fno-implement-inlines  -fms-extensions 
          -fno-nonansi-builtins  -fno-operator-names 
          -fno-optional-diags  -fpermissive 
          -frepo  -fno-rtti  -fstats  -ftemplate-depth-n 
          -fno-threadsafe-statics -fuse-cxa-atexit  -fno-weak  -nostdinc++ 
          -fno-default-inline  -fvisibility-inlines-hidden 
          -fvisibility-ms-compat 
          -Wabi  -Wctor-dtor-privacy 
          -Wnon-virtual-dtor  -Wreorder 
          -Weffc++  -Wstrict-null-sentinel 
          -Wno-non-template-friend  -Wold-style-cast 
          -Woverloaded-virtual  -Wno-pmf-conversions 
          -Wsign-promo

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-call-cxx-cdtors 
          -fobjc-direct-dispatch 
          -fobjc-exceptions 
          -fobjc-gc 
          -freplace-objc-classes 
          -fzero-link 
          -gen-decls 
          -Wassign-intercept 
          -Wno-protocol  -Wselector 
          -Wstrict-selector-match 
          -Wundeclared-selector

Language Independent Options

See Options to Control Diagnostic Messages Formatting.

          -fmessage-length=n  
          -fdiagnostics-show-location=[once|every-line]  
          -fdiagnostics-show-option

Warning Options

See Options to Request or Suppress Warnings.

          -fsyntax-only  -pedantic  -pedantic-errors 
          -w  -Wextra  -Wall  -Waddress  -Waggregate-return  -Warray-bounds 
          -Wno-attributes -Wno-builtin-macro-redefined 
          -Wc++-compat -Wc++0x-compat -Wcast-align  -Wcast-qual  
          -Wchar-subscripts -Wclobbered  -Wcomment 
          -Wconversion  -Wcoverage-mismatch  -Wno-deprecated  
          -Wno-deprecated-declarations -Wdisabled-optimization  
          -Wno-div-by-zero -Wempty-body  -Wenum-compare -Wno-endif-labels 
          -Werror  -Werror=* 
          -Wfatal-errors  -Wfloat-equal  -Wformat  -Wformat=2 
          -Wno-format-contains-nul -Wno-format-extra-args -Wformat-nonliteral 
          -Wformat-security  -Wformat-y2k 
          -Wframe-larger-than=len -Wignored-qualifiers 
          -Wimplicit  -Wimplicit-function-declaration  -Wimplicit-int 
          -Winit-self  -Winline 
          -Wno-int-to-pointer-cast -Wno-invalid-offsetof 
          -Winvalid-pch -Wlarger-than=len  -Wunsafe-loop-optimizations 
          -Wlogical-op -Wlong-long 
          -Wmain  -Wmissing-braces  -Wmissing-field-initializers 
          -Wmissing-format-attribute  -Wmissing-include-dirs 
          -Wmissing-noreturn  -Wno-mudflap 
          -Wno-multichar  -Wnonnull  -Wno-overflow 
          -Woverlength-strings  -Wpacked  -Wpacked-bitfield-compat  -Wpadded 
          -Wparentheses  -Wpedantic-ms-format -Wno-pedantic-ms-format 
          -Wpointer-arith  -Wno-pointer-to-int-cast 
          -Wredundant-decls 
          -Wreturn-type  -Wsequence-point  -Wshadow 
          -Wsign-compare  -Wsign-conversion  -Wstack-protector 
          -Wstrict-aliasing -Wstrict-aliasing=n 
          -Wstrict-overflow -Wstrict-overflow=n 
          -Wswitch  -Wswitch-default  -Wswitch-enum -Wsync-nand 
          -Wsystem-headers  -Wtrigraphs  -Wtype-limits  -Wundef  -Wuninitialized 
          -Wunknown-pragmas  -Wno-pragmas -Wunreachable-code 
          -Wunused  -Wunused-function  -Wunused-label  -Wunused-parameter 
          -Wno-unused-result -Wunused-value  -Wunused-variable 
          -Wvariadic-macros -Wvla 
          -Wvolatile-register-var  -Wwrite-strings

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 or GCC.

          -dletters  -dumpspecs  -dumpmachine  -dumpversion 
          -fdbg-cnt-list -fdbg-cnt=counter-value-list 
          -fdump-noaddr -fdump-unnumbered 
          -fdump-translation-unit[-n] 
          -fdump-class-hierarchy[-n] 
          -fdump-ipa-all -fdump-ipa-cgraph -fdump-ipa-inline 
          -fdump-statistics 
          -fdump-tree-all 
          -fdump-tree-original[-n]  
          -fdump-tree-optimized[-n] 
          -fdump-tree-cfg -fdump-tree-vcg -fdump-tree-alias 
          -fdump-tree-ch 
          -fdump-tree-ssa[-n] -fdump-tree-pre[-n] 
          -fdump-tree-ccp[-n] -fdump-tree-dce[-n] 
          -fdump-tree-gimple[-raw] -fdump-tree-mudflap[-n] 
          -fdump-tree-dom[-n] 
          -fdump-tree-dse[-n] 
          -fdump-tree-phiopt[-n] 
          -fdump-tree-forwprop[-n] 
          -fdump-tree-copyrename[-n] 
          -fdump-tree-nrv -fdump-tree-vect 
          -fdump-tree-sink 
          -fdump-tree-sra[-n] 
          -fdump-tree-fre[-n] 
          -fdump-tree-vrp[-n] 
          -ftree-vectorizer-verbose=n 
          -fdump-tree-storeccp[-n] 
          -feliminate-dwarf2-dups -feliminate-unused-debug-types 
          -feliminate-unused-debug-symbols -femit-class-debug-always 
          -fmem-report -fpre-ipa-mem-report -fpost-ipa-mem-report -fprofile-arcs 
          -frandom-seed=string -fsched-verbose=n 
          -fsel-sched-verbose -fsel-sched-dump-cfg -fsel-sched-pipelining-verbose 
          -ftest-coverage  -ftime-report -fvar-tracking 
          -g  -glevel  -gcoff -gdwarf-2 
          -ggdb  -gstabs  -gstabs+  -gvms  -gxcoff  -gxcoff+ 
          -fno-merge-debug-strings -fno-dwarf2-cfi-asm 
          -fdebug-prefix-map=old=new 
          -femit-struct-debug-baseonly -femit-struct-debug-reduced 
          -femit-struct-debug-detailed[=spec-list] 
          -p  -pg  -print-file-name=library  -print-libgcc-file-name 
          -print-multi-directory  -print-multi-lib 
          -print-prog-name=program  -print-search-dirs  -Q 
          -print-sysroot -print-sysroot-headers-suffix 
          -save-temps  -time

Optimization Options

See Options that Control Optimization.

          -falign-functions[=n] -falign-jumps[=n] 
          -falign-labels[=n] -falign-loops[=n] -fassociative-math 
          -fauto-inc-dec -fbranch-probabilities -fbranch-target-load-optimize 
          -fbranch-target-load-optimize2 -fbtr-bb-exclusive -fcaller-saves 
          -fcheck-data-deps -fconserve-stack -fcprop-registers -fcrossjumping 
          -fcse-follow-jumps -fcse-skip-blocks -fcx-fortran-rules -fcx-limited-range 
          -fdata-sections -fdce -fdce 
          -fdelayed-branch -fdelete-null-pointer-checks -fdse -fdse 
          -fearly-inlining -fexpensive-optimizations -ffast-math 
          -ffinite-math-only -ffloat-store -fforward-propagate 
          -ffunction-sections -fgcse -fgcse-after-reload -fgcse-las -fgcse-lm 
          -fgcse-sm -fif-conversion -fif-conversion2 -findirect-inlining 
          -finline-functions -finline-functions-called-once -finline-limit=n 
          -finline-small-functions -fipa-cp -fipa-cp-clone -fipa-matrix-reorg -fipa-pta 
          -fipa-pure-const -fipa-reference -fipa-struct-reorg 
          -fipa-type-escape -fira-algorithm=algorithm 
          -fira-region=region -fira-coalesce -fno-ira-share-save-slots 
          -fno-ira-share-spill-slots -fira-verbose=n 
          -fivopts -fkeep-inline-functions -fkeep-static-consts 
          -floop-block -floop-interchange -floop-strip-mine 
          -fmerge-all-constants -fmerge-constants -fmodulo-sched 
          -fmodulo-sched-allow-regmoves -fmove-loop-invariants -fmudflap 
          -fmudflapir -fmudflapth -fno-branch-count-reg -fno-default-inline 
          -fno-defer-pop -fno-function-cse -fno-guess-branch-probability 
          -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 
          -fno-sched-interblock -fno-sched-spec -fno-signed-zeros 
          -fno-toplevel-reorder -fno-trapping-math -fno-zero-initialized-in-bss 
          -fomit-frame-pointer -foptimize-register-move -foptimize-sibling-calls 
          -fpeel-loops -fpredictive-commoning -fprefetch-loop-arrays 
          -fprofile-correction -fprofile-dir=path -fprofile-generate 
          -fprofile-generate=path 
          -fprofile-use -fprofile-use=path -fprofile-values 
          -freciprocal-math -fregmove -frename-registers -freorder-blocks 
          -freorder-blocks-and-partition -freorder-functions 
          -frerun-cse-after-loop -freschedule-modulo-scheduled-loops 
          -frounding-math -frtl-abstract-sequences -fsched2-use-superblocks 
          -fsched2-use-traces -fsched-spec-load -fsched-spec-load-dangerous 
          -fsched-stalled-insns-dep[=n] -fsched-stalled-insns[=n] 
          -fschedule-insns -fschedule-insns2 -fsection-anchors -fsee 
          -fselective-scheduling -fselective-scheduling2 
          -fsel-sched-pipelining -fsel-sched-pipelining-outer-loops 
          -fsignaling-nans -fsingle-precision-constant -fsplit-ivs-in-unroller 
          -fsplit-wide-types -fstack-protector -fstack-protector-all 
          -fstrict-aliasing -fstrict-overflow -fthread-jumps -ftracer 
          -ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-copy-prop 
          -ftree-copyrename -ftree-dce 
          -ftree-dominator-opts -ftree-dse -ftree-fre -ftree-loop-im 
          -ftree-loop-distribution 
          -ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize 
          -ftree-parallelize-loops=n -ftree-pre -ftree-reassoc 
          -ftree-sink -ftree-sra -ftree-switch-conversion 
          -ftree-ter -ftree-vect-loop-version -ftree-vectorize -ftree-vrp 
          -funit-at-a-time -funroll-all-loops -funroll-loops 
          -funsafe-loop-optimizations -funsafe-math-optimizations -funswitch-loops 
          -fvariable-expansion-in-unroller -fvect-cost-model -fvpt -fweb 
          -fwhole-program -fuse-ld 
          --param name=value
          -O  -O0  -O1  -O2  -O3  -Os

Preprocessor Options

See Options Controlling the Preprocessor.

          -Aquestion=answer 
          -A-question[=answer] 
          -C  -dD  -dI  -dM  -dN 
          -Dmacro[=defn]  -E  -H 
          -idirafter dir 
          -include file  -imacros file 
          -iprefix file  -iwithprefix dir 
          -iwithprefixbefore dir  -isystem dir 
          -imultilib dir -isysroot dir 
          -M  -MM  -MF  -MG  -MP  -MQ  -MT  -nostdinc  
          -P  -fworking-directory  -remap 
          -trigraphs  -undef  -Umacro  -Wp,option 
          -Xpreprocessor option

Assembler Option

See Passing Options to the Assembler.

