Using the GNU Compiler Collection (GCC)

This file documents the use of the GNU compilers.

Copyright © 1988-2014 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being “Funding Free Software”, the Front-Cover Texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled “GNU Free Documentation License”.

(a) The FSF’s Front-Cover Text is:

A GNU Manual

(b) The FSF’s Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.

Short Table of Contents

Table of Contents

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 version 4.9.1. The internals of the GNU compilers, including how to port them to new targets and some information about how to write front ends for new languages, are documented in a separate manual. See Introduction in GNU Compiler Collection (GCC) Internals.

1 Programming Languages Supported by GCC

GCC stands for “GNU Compiler Collection”. GCC is an integrated distribution of compilers for several major programming languages. These languages currently include C, C++, Objective-C, Objective-C++, Java, Fortran, Ada, D and Go.

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=c90 or -std=iso9899:1990; to obtain all the diagnostics required by the standard, you should also specify -pedantic (or -pedantic-errors if you want them to be errors rather than warnings). See Options Controlling C Dialect.

Errors in the 1990 ISO C standard were corrected in two Technical Corrigenda published in 1994 and 1996. GCC does not support the uncorrected version.

An amendment to the 1990 standard was published in 1995. This amendment added digraphs and __STDC_VERSION__ to the language, but otherwise concerned the library. This amendment is commonly known as AMD1; the amended standard is sometimes known as C94 or C95. To select this standard in GCC, use the option -std=iso9899:199409 (with, as for other standard versions, -pedantic to receive all required diagnostics).

A new edition of the ISO C standard was published in 1999 as ISO/IEC 9899:1999, and is commonly known as C99. GCC has substantially complete support for this standard version; see http://gcc.gnu.org/c99status.html for details. To select this standard, use -std=c99 or -std=iso9899:1999. (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.

A fourth version of the C standard, known as C11, was published in 2011 as ISO/IEC 9899:2011. GCC has substantially complete support for this standard, enabled with -std=c11 or -std=iso9899:2011. (While in development, drafts of this standard version were referred to as C1X.)

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=gnu90 (for C90 with GNU extensions), -std=gnu99 (for C99 with GNU extensions) or -std=gnu11 (for C11 with GNU extensions). The default, if no C language dialect options are given, is -std=gnu90; this is intended to change to -std=gnu11 in some future release. Some features that are part of the C99 standard are accepted as extensions in C90 mode, and some features that are part of the C11 standard are accepted as extensions in C90 and C99 modes.

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>; since C99, also those in <stdbool.h> and <stdint.h>; and since C11, also those in <stdalign.h> and <stdnoreturn.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 on all platforms; 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 original ISO C++ standard (1998) and contains experimental support for the second ISO C++ standard (2011).

The original ISO C++ standard was published as the ISO standard (ISO/IEC 14882:1998) and amended by a Technical Corrigenda published in 2003 (ISO/IEC 14882:2003). These standards are referred to as C++98 and C++03, respectively. GCC implements the majority of C++98 (export is a notable exception) and most of the changes in C++03. To select this standard in GCC, use one of the options -ansi, -std=c++98, or -std=c++03; to obtain all the diagnostics required by the standard, you should also specify -pedantic (or -pedantic-errors if you want them to be errors rather than warnings).

A revised ISO C++ standard was published in 2011 as ISO/IEC 14882:2011, and is referred to as C++11; before its publication it was commonly referred to as C++0x. C++11 contains several changes to the C++ language, most of which have been implemented in an experimental C++11 mode in GCC. For information regarding the C++11 features available in the experimental C++11 mode, see http://gcc.gnu.org/projects/cxx0x.html. To select this standard in GCC, use the option -std=c++11; 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).

More information about the C++ standards is available on the ISO C++ committee’s web site at http://www.open-std.org/jtc1/sc22/wg21/.

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++11 (for C++11 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

GCC supports “traditional” Objective-C (also known as “Objective-C 1.0”) and contains support for the Objective-C exception and synchronization syntax. It has also support for a number of “Objective-C 2.0” language extensions, including properties, fast enumeration (only for Objective-C), method attributes and the @optional and @required keywords in protocols. GCC supports Objective-C++ and features available in Objective-C are also available in Objective-C++.

GCC by default uses the GNU Objective-C runtime library, which is part of GCC and is not the same as the Apple/NeXT Objective-C runtime library used on Apple systems. There are a number of differences documented in this manual. The options -fgnu-runtime and -fnext-runtime allow you to switch between producing output that works with the GNU Objective-C runtime library and output that works with the Apple/NeXT Objective-C runtime library.

There is no formal written standard for Objective-C or Objective-C++. The authoritative manual on traditional Objective-C (1.0) is “Object-Oriented Programming and the Objective-C Language”, available at a number of web sites:

• http://www.gnustep.org/resources/documentation/ObjectivCBook.pdf is the original NeXTstep document;
• http://objc.toodarkpark.net is the same document in another format;
• http://developer.apple.com/mac/library/documentation/Cocoa/Conceptual/ObjectiveC/ has an updated version but make sure you search for “Object Oriented Programming and the Objective-C Programming Language 1.0”, not documentation on the newer “Objective-C 2.0” language

The Objective-C exception and synchronization syntax (that is, the keywords @try, @throw, @catch, @finally and @synchronized) is supported by GCC and is enabled with the option -fobjc-exceptions. The syntax is briefly documented in this manual and in the Objective-C 2.0 manuals from Apple.

The Objective-C 2.0 language extensions and features are automatically enabled; they include properties (via the @property, @synthesize and @dynamic keywords), fast enumeration (not available in Objective-C++), attributes for methods (such as deprecated, noreturn, sentinel, format), the unused attribute for method arguments, the @package keyword for instance variables and the @optional and @required keywords in protocols. You can disable all these Objective-C 2.0 language extensions with the option -fobjc-std=objc1, which causes the compiler to recognize the same Objective-C language syntax recognized by GCC 4.0, and to produce an error if one of the new features is used.

GCC has currently no support for non-fragile instance variables.

The authoritative manual on Objective-C 2.0 is available from Apple:

• http://developer.apple.com/mac/library/documentation/Cocoa/Conceptual/ObjectiveC/

For more information concerning the history of Objective-C that is available online, see http://gcc.gnu.org/readings.html

2.4 Go language

As of the GCC 4.7.1 release, GCC supports the Go 1 language standard, described at http://golang.org/doc/go1.html.

2.5 D language

The D language is under development as of this writing; see the current language reference. At present the current major version of D is 2.0, and there is no way to describe the language supported by GCC in terms of a specific minor version. In general GCC follows the D frontend releases closely, and any given GCC release will support the current language as of the date that the release was frozen.

2.6 References for other languages

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

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

See Compatibility with the Java Platform in GNU gcj, 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 is -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.

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  -no-canonical-prefixes  
-pipe  -pass-exit-codes  
-x language  -v  -###  --help[=class[,…]]  --target-help  
--version -wrapper @file -fplugin=file -fplugin-arg-name=arg  
-fdump-ada-spec[-slim] -fada-spec-parent=unit -fdump-go-spec=file

C Language Options

See Options Controlling C Dialect.

-ansi  -std=standard  -fgnu89-inline 
-aux-info filename -fallow-parameterless-variadic-functions 
-fno-asm  -fno-builtin  -fno-builtin-function 
-fhosted  -ffreestanding -fopenmp -fopenmp-simd -fms-extensions 
-fplan9-extensions -trigraphs  -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 
-fconstexpr-depth=n  -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  -fnothrow-opt  -fno-operator-names 
-fno-optional-diags  -fpermissive 
-fno-pretty-templates 
-frepo  -fno-rtti  -fstats  -ftemplate-backtrace-limit=n 
-ftemplate-depth=n 
-fno-threadsafe-statics -fuse-cxa-atexit  -fno-weak  -nostdinc++ 
-fvisibility-inlines-hidden 
-fvtable-verify=std|preinit|none 
-fvtv-counts -fvtv-debug 
-fvisibility-ms-compat 
-fext-numeric-literals 
-Wabi  -Wconversion-null  -Wctor-dtor-privacy 
-Wdelete-non-virtual-dtor -Wliteral-suffix -Wnarrowing 
-Wnoexcept -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-abi-version=n 
-fobjc-call-cxx-cdtors 
-fobjc-direct-dispatch 
-fobjc-exceptions 
-fobjc-gc 
-fobjc-nilcheck 
-fobjc-std=objc1 
-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-color=[auto|never|always]  
-fno-diagnostics-show-option -fno-diagnostics-show-caret

Warning Options

See Options to Request or Suppress Warnings.

-fsyntax-only  -fmax-errors=n  -Wpedantic 
-pedantic-errors 
-w  -Wextra  -Wall  -Waddress  -Waggregate-return  
-Waggressive-loop-optimizations -Warray-bounds 
-Wno-attributes -Wno-builtin-macro-redefined 
-Wc++-compat -Wc++11-compat -Wcast-align  -Wcast-qual  
-Wchar-subscripts -Wclobbered  -Wcomment -Wconditionally-supported  
-Wconversion -Wcoverage-mismatch -Wdate-time -Wdelete-incomplete -Wno-cpp  
-Wno-deprecated -Wno-deprecated-declarations -Wdisabled-optimization  
-Wno-div-by-zero -Wdouble-promotion -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 -Wno-free-nonheap-object -Wjump-misses-init 
-Wignored-qualifiers 
-Wimplicit  -Wimplicit-function-declaration  -Wimplicit-int 
-Winit-self  -Winline -Wmaybe-uninitialized 
-Wno-int-to-pointer-cast -Wno-invalid-offsetof 
-Winvalid-pch -Wlarger-than=len  -Wunsafe-loop-optimizations 
-Wlogical-op -Wlong-long 
-Wmain -Wmaybe-uninitialized -Wmemset-transposed-args -Wmissing-braces 
-Wmissing-field-initializers -Wmissing-include-dirs 
-Wno-multichar  -Wnonnull  -Wno-overflow -Wopenmp-simd 
-Woverlength-strings  -Wpacked  -Wpacked-bitfield-compat  -Wpadded 
-Wparentheses  -Wpedantic-ms-format -Wno-pedantic-ms-format 
-Wpointer-arith  -Wno-pointer-to-int-cast 
-Wredundant-decls  -Wno-return-local-addr 
-Wreturn-type  -Wsequence-point  -Wshadow 
-Wsign-compare  -Wsign-conversion -Wfloat-conversion 
-Wsizeof-pointer-memaccess 
-Wstack-protector -Wstack-usage=len -Wstrict-aliasing 
-Wstrict-aliasing=n  -Wstrict-overflow -Wstrict-overflow=n 
-Wsuggest-attribute=[pure|const|noreturn|format] 
-Wmissing-format-attribute 
-Wswitch  -Wswitch-default  -Wswitch-enum -Wsync-nand 
-Wsystem-headers  -Wtrampolines  -Wtrigraphs  -Wtype-limits  -Wundef 
-Wuninitialized  -Wunknown-pragmas  -Wno-pragmas 
-Wunsuffixed-float-constants  -Wunused  -Wunused-function 
-Wunused-label  -Wunused-local-typedefs -Wunused-parameter 
-Wno-unused-result -Wunused-value  -Wunused-variable 
-Wunused-but-set-parameter -Wunused-but-set-variable 
-Wuseless-cast -Wvariadic-macros -Wvector-operation-performance 
-Wvla -Wvolatile-register-var  -Wwrite-strings -Wzero-as-null-pointer-constant

C and Objective-C-only Warning Options

-Wbad-function-cast  -Wmissing-declarations 
-Wmissing-parameter-type  -Wmissing-prototypes  -Wnested-externs 
-Wold-style-declaration  -Wold-style-definition 
-Wstrict-prototypes  -Wtraditional  -Wtraditional-conversion 
-Wdeclaration-after-statement -Wpointer-sign

Debugging Options

See Options for Debugging Your Program or GCC.

-dletters  -dumpspecs  -dumpmachine  -dumpversion 
-fsanitize=style 
-fdbg-cnt-list -fdbg-cnt=counter-value-list 
-fdisable-ipa-pass_name 
-fdisable-rtl-pass_name 
-fdisable-rtl-pass-name=range-list 
-fdisable-tree-pass_name 
-fdisable-tree-pass-name=range-list 
-fdump-noaddr -fdump-unnumbered -fdump-unnumbered-links 
-fdump-translation-unit[-n] 
-fdump-class-hierarchy[-n] 
-fdump-ipa-all -fdump-ipa-cgraph -fdump-ipa-inline 
-fdump-passes 
-fdump-statistics 
-fdump-tree-all 
-fdump-tree-original[-n]  
-fdump-tree-optimized[-n] 
-fdump-tree-cfg -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-dom[-n] 
-fdump-tree-dse[-n] 
-fdump-tree-phiprop[-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-forwprop[-n] 
-fdump-tree-fre[-n] 
-fdump-tree-vtable-verify 
-fdump-tree-vrp[-n] 
-fdump-tree-storeccp[-n] 
-fdump-final-insns=file 
-fcompare-debug[=opts]  -fcompare-debug-second 
-feliminate-dwarf2-dups -fno-eliminate-unused-debug-types 
-feliminate-unused-debug-symbols -femit-class-debug-always 
-fenable-kind-pass 
-fenable-kind-pass=range-list 
-fdebug-types-section -fmem-report-wpa 
-fmem-report -fpre-ipa-mem-report -fpost-ipa-mem-report -fprofile-arcs 
-fopt-info 
-fopt-info-options[=file] 
-frandom-seed=string -fsched-verbose=n 
-fsel-sched-verbose -fsel-sched-dump-cfg -fsel-sched-pipelining-verbose 
-fstack-usage  -ftest-coverage  -ftime-report -fvar-tracking 
-fvar-tracking-assignments  -fvar-tracking-assignments-toggle 
-g  -glevel  -gtoggle  -gcoff  -gdwarf-version 
-ggdb  -grecord-gcc-switches  -gno-record-gcc-switches 
-gstabs  -gstabs+  -gstrict-dwarf  -gno-strict-dwarf 
-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-multi-os-directory 
-print-prog-name=program  -print-search-dirs  -Q 
-print-sysroot -print-sysroot-headers-suffix 
-save-temps -save-temps=cwd -save-temps=obj -time[=file]

Optimization Options

See Options that Control Optimization.