          -Wa,option  -Xassembler option

Linker Options

See Options for Linking.

          object-file-name  -llibrary 
          -nostartfiles  -nodefaultlibs  -nostdlib -pie -rdynamic 
          -s  -static  -static-libgcc  -shared  -shared-libgcc  -symbolic 
          -T script  -Wl,option  -Xlinker option 
          -u symbol

Directory Options

See Options for Directory Search.

          -Bprefix  -Idir  -iquotedir  -Ldir
          -specs=file  -I- --sysroot=dir

Target Options

See Target Options.

          -V version  -b machine

Machine Dependent Options

See Hardware Models and Configurations.

ARC Options

          -EB  -EL 
          -mmangle-cpu  -mcpu=cpu  -mtext=text-section 
          -mdata=data-section  -mrodata=readonly-data-section

ARM Options

          -mapcs-frame  -mno-apcs-frame 
          -mabi=name 
          -mapcs-stack-check  -mno-apcs-stack-check 
          -mapcs-float  -mno-apcs-float 
          -mapcs-reentrant  -mno-apcs-reentrant 
          -msched-prolog  -mno-sched-prolog 
          -mlittle-endian  -mbig-endian  -mwords-little-endian 
          -mfloat-abi=name  -msoft-float  -mhard-float  -mfpe 
          -mthumb-interwork  -mno-thumb-interwork 
          -mcpu=name  -march=name  -mfpu=name  
          -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 
          -mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns 
          -mpoke-function-name 
          -mthumb  -marm 
          -mtpcs-frame  -mtpcs-leaf-frame 
          -mcaller-super-interworking  -mcallee-super-interworking 
          -mtp=name 
          -mword-relocations 
          -mfix-cortex-m3-ldrd

AVR Options

          -mmcu=mcu  -msize  -mno-interrupts 
          -mcall-prologues  -mno-tablejump  -mtiny-stack  -mint8

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

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

CRX Options

          -mmac -mpush-args

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  -malpha-as  -mgas 
          -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

DEC Alpha/VMS Options

          -mvms-return-codes

FR30 Options

          -msmall-model -mno-lsim

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

          -muclibc

H8/300 Options

          -mrelax  -mh  -ms  -mn  -mint32  -malign-300

HPPA Options

          -march=architecture-type 
          -mbig-switch  -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-big-switch  -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

i386 and x86-64 Options

          -mtune=cpu-type  -march=cpu-type 
          -mfpmath=unit 
          -masm=dialect  -mno-fancy-math-387 
          -mno-fp-ret-in-387  -msoft-float 
          -mno-wide-multiply  -mrtd  -malign-double 
          -mpreferred-stack-boundary=num
          -mincoming-stack-boundary=num
          -mcld -mcx16 -msahf -mrecip 
          -mmmx  -msse  -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx 
          -maes -mpclmul 
          -msse4a -m3dnow -mpopcnt -mabm -msse5 
          -mthreads  -mno-align-stringops  -minline-all-stringops 
          -minline-stringops-dynamically -mstringop-strategy=alg 
          -mpush-args  -maccumulate-outgoing-args  -m128bit-long-double 
          -m96bit-long-double  -mregparm=num  -msseregparm 
          -mveclibabi=type -mpc32 -mpc64 -mpc80 -mstackrealign 
          -momit-leaf-frame-pointer  -mno-red-zone -mno-tls-direct-seg-refs 
          -mcmodel=code-model 
          -m32  -m64 -mlarge-data-threshold=num 
          -mfused-madd -mno-fused-madd -msse2avx

IA-64 Options

          -mbig-endian  -mlittle-endian  -mgnu-as  -mgnu-ld  -mno-pic 
          -mvolatile-asm-stop  -mregister-names  -mno-sdata 
          -mconstant-gp  -mauto-pic  -minline-float-divide-min-latency 
          -minline-float-divide-max-throughput 
          -minline-int-divide-min-latency 
          -minline-int-divide-max-throughput  
          -minline-sqrt-min-latency -minline-sqrt-max-throughput 
          -mno-dwarf2-asm -mearly-stop-bits 
          -mfixed-range=register-range -mtls-size=tls-size 
          -mtune=cpu-type -mt -pthread -milp32 -mlp64 
          -mno-sched-br-data-spec -msched-ar-data-spec -mno-sched-control-spec 
          -msched-br-in-data-spec -msched-ar-in-data-spec -msched-in-control-spec 
          -msched-ldc -mno-sched-control-ldc -mno-sched-spec-verbose 
          -mno-sched-prefer-non-data-spec-insns 
          -mno-sched-prefer-non-control-spec-insns 
          -mno-sched-count-spec-in-critical-path

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

M68hc1x Options

          -m6811  -m6812  -m68hc11  -m68hc12   -m68hcs12 
          -mauto-incdec  -minmax  -mlong-calls  -mshort 
          -msoft-reg-count=count

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

MIPS Options

          -EL  -EB  -march=arch  -mtune=arch 
          -mips1  -mips2  -mips3  -mips4  -mips32  -mips32r2 
          -mips64  -mips64r2 
          -mips16  -mno-mips16  -mflip-mips16 
          -minterlink-mips16  -mno-interlink-mips16 
          -mabi=abi  -mabicalls  -mno-abicalls 
          -mshared  -mno-shared  -mplt  -mno-plt  -mxgot  -mno-xgot 
          -mgp32  -mgp64  -mfp32  -mfp64  -mhard-float  -msoft-float 
          -msingle-float  -mdouble-float  -mdsp  -mno-dsp  -mdspr2  -mno-dspr2 
          -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 
          -mmemcpy  -mno-memcpy  -mlong-calls  -mno-long-calls 
          -mmad  -mno-mad  -mfused-madd  -mno-fused-madd  -nocpp 
          -mfix-r4000  -mno-fix-r4000  -mfix-r4400  -mno-fix-r4400 
          -mfix-r10000 -mno-fix-r10000  -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 
          -mfp-exceptions -mno-fp-exceptions 
          -mvr4130-align -mno-vr4130-align

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 
          -mam33  -mno-am33 
          -mam33-2  -mno-am33-2 
          -mreturn-pointer-on-d0 
          -mno-crt0  -mrelax

PDP-11 Options

          -mfpu  -msoft-float  -mac0  -mno-ac0  -m40  -m45  -m10 
          -mbcopy  -mbcopy-builtin  -mint32  -mno-int16 
          -mint16  -mno-int32  -mfloat32  -mno-float64 
          -mfloat64  -mno-float32  -mabshi  -mno-abshi 
          -mbranch-expensive  -mbranch-cheap 
          -msplit  -mno-split  -munix-asm  -mdec-asm

picoChip Options

          -mae=ae_type -mvliw-lookahead=N
          -msymbol-as-address -mno-inefficient-warnings

PowerPC Options See RS/6000 and PowerPC Options.

RS/6000 and PowerPC Options

          -mcpu=cpu-type 
          -mtune=cpu-type 
          -mpower  -mno-power  -mpower2  -mno-power2 
          -mpowerpc  -mpowerpc64  -mno-powerpc 
          -maltivec  -mno-altivec 
          -mpowerpc-gpopt  -mno-powerpc-gpopt 
          -mpowerpc-gfxopt  -mno-powerpc-gfxopt 
          -mmfcrf  -mno-mfcrf  -mpopcntb  -mno-popcntb  -mfprnd  -mno-fprnd 
          -mcmpb -mno-cmpb -mmfpgpr -mno-mfpgpr -mhard-dfp -mno-hard-dfp 
          -mnew-mnemonics  -mold-mnemonics 
          -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 
          -msingle-float -mdouble-float -msimple-fpu 
          -mstring  -mno-string  -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  -maltivec  -mswdiv 
          -mprioritize-restricted-insns=priority 
          -msched-costly-dep=dependence_type 
          -minsert-sched-nops=scheme 
          -mcall-sysv  -mcall-netbsd 
          -maix-struct-return  -msvr4-struct-return 
          -mabi=abi-type -msecure-plt -mbss-plt 
          -misel -mno-isel 
          -misel=yes  -misel=no 
          -mspe -mno-spe 
          -mspe=yes  -mspe=no 
          -mpaired 
          -mgen-cell-microcode -mwarn-cell-microcode 
          -mvrsave -mno-vrsave 
          -mmulhw -mno-mulhw 
          -mdlmzb -mno-dlmzb 
          -mfloat-gprs=yes  -mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double 
          -mprototype  -mno-prototype 
          -msim  -mmvme  -mads  -myellowknife  -memb  -msdata 
          -msdata=opt  -mvxworks  -G num  -pthread

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 
          -mtpf-trace -mno-tpf-trace  -mfused-madd  -mno-fused-madd 
          -mwarn-framesize  -mwarn-dynamicstack  -mstack-size -mstack-guard

Score Options

          -meb -mel 
          -mnhwloop 
          -muls 
          -mmac 
          -mscore5 -mscore5u -mscore7 -mscore7d

SH Options

          -m1  -m2  -m2e  -m3  -m3e 
          -m4-nofpu  -m4-single-only  -m4-single  -m4 
          -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al 
          -m5-64media  -m5-64media-nofpu 
          -m5-32media  -m5-32media-nofpu 
          -m5-compact  -m5-compact-nofpu 
          -mb  -ml  -mdalign  -mrelax 
          -mbigtable  -mfmovd  -mhitachi -mrenesas -mno-renesas -mnomacsave 
          -mieee  -mbitops  -misize  -minline-ic_invalidate -mpadstruct  -mspace 
          -mprefergot  -musermode -multcost=number -mdiv=strategy 
          -mdivsi3_libfunc=name -mfixed-range=register-range 
          -madjust-unroll -mindexed-addressing -mgettrcost=number -mpt-fixed 
          -minvalid-symbols

SPARC Options

          -mcpu=cpu-type 
          -mtune=cpu-type 
          -mcmodel=code-model 
          -m32  -m64  -mapp-regs  -mno-app-regs 
          -mfaster-structs  -mno-faster-structs 
          -mfpu  -mno-fpu  -mhard-float  -msoft-float 
          -mhard-quad-float  -msoft-quad-float 
          -mimpure-text  -mno-impure-text  -mlittle-endian 
          -mstack-bias  -mno-stack-bias 
          -munaligned-doubles  -mno-unaligned-doubles 
          -mv8plus  -mno-v8plus  -mvis  -mno-vis
          -threads -pthreads -pthread

SPU Options

          -mwarn-reloc -merror-reloc 
          -msafe-dma -munsafe-dma 
          -mbranch-hints 
          -msmall-mem -mlarge-mem -mstdmain 
          -mfixed-range=register-range

System V Options

          -Qy  -Qn  -YP,paths  -Ym,dir

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 
          -mv850e1 
          -mv850e 
          -mv850  -mbig-switch

VAX Options

          -mg  -mgnu  -munix

VxWorks Options

          -mrtp  -non-static  -Bstatic  -Bdynamic 
          -Xbind-lazy  -Xbind-now

x86-64 Options See i386 and x86-64 Options.

i386 and x86-64 Windows Options

          -mconsole -mcygwin -mno-cygwin -mdll
          -mnop-fun-dllimport -mthread -mwin32 -mwindows

Xstormy16 Options

          -msim

Xtensa Options

          -mconst16 -mno-const16 
          -mfused-madd  -mno-fused-madd 
          -mserialize-volatile  -mno-serialize-volatile 
          -mtext-section-literals  -mno-text-section-literals 
          -mtarget-align  -mno-target-align 
          -mlongcalls  -mno-longcalls

zSeries Options See S/390 and zSeries Options.

Code Generation Options

See Options for Code Generation Conventions.

          -fcall-saved-reg  -fcall-used-reg 
          -ffixed-reg  -fexceptions 
          -fnon-call-exceptions  -funwind-tables 
          -fasynchronous-unwind-tables 
          -finhibit-size-directive  -finstrument-functions 
          -finstrument-functions-exclude-function-list=sym,sym,... 
          -finstrument-functions-exclude-file-list=file,file,... 
          -fno-common  -fno-ident 
          -fpcc-struct-return  -fpic  -fPIC -fpie -fPIE 
          -fno-jump-tables 
          -frecord-gcc-switches 
          -freg-struct-return  -fshort-enums 
          -fshort-double  -fshort-wchar 
          -fverbose-asm  -fpack-struct[=n]  -fstack-check 
          -fstack-limit-register=reg  -fstack-limit-symbol=sym 
          -fno-stack-limit  -fargument-alias  -fargument-noalias 
          -fargument-noalias-global  -fargument-noalias-anything 
          -fleading-underscore  -ftls-model=model 
          -ftrapv  -fwrapv  -fbounds-check 
          -fvisibility

• Overall Options: Controlling the kind of output: an executable, object files, assembler files, or preprocessed source.
• C Dialect Options: Controlling the variant of C language compiled.
• C++ Dialect Options: Variations on C++.
• Objective-C and Objective-C++ Dialect Options: Variations on Objective-C and Objective-C++.
• Language Independent Options: Controlling how diagnostics should be formatted.
• Warning Options: How picky should the compiler be?
• Debugging Options: Symbol tables, measurements, and debugging dumps.
• Optimize Options: How much optimization?
• 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.
• Spec Files: How to pass switches to sub-processes.
• Target Options: Running a cross-compiler, or an old version of 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 which must be preprocessed.

file.i

C source code which should not be preprocessed.

file.ii

C++ source code which 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 which 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 which should not be preprocessed.

file.h

C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header.

file.cc

file.cp

file.cxx

file.cpp

file.CPP

file.c++

file.C

C++ source code which 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 which must be preprocessed.

file.mii

Objective-C++ source code which 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.

file.f

file.for

file.ftn

Fixed form Fortran source code which should not be preprocessed.

file.F

file.FOR

file.fpp

file.FPP

file.FTN

Fixed form Fortran source code which must be preprocessed (with the traditional preprocessor).

file.f90

file.f95

file.f03

file.f08

Free form Fortran source code which should not be preprocessed.

file.F90

file.F95

file.F03

file.F08

Free form Fortran source code which must be preprocessed (with the traditional preprocessor).

file.ads

Ada source code file which 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 which 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  c-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
          f77  f77-cpp-input f95  f95-cpp-input
          java

-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).

-pass-exit-codes

Normally the gcc program will exit 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 will instead return with numerically highest error produced by any phase that returned an error indication. The C, C++, and Fortran frontends return 4, if an internal compiler error is encountered.

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 which don't require preprocessing are ignored.

-o file

Place output in file file. This applies regardless 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 all command arguments are quoted. This is useful for shell scripts to capture the driver-generated command lines.

-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.

-combine

If you are compiling multiple source files, this option tells the driver to pass all the source files to the compiler at once (for those languages for which the compiler can handle this). This will allow intermodule analysis (IMA) to be performed by the compiler. Currently the only language for which this is supported is C. If you pass source files for multiple languages to the driver, using this option, the driver will invoke the compiler(s) that support IMA once each, passing each compiler all the source files appropriate for it. For those languages that do not support IMA this option will be ignored, and the compiler will be invoked once for each source file in that language. If you use this option in conjunction with -save-temps, the compiler will generate multiple pre-processed files (one for each source file), but only one (combined) .o or .s file.

--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 will also be 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 which have no documentation associated with them will also be 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’

This will display all of the optimization options supported by the compiler.

‘warnings’

This will display all of the options controlling warning messages produced by the compiler.

‘target’

This will display target-specific options. Unlike the --target-help option however, target-specific options of the linker and assembler will not be displayed. This is because those tools do not currently support the extended --help= syntax.

‘params’

This will display the values recognized by the --param option.

language

This will display the options supported for language, where language is the name of one of the languages supported in this version of GCC.

‘common’

This will display the options that are common to all languages.

These are the supported qualifiers:

‘undocumented’

Display only those options which are undocumented.

‘joined’

Display options which take an argument that appears after an equal sign in the same continuous piece of text, such as: ‘--help=target’.

‘separate’

Display options which take 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 the following can be used:

          --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), which have a description the following can be used:

          --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 by so much that there is nothing to display. One case where it does work however is when one of the classes is target. So for example to display all the target-specific optimization options the following can be used:

          --help=target,optimizers

The --help= option can be repeated on the command line. Each successive use will display its requested class of options, skipping those that have already been displayed.

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.

-wrapper

Invoke all subcommands under a wrapper program. It takes a single comma separated list as an argument, which will be used to invoke the wrapper:

          gcc -c t.c -wrapper gdb,--args

This will invoke all subprograms of gcc under "gdb –args", thus cc1 invocation will be "gdb –args cc1 ...".

-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.

@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.

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 treats ‘.c’, ‘.h’ and ‘.i’ files as C++ source files instead of C source files unless -x is used, and automatically specifies linking against the C++ library. 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.

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=c89’. 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, -pedantic 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 would normally be 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 ‘c89’ or ‘c++98’, and GNU dialects of those standards, such as ‘gnu89’ or ‘gnu++98’. By specifying a base standard, the compiler will accept all programs following that standard and those using GNU extensions that do not contradict it. For example, ‘-std=c89’ 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, by specifying a GNU dialect of a standard, all features the compiler support are enabled, even when those features change the meaning of the base standard and some strict-conforming programs may be rejected. The particular standard is used by -pedantic to identify which features are GNU extensions given that version of the standard. For example ‘-std=gnu89 -pedantic’ would warn about C++ style ‘//’ comments, while ‘-std=gnu99 -pedantic’ would not.

A value for this option must be provided; possible values are

‘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. Note that this standard is not yet fully supported; see http://gcc.gnu.org/gcc-4.4/c99status.html for more information. The names ‘c9x’ and ‘iso9899:199x’ are deprecated.

‘gnu89’

GNU dialect of ISO C90 (including some C99 features). This is the default for C code.

‘gnu99’

‘gnu9x’

GNU dialect of ISO C99. When ISO C99 is fully implemented in GCC, this will become the default. The name ‘gnu9x’ is deprecated.

‘c++98’

The 1998 ISO C++ standard plus amendments. Same as -ansi for C++ code.

‘gnu++98’

GNU dialect of -std=c++98. This is the default for C++ code.

‘c++0x’

The working draft of the upcoming ISO C++0x standard. This option enables experimental features that are likely to be included in C++0x. The working draft is constantly changing, and any feature that is enabled by this flag may be removed from future versions of GCC if it is not part of the C++0x standard.

‘gnu++0x’

GNU dialect of -std=c++0x. This option enables experimental features that may be removed in future versions of GCC.

-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. This option is accepted and ignored by GCC versions 4.1.3 up to but not including 4.3. In GCC versions 4.3 and later it changes the behavior of GCC in C99 mode. 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 was first supported in GCC 4.3. This option is not supported in C89 or gnu89 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.

-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.

-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 that 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))

-fhosted

Assert that compilation takes place in 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 takes place in 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.

-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 v2.5 http://www.openmp.org/. This option implies -pthread, and thus is only supported on targets that have support for -pthread.

-fms-extensions

Accept some non-standard constructs used in Microsoft header files.

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.

-trigraphs

Support ISO C trigraphs. The -ansi option (and -std options for strict ISO C conformance) implies -trigraphs.

-no-integrated-cpp

Performs a compilation in two passes: preprocessing and compiling. This option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the -B option. The user supplied compilation step can then add in an additional preprocessing step after normal preprocessing but before compiling. The default is to use the integrated cpp (internal cpp)

The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged.

-traditional

-traditional-cpp

Formerly, these options caused GCC to attempt to emulate a pre-standard C compiler. They are now only supported with the -E switch. The preprocessor continues to support a pre-standard mode. See the GNU CPP manual for details.

-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.

3.5 Options Controlling C++ Dialect

This section describes the command-line options that are only meaningful for C++ programs; but 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 -frepo -O -c firstClass.C

In this example, only -frepo is an option meant only for C++ programs; you can use the other options with any language supported by GCC.

Here is a list of options that are only for compiling C++ programs:

-fabi-version=n

Use version n of the C++ ABI. Version 2 is the version of the C++ ABI that first appeared in G++ 3.4. Version 1 is the version of the C++ ABI that first appeared in G++ 3.2. Version 0 will always be the version that conforms most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change as ABI bugs are fixed.

The default is version 2.

-fno-access-control

Turn off all access checking. This switch is mainly useful for working around bugs in the access control code.

-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 will only return 0 if it is declared ‘throw()’, in which case the compiler will always check 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)’.

-fconserve-space

Put uninitialized or runtime-initialized global variables into the common segment, as C does. This saves space in the executable at the cost of not diagnosing duplicate definitions. If you compile with this flag and your program mysteriously crashes after main() has completed, you may have an object that is being destroyed twice because two definitions were merged.

This option is no longer useful on most targets, now that support has been added for putting variables into BSS without making them common.

-fno-deduce-init-list

Disable deduction of a template type parameter as std::initializer_list from a brace-enclosed initializer list, i.e.

          template <class T> auto forward(T t) -> decltype (realfn (t))
          {
            return realfn (t);
          }
          
          void f()
          {
            forward({1,2}); // call forward<std::initializer_list<int>>
          }

This option is present because this deduction is an extension to the current specification in the C++0x working draft, and there was some concern about potential overload resolution problems.

-ffriend-injection

Inject friend functions into the enclosing namespace, so that they are visible outside the scope of the class in which they are declared. Friend functions were documented to work this way in the old Annotated C++ Reference Manual, and versions of G++ before 4.1 always worked that way. However, in ISO C++ a friend function which is not declared in an enclosing scope can only be found using argument dependent lookup. This option causes friends to be injected as they were in earlier releases.

This option is for compatibility, and may be removed in a future release of G++.

-fno-elide-constructors

The C++ standard allows an implementation to omit creating a temporary which 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.

-fno-enforce-eh-specs

Don't generate code to check for violation of exception specifications at runtime. 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 will still optimize based on the specifications, so throwing an unexpected exception will result in undefined behavior.