-faggressive-loop-optimizations -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 -fcombine-stack-adjustments -fconserve-stack 
-fcompare-elim -fcprop-registers -fcrossjumping 
-fcse-follow-jumps -fcse-skip-blocks -fcx-fortran-rules 
-fcx-limited-range 
-fdata-sections -fdce -fdelayed-branch 
-fdelete-null-pointer-checks -fdevirtualize -fdevirtualize-speculatively -fdse 
-fearly-inlining -fipa-sra -fexpensive-optimizations -ffat-lto-objects 
-ffast-math -ffinite-math-only -ffloat-store -fexcess-precision=style 
-fforward-propagate -ffp-contract=style -ffunction-sections 
-fgcse -fgcse-after-reload -fgcse-las -fgcse-lm -fgraphite-identity 
-fgcse-sm -fhoist-adjacent-loads -fif-conversion 
-fif-conversion2 -findirect-inlining 
-finline-functions -finline-functions-called-once -finline-limit=n 
-finline-small-functions -fipa-cp -fipa-cp-clone 
-fipa-pta -fipa-profile -fipa-pure-const -fipa-reference 
-fira-algorithm=algorithm 
-fira-region=region -fira-hoist-pressure 
-fira-loop-pressure -fno-ira-share-save-slots 
-fno-ira-share-spill-slots -fira-verbose=n 
-fisolate-erroneous-paths-dereference -fisolate-erroneous-paths-attribute
-fivopts -fkeep-inline-functions -fkeep-static-consts -flive-range-shrinkage 
-floop-block -floop-interchange -floop-strip-mine -floop-nest-optimize 
-floop-parallelize-all -flto -flto-compression-level 
-flto-partition=alg -flto-report -flto-report-wpa -fmerge-all-constants 
-fmerge-constants -fmodulo-sched -fmodulo-sched-allow-regmoves 
-fmove-loop-invariants -fno-branch-count-reg 
-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-sibling-calls 
-fpartial-inlining -fpeel-loops -fpredictive-commoning 
-fprefetch-loop-arrays -fprofile-report 
-fprofile-correction -fprofile-dir=path -fprofile-generate 
-fprofile-generate=path 
-fprofile-use -fprofile-use=path -fprofile-values -fprofile-reorder-functions 
-freciprocal-math -free -frename-registers -freorder-blocks 
-freorder-blocks-and-partition -freorder-functions 
-frerun-cse-after-loop -freschedule-modulo-scheduled-loops 
-frounding-math -fsched2-use-superblocks -fsched-pressure 
-fsched-spec-load -fsched-spec-load-dangerous 
-fsched-stalled-insns-dep[=n] -fsched-stalled-insns[=n] 
-fsched-group-heuristic -fsched-critical-path-heuristic 
-fsched-spec-insn-heuristic -fsched-rank-heuristic 
-fsched-last-insn-heuristic -fsched-dep-count-heuristic 
-fschedule-insns -fschedule-insns2 -fsection-anchors 
-fselective-scheduling -fselective-scheduling2 
-fsel-sched-pipelining -fsel-sched-pipelining-outer-loops 
-fshrink-wrap -fsignaling-nans -fsingle-precision-constant 
-fsplit-ivs-in-unroller -fsplit-wide-types -fstack-protector 
-fstack-protector-all -fstack-protector-strong -fstrict-aliasing 
-fstrict-overflow -fthread-jumps -ftracer -ftree-bit-ccp 
-ftree-builtin-call-dce -ftree-ccp -ftree-ch 
-ftree-coalesce-inline-vars -ftree-coalesce-vars -ftree-copy-prop 
-ftree-copyrename -ftree-dce -ftree-dominator-opts -ftree-dse 
-ftree-forwprop -ftree-fre -ftree-loop-if-convert 
-ftree-loop-if-convert-stores -ftree-loop-im 
-ftree-phiprop -ftree-loop-distribution -ftree-loop-distribute-patterns 
-ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize 
-ftree-loop-vectorize 
-ftree-parallelize-loops=n -ftree-pre -ftree-partial-pre -ftree-pta 
-ftree-reassoc -ftree-sink -ftree-slsr -ftree-sra 
-ftree-switch-conversion -ftree-tail-merge -ftree-ter 
-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 -fwpa -fuse-ld=linker -fuse-linker-plugin 
--param name=value
-O  -O0  -O1  -O2  -O3  -Os -Ofast -Og

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  -fdebug-cpp -ftrack-macro-expansion -fworking-directory 
-remap -trigraphs  -undef  -Umacro  
-Wp,option -Xpreprocessor option -no-integrated-cpp

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 -static-libstdc++ 
-static-libasan -static-libtsan -static-liblsan -static-libubsan 
-shared -shared-libgcc  -symbolic 
-T script  -Wl,option  -Xlinker option 
-u symbol

Directory Options

See Options for Directory Search.

-Bprefix -Idir -iplugindir=dir 
-iquotedir -Ldir -specs=file -I- 
--sysroot=dir --no-sysroot-suffix

Machine Dependent Options

See Hardware Models and Configurations.

AArch64 Options

-mabi=name  -mbig-endian  -mlittle-endian 
-mgeneral-regs-only 
-mcmodel=tiny  -mcmodel=small  -mcmodel=large 
-mstrict-align 
-momit-leaf-frame-pointer  -mno-omit-leaf-frame-pointer 
-mtls-dialect=desc  -mtls-dialect=traditional 
-march=name  -mcpu=name  -mtune=name

Adapteva Epiphany Options

-mhalf-reg-file -mprefer-short-insn-regs 
-mbranch-cost=num -mcmove -mnops=num -msoft-cmpsf 
-msplit-lohi -mpost-inc -mpost-modify -mstack-offset=num 
-mround-nearest -mlong-calls -mshort-calls -msmall16 
-mfp-mode=mode -mvect-double -max-vect-align=num 
-msplit-vecmove-early -m1reg-reg

ARC Options

-mbarrel-shifter 
-mcpu=cpu -mA6 -mARC600 -mA7 -mARC700 
-mdpfp -mdpfp-compact -mdpfp-fast -mno-dpfp-lrsr 
-mea -mno-mpy -mmul32x16 -mmul64 
-mnorm -mspfp -mspfp-compact -mspfp-fast -msimd -msoft-float -mswap 
-mcrc -mdsp-packa -mdvbf -mlock -mmac-d16 -mmac-24 -mrtsc -mswape 
-mtelephony -mxy -misize -mannotate-align -marclinux -marclinux_prof 
-mepilogue-cfi -mlong-calls -mmedium-calls -msdata 
-mucb-mcount -mvolatile-cache 
-malign-call -mauto-modify-reg -mbbit-peephole -mno-brcc 
-mcase-vector-pcrel -mcompact-casesi -mno-cond-exec -mearly-cbranchsi 
-mexpand-adddi -mindexed-loads -mlra -mlra-priority-none 
-mlra-priority-compact mlra-priority-noncompact -mno-millicode 
-mmixed-code -mq-class -mRcq -mRcw -msize-level=level 
-mtune=cpu -mmultcost=num -munalign-prob-threshold=probability

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 
-mfp16-format=name
-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 
-mpoke-function-name 
-mthumb  -marm 
-mtpcs-frame  -mtpcs-leaf-frame 
-mcaller-super-interworking  -mcallee-super-interworking 
-mtp=name -mtls-dialect=dialect 
-mword-relocations 
-mfix-cortex-m3-ldrd 
-munaligned-access 
-mneon-for-64bits 
-mslow-flash-data 
-mrestrict-it

AVR Options

-mmcu=mcu -maccumulate-args -mbranch-cost=cost 
-mcall-prologues -mint8 -mno-interrupts -mrelax 
-mstrict-X -mtiny-stack -Waddr-space-convert

Blackfin Options

-mcpu=cpu[-sirevision] 
-msim -momit-leaf-frame-pointer  -mno-omit-leaf-frame-pointer 
-mspecld-anomaly  -mno-specld-anomaly  -mcsync-anomaly  -mno-csync-anomaly 
-mlow-64k -mno-low64k  -mstack-check-l1  -mid-shared-library 
-mno-id-shared-library  -mshared-library-id=n 
-mleaf-id-shared-library  -mno-leaf-id-shared-library 
-msep-data  -mno-sep-data  -mlong-calls  -mno-long-calls 
-mfast-fp -minline-plt -mmulticore  -mcorea  -mcoreb  -msdram 
-micplb

C6X Options

-mbig-endian  -mlittle-endian -march=cpu 
-msim -msdata=sdata-type

CRIS Options

-mcpu=cpu  -march=cpu  -mtune=cpu 
-mmax-stack-frame=n  -melinux-stacksize=n 
-metrax4  -metrax100  -mpdebug  -mcc-init  -mno-side-effects 
-mstack-align  -mdata-align  -mconst-align 
-m32-bit  -m16-bit  -m8-bit  -mno-prologue-epilogue  -mno-gotplt 
-melf  -maout  -melinux  -mlinux  -sim  -sim2 
-mmul-bug-workaround  -mno-mul-bug-workaround

CR16 Options

-mmac 
-mcr16cplus -mcr16c 
-msim -mint32 -mbit-ops
-mdata-model=model

Darwin Options

-all_load  -allowable_client  -arch  -arch_errors_fatal 
-arch_only  -bind_at_load  -bundle  -bundle_loader 
-client_name  -compatibility_version  -current_version 
-dead_strip 
-dependency-file  -dylib_file  -dylinker_install_name 
-dynamic  -dynamiclib  -exported_symbols_list 
-filelist  -flat_namespace  -force_cpusubtype_ALL 
-force_flat_namespace  -headerpad_max_install_names 
-iframework 
-image_base  -init  -install_name  -keep_private_externs 
-multi_module  -multiply_defined  -multiply_defined_unused 
-noall_load   -no_dead_strip_inits_and_terms 
-nofixprebinding -nomultidefs  -noprebind  -noseglinkedit 
-pagezero_size  -prebind  -prebind_all_twolevel_modules 
-private_bundle  -read_only_relocs  -sectalign 
-sectobjectsymbols  -whyload  -seg1addr 
-sectcreate  -sectobjectsymbols  -sectorder 
-segaddr -segs_read_only_addr -segs_read_write_addr 
-seg_addr_table  -seg_addr_table_filename  -seglinkedit 
-segprot  -segs_read_only_addr  -segs_read_write_addr 
-single_module  -static  -sub_library  -sub_umbrella 
-twolevel_namespace  -umbrella  -undefined 
-unexported_symbols_list  -weak_reference_mismatches 
-whatsloaded -F -gused -gfull -mmacosx-version-min=version 
-mkernel -mone-byte-bool

DEC Alpha Options

-mno-fp-regs  -msoft-float 
-mieee  -mieee-with-inexact  -mieee-conformant 
-mfp-trap-mode=mode  -mfp-rounding-mode=mode 
-mtrap-precision=mode  -mbuild-constants 
-mcpu=cpu-type  -mtune=cpu-type 
-mbwx  -mmax  -mfix  -mcix 
-mfloat-vax  -mfloat-ieee 
-mexplicit-relocs  -msmall-data  -mlarge-data 
-msmall-text  -mlarge-text 
-mmemory-latency=time

FR30 Options

-msmall-model -mno-lsim

FRV Options

-mgpr-32  -mgpr-64  -mfpr-32  -mfpr-64 
-mhard-float  -msoft-float 
-malloc-cc  -mfixed-cc  -mdword  -mno-dword 
-mdouble  -mno-double 
-mmedia  -mno-media  -mmuladd  -mno-muladd 
-mfdpic  -minline-plt -mgprel-ro  -multilib-library-pic 
-mlinked-fp  -mlong-calls  -malign-labels 
-mlibrary-pic  -macc-4  -macc-8 
-mpack  -mno-pack  -mno-eflags  -mcond-move  -mno-cond-move 
-moptimize-membar -mno-optimize-membar 
-mscc  -mno-scc  -mcond-exec  -mno-cond-exec 
-mvliw-branch  -mno-vliw-branch 
-mmulti-cond-exec  -mno-multi-cond-exec  -mnested-cond-exec 
-mno-nested-cond-exec  -mtomcat-stats 
-mTLS -mtls 
-mcpu=cpu

GNU/Linux Options

-mglibc -muclibc -mbionic -mandroid 
-tno-android-cc -tno-android-ld

H8/300 Options

-mrelax  -mh  -ms  -mn  -mexr -mno-exr  -mint32  -malign-300

HPPA Options

-march=architecture-type 
-mdisable-fpregs  -mdisable-indexing 
-mfast-indirect-calls  -mgas  -mgnu-ld   -mhp-ld 
-mfixed-range=register-range 
-mjump-in-delay -mlinker-opt -mlong-calls 
-mlong-load-store  -mno-disable-fpregs 
-mno-disable-indexing  -mno-fast-indirect-calls  -mno-gas 
-mno-jump-in-delay  -mno-long-load-store 
-mno-portable-runtime  -mno-soft-float 
-mno-space-regs  -msoft-float  -mpa-risc-1-0 
-mpa-risc-1-1  -mpa-risc-2-0  -mportable-runtime 
-mschedule=cpu-type  -mspace-regs  -msio  -mwsio 
-munix=unix-std  -nolibdld  -static  -threads

i386 and x86-64 Options

-mtune=cpu-type  -march=cpu-type 
-mtune-ctrl=feature-list -mdump-tune-features -mno-default 
-mfpmath=unit 
-masm=dialect  -mno-fancy-math-387 
-mno-fp-ret-in-387  -msoft-float 
-mno-wide-multiply  -mrtd  -malign-double 
-mpreferred-stack-boundary=num 
-mincoming-stack-boundary=num 
-mcld -mcx16 -msahf -mmovbe -mcrc32 
-mrecip -mrecip=opt 
-mvzeroupper -mprefer-avx128 
-mmmx  -msse  -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx 
-mavx2 -mavx512f -mavx512pf -mavx512er -mavx512cd -msha 
-maes -mpclmul -mfsgsbase -mrdrnd -mf16c -mfma -mprefetchwt1 
-msse4a -m3dnow -mpopcnt -mabm -mbmi -mtbm -mfma4 -mxop -mlzcnt 
-mbmi2 -mfxsr -mxsave -mxsaveopt -mrtm -mlwp -mthreads 
-mno-align-stringops  -minline-all-stringops 
-minline-stringops-dynamically -mstringop-strategy=alg 
-mmemcpy-strategy=strategy -mmemset-strategy=strategy
-mpush-args  -maccumulate-outgoing-args  -m128bit-long-double 
-m96bit-long-double -mlong-double-64 -mlong-double-80 -mlong-double-128 
-mregparm=num  -msseregparm 
-mveclibabi=type -mvect8-ret-in-mem 
-mpc32 -mpc64 -mpc80 -mstackrealign 
-momit-leaf-frame-pointer  -mno-red-zone -mno-tls-direct-seg-refs 
-mcmodel=code-model -mabi=name -maddress-mode=mode 
-m32 -m64 -mx32 -m16 -mlarge-data-threshold=num 
-msse2avx -mfentry -m8bit-idiv 
-mavx256-split-unaligned-load -mavx256-split-unaligned-store 
-mstack-protector-guard=guard

i386 and x86-64 Windows Options

-mconsole -mcygwin -mno-cygwin -mdll 
-mnop-fun-dllimport -mthread 
-municode -mwin32 -mwindows -fno-set-stack-executable

IA-64 Options

-mbig-endian  -mlittle-endian  -mgnu-as  -mgnu-ld  -mno-pic 
-mvolatile-asm-stop  -mregister-names  -msdata -mno-sdata 
-mconstant-gp  -mauto-pic  -mfused-madd 
-minline-float-divide-min-latency 
-minline-float-divide-max-throughput 
-mno-inline-float-divide 
-minline-int-divide-min-latency 
-minline-int-divide-max-throughput  
-mno-inline-int-divide 
-minline-sqrt-min-latency -minline-sqrt-max-throughput 
-mno-inline-sqrt 
-mdwarf2-asm -mearly-stop-bits 
-mfixed-range=register-range -mtls-size=tls-size 
-mtune=cpu-type -milp32 -mlp64 
-msched-br-data-spec -msched-ar-data-spec -msched-control-spec 
-msched-br-in-data-spec -msched-ar-in-data-spec -msched-in-control-spec 
-msched-spec-ldc -msched-spec-control-ldc 
-msched-prefer-non-data-spec-insns -msched-prefer-non-control-spec-insns 
-msched-stop-bits-after-every-cycle -msched-count-spec-in-critical-path 
-msel-sched-dont-check-control-spec -msched-fp-mem-deps-zero-cost 
-msched-max-memory-insns-hard-limit -msched-max-memory-insns=max-insns

LM32 Options

-mbarrel-shift-enabled -mdivide-enabled -mmultiply-enabled 
-msign-extend-enabled -muser-enabled

M32R/D Options

-m32r2 -m32rx -m32r 
-mdebug 
-malign-loops -mno-align-loops 
-missue-rate=number 
-mbranch-cost=number 
-mmodel=code-size-model-type 
-msdata=sdata-type 
-mno-flush-func -mflush-func=name 
-mno-flush-trap -mflush-trap=number 
-G num

M32C Options

-mcpu=cpu -msim -memregs=number

M680x0 Options

-march=arch  -mcpu=cpu  -mtune=tune
-m68000  -m68020  -m68020-40  -m68020-60  -m68030  -m68040 
-m68060  -mcpu32  -m5200  -m5206e  -m528x  -m5307  -m5407 
-mcfv4e  -mbitfield  -mno-bitfield  -mc68000  -mc68020 
-mnobitfield  -mrtd  -mno-rtd  -mdiv  -mno-div  -mshort 
-mno-short  -mhard-float  -m68881  -msoft-float  -mpcrel 
-malign-int  -mstrict-align  -msep-data  -mno-sep-data 
-mshared-library-id=n  -mid-shared-library  -mno-id-shared-library 
-mxgot -mno-xgot

MCore Options

-mhardlit  -mno-hardlit  -mdiv  -mno-div  -mrelax-immediates 
-mno-relax-immediates  -mwide-bitfields  -mno-wide-bitfields 
-m4byte-functions  -mno-4byte-functions  -mcallgraph-data 
-mno-callgraph-data  -mslow-bytes  -mno-slow-bytes  -mno-lsim 
-mlittle-endian  -mbig-endian  -m210  -m340  -mstack-increment

MeP Options

-mabsdiff -mall-opts -maverage -mbased=n -mbitops 
-mc=n -mclip -mconfig=name -mcop -mcop32 -mcop64 -mivc2 
-mdc -mdiv -meb -mel -mio-volatile -ml -mleadz -mm -mminmax 
-mmult -mno-opts -mrepeat -ms -msatur -msdram -msim -msimnovec -mtf 
-mtiny=n

MicroBlaze Options

-msoft-float -mhard-float -msmall-divides -mcpu=cpu 
-mmemcpy -mxl-soft-mul -mxl-soft-div -mxl-barrel-shift 
-mxl-pattern-compare -mxl-stack-check -mxl-gp-opt -mno-clearbss 
-mxl-multiply-high -mxl-float-convert -mxl-float-sqrt 
-mbig-endian -mlittle-endian -mxl-reorder -mxl-mode-app-model

MIPS Options

-EL  -EB  -march=arch  -mtune=arch 
-mips1  -mips2  -mips3  -mips4  -mips32  -mips32r2 
-mips64  -mips64r2 
-mips16  -mno-mips16  -mflip-mips16 
-minterlink-compressed -mno-interlink-compressed 
-minterlink-mips16  -mno-interlink-mips16 
-mabi=abi  -mabicalls  -mno-abicalls 
-mshared  -mno-shared  -mplt  -mno-plt  -mxgot  -mno-xgot 
-mgp32  -mgp64  -mfp32  -mfp64  -mhard-float  -msoft-float 
-mno-float  -msingle-float  -mdouble-float 
-mabs=mode  -mnan=encoding 
-mdsp  -mno-dsp  -mdspr2  -mno-dspr2 
-mmcu -mmno-mcu 
-meva -mno-eva 
-mvirt -mno-virt 
-mmicromips -mno-micromips 
-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 -mimadd -mno-imadd -mfused-madd  -mno-fused-madd  -nocpp 
-mfix-24k -mno-fix-24k 
-mfix-r4000  -mno-fix-r4000  -mfix-r4400  -mno-fix-r4400 
-mfix-r10000 -mno-fix-r10000  -mfix-rm7000 -mno-fix-rm7000 
-mfix-vr4120  -mno-fix-vr4120 
-mfix-vr4130  -mno-fix-vr4130  -mfix-sb1  -mno-fix-sb1 
-mflush-func=func  -mno-flush-func 
-mbranch-cost=num  -mbranch-likely  -mno-branch-likely 
-mfp-exceptions -mno-fp-exceptions 
-mvr4130-align -mno-vr4130-align -msynci -mno-synci 
-mrelax-pic-calls -mno-relax-pic-calls -mmcount-ra-address

MMIX Options

-mlibfuncs  -mno-libfuncs  -mepsilon  -mno-epsilon  -mabi=gnu 
-mabi=mmixware  -mzero-extend  -mknuthdiv  -mtoplevel-symbols 
-melf  -mbranch-predict  -mno-branch-predict  -mbase-addresses 
-mno-base-addresses  -msingle-exit  -mno-single-exit

MN10300 Options

-mmult-bug  -mno-mult-bug 
-mno-am33 -mam33 -mam33-2 -mam34 
-mtune=cpu-type 
-mreturn-pointer-on-d0 
-mno-crt0  -mrelax -mliw -msetlb

Moxie Options

-meb -mel -mno-crt0

MSP430 Options

-msim -masm-hex -mmcu= -mcpu= -mlarge -msmall -mrelax 
-mhwmult= -minrt

NDS32 Options

-mbig-endian -mlittle-endian 
-mreduced-regs -mfull-regs 
-mcmov -mno-cmov 
-mperf-ext -mno-perf-ext 
-mv3push -mno-v3push 
-m16bit -mno-16bit 
-mgp-direct -mno-gp-direct 
-misr-vector-size=num 
-mcache-block-size=num 
-march=arch 
-mforce-fp-as-gp -mforbid-fp-as-gp 
-mex9 -mctor-dtor -mrelax

Nios II Options

-G num -mgpopt -mno-gpopt -mel -meb 
-mno-bypass-cache -mbypass-cache 
-mno-cache-volatile -mcache-volatile 
-mno-fast-sw-div -mfast-sw-div 
-mhw-mul -mno-hw-mul -mhw-mulx -mno-hw-mulx -mno-hw-div -mhw-div 
-mcustom-insn=N -mno-custom-insn 
-mcustom-fpu-cfg=name 
-mhal -msmallc -msys-crt0=name -msys-lib=name

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 
-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.

RL78 Options

-msim -mmul=none -mmul=g13 -mmul=rl78

RS/6000 and PowerPC Options

-mcpu=cpu-type 
-mtune=cpu-type 
-mcmodel=code-model 
-mpowerpc64 
-maltivec  -mno-altivec 
-mpowerpc-gpopt  -mno-powerpc-gpopt 
-mpowerpc-gfxopt  -mno-powerpc-gfxopt 
-mmfcrf  -mno-mfcrf  -mpopcntb  -mno-popcntb -mpopcntd -mno-popcntd 
-mfprnd  -mno-fprnd 
-mcmpb -mno-cmpb -mmfpgpr -mno-mfpgpr -mhard-dfp -mno-hard-dfp 
-mfull-toc   -mminimal-toc  -mno-fp-in-toc  -mno-sum-in-toc 
-m64  -m32  -mxl-compat  -mno-xl-compat  -mpe 
-malign-power  -malign-natural 
-msoft-float  -mhard-float  -mmultiple  -mno-multiple 
-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  -msingle-pic-base 
-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 
-mblock-move-inline-limit=num 
-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 
-mrecip -mrecip=opt -mno-recip -mrecip-precision 
-mno-recip-precision 
-mveclibabi=type -mfriz -mno-friz 
-mpointers-to-nested-functions -mno-pointers-to-nested-functions 
-msave-toc-indirect -mno-save-toc-indirect 
-mpower8-fusion -mno-mpower8-fusion -mpower8-vector -mno-power8-vector 
-mcrypto -mno-crypto -mdirect-move -mno-direct-move 
-mquad-memory -mno-quad-memory 
-mquad-memory-atomic -mno-quad-memory-atomic 
-mcompat-align-parm -mno-compat-align-parm

RX Options

-m64bit-doubles  -m32bit-doubles  -fpu  -nofpu
-mcpu=
-mbig-endian-data -mlittle-endian-data 
-msmall-data 
-msim  -mno-sim
-mas100-syntax -mno-as100-syntax
-mrelax
-mmax-constant-size=
-mint-register=
-mpid
-mno-warn-multiple-fast-interrupts
-msave-acc-in-interrupts

S/390 and zSeries Options

-mtune=cpu-type  -march=cpu-type 
-mhard-float  -msoft-float  -mhard-dfp -mno-hard-dfp 
-mlong-double-64 -mlong-double-128 
-mbackchain  -mno-backchain -mpacked-stack  -mno-packed-stack 
-msmall-exec  -mno-small-exec  -mmvcle -mno-mvcle 
-m64  -m31  -mdebug  -mno-debug  -mesa  -mzarch 
-mtpf-trace -mno-tpf-trace  -mfused-madd  -mno-fused-madd 
-mwarn-framesize  -mwarn-dynamicstack  -mstack-size -mstack-guard 
-mhotpatch[=halfwords] -mno-hotpatch

Score Options

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

SH Options

-m1  -m2  -m2e 
-m2a-nofpu -m2a-single-only -m2a-single -m2a 
-m3  -m3e 
-m4-nofpu  -m4-single-only  -m4-single  -m4 
-m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al 
-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 -mno-ieee -mbitops  -misize  -minline-ic_invalidate -mpadstruct 
-mspace -mprefergot  -musermode -multcost=number -mdiv=strategy 
-mdivsi3_libfunc=name -mfixed-range=register-range 
-mindexed-addressing -mgettrcost=number -mpt-fixed 
-maccumulate-outgoing-args -minvalid-symbols 
-matomic-model=atomic-model 
-mbranch-cost=num -mzdcbranch -mno-zdcbranch 
-mfused-madd -mno-fused-madd -mfsca -mno-fsca -mfsrra -mno-fsrra 
-mpretend-cmove -mtas

Solaris 2 Options

-mclear-hwcap -mno-clear-hwcap -mimpure-text  -mno-impure-text 
-pthreads -pthread

SPARC Options

-mcpu=cpu-type 
-mtune=cpu-type 
-mcmodel=code-model 
-mmemory-model=mem-model 
-m32  -m64  -mapp-regs  -mno-app-regs 
-mfaster-structs  -mno-faster-structs  -mflat  -mno-flat 
-mfpu  -mno-fpu  -mhard-float  -msoft-float 
-mhard-quad-float  -msoft-quad-float 
-mstack-bias  -mno-stack-bias 
-munaligned-doubles  -mno-unaligned-doubles 
-muser-mode  -mno-user-mode 
-mv8plus  -mno-v8plus  -mvis  -mno-vis 
-mvis2  -mno-vis2  -mvis3  -mno-vis3 
-mcbcond -mno-cbcond 
-mfmaf  -mno-fmaf  -mpopc  -mno-popc 
-mfix-at697f -mfix-ut699

SPU Options

-mwarn-reloc -merror-reloc 
-msafe-dma -munsafe-dma 
-mbranch-hints 
-msmall-mem -mlarge-mem -mstdmain 
-mfixed-range=register-range 
-mea32 -mea64 
-maddress-space-conversion -mno-address-space-conversion 
-mcache-size=cache-size 
-matomic-updates -mno-atomic-updates

System V Options

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

TILE-Gx Options

-mcpu=CPU -m32 -m64 -mbig-endian -mlittle-endian 
-mcmodel=code-model

TILEPro Options

-mcpu=cpu -m32

V850 Options

-mlong-calls  -mno-long-calls  -mep  -mno-ep 
-mprolog-function  -mno-prolog-function  -mspace 
-mtda=n  -msda=n  -mzda=n 
-mapp-regs  -mno-app-regs 
-mdisable-callt  -mno-disable-callt 
-mv850e2v3 -mv850e2 -mv850e1 -mv850es 
-mv850e -mv850 -mv850e3v5 
-mloop 
-mrelax 
-mlong-jumps 
-msoft-float 
-mhard-float 
-mgcc-abi 
-mrh850-abi 
-mbig-switch

VAX Options

-mg  -mgnu  -munix

VMS Options

-mvms-return-codes -mdebug-main=prefix -mmalloc64 
-mpointer-size=size

VxWorks Options

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

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

Xstormy16 Options

-msim

Xtensa Options

-mconst16 -mno-const16 
-mfused-madd  -mno-fused-madd 
-mforce-no-pic 
-mserialize-volatile  -mno-serialize-volatile 
-mtext-section-literals  -mno-text-section-literals 
-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  -fdelete-dead-exceptions  -funwind-tables 
-fasynchronous-unwind-tables 
-fno-gnu-unique 
-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 -fsplit-stack 
-fleading-underscore  -ftls-model=model 
-fstack-reuse=reuse_level 
-ftrapv  -fwrapv  -fbounds-check 
-fvisibility -fstrict-volatile-bitfields -fsync-libcalls

3.2 Options Controlling the Kind of Output

Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in that order. GCC is capable of preprocessing and compiling several files either into several assembler input files, or into one assembler input file; then each assembler input file produces an object file, and linking combines all the object files (those newly compiled, and those specified as input) into an executable file.