-ffor-scope

-fno-for-scope

If -ffor-scope is specified, the scope of variables declared in a for-init-statement is limited to the ‘for’ loop itself, as specified by the C++ standard. If -fno-for-scope is specified, the scope of variables declared in a for-init-statement extends to the end of the enclosing scope, as was the case in old versions of G++, and other (traditional) implementations of C++.

The default if neither flag is given to follow the standard, but to allow and give a warning for old-style code that would otherwise be invalid, or have different behavior.

-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. -ansi implies -fno-gnu-keywords.

-fno-implicit-templates

Never emit code for non-inline templates which are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. 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 will 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 will cause linker errors if these functions are not inlined everywhere they are called.

-fms-extensions

Disable pedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax.

-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.

-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 will allow some nonconforming code to compile.

-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++ runtime 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 it will generate it as needed. The ‘dynamic_cast’ operator can still be used for casts that do not require runtime type information, i.e. casts to void * or to unambiguous base classes.

-fstats

Emit statistics about front-end processing at the end of the compilation. This information is generally only useful to the G++ development team.

-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.

-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 will only work if your C library supports __cxa_atexit.

-fno-use-cxa-get-exception-ptr

Don't use the __cxa_get_exception_ptr runtime routine. This will cause 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 methods where the addresses of the two functions were 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 will have 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 which are defined in more than one different shared object: those declarations are permitted if they would have been permitted when this option was not used.

In new code it is better to use -fvisibility=hidden and export those classes which 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 will be different, so changing one will 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++ will use weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it will result 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:

-fno-default-inline

Do not assume ‘inline’ for functions defined inside a class scope. See Options That Control Optimization. Note that these functions will have linkage like inline functions; they just won't be inlined by default.

-Wabi (C, Objective-C, C++ and Objective-C++ only)

Warn when G++ generates code that is probably not compatible with the vendor-neutral C++ ABI. 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 will be 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.

The known incompatibilities at this point include:

• Incorrect handling of tail-padding for bit-fields. G++ may attempt to pack data into the same byte as a base class. For example:

                 struct A { virtual void f(); int f1 : 1; };
                 struct B : public A { int f2 : 1; };
  

  In this case, G++ will place B::f2 into the same byte asA::f1; other compilers will not. You can avoid this problem by explicitly padding A so that its size is a multiple of the byte size on your platform; that will cause G++ and other compilers to layout B identically.

• Incorrect handling of tail-padding for virtual bases. G++ does not use tail padding when laying out virtual bases. For example:

                 struct A { virtual void f(); char c1; };
                 struct B { B(); char c2; };
                 struct C : public A, public virtual B {};
  

  In this case, G++ will not place B into the tail-padding for A; other compilers will. You can avoid this problem by explicitly padding A so that its size is a multiple of its alignment (ignoring virtual base classes); that will cause G++ and other compilers to layout C identically.

• Incorrect handling of bit-fields with declared widths greater than that of their underlying types, when the bit-fields appear in a union. For example:

                 union U { int i : 4096; };
  

  Assuming that an int does not have 4096 bits, G++ will make the union too small by the number of bits in an int.

• Empty classes can be placed at incorrect offsets. For example:

                 struct A {};
                 
                 struct B {
                   A a;
                   virtual void f ();
                 };
                 
                 struct C : public B, public A {};
  

  G++ will place the A base class of C at a nonzero offset; it should be placed at offset zero. G++ mistakenly believes that the A data member of B is already at offset zero.

• Names of template functions whose types involve typename or template template parameters can be mangled incorrectly.

                 template <typename Q>
                 void f(typename Q::X) {}
                 
                 template <template <typename> class Q>
                 void f(typename Q<int>::X) {}
  

  Instantiations of these templates may be mangled incorrectly.

It also warns psABI related changes. The known psABI changes at this point include:

• For SYSV/x86-64, when passing union with long double, it is changed to pass in memory as specified in psABI. For example:

                 union U {
                   long double ld;
                   int i;
                 };
  

  union U will always be passed in memory.

-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.

-Wnon-virtual-dtor (C++ and Objective-C++ only)

Warn when a class has virtual functions and accessible non-virtual destructor, in which case it would be possible but unsafe to delete an instance of a derived class through a pointer to the base class. This warning is also enabled if -Weffc++ is specified.

-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 will rearrange 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.

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++ book:

• Item 11: Define a copy constructor and an assignment operator for classes with dynamically allocated memory.
• Item 12: Prefer initialization to assignment in constructors.
• Item 14: Make destructors virtual in base classes.
• Item 15: Have operator= return a reference to *this.
• Item 23: Don't try to return a reference when you must return an object.

Also warn about violations of the following style guidelines from Scott Meyers' More Effective C++ book:

• Item 6: Distinguish between prefix and postfix forms of increment and decrement operators.
• Item 7: Never overload &&, ||, or ,.

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 also 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 not 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-templatized friend functions are declared within a template. Since the advent of explicit template specification support in G++, if the name of the friend is an unqualified-id (i.e., ‘friend foo(int)’), the C++ language specification demands that the friend declare or define an ordinary, nontemplate function. (Section 14.5.3). Before G++ implemented explicit specification, unqualified-ids could be interpreted as a particular specialization of a templatized function. Because this non-conforming behavior is no longer the default behavior for G++, -Wnon-template-friend allows the compiler to check existing code for potential trouble spots and is on by default. This new compiler behavior can be turned off with -Wno-non-template-friend which keeps the conformant compiler code but disables the helpful warning.

-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();

will fail 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++ would try to preserve unsignedness, but the standard mandates the current behavior.

          struct A {
            operator int ();
            A& operator = (int);
          };
          
          main ()
          {
            A a,b;
            a = b;
          }

In this example, G++ will synthesize a default ‘A& operator = (const A&);’, while cfront will use the user-defined ‘operator =’.

3.6 Options Controlling Objective-C and Objective-C++ Dialects

(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves. See See Language Standards Supported by GCC, for references.)

This section describes the command-line options that are only meaningful for Objective-C and Objective-C++ programs, but you can also use most of the language-independent GNU compiler options. For example, you might compile a file some_class.m like this:

     gcc -g -fgnu-runtime -O -c some_class.m

In this example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; you can use the other options with any language supported by GCC.

Note that since Objective-C is an extension of the C language, Objective-C compilations may also use options specific to the C front-end (e.g., -Wtraditional). Similarly, Objective-C++ compilations may use C++-specific options (e.g., -Wabi).

Here is a list of options that are only for compiling Objective-C and Objective-C++ programs:

-fconstant-string-class=class-name

Use class-name as the name of the class to instantiate for each literal string specified with the syntax @"...". The default class name is NXConstantString if the GNU runtime is being used, and NSConstantString if the NeXT runtime is being used (see below). The -fconstant-cfstrings option, if also present, will override the -fconstant-string-class setting and cause @"..." literals to be laid out as constant CoreFoundation strings.

-fgnu-runtime

Generate object code compatible with the standard GNU Objective-C runtime. This is the default for most types of systems.

-fnext-runtime

Generate output compatible with the NeXT runtime. This is the default for NeXT-based systems, including Darwin and Mac OS X. The macro __NEXT_RUNTIME__ is predefined if (and only if) this option is used.

-fno-nil-receivers

Assume that all Objective-C message dispatches (e.g., [receiver message:arg]) in this translation unit ensure that the receiver is not nil. This allows for more efficient entry points in the runtime to be used. Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.

-fobjc-call-cxx-cdtors

For each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial default constructor. If so, synthesize a special - (id) .cxx_construct instance method that will run non-trivial default constructors on any such instance variables, in order, and then return self. Similarly, check if any instance variable is a C++ object with a non-trivial destructor, and if so, synthesize a special - (void) .cxx_destruct method that will run all such default destructors, in reverse order.

The - (id) .cxx_construct and/or - (void) .cxx_destruct methods thusly generated will only operate on instance variables declared in the current Objective-C class, and not those inherited from superclasses. It is the responsibility of the Objective-C runtime to invoke all such methods in an object's inheritance hierarchy. The - (id) .cxx_construct methods will be invoked by the runtime immediately after a new object instance is allocated; the - (void) .cxx_destruct methods will be invoked immediately before the runtime deallocates an object instance.

As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the - (id) .cxx_construct and - (void) .cxx_destruct methods.

-fobjc-direct-dispatch

Allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page.

-fobjc-exceptions

Enable syntactic support for structured exception handling in Objective-C, similar to what is offered by C++ and Java. This option is unavailable in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier.

            @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:

• 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).

The -fobjc-exceptions switch also enables the use of synchronization blocks for thread-safe execution:

            @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.

-fobjc-gc

Enable garbage collection (GC) in Objective-C and Objective-C++ programs.

-freplace-objc-classes

Emit a special marker instructing ld(1) not to statically link in the resulting object file, and allow dyld(1) to load it in at run time instead. This is used in conjunction with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded in the course of program execution, without the need to restart the program itself. Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.

-fzero-link

When compiling for the NeXT runtime, the compiler ordinarily replaces calls to objc_getClass("...") (when the name of the class is known at compile time) with static class references that get initialized at load time, which improves run-time performance. Specifying the -fzero-link flag suppresses this behavior and causes calls to objc_getClass("...") to be retained. This is useful in Zero-Link debugging mode, since it allows for individual class implementations to be modified during program execution.

-gen-decls

Dump interface declarations for all classes seen in the source file to a file named sourcename.decl.

-Wassign-intercept (Objective-C and Objective-C++ only)

Warn whenever an Objective-C assignment is being intercepted by the garbage collector.

-Wno-protocol (Objective-C and Objective-C++ only)

If a class is declared to implement a protocol, a warning is issued for every method in the protocol that is not implemented by the class. The default behavior is to issue a warning for every method not explicitly implemented in the class, even if a method implementation is inherited from the superclass. If you use the -Wno-protocol option, then methods inherited from the superclass are considered to be implemented, and no warning is issued for them.

-Wselector (Objective-C and Objective-C++ only)

Warn if multiple methods of different types for the same selector are found during compilation. The check is performed on the list of methods in the final stage of compilation. Additionally, a check is performed for each selector appearing in a @selector(...) expression, and a corresponding method for that selector has been found during compilation. Because these checks scan the method table only at the end of compilation, these warnings are not produced if the final stage of compilation is not reached, for example because an error is found during compilation, or because the -fsyntax-only option is being used.

-Wstrict-selector-match (Objective-C and Objective-C++ only)

Warn if multiple methods with differing argument and/or return types are found for a given selector when attempting to send a message using this selector to a receiver of type id or Class. When this flag is off (which is the default behavior), the compiler will omit such warnings if any differences found are confined to types which share the same size and alignment.

-Wundeclared-selector (Objective-C and Objective-C++ only)

Warn if a @selector(...) expression referring to an undeclared selector is found. A selector is considered undeclared if no method with that name has been declared before the @selector(...) expression, either explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section. This option always performs its checks as soon as a @selector(...) expression is found, while -Wselector only performs its checks in the final stage of compilation. This also enforces the coding style convention that methods and selectors must be declared before being used.

-print-objc-runtime-info

Generate C header describing the largest structure that is passed by value, if any.

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, ...). The options described below can be used to control the diagnostic messages formatting algorithm, e.g. how many characters per line, how often source location information should be reported. Right now, only the C++ front end can honor these options. However it is expected, in the near future, that the remaining front ends would be able to digest them correctly.