For any given input file, the file name suffix determines what kind of compilation is done:

file.c

C source code that must be preprocessed.

file.i

C source code that should not be preprocessed.

file.ii

C++ source code that should not be preprocessed.

file.m

Objective-C source code. Note that you must link with the libobjc library to make an Objective-C program work.

file.mi

Objective-C source code that should not be preprocessed.

file.mm

file.M

Objective-C++ source code. Note that you must link with the libobjc library to make an Objective-C++ program work. Note that ‘.M’ refers to a literal capital M.

file.mii

Objective-C++ source code that should not be preprocessed.

file.h

C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (default), or C, C++ header file to be turned into an Ada spec (via the -fdump-ada-spec switch).

file.cc

file.cp

file.cxx

file.cpp

file.CPP

file.c++

file.C

C++ source code that must be preprocessed. Note that in ‘.cxx’, the last two letters must both be literally ‘x’. Likewise, ‘.C’ refers to a literal capital C.

file.mm

file.M

Objective-C++ source code that must be preprocessed.

file.mii

Objective-C++ source code that should not be preprocessed.

file.hh

file.H

file.hp

file.hxx

file.hpp

file.HPP

file.h++

file.tcc

C++ header file to be turned into a precompiled header or Ada spec.

file.f

file.for

file.ftn

Fixed form Fortran source code that should not be preprocessed.

file.F

file.FOR

file.fpp

file.FPP

file.FTN

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

file.f90

file.f95

file.f03

file.f08

Free form Fortran source code that should not be preprocessed.

file.F90

file.F95

file.F03

file.F08

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

file.go

Go source code.

file.ads

Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). Such files are also called specs.

file.adb

Ada source code file containing a library unit body (a subprogram or package body). Such files are also called bodies.

file.d

D source code file.

file.di

D interface code file.

file.dd

D documentation code file.

file.s

Assembler code.

file.S

file.sx

Assembler code that must be preprocessed.

other

An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.

You can specify the input language explicitly with the -x option:

-x language

Specify explicitly the language for the following input files (rather than letting the compiler choose a default based on the file name suffix). This option applies to all following input files until the next -x option. Possible values for language are:

c  c-header  cpp-output
c++  c++-header  c++-cpp-output
objective-c  objective-c-header  objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler  assembler-with-cpp
ada
d
f77  f77-cpp-input f95  f95-cpp-input
go
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 exits with the code of 1 if any phase of the compiler returns a non-success return code. If you specify -pass-exit-codes, the gcc program instead returns with the numerically highest error produced by any phase returning an error indication. The C, C++, and Fortran front ends return 4 if an internal compiler error is encountered.

If you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc where to start, and one of the options -c, -S, or -E to say where gcc is to stop. Note that some combinations (for example, ‘-x cpp-output -E’) instruct gcc to do nothing at all.

-c

Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in the form of an object file for each source file.

By default, the object file name for a source file is made by replacing the suffix ‘.c’, ‘.i’, ‘.s’, etc., with ‘.o’.

Unrecognized input files, not requiring compilation or assembly, are ignored.

-S

Stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler code file for each non-assembler input file specified.

By default, the assembler file name for a source file is made by replacing the suffix ‘.c’, ‘.i’, etc., with ‘.s’.

Input files that don’t require compilation are ignored.

-E

Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output.

Input files that don’t require preprocessing are ignored.

-o file

Place output in file file. This applies to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code.

If -o is not specified, the default is to put an executable file in a.out, the object file for source.suffix in source.o, its assembler file in source.s, a precompiled header file in source.suffix.gch, and all preprocessed C source on standard output.

-v

Print (on standard error output) the commands executed to run the stages of compilation. Also print the version number of the compiler driver program and of the preprocessor and the compiler proper.

-###

Like -v except the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or ./-_. This is useful for shell scripts to capture the driver-generated command lines.

-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.

--help

Print (on the standard output) a description of the command-line options understood by gcc. If the -v option is also specified then --help is also passed on to the various processes invoked by gcc, so that they can display the command-line options they accept. If the -Wextra option has also been specified (prior to the --help option), then command-line options that have no documentation associated with them are also displayed.

--target-help

Print (on the standard output) a description of target-specific command-line options for each tool. For some targets extra target-specific information may also be printed.

--help={class|[^]qualifier}[,…]

Print (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers. These are the supported classes:

‘optimizers’

Display all of the optimization options supported by the compiler.

‘warnings’

Display all of the options controlling warning messages produced by the compiler.

‘target’

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

‘params’

Display the values recognized by the --param option.

language

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

‘common’

Display the options that are common to all languages.

These are the supported qualifiers:

‘undocumented’

Display only those options that are undocumented.

‘joined’

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

‘separate’

Display options taking an argument that appears as a separate word following the original option, such as: ‘-o output-file’.

Thus for example to display all the undocumented target-specific switches supported by the compiler, use:

--help=target,undocumented

The sense of a qualifier can be inverted by prefixing it with the ‘^’ character, so for example to display all binary warning options (i.e., ones that are either on or off and that do not take an argument) that have a description, use:

--help=warnings,^joined,^undocumented

The argument to --help= should not consist solely of inverted qualifiers.

Combining several classes is possible, although this usually restricts the output so much that there is nothing to display. One case where it does work, however, is when one of the classes is target. For example, to display all the target-specific optimization options, use:

--help=target,optimizers

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

If 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

-no-canonical-prefixes

Do not expand any symbolic links, resolve references to ‘/../’ or ‘/./’, or make the path absolute when generating a relative prefix.

--version

Display the version number and copyrights of the invoked GCC.

-wrapper

Invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma separated list.

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

This invokes all subprograms of gcc under ‘gdb --args’, thus the invocation of cc1 is ‘gdb --args cc1 …’.

-fplugin=name.so

Load the plugin code in file name.so, assumed to be a shared object to be dlopen’d by the compiler. The base name of the shared object file is used to identify the plugin for the purposes of argument parsing (See -fplugin-arg-name-key=value below). Each plugin should define the callback functions specified in the Plugins API.

-fplugin-arg-name-key=value

Define an argument called key with a value of value for the plugin called name.

-fdump-ada-spec[-slim]

For C and C++ source and include files, generate corresponding Ada specs. See Generating Ada Bindings for C and C++ headers in GNAT User’s Guide, which provides detailed documentation on this feature.

-fada-spec-parent=unit

In conjunction with -fdump-ada-spec[-slim] above, generate Ada specs as child units of parent unit.

-fdump-go-spec=file

For input files in any language, generate corresponding Go declarations in file. This generates Go const, type, var, and func declarations which may be a useful way to start writing a Go interface to code written in some other language.

@file

Read command-line options from file. The options read are inserted in place of the original @file option. If file does not exist, or cannot be read, then the option will be treated literally, and not removed.

Options in file are separated by whitespace. A whitespace character may be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) may be included by prefixing the character to be included with a backslash. The file may itself contain additional @file options; any such options will be processed recursively.

3.3 Compiling C++ Programs

C++ source files conventionally use one of the suffixes ‘.C’, ‘.cc’, ‘.cpp’, ‘.CPP’, ‘.c++’, ‘.cp’, or ‘.cxx’; C++ header files often use ‘.hh’, ‘.hpp’, ‘.H’, or (for shared template code) ‘.tcc’; and preprocessed C++ files use the suffix ‘.ii’. GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name gcc).

However, the use of gcc does not add the C++ library. g++ is a program that calls GCC and automatically specifies linking against the C++ library. It treats ‘.c’, ‘.h’ and ‘.i’ files as C++ source files instead of C source files unless -x is used. This program is also useful when precompiling a C header file with a ‘.h’ extension for use in C++ compilations. On many systems, g++ is also installed with the name c++.

When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs. See Options Controlling C Dialect, for explanations of options for languages related to C. See Options Controlling C++ Dialect, for explanations of options that are meaningful only for C++ programs.

3.4 Options Controlling C Dialect

The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that the compiler accepts:

-ansi

In C mode, this is equivalent to -std=c90. In C++ mode, it is equivalent to -std=c++98.

This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of standard C++ (when compiling C++ code), such as the asm and typeof keywords, and predefined macros such as unix and vax that identify the type of system you are using. It also enables the undesirable and rarely used ISO trigraph feature. For the C compiler, it disables recognition of C++ style ‘//’ comments as well as the inline keyword.

The alternate keywords __asm__, __extension__, __inline__ and __typeof__ continue to work despite -ansi. You would not want to use them in an ISO C program, of course, but it is useful to put them in header files that might be included in compilations done with -ansi. Alternate predefined macros such as __unix__ and __vax__ are also available, with or without -ansi.

The -ansi option does not cause non-ISO programs to be rejected gratuitously. For that, -Wpedantic is required in addition to -ansi. See Warning Options.

The macro __STRICT_ANSI__ is predefined when the -ansi option is used. Some header files may notice this macro and refrain from declaring certain functions or defining certain macros that the ISO standard doesn’t call for; this is to avoid interfering with any programs that might use these names for other things.

Functions that are normally built in but do not have semantics defined by ISO C (such as alloca and ffs) are not built-in functions when -ansi is used. See Other built-in functions provided by GCC, for details of the functions affected.

-std=

Determine the language standard. See Language Standards Supported by GCC, for details of these standard versions. This option is currently only supported when compiling C or C++.

The compiler can accept several base standards, such as ‘c90’ or ‘c++98’, and GNU dialects of those standards, such as ‘gnu90’ or ‘gnu++98’. When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that do not contradict it. For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the asm and typeof keywords, but not other GNU extensions that do not have a meaning in ISO C90, such as omitting the middle term of a ?: expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard. As a result, some strict-conforming programs may be rejected. The particular standard is used by -Wpedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style ‘//’ comments, while -std=gnu99 -Wpedantic does not.

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

‘c90’

‘c89’

‘iso9899:1990’

Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code.

‘iso9899:199409’

ISO C90 as modified in amendment 1.

‘c99’

‘c9x’

‘iso9899:1999’

‘iso9899:199x’

ISO C99. This standard is substantially completely supported, modulo bugs, extended identifiers (supported except for corner cases when -fextended-identifiers is used) and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See http://gcc.gnu.org/c99status.html for more information. The names ‘c9x’ and ‘iso9899:199x’ are deprecated.

‘c11’

‘c1x’

‘iso9899:2011’

ISO C11, the 2011 revision of the ISO C standard. This standard is substantially completely supported, modulo bugs, extended identifiers (supported except for corner cases when -fextended-identifiers is used), floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The name ‘c1x’ is deprecated.

‘gnu90’

‘gnu89’

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

‘gnu99’

‘gnu9x’

GNU dialect of ISO C99. The name ‘gnu9x’ is deprecated.

‘gnu11’

‘gnu1x’

GNU dialect of ISO C11. This is intended to become the default in a future release of GCC. The name ‘gnu1x’ is deprecated.

‘c++98’

‘c++03’

The 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as -ansi for C++ code.

‘gnu++98’

‘gnu++03’

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

‘c++11’

‘c++0x’

The 2011 ISO C++ standard plus amendments. The name ‘c++0x’ is deprecated.

‘gnu++11’

‘gnu++0x’

GNU dialect of -std=c++11. The name ‘gnu++0x’ is deprecated.

‘c++1y’

The next revision of the ISO C++ standard, tentatively planned for 2014. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.

‘gnu++1y’

GNU dialect of -std=c++1y. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.

-fgnu89-inline

The option -fgnu89-inline tells GCC to use the traditional GNU semantics for inline functions when in C99 mode. See An Inline Function is As Fast As a Macro. 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 -std=c90 or -std=gnu90 mode.

The preprocessor macros __GNUC_GNU_INLINE__ and __GNUC_STDC_INLINE__ may be used to check which semantics are in effect for inline functions. See Common Predefined Macros in The C Preprocessor.

-aux-info filename

Output to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files. This option is silently ignored in any language other than C.

Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (‘I’, ‘N’ for new or ‘O’ for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (‘C’ or ‘F’, respectively, in the following character). In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration.

-fallow-parameterless-variadic-functions

Accept variadic functions without named parameters.

Although it is possible to define such a function, this is not very useful as it is not possible to read the arguments. This is only supported for C as this construct is allowed by C++.

-fno-asm

Do not recognize asm, inline or typeof as a keyword, so that code can use these words as identifiers. You can use the keywords __asm__, __inline__ and __typeof__ instead. -ansi implies -fno-asm.

In C++, this switch only affects the typeof keyword, since asm and inline are standard keywords. You may want to use the -fno-gnu-keywords flag instead, which has the same effect. In C99 mode (-std=c99 or -std=gnu99), this switch only affects the asm and typeof keywords, since inline is a standard keyword in ISO C99.

-fno-builtin

-fno-builtin-function

Don’t recognize built-in functions that do not begin with ‘__builtin_’ as prefix. See Other built-in functions provided by GCC, for details of the functions affected, including those which are not built-in functions when -ansi or -std options for strict ISO C conformance are used because they do not have an ISO standard meaning.

GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to alloca may become single instructions which adjust the stack directly, and calls to memcpy may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library. In addition, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function. For example, warnings are given with -Wformat for bad calls to printf when printf is built in and strlen is known not to modify global memory.

With the -fno-builtin-function option only the built-in function function is disabled. function must not begin with ‘__builtin_’. If a function is named that is not built-in in this version of GCC, this option is ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:

#define abs(n)          __builtin_abs ((n))
#define strcpy(d, s)    __builtin_strcpy ((d), (s))

-fhosted

Assert that compilation targets a hosted environment. This implies -fbuiltin. A hosted environment is one in which the entire standard library is available, and in which main has a return type of int. Examples are nearly everything except a kernel. This is equivalent to -fno-freestanding.

-ffreestanding

Assert that compilation targets a freestanding environment. This implies -fno-builtin. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at main. The most obvious example is an OS kernel. This is equivalent to -fno-hosted.

See Language Standards Supported by GCC, for details of freestanding and hosted environments.

-fopenmp

Enable handling of OpenMP directives #pragma omp in C/C++ and !$omp in Fortran. When -fopenmp is specified, the compiler generates parallel code according to the OpenMP Application Program Interface v4.0 http://www.openmp.org/. This option implies -pthread, and thus is only supported on targets that have support for -pthread. -fopenmp implies -fopenmp-simd.