-fmessage-length=n

Try to format error messages so that they fit on lines of about n characters. The default is 72 characters for g++ and 0 for the rest of the front ends supported by GCC. If n is zero, then no line-wrapping will be done; each error message will appear on a single line.

-fdiagnostics-show-location=once

Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit once source location information; 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-show-option

This option instructs the diagnostic machinery to add text to each diagnostic emitted, which indicates which command line option directly controls that diagnostic, when such an option is known to the diagnostic machinery.

-Wcoverage-mismatch

Warn if feedback profiles do not match when using the -fprofile-use option. If a source file was changed between -fprofile-gen and -fprofile-use, the files with the profile feedback can fail to match the source file and GCC can not use the profile feedback information. By default, GCC emits an error message in this case. The option -Wcoverage-mismatch emits a warning instead of an error. GCC does not use appropriate feedback profiles, so using this option can result in poorly optimized code. This option is useful only in the case of very minor changes such as bug fixes to an existing code-base.

3.8 Options to Request or Suppress Warnings

Warnings are diagnostic messages that report constructions which are not inherently erroneous but which 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.

-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. You can use the -fdiagnostics-show-option option to have each controllable warning amended with the option which controls it, to determine what to use with this option.

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 ‘-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.

-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 will 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.

-pedantic 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 -pedantic 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 -pedantic. 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 ‘gnu89’ or ‘gnu99’, there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from -pedantic are given where they are required by the base standard. (It would 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

Like -pedantic, except that errors are produced rather than warnings.

-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 (only with -O2)  
          -Wc++0x-compat  
          -Wchar-subscripts  
          -Wimplicit-int  
          -Wimplicit-function-declaration  
          -Wcomment  
          -Wformat   
          -Wmain (only for C/ObjC and unless -ffreestanding)  
          -Wmissing-braces  
          -Wnonnull  
          -Wparentheses  
          -Wpointer-sign  
          -Wreorder   
          -Wreturn-type  
          -Wsequence-point  
          -Wsign-compare (only in C++)  
          -Wstrict-aliasing  
          -Wstrict-overflow=1  
          -Wswitch  
          -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  
          -Wempty-body  
          -Wignored-qualifiers 
          -Wmissing-field-initializers  
          -Wmissing-parameter-type (C only)  
          -Wold-style-declaration (C only)  
          -Woverride-init  
          -Wsign-compare  
          -Wtype-limits  
          -Wuninitialized  
          -Wunused-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 which has been declared ‘register’.
• (C++ only) Taking the address of a variable which has been declared ‘register’.
• (C++ only) A base class is not initialized in a derived class' copy constructor.

-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.

-Wcomment

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.

-Wformat

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 -pedantic is used with -Wformat, warnings will be 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.

Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull.

-Wformat is included in -Wall. For more control over some aspects of format checking, the options -Wformat-y2k, -Wno-format-extra-args, -Wno-format-zero-length, -Wformat-nonliteral, -Wformat-security, and -Wformat=2 are available, but are not included in -Wall.

NOTE: In Ubuntu 8.10 and later versions this option is enabled by default for C, C++, ObjC, ObjC++. To disable, use -Wformat=0.

-Wformat-y2k

If -Wformat is specified, also warn about strftime formats which may yield only a two-digit year.

-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 will suppress the warning if the unused arguments are all pointers, since the Single Unix Specification says that such unused arguments are allowed.

-Wno-format-zero-length (C and Objective-C only)

If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed.

-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.)

NOTE: In Ubuntu 8.10 and later versions this option is enabled by default for C, C++, ObjC, ObjC++. To disable, use -Wno-format-security, or disable all format warnings with -Wformat=0. To make format security warnings fatal, specify -Werror=format-security.

-Wformat=2

Enable -Wformat plus format checks not included in -Wformat. Currently equivalent to ‘-Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k’.

-Wnonnull (C and Objective-C only)

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.

-Winit-self (C, C++, Objective-C and Objective-C++ only)

Warn about uninitialized variables which are initialized with themselves. Note this option can only be used with the -Wuninitialized option.

For example, GCC will warn about i being uninitialized in the following snippet only when -Winit-self has been specified:

          int f()
          {
            int i = i;
            return i;
          }

-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

Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall.

-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.

-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 -pedantic.

-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.

          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.

-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 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 will issue 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 could belong to the enclosing if. The resulting code would look like this:

          {
            if (a)
              {
                if (b)
                  foo ();
                else
                  bar ();
              }
          }

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 defines 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 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++.

-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), and about a return statement with a expression in a function whose return-type is void.

For C++, a function without return type always produces a diagnostic message, even when -Wno-return-type is specified. The only exceptions are ‘main’ and functions defined in system headers.

This warning is enabled by -Wall.

-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. 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.

-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.

-Wtrigraphs

Warn if any trigraphs are encountered that might change the meaning of the program (trigraphs within comments are not warned about). This warning 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-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 Variable Attributes) does not use its return value. The default is -Wunused-result.

-Wunused-variable

Warn whenever a local variable or non-constant static variable is unused aside from its declaration. This warning is enabled by -Wall.

To suppress this warning use the ‘unused’ attribute (see Variable Attributes).

-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]’ will cause a warning, while ‘x[(void)i,j]’ will 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 which 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 which 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 will depend 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.

These warnings are made optional because GCC is not smart enough to see all the reasons why the code might be correct despite 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. Here is another common case:

          {
            int save_y;
            if (change_y) save_y = y, y = new_y;
            ...
            if (change_y) y = save_y;
          }

This has no bug because save_y is used only if it is set.

This option also warns when a non-volatile automatic variable might be changed by a call to longjmp. These warnings as well are possible only in optimizing compilation.

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 which 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 which is not understood by GCC. If this command line option is used, warnings will even be issued for unknown pragmas in system header files. This is not the case if the warnings were 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’.

-Wstrict-aliasing

This option is only active when -fstrict-aliasing is active. It warns about code which 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 which 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=n, with n=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 frontend 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 frontend 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 punn+dereference pattern in the frontend: *(int*)&some_float. If optimization is enabled, it also runs in the backend, 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 -fstrict-overflow is active. 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 which 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 which 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 will require, in particular when determining whether a loop will be executed at all.

-Wstrict-overflow=1

Warn about cases which are both questionable and easy to avoid. For example: x + 1 > x; with -fstrict-overflow, the compiler will simplify this 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 -fstrict-overflow is in effect, 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 will be simplified to x > 0.

-Wstrict-overflow=4

Also warn about other simplifications not covered by the above cases. For example: (x * 10) / 5 will be 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 will be 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 will give a very large number of false positives.

-Warray-bounds

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.

-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 will not warn about unknown pragmas in system headers—for that, -Wunknown-pragmas must also be used.

-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 would 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 which 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 does not in ISO C.
• In traditional C, some preprocessor directives did not exist. Traditional preprocessors would only consider 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 would ignore 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 would not recognize ‘#elif’, so it 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 <limits.h>. 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 would cause 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 will 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 and was not supported by GCC versions before GCC 3.0. See Mixed Declarations.

-Wundef

Warn if an undefined identifier is evaluated in an ‘#if’ directive.

-Wno-endif-labels

Do not warn whenever an ‘#else’ or an ‘#endif’ are followed by text.

-Wshadow

Warn whenever a local variable shadows another local variable, parameter or global variable or whenever a built-in function is shadowed.

-Wlarger-than=len

Warn whenever an object of larger than len bytes is defined.

-Wframe-larger-than=len

Warn if the size of a function frame is larger than len bytes. The computation done to determine the stack frame size is approximate and not conservative. The actual requirements may be somewhat greater than len 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.

-Wunsafe-loop-optimizations

Warn if the loop cannot be optimized because the compiler could not assume anything on the bounds of the loop indices. With -funsafe-loop-optimizations warn if the compiler made such assumptions.

-Wno-pedantic-ms-format (MinGW targets only)

Disables the warnings about non-ISO printf / scanf format width specifiers I32, I64, and I used on Windows targets depending on the MS runtime, when you are using the options -Wformat and -pedantic without gnu-extensions.

-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 -pedantic.

-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.

-Wbad-function-cast (C and Objective-C only)

Warn whenever a function call is cast to a non-matching type. For example, warn if int malloc() is cast to anything *.

-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++0x-compat (C++ and Objective-C++ only)

Warn about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 200x, e.g., identifiers in ISO C++ 1998 that will become keywords in ISO C++ 200x. 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 *.

-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.

-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 will get a warning. These warnings will 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 will just be 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.

-Wclobbered

Warn for variables that might be changed by ‘longjmp’ or ‘vfork’. This warning is also enabled by -Wextra.

-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 conversions between NULL and non-pointer types; confusing overload resolution for user-defined conversions; and conversions that will 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.

-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 (C++ and Objective-C++ only)

Warn about a comparison between values of different enum types. This warning is enabled by default.

-Wsign-compare

Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. This warning is also enabled by -Wextra; to get the other warnings of -Wextra without this warning, use ‘-Wextra -Wno-sign-compare’.

-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.

-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.

-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.

-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-attributes

Do not warn if an unexpected __attribute__ is used, such as unrecognized attributes, function attributes applied to variables, etc. This will not stop errors for incorrect use of supported attributes.

-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 which 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. The aim is to detect global functions that fail to be declared in header files.

-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 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 would cause 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 would not trigger a warning:

          struct s { int f, g, h; };
          struct s x = { .f = 3, .g = 4 };

This warning is included in -Wextra. To get other -Wextra warnings without this one, use ‘-Wextra -Wno-missing-field-initializers’.

-Wmissing-noreturn

Warn about functions which might be candidates for attribute noreturn. Note these are only possible candidates, not absolute ones. Care should be taken to manually verify functions actually do not ever return before adding the noreturn attribute, otherwise subtle code generation bugs could be introduced. You will not get a warning for main in hosted C environments.

-Wmissing-format-attribute

Warn about function pointers which might be candidates for format attributes. Note these are only possible candidates, not absolute ones. GCC will guess 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 will also warn about function definitions which might be candidates for format attributes. Again, these are only possible candidates. GCC will guess 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.

-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 which have not been normalized; this option controls that warning.

There are four levels of warning that GCC supports. The default is -Wnormalized=nfc, which warns about any identifier which is not in the ISO 10646 “C” normalized form, NFC. NFC is the recommended form for most uses.

Unfortunately, there are some characters which ISO C and ISO C++ allow in identifiers that when turned into NFC aren't allowable as 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. You would only want to do this if you were 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”, will display just like a regular n which 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 will warn 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 is unable to be fixed to display these characters distinctly.

-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.

-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’.

-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 will be 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.

-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.

-Wnested-externs (C and Objective-C only)

Warn if an extern declaration is encountered within a function.

-Wunreachable-code

Warn if the compiler detects that code will never be executed.

This option is intended to warn when the compiler detects that at least a whole line of source code will never be executed, because some condition is never satisfied or because it is after a procedure that never returns.

It is possible for this option to produce a warning even though there are circumstances under which part of the affected line can be executed, so care should be taken when removing apparently-unreachable code.

For instance, when a function is inlined, a warning may mean that the line is unreachable in only one inlined copy of the function.

This option is not made part of -Wall because in a debugging version of a program there is often substantial code which checks correct functioning of the program and is, hopefully, unreachable because the program does work. Another common use of unreachable code is to provide behavior which is selectable at compile-time.

-Winline

Warn if a function can not be inlined and it was declared as inline. Even with this option, the compiler will 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 1998 ISO C++ standard, applying ‘offsetof’ to a non-POD type is undefined. In existing C++ implementations, however, ‘offsetof’ typically gives meaningful results even when applied to certain kinds of non-POD types. (Such as a simple ‘struct’ that fails to be a POD type only by virtue of having a constructor.) 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.

-Wno-int-to-pointer-cast (C and Objective-C only)

Suppress warnings from casts to pointer type of an integer of a different size.

-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 can't be used.

-Wlong-long

Warn if ‘long long’ type is used. This is default. To inhibit the warning messages, use -Wno-long-long. Flags -Wlong-long and -Wno-long-long are taken into account only when -pedantic flag is used.

-Wvariadic-macros

Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU alternate syntax when in pedantic ISO C99 mode. This is default. To inhibit the warning messages, use -Wno-variadic-macros.

-Wvla

Warn if variable length array is used in the code. -Wno-vla will prevent the -pedantic warning of the variable length array.

-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 were unable to handle the code effectively. Often, the problem is that your code is too big or too complex; GCC will refuse 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 -pedantic, 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 will not be protected against stack smashing.

-Wno-mudflap

Suppress warnings about constructs that cannot be instrumented by -fmudflap.

-Woverlength-strings

Warn about string constants which are longer than the “minimum maximum” length specified in the C standard. Modern compilers generally allow string constants which 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 C89, 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 -pedantic, and can be disabled with -Wno-overlength-strings.

3.9 Options for Debugging Your Program or GCC

GCC has various special options that are used for debugging either your program or GCC:

-g

Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF 2). 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 will probably make 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).

GCC allows you to use -g with -O. The shortcuts taken by optimized code may occasionally produce surprising results: 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 were already at hand; some statements may execute in different places because they were moved out of loops.

Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs.

The following options are useful when GCC is generated with the capability for more than one debugging format.

-ggdb

Produce debugging information for use by GDB. This means to use the most expressive format available (DWARF 2, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible.

-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 which is not understood by DBX or SDB. On System V Release 4 systems this option requires the GNU assembler.

-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 will increase the size of debugging information by as much as a factor of two.

-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.

-gcoff

Produce debugging information in COFF format (if that is supported). This is the format used by SDB on most System V systems prior to System V Release 4.

-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.

-gdwarf-2

Produce debugging information in DWARF version 2 format (if that is supported). This is the format used by DBX on IRIX 6. With this option, GCC uses features of DWARF version 3 when they are useful; version 3 is upward compatible with version 2, but may still cause problems for older debuggers.

-gvms

Produce debugging information in VMS debug format (if that is supported). This is the format used by DEBUG on VMS systems.

-glevel

-ggdblevel

-gstabslevel

-gcofflevel

-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, but no information about local variables and no line numbers.

Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3.

-gdwarf-2 does not accept a concatenated debug level, because GCC used to support an option -gdwarf that meant to generate debug information in version 1 of the DWARF format (which is very different from version 2), and it would have been too confusing. That debug format is long obsolete, but the option cannot be changed now. Instead use an additional -glevel option to change the debug level for DWARF2.

-feliminate-dwarf2-dups

Compress DWARF2 debugging information by eliminating duplicated information about each symbol. This option only makes sense when generating DWARF2 debugging information with -gdwarf-2.

-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 was 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 2.

-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 was 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 2.

-femit-struct-debug-detailed[=spec-list]

Specify the struct-like types for which the compiler will generate 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 will serve 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 would be legal, 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 will 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 types declared in foo.c and foo.h will have debug information, but types declared in other header will not. 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 2.

-fno-merge-debug-strings

Direct the linker to not merge together strings in the debugging information which 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 in directory old, record debugging information describing them as in new instead.

-fno-dwarf2-cfi-asm

Emit DWARF 2 unwind info as compiler generated .eh_frame section instead of using GAS .cfi_* directives.

-p

Generate extra code to write profile information suitable for the analysis program prof. You must use this option when compiling the source files you want data about, and you must also use it when linking.

-pg

Generate extra code to write profile information suitable for the analysis program gprof. You must use this option when compiling the source files you want data about, and you must also use it when linking.

-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.

-fmem-report

Makes the compiler print some statistics about permanent memory allocation when it finishes.

-fpre-ipa-mem-report

-fpost-ipa-mem-report

Makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization.

-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. 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.
• 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. Also fork calls are detected and correctly handled (double counting will not happen).
• 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 will match the source files more closely, if you do not optimize.

-fdbg-cnt-list

Print the name and the counter upperbound for all debug counters.

-fdbg-cnt=counter-value-list

Set the internal debug counter upperbound. counter-value-list is a comma-separated list of name:value pairs which sets the upperbound of each debug counter name to value. All debug counters have the initial upperbound of UINT_MAX, thus dbg_cnt() returns true always unless the upperbound is set by this option. e.g. With -fdbg-cnt=dce:10,tail_call:0 dbg_cnt(dce) will return true only for first 10 invocations and dbg_cnt(tail_call) will return false always.

-dletters

-fdump-rtl-pass

Says to make debugging dumps during compilation at times specified by letters. This is used for debugging the RTL-based passes of the compiler. The file names for most of the dumps are made by appending a pass number and a word to the dumpname. dumpname 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. These switches may have different effects when -E is used for preprocessing.

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 sub-expression 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 pro and epilogues.

-fdump-rtl-regmove

Dump after the register move pass.

-fdump-rtl-sched1

-fdump-rtl-sched2

-fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic block scheduling passes.

-fdump-rtl-see

Dump after sign 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

-fdump-rtl-split1, -fdump-rtl-split2, -fdump-rtl-split3, -fdump-rtl-split4 and -fdump-rtl-split5 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.

-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.

-dm

Print statistics on memory usage, at the end of the run, to standard error.

-dp

Annotate the assembler output with a comment indicating which pattern and alternative was used. The length of each instruction is also printed.

-dP

Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp annotation.

-dv

For each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with VCG to file.pass.vcg.

-dx

Just generate RTL for a function instead of compiling it. Usually used with -fdump-rtl-expand.

-dy

Dump debugging information during parsing, to standard error.

-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.

-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-translation-unit (C++ only)

-fdump-translation-unit-options (C++ only)

Dump a representation of the tree structure for the entire translation unit to a file. The file name is made by appending .tu to the source file name. If the ‘-options’ form is used, options controls the details of the dump as described for the -fdump-tree options.

-fdump-class-hierarchy (C++ only)

-fdump-class-hierarchy-options (C++ only)

Dump a representation of each class's hierarchy and virtual function table layout to a file. The file name is made by appending .class to the source file name. If the ‘-options’ form is used, options controls the details of the dump as described for the -fdump-tree options.

-fdump-ipa-switch

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. 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.

-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. If the ‘-option’ form is used, ‘-stats’ will cause counters to be summed over the whole compilation unit while ‘-details’ will dump every event as the passes generate them. The default with no option is to sum counters for each function compiled.

-fdump-tree-switch

-fdump-tree-switch-options

Control the dumping at various stages of processing the intermediate language tree to a file. The file name is generated by appending a switch specific suffix to the source file name. If the ‘-options’ form is used, options is a list of ‘-’ separated options that control the details of the dump. Not all options are applicable to all dumps, those which are not meaningful will be 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.

‘slim’

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.

‘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).

‘stats’

Enable dumping various statistics about the pass (not honored by every dump option).

‘blocks’

Enable showing basic block boundaries (disabled in raw dumps).

‘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.

‘all’

Turn on all options, except raw, slim, verbose and lineno.

The following tree dumps are possible:

‘original’

Dump before any tree based optimization, to file.original.

‘optimized’

Dump after all tree based optimization, to file.optimized.

‘gimple’

Dump each function before and after the gimplification pass to a file. The file name is made by appending .gimple to the source file name.

‘cfg’

Dump the control flow graph of each function to a file. The file name is made by appending .cfg to the source file name.

‘vcg’

Dump the control flow graph of each function to a file in VCG format. The file name is made by appending .vcg to the source file name. Note that if the file contains more than one function, the generated file cannot be used directly by VCG. You will need to cut and paste each function's graph into its own separate file first.

‘ch’

Dump each function after copying loop headers. The file name is made by appending .ch to the source file name.

‘ssa’

Dump SSA related information to a file. The file name is made by appending .ssa to the source file name.

‘alias’

Dump aliasing information for each function. The file name is made by appending .alias to the source file name.

‘ccp’

Dump each function after CCP. The file name is made by appending .ccp to the source file name.

‘storeccp’

Dump each function after STORE-CCP. The file name is made by appending .storeccp to the source file name.

‘pre’

Dump trees after partial redundancy elimination. The file name is made by appending .pre to the source file name.

‘fre’

Dump trees after full redundancy elimination. The file name is made by appending .fre to the source file name.

‘copyprop’

Dump trees after copy propagation. The file name is made by appending .copyprop to the source file name.

‘store_copyprop’

Dump trees after store copy-propagation. The file name is made by appending .store_copyprop to the source file name.

‘dce’

Dump each function after dead code elimination. The file name is made by appending .dce to the source file name.

‘mudflap’

Dump each function after adding mudflap instrumentation. The file name is made by appending .mudflap to the source file name.

‘sra’

Dump each function after performing scalar replacement of aggregates. The file name is made by appending .sra to the source file name.

‘sink’

Dump each function after performing code sinking. The file name is made by appending .sink to the source file name.

‘dom’

Dump each function after applying dominator tree optimizations. The file name is made by appending .dom to the source file name.

‘dse’

Dump each function after applying dead store elimination. The file name is made by appending .dse to the source file name.

‘phiopt’

Dump each function after optimizing PHI nodes into straightline code. The file name is made by appending .phiopt to the source file name.

‘forwprop’

Dump each function after forward propagating single use variables. The file name is made by appending .forwprop to the source file name.

‘copyrename’

Dump each function after applying the copy rename optimization. The file name is made by appending .copyrename to the source file name.

‘nrv’

Dump each function after applying the named return value optimization on generic trees. The file name is made by appending .nrv to the source file name.

‘vect’

Dump each function after applying vectorization of loops. The file name is made by appending .vect to the source file name.

‘vrp’

Dump each function after Value Range Propagation (VRP). The file name is made by appending .vrp to the source file name.

‘all’

Enable all the available tree dumps with the flags provided in this option.

-ftree-vectorizer-verbose=n

This option controls the amount of debugging output the vectorizer prints. This information is written to standard error, unless -fdump-tree-all or -fdump-tree-vect is specified, in which case it is output to the usual dump listing file, .vect. For n=0 no diagnostic information is reported. If n=1 the vectorizer reports each loop that got vectorized, and the total number of loops that got vectorized. If n=2 the vectorizer also reports non-vectorized loops that passed the first analysis phase (vect_analyze_loop_form) - i.e. countable, inner-most, single-bb, single-entry/exit loops. This is the same verbosity level that -fdump-tree-vect-stats uses. Higher verbosity levels mean either more information dumped for each reported loop, or same amount of information reported for more loops: If n=3, alignment related information is added to the reports. If n=4, data-references related information (e.g. memory dependences, memory access-patterns) is added to the reports. If n=5, the vectorizer reports also non-vectorized inner-most loops that did not pass the first analysis phase (i.e., may not be countable, or may have complicated control-flow). If n=6, the vectorizer reports also non-vectorized nested loops. For n=7, all the information the vectorizer generates during its analysis and transformation is reported. This is the same verbosity level that -fdump-tree-vect-details uses.