-fopenmp-simd

Enable handling of OpenMP’s SIMD directives with #pragma omp in C/C++ and !$omp in Fortran. Other OpenMP directives are ignored.

-fcilkplus

Enable the usage of Cilk Plus language extension features for C/C++. When the option -fcilkplus is specified, enable the usage of the Cilk Plus Language extension features for C/C++. The present implementation follows ABI version 1.2. This is an experimental feature that is only partially complete, and whose interface may change in future versions of GCC as the official specification changes. Currently, all features but _Cilk_for have been implemented.

-fgnu-tm

When the option -fgnu-tm is specified, the compiler generates code for the Linux variant of Intel’s current Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature.

For more information on GCC’s support for transactional memory, See The GNU Transactional Memory Library in GNU Transactional Memory Library.

Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions).

-fms-extensions

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

In C++ code, this allows member names in structures to be similar to previous types declarations.

typedef int UOW;
struct ABC {
  UOW UOW;
};

Some cases of unnamed fields in structures and unions are only accepted with this option. See Unnamed struct/union fields within structs/unions, for details.

Note that this option is off for all targets but i?86 and x86_64 targets using ms-abi.

-fplan9-extensions

Accept some non-standard constructs used in Plan 9 code.

This enables -fms-extensions, permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field, and permits referring to anonymous fields declared using a typedef. See Unnamed struct/union fields within structs/unions, for details. This is only supported for C, not C++.

-trigraphs

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

-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. 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. The default is version 2.

Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed.

Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.

Version 2 is the version of the C++ ABI that first appeared in G++ 3.4.

Version 3 corrects an error in mangling a constant address as a template argument.

Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.

Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.

Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and –, and a class scope function used as a template argument.

See also -Wabi.

-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 only returns 0 if it is declared ‘throw()’, in which case the compiler always checks the return value even without this option. In all other cases, when operator new has a non-empty exception specification, memory exhaustion is signalled by throwing std::bad_alloc. See also ‘new (nothrow)’.

-fconstexpr-depth=n

Set the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed to detect endless recursion during constant expression evaluation. The minimum specified by the standard is 512.

-fdeduce-init-list

Enable 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 deduction was implemented as a possible extension to the originally proposed semantics for the C++11 standard, but was not part of the final standard, so it is disabled by default. This option is deprecated, and may be removed in a future version of G++.

-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 that 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 that is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases.

-fno-enforce-eh-specs

Don’t generate code to check for violation of exception specifications at run time. This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining ‘NDEBUG’. This does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time.

-fextern-tls-init

-fno-extern-tls-init

The C++11 and OpenMP standards allow ‘thread_local’ and ‘threadprivate’ variables to have dynamic (runtime) initialization. To support this, any use of such a variable goes through a wrapper function that performs any necessary initialization. When the use and definition of the variable are in the same translation unit, this overhead can be optimized away, but when the use is in a different translation unit there is significant overhead even if the variable doesn’t actually need dynamic initialization. If the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or a use of the variable in the defining TU will be executed before any uses in another TU), they can avoid this overhead with the -fno-extern-tls-init option.

On targets that support symbol aliases, the default is -fextern-tls-init. On targets that do not support symbol aliases, the default is -fno-extern-tls-init.

-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++.

If neither flag is given, the default is 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 that 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 need the same set of explicit instantiations.

-fno-implement-inlines

To save space, do not emit out-of-line copies of inline functions controlled by ‘#pragma implementation’. This causes linker errors if these functions are not inlined everywhere they are called.

-fms-extensions

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

-fno-nonansi-builtins

Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include ffs, alloca, _exit, index, bzero, conjf, and other related functions.

-fnothrow-opt

Treat a throw() exception specification as if it were a noexcept specification to reduce or eliminate the text size overhead relative to a function with no exception specification. If the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller because the EH cleanups for those variables can be optimized away. The semantic effect is that an exception thrown out of a function with such an exception specification results in a call to terminate rather than unexpected.

-fno-operator-names

Do not treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms as keywords.

-fno-optional-diags

Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class.

-fpermissive

Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using -fpermissive allows some nonconforming code to compile.

-fno-pretty-templates

When an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g. void f(T) [with T = int] rather than void f(int)) so that it’s clear which template is involved. When an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. If either of these behaviors make it harder to understand the error message rather than easier, you can use -fno-pretty-templates to disable them.

-frepo

Enable automatic template instantiation at link time. This option also implies -fno-implicit-templates. See Template Instantiation, for more information.

-fno-rtti

Disable generation of information about every class with virtual functions for use by the C++ run-time type identification features (‘dynamic_cast’ and ‘typeid’). If you don’t use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but G++ generates it as needed. The ‘dynamic_cast’ operator can still be used for casts that do not require run-time type information, i.e. casts to void * or to unambiguous base classes.

-fstats

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

-fstrict-enums

Allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type.

-ftemplate-backtrace-limit=n

Set the maximum number of template instantiation notes for a single warning or error to n. The default value is 10.

-ftemplate-depth=n

Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11). The default value is 900, as the compiler can run out of stack space before hitting 1024 in some situations.

-fno-threadsafe-statics

Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn’t need to be thread-safe.

-fuse-cxa-atexit

Register destructors for objects with static storage duration with the __cxa_atexit function rather than the atexit function. This option is required for fully standards-compliant handling of static destructors, but only works if your C library supports __cxa_atexit.

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

Don’t use the __cxa_get_exception_ptr runtime routine. This causes std::uncaught_exception to be incorrect, but is necessary if the runtime routine is not available.

-fvisibility-inlines-hidden

This switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.

The effect of this is that GCC may, effectively, mark inline methods with __attribute__ ((visibility ("hidden"))) so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates.

The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object.

You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility has no effect.

Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary. See Template Instantiation.

-fvisibility-ms-compat

This flag attempts to use visibility settings to make GCC’s C++ linkage model compatible with that of Microsoft Visual Studio.

The flag makes these changes to GCC’s linkage model:

1. It sets the default visibility to hidden, like -fvisibility=hidden.
2. Types, but not their members, are not hidden by default.
3. The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used.

In new code it is better to use -fvisibility=hidden and export those classes that are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior.

Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently.

-fvtable-verify=std|preinit|none

Turn on (or off, if using -fvtable-verify=none) the security feature that verifies at runtime, for every virtual call that is made, that the vtable pointer through which the call is made is valid for the type of the object, and has not been corrupted or overwritten. If an invalid vtable pointer is detected (at runtime), an error is reported and execution of the program is immediately halted.

This option causes runtime data structures to be built, at program start up, for verifying the vtable pointers. The options std and preinit control the timing of when these data structures are built. In both cases the data structures are built before execution reaches ’main’. The -fvtable-verify=std causes these data structure to be built after the shared libraries have been loaded and initialized. -fvtable-verify=preinit causes them to be built before the shared libraries have been loaded and initialized.

If this option appears multiple times in the compiler line, with different values specified, ’none’ will take highest priority over both ’std’ and ’preinit’; ’preinit’ will take priority over ’std’.

-fvtv-debug

Causes debug versions of the runtime functions for the vtable verification feature to be called. This assumes the -fvtable-verify=std or -fvtable-verify=preinit has been used. This flag will also cause the compiler to keep track of which vtable pointers it found for each class, and record that information in the file “vtv_set_ptr_data.log”, in the dump file directory on the user’s machine.

Note: This feature APPENDS data to the log file. If you want a fresh log file, be sure to delete any existing one.

-fvtv-counts

This is a debugging flag. When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, this causes the compiler to keep track of the total number of virtual calls it encountered and the number of verifications it inserted. It also counts the number of calls to certain runtime library functions that it inserts. This information, for each compilation unit, is written to a file named “vtv_count_data.log”, in the dump_file directory on the user’s machine. It also counts the size of the vtable pointer sets for each class, and writes this information to “vtv_class_set_sizes.log” in the same directory.

Note: This feature APPENDS data to the log files. To get a fresh log files, be sure to delete any existing ones.

-fno-weak

Do not use weak symbol support, even if it is provided by the linker. By default, G++ uses weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++.

-nostdinc++

Do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library.)

In addition, these optimization, warning, and code generation options have meanings only for C++ programs:

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

Warn when G++ 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 is compatible.

You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.

The known incompatibilities in -fabi-version=2 (the default) include:

• A template with a non-type template parameter of reference type is mangled incorrectly:

  extern int N;
  template <int &> struct S {};
  void n (S<N>) {2}
  

  This is fixed in -fabi-version=3.

• SIMD vector types declared using __attribute ((vector_size)) are mangled in a non-standard way that does not allow for overloading of functions taking vectors of different sizes.

  The mangling is changed in -fabi-version=4.

The known incompatibilities in -fabi-version=1 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++ places B::f2 into the same byte as A::f1; other compilers do 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 causes G++ and other compilers to lay out 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++ does not place B into the tail-padding for A; other compilers do. You can avoid this problem by explicitly padding A so that its size is a multiple of its alignment (ignoring virtual base classes); that causes G++ and other compilers to lay out 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++ makes 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++ places 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 about psABI-related changes. The known psABI changes at this point include:

• For SysV/x86-64, unions with long double members are passed in memory as specified in psABI. For example:

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

  union U is always passed in memory.

-Wctor-dtor-privacy (C++ and Objective-C++ only)

Warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions. Also warn if there are no non-private methods, and there’s at least one private member function that isn’t a constructor or destructor.

-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)

Warn when ‘delete’ is used to destroy an instance of a class that has virtual functions and non-virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor. This warning is enabled by -Wall.

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

Warn when a string or character literal is followed by a ud-suffix which does not begin with an underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens in order to maintain backwards compatibility with code that uses formatting macros from <inttypes.h>. For example:

#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>

int main() {
  int64_t i64 = 123;
  printf("My int64: %"PRId64"\n", i64);
}

In this case, PRId64 is treated as a separate preprocessing token.

This warning is enabled by default.

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

Warn when a narrowing conversion prohibited by C++11 occurs within ‘{ }’, e.g.

int i = { 2.2 }; // error: narrowing from double to int

This flag is included in -Wall and -Wc++11-compat.

With -std=c++11, -Wno-narrowing suppresses the diagnostic required by the standard. Note that this does not affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE context.

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

Warn when a noexcept-expression evaluates to false because of a call to a function that does not have a non-throwing exception specification (i.e. ‘throw()’ or ‘noexcept’) but is known by the compiler to never throw an exception.

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

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

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

Warn when the order of member initializers given in the code does not match the order in which they must be executed. For instance:

struct A {
  int i;
  int j;
  A(): j (0), i (1) { }
};

The compiler rearranges the member initializers for ‘i’ and ‘j’ to match the declaration order of the members, emitting a warning to that effect. This warning is enabled by -Wall.

-fext-numeric-literals (C++ and Objective-C++ only)

Accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off these suffixes are treated as C++11 user-defined literal numeric suffixes. This is on by default for all pre-C++11 dialects and all GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++1y. This option is off by default for ISO C++11 onwards (-std=c++11, ...).

The following -W… options are not affected by -Wall.

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

Warn about violations of the following style guidelines from Scott Meyers’ Effective C++ series of books:

• Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
• Prefer initialization to assignment in constructors.
• Have operator= return a reference to *this.
• Don’t try to return a reference when you must return an object.
• Distinguish between prefix and postfix forms of increment and decrement operators.
• Never overload &&, ||, or ,.

This option also enables -Wnon-virtual-dtor, which is also one of the effective C++ recommendations. However, the check is extended to warn about the lack of virtual destructor in accessible non-polymorphic bases classes too.

When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use ‘grep -v’ to filter out those warnings.

-Wstrict-null-sentinel (C++ and Objective-C++ only)

Warn about the use of an uncasted NULL as sentinel. When compiling only with GCC this is a valid sentinel, as NULL is defined to __null. Although it is a null pointer constant rather than a null pointer, it is guaranteed to be of the same size as a pointer. But this use is not portable across different compilers.

-Wno-non-template-friend (C++ and Objective-C++ only)

Disable warnings when non-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();

fails to compile.

-Wno-pmf-conversions (C++ and Objective-C++ only)

Disable the diagnostic for converting a bound pointer to member function to a plain pointer.

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

Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. Previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior.

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 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. 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, overrides 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 ([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. This option is only available in conjunction with the NeXT runtime and ABI version 0 or 1.

-fobjc-abi-version=n

Use version n of the Objective-C ABI for the selected runtime. This option is currently supported only for the NeXT runtime. In that case, Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C 2.0 additions. Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI. If nothing is specified, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit target machines.

-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 which runs 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 which runs all such default destructors, in reverse order.

The - (id) .cxx_construct and - (void) .cxx_destruct methods thusly generated 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 are invoked by the runtime immediately after a new object instance is allocated; the - (void) .cxx_destruct methods are 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 required to use the Objective-C keywords @try, @throw, @catch, @finally and @synchronized. This option is available with both the GNU runtime and the NeXT runtime (but not available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier).

-fobjc-gc

Enable garbage collection (GC) in Objective-C and Objective-C++ programs. This option is only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags.

-fobjc-nilcheck

For the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual method call. This is the default and can be disabled using -fno-objc-nilcheck. Class methods and super calls are never checked for nil in this way no matter what this flag is set to. Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used.

-fobjc-std=objc1

Conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards, which is controlled by the separate C/C++ dialect option flags. When this option is used with the Objective-C or Objective-C++ compiler, any Objective-C syntax that is not recognized by GCC 4.0 is rejected. This is useful if you need to make sure that your Objective-C code can be compiled with older versions of GCC.

-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. The GNU runtime currently always retains calls to objc_get_class("…") regardless of command-line options.

-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 omits such warnings if any differences found are confined to types that 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, …). You can use the options described below to control the formatting algorithm for diagnostic messages, e.g. how many characters per line, how often source location information should be reported. Note that some language front ends may not honor these options.

-fmessage-length=n

Try to format error messages so that they fit on lines of about n characters. 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 is done; each error message appears on a single line.

-fdiagnostics-show-location=once

Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit source location information once; that is, in case the message is too long to fit on a single physical line and has to be wrapped, the source location won’t be emitted (as prefix) again, over and over, in subsequent continuation lines. This is the default behavior.

-fdiagnostics-show-location=every-line

Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the same source location information (as prefix) for physical lines that result from the process of breaking a message which is too long to fit on a single line.