-frandom-seed=string

This option provides a seed that GCC uses when it would otherwise use random numbers. It is used to generate 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 should be different for every file you compile.

-fsched-verbose=n

On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints. This information is written to standard error, unless -fdump-rtl-sched1 or -fdump-rtl-sched2 is specified, in which case it is output to the usual dump listing file, .sched or .sched2 respectively. However for n greater than nine, the output is always printed to standard error.

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.

-save-temps

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’ would produce 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.

-time

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). 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.

-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.

-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-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 will search—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 will be 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’)—and don't do anything else.

-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.

-feliminate-unused-debug-types

Normally, when producing DWARF2 output, GCC will emit debugging information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit. Sometimes this is useful, such as 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. With this option, GCC will avoid producing debug symbol output for types that are nowhere used in the source file being compiled.

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 would 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.

-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 
          -fcprop-registers 
          -fdce 
          -fdefer-pop 
          -fdelayed-branch 
          -fdse 
          -fguess-branch-probability 
          -fif-conversion2 
          -fif-conversion 
          -finline-small-functions 
          -fipa-pure-const 
          -fipa-reference 
          -fmerge-constants
          -fsplit-wide-types 
          -ftree-builtin-call-dce 
          -ftree-ccp 
          -ftree-ch 
          -ftree-copyrename 
          -ftree-dce 
          -ftree-dominator-opts 
          -ftree-dse 
          -ftree-fre 
          -ftree-sra 
          -ftree-ter 
          -funit-at-a-time

-O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.

-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:

          -fthread-jumps 
          -falign-functions  -falign-jumps 
          -falign-loops  -falign-labels 
          -fcaller-saves 
          -fcrossjumping 
          -fcse-follow-jumps  -fcse-skip-blocks 
          -fdelete-null-pointer-checks 
          -fexpensive-optimizations 
          -fgcse  -fgcse-lm  
          -findirect-inlining 
          -foptimize-sibling-calls 
          -fpeephole2 
          -fregmove 
          -freorder-blocks  -freorder-functions 
          -frerun-cse-after-loop  
          -fsched-interblock  -fsched-spec 
          -fschedule-insns  -fschedule-insns2 
          -fstrict-aliasing -fstrict-overflow 
          -ftree-switch-conversion 
          -ftree-pre 
          -ftree-vrp

Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos.

NOTE: In Ubuntu 8.10 and later versions, -D_FORTIFY_SOURCE=2 is set by default, and is activated when -O is set to 2 or higher. This enables additional compile-time and run-time checks for several libc functions. To disable, specify either -U_FORTIFY_SOURCE or -D_FORTIFY_SOURCE=0.

-O3

Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the -finline-functions, -funswitch-loops, -fpredictive-commoning, -fgcse-after-reload and -ftree-vectorize options.

-O0

Reduce compilation time and make debugging produce the expected results. This is the default.

-Os

Optimize for size. -Os enables all -O2 optimizations that do not typically increase code size. It also performs further optimizations designed to reduce code size.

-Os disables the following optimization flags:

          -falign-functions  -falign-jumps  -falign-loops 
          -falign-labels  -freorder-blocks  -freorder-blocks-and-partition 
          -fprefetch-loop-arrays  -ftree-vect-loop-version

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 would be -fno-foo. In the table below, only one of the forms is listed—the one you typically will 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-default-inline

Do not make member functions inline by default merely because they are defined inside the class scope (C++ only). Otherwise, when you specify -O, member functions defined inside class scope are compiled inline by default; i.e., you don't need to add ‘inline’ in front of the member function name.

-fno-defer-pop

Always pop the arguments to each function call as soon as that function returns. For machines which must pop arguments after a function call, the compiler normally lets arguments accumulate on the stack for several function calls and pops them all at once.

Disabled at levels -O, -O2, -O3, -Os.

-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 -O2, -O3, -Os.

-fomit-frame-pointer

Don't keep the frame pointer in a register for functions that don't need one. This avoids the instructions to save, set up and restore frame pointers; it also makes an extra register available in many functions. It also makes debugging impossible on some machines.

On some machines, such as the VAX, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't exist. The machine-description macro FRAME_POINTER_REQUIRED controls whether a target machine supports this flag. See Register Usage.

Enabled at levels -O, -O2, -O3, -Os.

-foptimize-sibling-calls

Optimize sibling and tail recursive calls.

Enabled at levels -O2, -O3, -Os.

-fno-inline

Don't pay attention to the inline keyword. Normally this option is used to keep the compiler from expanding any functions inline. Note that if you are not optimizing, no functions can be expanded inline.

-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.

Enabled at level -O2.

-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 level -O2.

-finline-functions

Integrate all simple functions into their callers. The compiler heuristically decides which functions are simple enough to be 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 level -O3.

-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.

-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.

-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.

-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 C89. In C++, emit any and all inline functions into the object file.

-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 the variable was 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 will result 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 will be deleted which will trigger the generation of reg-moves based on the life-range analysis. This option is effective only with -fmodulo-sched enabled.

-fno-branch-count-reg

Do not use “decrement and branch” instructions on a count register, but instead generate a sequence of instructions that decrement a register, compare it against zero, 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.

The default is -fbranch-count-reg.

-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.

-fmudflap -fmudflapth -fmudflapir

For front-ends that support it (C and C++), instrument all risky pointer/array dereferencing operations, some standard library string/heap functions, and some other associated constructs with range/validity tests. Modules so instrumented should be immune to buffer overflows, invalid heap use, and some other classes of C/C++ programming errors. The instrumentation relies on a separate runtime library (libmudflap), which will be linked into a program if -fmudflap is given at link time. Run-time behavior of the instrumented program is controlled by the MUDFLAP_OPTIONS environment variable. See env MUDFLAP_OPTIONS=-help a.out for its options.

Use -fmudflapth instead of -fmudflap to compile and to link if your program is multi-threaded. Use -fmudflapir, in addition to -fmudflap or -fmudflapth, if instrumentation should ignore pointer reads. This produces less instrumentation (and therefore faster execution) and still provides some protection against outright memory corrupting writes, but allows erroneously read data to propagate within a program.

-fthread-jumps

Perform optimizations where we 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 will follow 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 which 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 has been 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 runtime 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 will attempt to move loads which 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 gcse is enabled.

-fgcse-sm

When -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination. This pass will attempt 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 cleanup redundant spilling.

-funsafe-loop-optimizations

If given, the loop optimizer will assume that loop indices do not overflow, and that the loops with nontrivial exit condition are not infinite. This enables a wider range of loop optimizations even if the loop optimizer itself cannot prove that these assumptions are valid. Using -Wunsafe-loop-optimizations, the compiler will warn you if it finds this kind of loop.

-fcrossjumping

Perform cross-jumping transformation. This transformation unifies equivalent code and save 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 include 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 if-conversion2.

Enabled at levels -O, -O2, -O3, -Os.

-fif-conversion2

Use conditional execution (where available) to transform conditional jumps into branch-less equivalents.

Enabled at levels -O, -O2, -O3, -Os.

-fdelete-null-pointer-checks

Use global dataflow analysis to identify and eliminate useless checks for null pointers. The compiler assumes that dereferencing a null pointer would have halted the program. If a pointer is checked after it has already been dereferenced, it cannot be null.

In some environments, this assumption is not true, and programs can safely dereference null pointers. Use -fno-delete-null-pointer-checks to disable this optimization for programs which depend on that behavior.

Enabled at levels -O2, -O3, -Os.

-fexpensive-optimizations

Perform a number of minor optimizations that are relatively expensive.

Enabled at levels -O2, -O3, -Os.

-foptimize-register-move

-fregmove

Attempt to reassign register numbers in move instructions and as operands of other simple instructions in order to maximize the amount of register tying. This is especially helpful on machines with two-operand instructions.

Note -fregmove and -foptimize-register-move are the same optimization.

Enabled at levels -O2, -O3, -Os.

-fira-algorithm=algorithm

Use specified coloring algorithm for the integrated register allocator. The algorithm argument should be priority or CB. The first algorithm specifies Chow's priority coloring, the second one specifies Chaitin-Briggs coloring. The second algorithm can be unimplemented for some architectures. If it is implemented, it is the default because Chaitin-Briggs coloring as a rule generates a better code.

-fira-region=region

Use specified regions for the integrated register allocator. The region argument should be one of all, mixed, or one. The first value means using all loops as register allocation regions, the second value which is the default means using all loops except for loops with small register pressure as the regions, and third one means using all function as a single region. The first value can give best result for machines with small size and irregular register set, the third one results in faster and generates decent code and the smallest size code, and the default value usually give the best results in most cases and for most architectures.

-fira-coalesce

Do optimistic register coalescing. This option might be profitable for architectures with big regular register files.

-fno-ira-share-save-slots

Switch off sharing stack slots used for saving call used hard registers living through a call. Each hard register will get a separate stack slot and as a result function stack frame will be bigger.

-fno-ira-share-spill-slots

Switch off sharing stack slots allocated for pseudo-registers. Each pseudo-register which did not get a hard register will get a separate stack slot and as a result function stack frame will be bigger.

-fira-verbose=n

Set up how verbose dump file for the integrated register allocator will be. Default value is 5. If the value is greater or equal to 10, the dump file will be stderr as if the value were n minus 10.

-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.

-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, -Os.

-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

Don't schedule instructions across basic blocks. This is normally enabled by default when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-fno-sched-spec

Don't allow speculative motion of non-load instructions. This is normally enabled by default when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-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 will be 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) will be examined for a dependency on a stalled insn that is 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, do use superblock scheduling algorithm. Superblock scheduling allows motion across basic block boundaries resulting on 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.

-fsched2-use-traces

Use -fsched2-use-superblocks algorithm when scheduling after register allocation and additionally perform code duplication in order to increase the size of superblocks using tracer pass. See -ftracer for details on trace formation.

This mode should produce faster but significantly longer programs. Also without -fbranch-probabilities the traces constructed may not match the reality and hurt the performance. This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.

-fsee

Eliminate redundant sign extension instructions and move the non-redundant ones to optimal placement using lazy code motion (LCM).

-freschedule-modulo-scheduled-loops

The modulo scheduling comes before the traditional scheduling, if a loop was modulo scheduled we may want to prevent the later scheduling passes from changing its schedule, we use this option to control that.

-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 until 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 until -fsel-sched-pipelining is turned on.

-fcaller-saves

Enable values to be allocated in registers that will be 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 than would otherwise be produced.

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.

-fconserve-stack

Attempt to minimize stack usage. The compiler will attempt 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.

-ftree-pre

Perform partial redundancy elimination (PRE) on trees. This flag is enabled by default at -O2 and -O3.

-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-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 cannot escape the compilation unit. Enabled by default at -O and higher.

-fipa-struct-reorg

Perform structure reorganization optimization, that change C-like structures layout in order to better utilize spatial locality. This transformation is affective for programs containing arrays of structures. Available in two compilation modes: profile-based (enabled with -fprofile-generate) or static (which uses built-in heuristics). Require -fipa-type-escape to provide the safety of this transformation. It works only in whole program mode, so it requires -fwhole-program and -combine to be enabled. Structures considered ‘cold’ by this transformation are not affected (see --param struct-reorg-cold-struct-ratio=value).

With this flag, the program debug info reflects a new structure layout.

-fipa-pta

Perform interprocedural pointer analysis. This option is experimental and does not affect generated code.

-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.

-fipa-cp-clone

Perform function cloning to make interprocedural constant propagation stronger. When enabled, interprocedural constant propagation will perform 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.

-fipa-matrix-reorg

Perform matrix flattening and transposing. Matrix flattening tries to replace a m-dimensional matrix with its equivalent n-dimensional matrix, where n < m. This reduces the level of indirection needed for accessing the elements of the matrix. The second optimization is matrix transposing that attempts to change the order of the matrix's dimensions in order to improve cache locality. Both optimizations need the -fwhole-program flag. Transposing is enabled only if profiling information is available.

-ftree-sink

Perform forward store motion on trees. This flag is enabled by default at -O and higher.

-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.

-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-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 builtin functions that may set errno but are otherwise side-effect free. 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 which will later be 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

Perform linear loop transformations on tree. This flag can improve cache performance and allow further loop optimizations to take place.

-floop-interchange

Perform loop interchange transformations on loops. Interchanging two nested loops switches the inner and outer loops. For example, given a loop like:

          DO J = 1, M
            DO I = 1, N
              A(J, I) = A(J, I) * C
            ENDDO
          ENDDO

loop interchange will transform the loop as if the user had written:

          DO I = 1, N
            DO J = 1, M
              A(J, I) = A(J, I) * C
            ENDDO
          ENDDO

which can be beneficial when N is larger than the caches, because in Fortran, the elements of an array are stored in memory contiguously by column, and the original loop iterates over rows, potentially creating at each access a cache miss. This optimization applies to all the languages supported by GCC and is not limited to Fortran. To use this code transformation, GCC has to be configured with --with-ppl and --with-cloog to enable the Graphite loop transformation infrastructure.

-floop-strip-mine

Perform loop strip mining transformations on loops. Strip mining splits a loop into two nested loops. The outer loop has strides equal to the strip size and the inner loop has strides of the original loop within a strip. For example, given a loop like:

          DO I = 1, N
            A(I) = A(I) + C
          ENDDO

loop strip mining will transform the loop as if the user had written:

          DO II = 1, N, 4
            DO I = II, min (II + 3, N)
              A(I) = A(I) + C
            ENDDO
          ENDDO

This optimization applies to all the languages supported by GCC and is not limited to Fortran. To use this code transformation, GCC has to be configured with --with-ppl and --with-cloog to enable the Graphite loop transformation infrastructure.

-floop-block

Perform loop blocking transformations on loops. Blocking strip mines each loop in the loop nest such that the memory accesses of the element loops fit inside caches. For example, given a loop like:

          DO I = 1, N
            DO J = 1, M
              A(J, I) = B(I) + C(J)
            ENDDO
          ENDDO

loop blocking will transform the loop as if the user had written:

          DO II = 1, N, 64
            DO JJ = 1, M, 64
              DO I = II, min (II + 63, N)
                DO J = JJ, min (JJ + 63, M)
                  A(J, I) = B(I) + C(J)
                ENDDO
              ENDDO
            ENDDO
          ENDDO

which can be beneficial when M is larger than the caches, because the innermost loop will iterate over a smaller amount of data that can be kept in the caches. This optimization applies to all the languages supported by GCC and is not limited to Fortran. To use this code transformation, GCC has to be configured with --with-ppl and --with-cloog to enable the Graphite loop transformation infrastructure.

-fcheck-data-deps

Compare the results of several data dependence analyzers. This option is used for debugging the data dependence analyzers.

-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

-ftree-loop-im

Perform loop invariant motion on trees. This pass moves only invariants that would be 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 the loop for that determining number of iterations requires complicated analysis. Later optimizations then may determine the number easily. Useful especially in connection with unrolling.

-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-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 -O and higher.

-ftree-copyrename

Perform copy renaming on trees. This pass attempts to rename compiler temporaries to other variables at copy locations, usually resulting in variable names which more closely resemble the original variables. This flag is enabled by default at -O and higher.

-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-vectorize

Perform loop vectorization on trees. This flag is enabled by default at -O3.

-ftree-vect-loop-version

Perform loop versioning when doing loop vectorization on trees. When a loop appears to be vectorizable except that data alignment or data dependence cannot be determined at compile time then vectorized and non-vectorized versions of the loop are generated along with runtime checks for alignment or dependence to control which version is executed. This option is enabled by default except at level -Os where it is disabled.

-fvect-cost-model

Enable cost model for vectorization.

-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.

-ftracer

Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing other optimizations to do better job.

-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. This option makes code larger, and may or may not make it run faster.

-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,

-fsplit-ivs-in-unroller

Enables expressing 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.

Combination of -fweb and CSE is often sufficient to obtain the same effect. However in cases the loop body is more complicated than a single basic block, this is not reliable. It also does not work at all on some of the architectures due to restrictions in the CSE pass.

This optimization is enabled by default.

-fvariable-expansion-in-unroller

With this option, the compiler will create multiple copies of some local variables when unrolling a loop which can result in superior code.

-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.

-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-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 will use 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 will be 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.

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 -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, for linkonce sections, for functions with a user-defined section attribute and on any architecture that does not support named sections.

-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.

Also profile feedback must be available in to make this option effective. See -fprofile-arcs for details.

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 will work 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.

-fstrict-overflow

Allow the compiler to assume strict signed overflow rules, depending on the language being compiled. For C (and C++) this means that overflow when doing arithmetic with signed numbers is undefined, which means that the compiler may assume that it will not happen. This permits various optimizations. For example, the compiler will assume that an expression like i + 10 > i will always be true for signed i. This assumption is only valid if signed overflow is undefined, as the expression is false if i + 10 overflows when using twos complement arithmetic. When this option is in effect any attempt to determine whether an operation on signed numbers will overflow must be written carefully to not actually involve overflow.

This option also allows the compiler to assume strict pointer semantics: given a pointer to an object, if adding an offset to that pointer does not produce a pointer to the same object, the addition is undefined. This permits the compiler to conclude that p + u > p is always true for a pointer p and unsigned integer u. This assumption is only valid because pointer wraparound is undefined, as the expression is false if p + u overflows using twos complement arithmetic.

See also the -fwrapv option. Using -fwrapv means that integer signed overflow is fully defined: it wraps. When -fwrapv is used, there is no difference between -fstrict-overflow and -fno-strict-overflow for integers. With -fwrapv certain types of overflow are permitted. For example, if the compiler gets an overflow when doing arithmetic on constants, the overflowed value can still be used with -fwrapv, but not otherwise.

The -fstrict-overflow option is enabled at levels -O2, -O3, -Os.

-falign-functions

-falign-functions=n

Align the start of functions to the next power-of-two greater than n, skipping up to n bytes. For instance, -falign-functions=32 aligns functions to the next 32-byte boundary, but -falign-functions=24 would align to the next 32-byte boundary only if this can be done by skipping 23 bytes or less.

-fno-align-functions and -falign-functions=1 are equivalent and mean that functions will not be aligned.

Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.

If n is not specified or is zero, use a machine-dependent default.

Enabled at levels -O2, -O3.

-falign-labels

-falign-labels=n

Align all branch targets to a power-of-two boundary, skipping up to n bytes like -falign-functions. This option can easily make code slower, because it must insert dummy operations for when the branch target is reached in the usual flow of the code.

-fno-align-labels and -falign-labels=1 are equivalent and mean that labels will not be 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.

Enabled at levels -O2, -O3.

-falign-loops

-falign-loops=n

Align loops to a power-of-two boundary, skipping up to n bytes like -falign-functions. The hope is that the loop will be executed many times, which will make up for any execution of the dummy operations.

-fno-align-loops and -falign-loops=1 are equivalent and mean that loops will not be aligned.

If n is not specified or is zero, use a machine-dependent default.

Enabled at levels -O2, -O3.

-falign-jumps

-falign-jumps=n

Align branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by jumping, skipping up to n bytes like -falign-functions. In this case, no dummy operations need be executed.

-fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops will not be aligned.

If n is not specified or is zero, use a machine-dependent default.

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 will not be removed. This option is intended to support existing code which relies on a particular ordering. For new code, it is better to use attributes.

Enabled at level -O0. When disabled explicitly, it also imply -fno-section-anchors that is otherwise enabled at -O0 on some targets.

-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 will no longer stay in a “home register”.

Enabled by default with -funroll-loops.

-fwhole-program

Assume that the current compilation unit represents 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 a affect gets more aggressively optimized by interprocedural optimizers. While this option is equivalent to proper use of static keyword for programs consisting of single file, in combination with option --combine this flag can be used to compile most of smaller scale C programs since the functions and variables become local for the whole combined compilation unit, not for the single source file itself.

This option is not supported for Fortran programs.

-fuse-ld=gold

Use the gold linker instead of the default linker. This option is only necessary if GCC has been configured with --enable-gold=both or --enable-gold=both/ld. Note: Backported for Debian/Ubuntu from GCC 4.5.

-fuse-ld=bfd

Use the ld.bfd linker instead of the default linker. This option is only necessary if GCC has been configured with --enable-gold=both/gold. Note: Backported for Debian/Ubuntu from GCC 4.5.

-fcprop-registers

After register allocation and post-register allocation instruction splitting, we 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 will use heuristics to correct or smooth out such inconsistencies. By default, GCC will emit an error message when an inconsistent profile is detected.

-fprofile-dir=path

Set the directory to search 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. By default, GCC will use the current directory as path thus the profile data file will appear in the same directory as the object file.

-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, -fvpt.

If path is specified, GCC will look at the path to find the profile feedback data files. See -fprofile-dir.

-fprofile-use

-fprofile-use=path

Enable profile feedback directed optimizations, and optimizations generally profitable only with profile feedback available.

The following options are enabled: -fbranch-probabilities, -fvpt, -funroll-loops, -fpeel-loops, -ftracer

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 -Wcoverage-mismatch. Note this may result in poorly optimized code.

If path is specified, GCC will look at the path to find the profile feedback data files. See -fprofile-dir.

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.

-ffast-math

Sets -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans and -fcx-limited-range.

This option causes the preprocessor macro __FAST_MATH__ to be defined.

This option is not turned on by any -O option since it can result in incorrect output for programs which 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 which 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 which 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 a code which 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.

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 which 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 which 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.

-frtl-abstract-sequences

It is a size optimization method. This option is to find identical sequences of code, which can be turned into pseudo-procedures and then replace all occurrences with calls to the newly created subroutine. It is kind of an opposite of -finline-functions. This optimization runs at RTL level.

-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.

-fsingle-precision-constant

Treat floating point constant as single precision constant instead of implicitly converting it to double precision constant.

-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.