-fdiagnostics-color[=WHEN]

-fno-diagnostics-color

Use color in diagnostics. WHEN is ‘never’, ‘always’, or ‘auto’. The default is ‘never’ if GCC_COLORS environment variable isn’t present in the environment, and ‘auto’ otherwise. ‘auto’ means to use color only when the standard error is a terminal. The forms -fdiagnostics-color and -fno-diagnostics-color are aliases for -fdiagnostics-color=always and -fdiagnostics-color=never, respectively.

The colors are defined by the environment variable GCC_COLORS. Its value is a colon-separated list of capabilities and Select Graphic Rendition (SGR) substrings. SGR commands are interpreted by the terminal or terminal emulator. (See the section in the documentation of your text terminal for permitted values and their meanings as character attributes.) These substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate include ‘1’ for bold, ‘4’ for underline, ‘5’ for blink, ‘7’ for inverse, ‘39’ for default foreground color, ‘30’ to ‘37’ for foreground colors, ‘90’ to ‘97’ for 16-color mode foreground colors, ‘38;5;0’ to ‘38;5;255’ for 88-color and 256-color modes foreground colors, ‘49’ for default background color, ‘40’ to ‘47’ for background colors, ‘100’ to ‘107’ for 16-color mode background colors, and ‘48;5;0’ to ‘48;5;255’ for 88-color and 256-color modes background colors.

The default GCC_COLORS is ‘error=01;31:warning=01;35:note=01;36:caret=01;32:locus=01:quote=01’ where ‘01;31’ is bold red, ‘01;35’ is bold magenta, ‘01;36’ is bold cyan, ‘01;32’ is bold green and ‘01’ is bold. Setting GCC_COLORS to the empty string disables colors. Supported capabilities are as follows.

error=

SGR substring for error: markers.

warning=

SGR substring for warning: markers.

note=

SGR substring for note: markers.

caret=

SGR substring for caret line.

locus=

SGR substring for location information, ‘file:line’ or ‘file:line:column’ etc.

quote=

SGR substring for information printed within quotes.

-fno-diagnostics-show-option

By default, each diagnostic emitted includes text indicating the command-line option that directly controls the diagnostic (if such an option is known to the diagnostic machinery). Specifying the -fno-diagnostics-show-option flag suppresses that behavior.

-fno-diagnostics-show-caret

By default, each diagnostic emitted includes the original source line and a caret ’^’ indicating the column. This option suppresses this information.

3.8 Options to Request or Suppress Warnings

Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error.

The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by GCC.

-fsyntax-only

Check the code for syntax errors, but don’t do anything beyond that.

-fmax-errors=n

Limits the maximum number of error messages to n, at which point GCC bails out rather than attempting to continue processing the source code. If n is 0 (the default), there is no limit on the number of error messages produced. If -Wfatal-errors is also specified, then -Wfatal-errors takes precedence over this option.

-w

Inhibit all warning messages.

-Werror

Make all warnings into errors.

-Werror=

Make the specified warning into an error. The specifier for a warning is appended; for example -Werror=switch turns the warnings controlled by -Wswitch into errors. This switch takes a negative form, to be used to negate -Werror for specific warnings; for example -Wno-error=switch makes -Wswitch warnings not be errors, even when -Werror is in effect.

The warning message for each controllable warning includes the option that controls the warning. That option can then be used with -Werror= and -Wno-error= as described above. (Printing of the option in the warning message can be disabled using the -fno-diagnostics-show-option flag.)

Note that specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply anything.

-Wfatal-errors

This option causes the compiler to abort compilation on the first error occurred rather than trying to keep going and printing further error messages.

You can request many specific warnings with options beginning with ‘-W’, for example -Wimplicit to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning ‘-Wno-’ to turn off warnings; for example, -Wno-implicit. This manual lists only one of the two forms, whichever is not the default. For further language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.

When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC emits a diagnostic stating that the option is not recognized. However, if the -Wno- form is used, the behavior is slightly different: no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being produced. This allows the use of new -Wno- options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present.

-Wpedantic

-pedantic

Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++. For ISO C, follows the version of the ISO C standard specified by any -std option used.

Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few require -ansi or a -std option specifying the required version of ISO C). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are rejected.

-Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with ‘__’. Pedantic warnings are also disabled in the expression that follows __extension__. However, only system header files should use these escape routes; application programs should avoid them. See Alternate Keywords.

Some users try to use -Wpedantic to check programs for strict ISO C conformance. They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all—only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added.

A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable additional work and would be quite different from -Wpedantic. We don’t have plans to support such a feature in the near future.

Where the standard specified with -std represents a GNU extended dialect of C, such as ‘gnu90’ or ‘gnu99’, there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from -Wpedantic are given where they are required by the base standard. (It does not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.)

-pedantic-errors

Like -Wpedantic, 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++11-compat  
-Wchar-subscripts  
-Wenum-compare (in C/ObjC; this is on by default in C++) 
-Wimplicit-int (C and Objective-C only) 
-Wimplicit-function-declaration (C and Objective-C only) 
-Wcomment  
-Wformat   
-Wmain (only for C/ObjC and unless -ffreestanding)  
-Wmaybe-uninitialized 
-Wmissing-braces (only for C/ObjC) 
-Wnonnull  
-Wopenmp-simd 
-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) 
-Wunused-but-set-parameter (only with -Wunused or -Wall)  

The option -Wextra also prints warning messages for the following cases:

• A pointer is compared against integer zero with ‘<’, ‘<=’, ‘>’, or ‘>=’.
• (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.
• (C++ only) Ambiguous virtual bases.
• (C++ only) Subscripting an array that has been declared ‘register’.
• (C++ only) Taking the address of a variable that has been declared ‘register’.
• (C++ only) A base class is not initialized in a derived class’s 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.

-Wno-coverage-mismatch

Warn if feedback profiles do not match when using the -fprofile-use option. If a source file is changed between compiling with -fprofile-gen and with -fprofile-use, the files with the profile feedback can fail to match the source file and GCC cannot use the profile feedback information. By default, this warning is enabled and is treated as an error. -Wno-coverage-mismatch can be used to disable the warning or -Wno-error=coverage-mismatch can be used to disable the error. Disabling the error for this warning can result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to an existing code-base. Completely disabling the warning is not recommended.

-Wno-cpp

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

Suppress warning messages emitted by #warning directives.

-Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)

Give a warning when a value of type float is implicitly promoted to double. CPUs with a 32-bit “single-precision” floating-point unit implement float in hardware, but emulate double in software. On such a machine, doing computations using double values is much more expensive because of the overhead required for software emulation.

It is easy to accidentally do computations with double because floating-point literals are implicitly of type double. For example, in:

float area(float radius)
{
   return 3.14159 * radius * radius;
}

the compiler performs the entire computation with double because the floating-point literal is a double.

-Wformat

-Wformat=n

Check calls to printf and scanf, etc., to make sure that the arguments supplied have types appropriate to the format string specified, and that the conversions specified in the format string make sense. This includes standard functions, and others specified by format attributes (see Function Attributes), in the printf, scanf, strftime and strfmon (an X/Open extension, not in the C standard) families (or other target-specific families). Which functions are checked without format attributes having been specified depends on the standard version selected, and such checks of functions without the attribute specified are disabled by -ffreestanding or -fno-builtin.

The formats are checked against the format features supported by GNU libc version 2.2. These include all ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU extensions. Other library implementations may not support all these features; GCC does not support warning about features that go beyond a particular library’s limitations. However, if -Wpedantic is used with -Wformat, warnings are given about format features not in the selected standard version (but not for strfmon formats, since those are not in any version of the C standard). See Options Controlling C Dialect.

-Wformat=1

-Wformat

Option -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0. Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull. Some aspects of this level of format checking can be disabled by the options: -Wno-format-contains-nul, -Wno-format-extra-args, and -Wno-format-zero-length. -Wformat is enabled by -Wall.

-Wno-format-contains-nul

If -Wformat is specified, do not warn about format strings that contain NUL bytes.

-Wno-format-extra-args

If -Wformat is specified, do not warn about excess arguments to a printf or scanf format function. The C standard specifies that such arguments are ignored.

Where the unused arguments lie between used arguments that are specified with ‘$’ operand number specifications, normally warnings are still given, since the implementation could not know what type to pass to va_arg to skip the unused arguments. However, in the case of scanf formats, this option suppresses the warning if the unused arguments are all pointers, since the Single Unix Specification says that such unused arguments are allowed.

-Wno-format-zero-length

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

-Wformat=2

Enable -Wformat plus additional format checks. Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k.

-Wformat-nonliteral

If -Wformat is specified, also warn if the format string is not a string literal and so cannot be checked, unless the format function takes its format arguments as a va_list.

-Wformat-security

If -Wformat is specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, as in printf (foo);. This may be a security hole if the format string came from untrusted input and contains ‘%n’. (This is currently a subset of what -Wformat-nonliteral warns about, but in future warnings may be added to -Wformat-security that are not included in -Wformat-nonliteral.)

-Wformat-y2k

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

-Wnonnull

Warn about passing a null pointer for arguments marked as requiring a non-null value by the nonnull function attribute.

-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option.

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

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

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

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

This warning is enabled by -Wall in C++.

-Wimplicit-int (C and Objective-C only)

Warn when a declaration does not specify a type. This warning is enabled by -Wall.

-Wimplicit-function-declaration (C and Objective-C only)

Give a warning whenever a function is used before being declared. In C99 mode (-std=c99 or -std=gnu99), this warning is enabled by default and it is made into an error by -pedantic-errors. This warning is also enabled by -Wall.

-Wimplicit (C and Objective-C only)

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

-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 -Wpedantic.

-Wmissing-braces

Warn if an aggregate or union initializer is not fully bracketed. In the following example, the initializer for ‘a’ is not fully bracketed, but that for ‘b’ is fully bracketed. This warning is enabled by -Wall in C.

int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };

This warning is enabled by -Wall.

-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)

Warn if a user-supplied include directory does not exist.

-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 issues a warning when this flag is specified. To eliminate the warning, add explicit braces around the innermost if statement so there is no way the else can belong to the enclosing if. The resulting code looks like this:

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

Also warn for dangerous uses of the GNU extension to ?: with omitted middle operand. When the condition in the ?: operator is a boolean expression, the omitted value is always 1. Often programmers expect it to be a value computed inside the conditional expression instead.

This warning is enabled by -Wall.

-Wsequence-point

Warn about code that may have undefined semantics because of violations of sequence point rules in the C and C++ standards.

The C and C++ standards define the order in which expressions in a C/C++ program are evaluated in terms of sequence points, which represent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those executed after it. These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a &&, ||, ? : or , (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is not specified. All these rules describe only a partial order rather than a total order, since, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified. However, the standards committee have ruled that function calls do not overlap.

It is not specified when between sequence points modifications to the values of objects take effect. Programs whose behavior depends on this have undefined behavior; the C and C++ standards specify that “Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.”. If a program breaks these rules, the results on any particular implementation are entirely unpredictable.

Examples of code with undefined behavior are a = a++;, a[n] = b[n++] and a[i++] = i;. Some more complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs.

The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at http://gcc.gnu.org/readings.html.

This warning is enabled by -Wall for C and C++.

-Wno-return-local-addr

Do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the function returns.

-Wreturn-type

Warn whenever a function is defined with a return type that defaults to int. Also warn about any return statement with no return value in a function whose return type is not void (falling off the end of the function body is considered returning without a value), and about a return statement with an 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 (even if there is a default label). This warning is enabled by -Wall.

-Wswitch-default

Warn whenever a switch statement does not have a default case.

-Wswitch-enum

Warn whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration. case labels outside the enumeration range also provoke warnings when this option is used. The only difference between -Wswitch and this option is that this option gives a warning about an omitted enumeration code even if there is a default label.

-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-but-set-parameter

Warn whenever a function parameter is assigned to, but otherwise unused (aside from its declaration).

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

This warning is also enabled by -Wunused together with -Wextra.

-Wunused-but-set-variable

Warn whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This warning is enabled by -Wall.

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

This warning is also enabled by -Wunused, which is enabled by -Wall.

-Wunused-function

Warn whenever a static function is declared but not defined or a non-inline static function is unused. This warning is enabled by -Wall.

-Wunused-label

Warn whenever a label is declared but not used. This warning is enabled by -Wall.

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

-Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only)

Warn when a typedef locally defined in a function is not used. This warning is enabled by -Wall.

-Wunused-parameter

Warn whenever a function parameter is unused aside from its declaration.

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

-Wno-unused-result

Do not warn if a caller of a function marked with attribute warn_unused_result (see Function Attributes) does not use its return value. The default is -Wunused-result.

-Wunused-variable

Warn whenever a local 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]’ causes a warning, while ‘x[(void)i,j]’ does not.

This warning is enabled by -Wall.

-Wunused

All the above -Wunused options combined.

In order to get a warning about an unused function parameter, you must either specify -Wextra -Wunused (note that -Wall implies -Wunused), or separately specify -Wunused-parameter.

-Wuninitialized

Warn if an automatic variable is used without first being initialized or if a variable may be clobbered by a setjmp call. In C++, warn if a non-static reference or non-static ‘const’ member appears in a class without constructors.

If you want to warn about code that uses the uninitialized value of the variable in its own initializer, use the -Winit-self option.

These warnings occur for individual uninitialized or clobbered elements of structure, union or array variables as well as for variables that are uninitialized or clobbered as a whole. They do not occur for variables or elements declared volatile. Because these warnings depend on optimization, the exact variables or elements for which there are warnings depends on the precise optimization options and version of GCC used.

Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed.

-Wmaybe-uninitialized

For an automatic variable, if there exists a path from the function entry to a use of the variable that is initialized, but there exist some other paths for which the variable is not initialized, the compiler emits a warning if it cannot prove the uninitialized paths are not executed at run time. These warnings are made optional because GCC is not smart enough to see all the reasons why the code might be correct in spite of appearing to have an error. Here is one example of how this can happen:

{
  int x;
  switch (y)
    {
    case 1: x = 1;
      break;
    case 2: x = 4;
      break;
    case 3: x = 5;
    }
  foo (x);
}

If the value of y is always 1, 2 or 3, then x is always initialized, but GCC doesn’t know this. To suppress the warning, you need to provide a default case with assert(0) or similar code.

This option also warns when a non-volatile automatic variable might be changed by a call to longjmp. 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 that would cause a problem.

Some spurious warnings can be avoided if you declare all the functions you use that never return as noreturn. See Function Attributes.

This warning is enabled by -Wall or -Wextra.

-Wunknown-pragmas

Warn when a #pragma directive is encountered that is not understood by GCC. If this command-line option is used, warnings are even issued for unknown pragmas in system header files. This is not the case if the warnings are only enabled by the -Wall command-line option.

-Wno-pragmas

Do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas. See also -Wunknown-pragmas.

-Wstrict-aliasing

This option is only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. The warning does not catch all cases, but does attempt to catch the more common pitfalls. It is included in -Wall. It is equivalent to -Wstrict-aliasing=3

-Wstrict-aliasing=n

This option is only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. Higher levels correspond to higher accuracy (fewer false positives). Higher levels also correspond to more effort, similar to the way -O works. -Wstrict-aliasing is equivalent to -Wstrict-aliasing=3.

Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but -fstrict-aliasing still breaks the code, as it has very few false negatives. However, it has many false positives. Warns for all pointer conversions between possibly incompatible types, even if never dereferenced. Runs in the front end only.

Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. Warns about incomplete types. Runs in the front end only.

Level 3 (default for -Wstrict-aliasing): Should have very few false positives and few false negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the front end: *(int*)&some_float. If optimization is enabled, it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types.

-Wstrict-overflow

-Wstrict-overflow=n

This option is only active when -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 that assumes that signed overflow does not occur is perfectly safe if the values of the variables involved are such that overflow never does, in fact, occur. Therefore this warning can easily give a false positive: a warning about code that is not actually a problem. To help focus on important issues, several warning levels are defined. No warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop requires, in particular when determining whether a loop will be executed at all.

-Wstrict-overflow=1

Warn about cases that are both questionable and easy to avoid. For example, with -fstrict-overflow, the compiler simplifies x + 1 > x to 1. This level of -Wstrict-overflow is enabled by -Wall; higher levels are not, and must be explicitly requested.

-Wstrict-overflow=2

Also warn about other cases where a comparison is simplified to a constant. For example: abs (x) >= 0. This can only be simplified when -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 is simplified to x > 0.

-Wstrict-overflow=4

Also warn about other simplifications not covered by the above cases. For example: (x * 10) / 5 is simplified to x * 2.

-Wstrict-overflow=5

Also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example: x + 2 > y is simplified to x + 1 >= y. This is reported only at the highest warning level because this simplification applies to many comparisons, so this warning level gives a very large number of false positives.

-Wsuggest-attribute=[pure|const|noreturn|format]

Warn for cases where adding an attribute may be beneficial. The attributes currently supported are listed below.

-Wsuggest-attribute=pure

-Wsuggest-attribute=const

-Wsuggest-attribute=noreturn

Warn about functions that might be candidates for attributes pure, const or noreturn. The compiler only warns for functions visible in other compilation units or (in the case of pure and const) if it cannot prove that the function returns normally. A function returns normally if it doesn’t contain an infinite loop or return abnormally by throwing, calling abort() or trapping. This analysis requires option -fipa-pure-const, which is enabled by default at -O and higher. Higher optimization levels improve the accuracy of the analysis.

-Wsuggest-attribute=format

-Wmissing-format-attribute

Warn about function pointers that might be candidates for format attributes. Note these are only possible candidates, not absolute ones. GCC guesses that function pointers with format attributes that are used in assignment, initialization, parameter passing or return statements should have a corresponding format attribute in the resulting type. I.e. the left-hand side of the assignment or initialization, the type of the parameter variable, or the return type of the containing function respectively should also have a format attribute to avoid the warning.

GCC also warns about function definitions that might be candidates for format attributes. Again, these are only possible candidates. GCC guesses that format attributes might be appropriate for any function that calls a function like vprintf or vscanf, but this might not always be the case, and some functions for which format attributes are appropriate may not be detected.

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

-Wtrampolines

Warn about trampolines generated for pointers to nested functions.

A trampoline is a small piece of data or code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. For some targets, it is made up of data only and thus requires no special treatment. But, for most targets, it is made up of code and thus requires the stack to be made executable in order for the program to work properly.

-Wfloat-equal

Warn if floating-point values are used in equality comparisons.

The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you are doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that’s a different problem). In particular, instead of testing for equality, you should check to see whether the two values have ranges that overlap; and this is done with the relational operators, so equality comparisons are probably mistaken.

-Wtraditional (C and Objective-C only)

Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided.

• Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes place within string literals, but in ISO C it does not.
• In traditional C, some preprocessor directives did not exist. Traditional preprocessors only considered a line to be a directive if the ‘#’ appeared in column 1 on the line. Therefore -Wtraditional warns about directives that traditional C understands but ignores because the ‘#’ does not appear as the first character on the line. It also suggests you hide directives like ‘#pragma’ not understood by traditional C by indenting them. Some traditional implementations do not recognize ‘#elif’, so this option suggests avoiding it altogether.
• A function-like macro that appears without arguments.
• The unary plus operator.
• The ‘U’ integer constant suffix, or the ‘F’ or ‘L’ floating-point constant suffixes. (Traditional C does support the ‘L’ suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the ‘_MIN’/‘_MAX’ macros in <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 causes serious problems. This is a subset of the possible conversion warnings; for the full set use -Wtraditional-conversion.
• Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features appear in your code when using libiberty’s traditional C compatibility macros, PARAMS and VPARAMS. This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility.

-Wtraditional-conversion (C and Objective-C only)

Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed-point argument except when the same as the default promotion.

-Wdeclaration-after-statement (C and Objective-C only)

Warn when a declaration is found after a statement in a block. This construct, known from C++, was introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90 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 or type declaration shadows another variable, parameter, type, or class member (in C++), or whenever a built-in function is shadowed. Note that in C++, the compiler warns if a local variable shadows an explicit typedef, but not if it shadows a struct/class/enum.

-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.

-Wno-free-nonheap-object

Do not warn when attempting to free an object that was not allocated on the heap.

-Wstack-usage=len

Warn if the stack usage of a function might be larger than len bytes. The computation done to determine the stack usage is conservative. Any space allocated via alloca, variable-length arrays, or related constructs is included by the compiler when determining whether or not to issue a warning.

The message is in keeping with the output of -fstack-usage.

• If the stack usage is fully static but exceeds the specified amount, it’s:

    warning: stack usage is 1120 bytes
  

• If the stack usage is (partly) dynamic but bounded, it’s:

    warning: stack usage might be 1648 bytes
  

• If the stack usage is (partly) dynamic and not bounded, it’s:

    warning: stack usage might be unbounded
  

-Wunsafe-loop-optimizations

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

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

When used in combination with -Wformat and -pedantic without GNU extensions, this option disables the warnings about non-ISO printf / scanf format width specifiers I32, I64, and I used on Windows targets, which depend on the MS runtime.

-Wpointer-arith

Warn about anything that depends on the “size of” a function type or of void. GNU C assigns these types a size of 1, for convenience in calculations with void * pointers and pointers to functions. In C++, warn also when an arithmetic operation involves NULL. This warning is also enabled by -Wpedantic.

-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++11-compat (C++ and Objective-C++ only)

Warn about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is enabled by -Wall.

-Wcast-qual

Warn whenever a pointer is cast so as to remove a type qualifier from the target type. For example, warn if a const char * is cast to an ordinary char *.

Also warn when making a cast that introduces a type qualifier in an unsafe way. For example, casting char ** to const char ** is unsafe, as in this example:

  /* p is char ** value.  */
  const char **q = (const char **) p;
  /* Assignment of readonly string to const char * is OK.  */
  *q = "string";
  /* Now char** pointer points to read-only memory.  */
  **p = 'b';

-Wcast-align

Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a char * is cast to an int * on machines where integers can only be accessed at two- or four-byte boundaries.

-Wwrite-strings

When compiling C, give string constants the type const char[length] so that copying the address of one into a non-const char * pointer produces a warning. These warnings help you find at compile time code that can try to write into a string constant, but only if you have been very careful about using const in declarations and prototypes. Otherwise, it is just a nuisance. This is why we did not make -Wall request these warnings.

When compiling C++, warn about the deprecated conversion from string literals to char *. This warning is enabled by default for C++ programs.

-Wclobbered

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

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

Warn for conditionally-supported (C++11 [intro.defs]) constructs.

-Wconversion

Warn for implicit conversions that may alter a value. This includes conversions between real and integer, like abs (x) when x is double; conversions between signed and unsigned, like unsigned ui = -1; and conversions to smaller types, like sqrtf (M_PI). Do not warn for explicit casts like abs ((int) x) and ui = (unsigned) -1, or if the value is not changed by the conversion like in abs (2.0). Warnings about conversions between signed and unsigned integers can be disabled by using -Wno-sign-conversion.

For C++, also warn for confusing overload resolution for user-defined conversions; and conversions that never use a type conversion operator: conversions to void, the same type, a base class or a reference to them. Warnings about conversions between signed and unsigned integers are disabled by default in C++ unless -Wsign-conversion is explicitly enabled.

-Wno-conversion-null (C++ and Objective-C++ only)

Do not warn for conversions between NULL and non-pointer types. -Wconversion-null is enabled by default.

-Wzero-as-null-pointer-constant (C++ and Objective-C++ only)

Warn when a literal ’0’ is used as null pointer constant. This can be useful to facilitate the conversion to nullptr in C++11.

-Wdate-time

Warn when macros __TIME__, __DATE__ or __TIMESTAMP__ are encountered as they might prevent bit-wise-identical reproducible compilations.

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

Warn when deleting a pointer to incomplete type, which may cause undefined behavior at runtime. This warning is enabled by default.

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

Warn when an expression is casted to its own type.

-Wempty-body

Warn if an empty body occurs in an ‘if’, ‘else’ or ‘do while’ statement. This warning is also enabled by -Wextra.

-Wenum-compare

Warn about a comparison between values of different enumerated types. In C++ enumeral mismatches in conditional expressions are also diagnosed and the warning is enabled by default. In C this warning is enabled by -Wall.

-Wjump-misses-init (C, Objective-C only)

Warn if a goto statement or a switch statement jumps forward across the initialization of a variable, or jumps backward to a label after the variable has been initialized. This only warns about variables that are initialized when they are declared. This warning is only supported for C and Objective-C; in C++ this sort of branch is an error in any case.

-Wjump-misses-init is included in -Wc++-compat. It can be disabled with the -Wno-jump-misses-init option.

-Wsign-compare

Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. 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.

-Wfloat-conversion

Warn for implicit conversions that reduce the precision of a real value. This includes conversions from real to integer, and from higher precision real to lower precision real values. This option is also enabled by -Wconversion.

-Wsizeof-pointer-memaccess

Warn for suspicious length parameters to certain string and memory built-in functions if the argument uses sizeof. This warning warns e.g. about memset (ptr, 0, sizeof (ptr)); if ptr is not an array, but a pointer, and suggests a possible fix, or about memcpy (&foo, ptr, sizeof (&foo));. This warning is enabled by -Wall.

-Wmemset-transposed-args

Warn for suspicious calls to the memset built-in function, if the second argument is not zero and the third argument is zero. This warns e.g. about memset (buf, sizeof buf, 0) where most probably memset (buf, 0, sizeof buf) was meant instead. The diagnostics is only emitted if the third argument is literal zero, if it is some expression that is folded to zero, or e.g. a cast of zero to some type etc., it is far less likely that user has mistakenly exchanged the arguments and no warning is emitted. This warning is enabled by -Wall.

-Waddress

Warn about suspicious uses of memory addresses. These include using the address of a function in a conditional expression, such as void func(void); if (func), and comparisons against the memory address of a string literal, such as if (x == "abc"). Such uses typically indicate a programmer error: the address of a function always evaluates to true, so their use in a conditional usually indicate that the programmer forgot the parentheses in a function call; and comparisons against string literals result in unspecified behavior and are not portable in C, so they usually indicate that the programmer intended to use strcmp. This warning is enabled by -Wall.

-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-aggressive-loop-optimizations

Warn if in a loop with constant number of iterations the compiler detects undefined behavior in some statement during one or more of the iterations.

-Wno-attributes

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

-Wno-builtin-macro-redefined

Do not warn if certain built-in macros are redefined. This suppresses warnings for redefinition of __TIMESTAMP__, __TIME__, __DATE__, __FILE__, and __BASE_FILE__.

-Wstrict-prototypes (C and Objective-C only)

Warn if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if preceded by a declaration that specifies the argument types.)

-Wold-style-declaration (C and Objective-C only)

Warn for obsolescent usages, according to the C Standard, in a declaration. For example, warn if storage-class specifiers like static are not the first things in a declaration. This warning is also enabled by -Wextra.

-Wold-style-definition (C and Objective-C only)

Warn if an old-style function definition is used. A warning is given even if there is a previous prototype.

-Wmissing-parameter-type (C and Objective-C only)

A function parameter is declared without a type specifier in K&R-style functions:

void foo(bar) { }

This warning is also enabled by -Wextra.

-Wmissing-prototypes (C and Objective-C only)

Warn if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. Use this option to detect global functions that do not have a matching prototype declaration in a header file. This option is not valid for C++ because all function declarations provide prototypes and a non-matching declaration will declare an overload rather than conflict with an earlier declaration. Use -Wmissing-declarations to detect missing declarations in C++.

-Wmissing-declarations

Warn if a global function is defined without a previous declaration. Do so even if the definition itself provides a prototype. Use this option to detect global functions that are not declared in header files. In C, no warnings are issued for functions with previous non-prototype declarations; use -Wmissing-prototype to detect missing prototypes. In C++, no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces.

-Wmissing-field-initializers

Warn if a structure’s initializer has some fields missing. For example, the following code causes such a warning, because x.h is implicitly zero:

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

This option does not warn about designated initializers, so the following modification does not trigger a warning:

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

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

-Wno-multichar

Do not warn if a multicharacter constant (‘'FOOF'’) is used. Usually they indicate a typo in the user’s code, as they have implementation-defined values, and should not be used in portable code.

-Wnormalized=<none|id|nfc|nfkc>

In ISO C and ISO C++, two identifiers are different if they are different sequences of characters. However, sometimes when characters outside the basic ASCII character set are used, you can have two different character sequences that look the same. To avoid confusion, the ISO 10646 standard sets out some normalization rules which when applied ensure that two sequences that look the same are turned into the same sequence. GCC can warn you if you are using identifiers that have not been normalized; this option controls that warning.

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

Unfortunately, there are some characters allowed in identifiers by ISO C and ISO C++ that, when turned into NFC, are not allowed in identifiers. That is, there’s no way to use these symbols in portable ISO C or C++ and have all your identifiers in NFC. -Wnormalized=id suppresses the warning for these characters. It is hoped that future versions of the standards involved will correct this, which is why this option is not the default.

You can switch the warning off for all characters by writing -Wnormalized=none. You should only do this if you are using some other normalization scheme (like “D”), because otherwise you can easily create bugs that are literally impossible to see.

Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies, especially once formatting has been applied. For instance \u207F, “SUPERSCRIPT LATIN SMALL LETTER N”, displays just like a regular n that has been placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert all these into a standard form as well, and GCC warns if your code is not in NFKC if you use -Wnormalized=nfkc. This warning is comparable to warning about every identifier that contains the letter O because it might be confused with the digit 0, and so is not the default, but may be useful as a local coding convention if the programming environment cannot be fixed to display these characters distinctly.

-Wno-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.

-Wopenmp-simd

Warn if the vectorizer cost model overrides the OpenMP or the Cilk Plus simd directive set by user. The -fsimd-cost-model=unlimited can be used to relax the cost model.

-Woverride-init (C and Objective-C only)

Warn if an initialized field without side effects is overridden when using designated initializers (see Designated Initializers).

This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-override-init.

-Wpacked

Warn if a structure is given the packed attribute, but the packed attribute has no effect on the layout or size of the structure. Such structures may be mis-aligned for little benefit. For instance, in this code, the variable f.x in struct bar is misaligned even though struct bar does not itself have the packed attribute:

struct foo {
  int x;
  char a, b, c, d;
} __attribute__((packed));
struct bar {
  char z;
  struct foo f;
};

-Wpacked-bitfield-compat

The 4.1, 4.2 and 4.3 series of GCC ignore the packed attribute on bit-fields of type char. This has been fixed in GCC 4.4 but the change can lead to differences in the structure layout. GCC informs you when the offset of such a field has changed in GCC 4.4. For example there is no longer a 4-bit padding between field a and b in this structure:

struct foo
{
  char a:4;
  char b:8;
} __attribute__ ((packed));

This warning is enabled by default. Use -Wno-packed-bitfield-compat to disable this warning.

-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.

-Wno-inherited-variadic-ctor

Suppress warnings about use of C++11 inheriting constructors when the base class inherited from has a C variadic constructor; the warning is on by default because the ellipsis is not inherited.

-Winline

Warn if a function that is declared as inline cannot be inlined. Even with this option, the compiler does not warn about failures to inline functions declared in system headers.

The compiler uses a variety of heuristics to determine whether or not to inline a function. For example, the compiler takes into account the size of the function being inlined and the amount of inlining that has already been done in the current function. Therefore, seemingly insignificant changes in the source program can cause the warnings produced by -Winline to appear or disappear.

-Wno-invalid-offsetof (C++ and Objective-C++ only)

Suppress warnings from applying the ‘offsetof’ macro to a non-POD type. According to the 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

Suppress warnings from casts to pointer type of an integer of a different size. In C++, casting to a pointer type of smaller size is an error. Wint-to-pointer-cast is enabled by default.

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

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

-Winvalid-pch

Warn if a precompiled header (see Precompiled Headers) is found in the search path but can’t be used.

-Wlong-long

Warn if ‘long long’ type is used. This is enabled by either -Wpedantic or -Wtraditional in ISO C90 and C++98 modes. To inhibit the warning messages, use -Wno-long-long.

-Wvariadic-macros

Warn if variadic macros are used in 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.

-Wvarargs

Warn upon questionable usage of the macros used to handle variable arguments like ‘va_start’. This is default. To inhibit the warning messages, use -Wno-varargs.

-Wvector-operation-performance

Warn if vector operation is not implemented via SIMD capabilities of the architecture. Mainly useful for the performance tuning. Vector operation can be implemented piecewise, which means that the scalar operation is performed on every vector element; in parallel, which means that the vector operation is implemented using scalars of wider type, which normally is more performance efficient; and as a single scalar, which means that vector fits into a scalar type.

-Wno-virtual-move-assign

Suppress warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. This is dangerous because if the virtual base is reachable along more than one path, it will be moved multiple times, which can mean both objects end up in the moved-from state. If the move assignment operator is written to avoid moving from a moved-from object, this warning can be disabled.

-Wvla

Warn if variable length array is used in the code. -Wno-vla prevents the -Wpedantic 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 are unable to handle the code effectively. Often, the problem is that your code is too big or too complex; GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time.

-Wpointer-sign (C and Objective-C only)

Warn for pointer argument passing or assignment with different signedness. This option is only supported for C and Objective-C. It is implied by -Wall and by -Wpedantic, which can be disabled with -Wno-pointer-sign.

-Wstack-protector

This option is only active when -fstack-protector is active. It warns about functions that are not protected against stack smashing.

-Woverlength-strings

Warn about string constants that are longer than the “minimum maximum” length specified in the C standard. Modern compilers generally allow string constants that are much longer than the standard’s minimum limit, but very portable programs should avoid using longer strings.

The limit applies after string constant concatenation, and does not count the trailing NUL. In C90, the limit was 509 characters; in C99, it was raised to 4095. C++98 does not specify a normative minimum maximum, so we do not diagnose overlength strings in C++.

This option is implied by -Wpedantic, and can be disabled with -Wno-overlength-strings.

-Wunsuffixed-float-constants (C and Objective-C only)

Issue a warning for any floating constant that does not have a suffix. When used together with -Wsystem-headers it warns about such constants in system header files. This can be useful when preparing code to use with the FLOAT_CONST_DECIMAL64 pragma from the decimal floating-point extension to C99.

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 probably makes other debuggers crash or refuse to read the program. If you want to control for certain whether to generate the extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).

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 are already at hand; some statements may execute in different places because they have been 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.

-gsplit-dwarf

Separate as much dwarf debugging information as possible into a separate output file with the extension .dwo. This option allows the build system to avoid linking files with debug information. To be useful, this option requires a debugger capable of reading .dwo files.

-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.

-gpubnames

Generate dwarf .debug_pubnames and .debug_pubtypes sections.

-ggnu-pubnames

Generate .debug_pubnames and .debug_pubtypes sections in a format suitable for conversion into a GDB index. This option is only useful with a linker that can produce GDB index version 7.

-gstabs

Produce debugging information in stabs format (if that is supported), without GDB extensions. This is the format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output that is not understood by DBX 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 increases the size of debugging information by as much as a factor of two.

-fdebug-types-section

When using DWARF Version 4 or higher, type DIEs can be put into their own .debug_types section instead of making them part of the .debug_info section. It is more efficient to put them in a separate comdat sections since the linker can then remove duplicates. But not all DWARF consumers support .debug_types sections yet and on some objects .debug_types produces larger instead of smaller debugging information.

-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-version

Produce debugging information in DWARF format (if that is supported). The value of version may be either 2, 3 or 4; the default version for most targets is 4.

Note that with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables.

Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit.

-grecord-gcc-switches

This switch causes the command-line options used to invoke the compiler that may affect code generation to be appended to the DW_AT_producer attribute in DWARF debugging information. The options are concatenated with spaces separating them from each other and from the compiler version. See also -frecord-gcc-switches for another way of storing compiler options into the object file. This is the default.

-gno-record-gcc-switches

Disallow appending command-line options to the DW_AT_producer attribute in DWARF debugging information.

-gstrict-dwarf

Disallow using extensions of later DWARF standard version than selected with -gdwarf-version. On most targets using non-conflicting DWARF extensions from later standard versions is allowed.

-gno-strict-dwarf

Allow using extensions of later DWARF standard version than selected with -gdwarf-version.

-gvms

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

-glevel

-ggdblevel

-gstabslevel

-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, and line number tables, but no information about local variables.

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

-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 DWARF.

-gtoggle

Turn off generation of debug info, if leaving out this option generates it, or turn it on at level 2 otherwise. The position of this argument in the command line does not matter; it takes effect after all other options are processed, and it does so only once, no matter how many times it is given. This is mainly intended to be used with -fcompare-debug.

-fsanitize=address

Enable AddressSanitizer, a fast memory error detector. Memory access instructions will be instrumented to detect out-of-bounds and use-after-free bugs. See http://code.google.com/p/address-sanitizer/ for more details. The run-time behavior can be influenced using the ASAN_OPTIONS environment variable; see https://code.google.com/p/address-sanitizer/wiki/Flags#Run-time_flags for a list of supported options.

-fsanitize=thread

Enable ThreadSanitizer, a fast data race detector. Memory access instructions will be instrumented to detect data race bugs. See http://code.google.com/p/thread-sanitizer/ for more details. The run-time behavior can be influenced using the TSAN_OPTIONS environment variable; see https://code.google.com/p/thread-sanitizer/wiki/Flags for a list of supported options.

-fsanitize=leak

Enable LeakSanitizer, a memory leak detector. This option only matters for linking of executables and if neither -fsanitize=address nor -fsanitize=thread is used. In that case it will link the executable against a library that overrides malloc and other allocator functions. See https://code.google.com/p/address-sanitizer/wiki/LeakSanitizer for more details. The run-time behavior can be influenced using the LSAN_OPTIONS environment variable.

-fsanitize=undefined

Enable UndefinedBehaviorSanitizer, a fast undefined behavior detector. Various computations will be instrumented to detect undefined behavior at runtime. Current suboptions are:

-fsanitize=shift

This option enables checking that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc.

-fsanitize=integer-divide-by-zero

Detect integer division by zero as well as INT_MIN / -1 division.

-fsanitize=unreachable

With this option, the compiler will turn the __builtin_unreachable call into a diagnostics message call instead. When reaching the __builtin_unreachable call, the behavior is undefined.

-fsanitize=vla-bound

This option instructs the compiler to check that the size of a variable length array is positive. This option does not have any effect in -std=c++1y mode, as the standard requires the exception be thrown instead.

-fsanitize=null

This option enables pointer checking. Particularly, the application built with this option turned on will issue an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue reference) is bound to a NULL pointer.

-fsanitize=return

This option enables return statement checking. Programs built with this option turned on will issue an error message when the end of a non-void function is reached without actually returning a value. This option works in C++ only.

-fsanitize=signed-integer-overflow

This option enables signed integer overflow checking. We check that the result of +, *, and both unary and binary - does not overflow in the signed arithmetics. Note, integer promotion rules must be taken into account. That is, the following is not an overflow:

signed char a = SCHAR_MAX;
a++;

While -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives a diagnostic message. This currently works only for the C family of languages.

-fdump-final-insns[=file]

Dump the final internal representation (RTL) to file. If the optional argument is omitted (or if file is .), the name of the dump file is determined by appending .gkd to the compilation output file name.

-fcompare-debug[=opts]

If no error occurs during compilation, run the compiler a second time, adding opts and -fcompare-debug-second to the arguments passed to the second compilation. Dump the final internal representation in both compilations, and print an error if they differ.

If the equal sign is omitted, the default -gtoggle is used.

The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and nonzero, implicitly enables -fcompare-debug. If GCC_COMPARE_DEBUG is defined to a string starting with a dash, then it is used for opts, otherwise the default -gtoggle is used.

-fcompare-debug=, with the equal sign but without opts, is equivalent to -fno-compare-debug, which disables the dumping of the final representation and the second compilation, preventing even GCC_COMPARE_DEBUG from taking effect.

To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say ‘-fcompare-debug-not-overridden’, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking). To get just a warning, setting GCC_COMPARE_DEBUG to ‘-w%n-fcompare-debug not overridden’ will do.

-fcompare-debug-second

This option is implicitly passed to the compiler for the second compilation requested by -fcompare-debug, along with options to silence warnings, and omitting other options that would cause side-effect compiler outputs to files or to the standard output. Dump files and preserved temporary files are renamed so as to contain the .gk additional extension during the second compilation, to avoid overwriting those generated by the first.

When this option is passed to the compiler driver, it causes the first compilation to be skipped, which makes it useful for little other than debugging the compiler proper.

-feliminate-dwarf2-dups

Compress DWARF 2 debugging information by eliminating duplicated information about each symbol. This option only makes sense when generating DWARF 2 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 is defined.

This option substantially reduces the size of debugging information, but at significant potential loss in type information to the debugger. See -femit-struct-debug-reduced for a less aggressive option. See -femit-struct-debug-detailed for more detailed control.

This option works only with DWARF 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 is defined, unless the struct is a template or defined in a system header.

This option significantly reduces the size of debugging information, with some potential loss in type information to the debugger. See -femit-struct-debug-baseonly for a more aggressive option. See -femit-struct-debug-detailed for more detailed control.

This option works only with DWARF 2.

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

Specify the struct-like types for which the compiler generates debug information. The intent is to reduce duplicate struct debug information between different object files within the same program.

This option is a detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, which serves for most needs.

A specification has the syntax
[‘dir:’|‘ind:’][‘ord:’|‘gen:’](‘any’|‘sys’|‘base’|‘none’)

The optional first word limits the specification to structs that are used directly (‘dir:’) or used indirectly (‘ind:’). A struct type is used directly when it is the type of a variable, member. Indirect uses arise through pointers to structs. That is, when use of an incomplete struct is valid, the use is indirect. An example is ‘struct one direct; struct two * indirect;’.

The optional second word limits the specification to ordinary structs (‘ord:’) or generic structs (‘gen:’). Generic structs are a bit complicated to explain. For C++, these are non-explicit specializations of template classes, or non-template classes within the above. Other programming languages have generics, but -femit-struct-debug-detailed does not yet implement them.

The third word specifies the source files for those structs for which the compiler should emit debug information. The values ‘none’ and ‘any’ have the normal meaning. The value ‘base’ means that the base of name of the file in which the type declaration appears must match the base of the name of the main compilation file. In practice, this means that when compiling foo.c, debug information is generated for types declared in that file and foo.h, but not other header files. The value ‘sys’ means those types satisfying ‘base’ or declared in system or compiler headers.

You may need to experiment to determine the best settings for your application.

The default is -femit-struct-debug-detailed=all.

This option works only with DWARF 2.

-fno-merge-debug-strings

Direct the linker to not merge together strings in the debugging information that are identical in different object files. Merging is not supported by all assemblers or linkers. Merging decreases the size of the debug information in the output file at the cost of increasing link processing time. Merging is enabled by default.

-fdebug-prefix-map=old=new

When compiling files 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.

-fmem-report-wpa

Makes the compiler print some statistics about permanent memory allocation for the WPA phase only.

-fpre-ipa-mem-report

-fpost-ipa-mem-report

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

-fprofile-report

Makes the compiler print some statistics about consistency of the (estimated) profile and effect of individual passes.

-fstack-usage

Makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending .su to the auxname. auxname is generated from the name of the output file, if explicitly specified and it is not an executable, otherwise it is the basename of the source file. An entry is made up of three fields:

• The name of the function.
• A number of bytes.
• One or more qualifiers: static, dynamic, bounded.

The qualifier static means that the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes.

The qualifier dynamic means that the function manipulates the stack dynamically: in addition to the static allocation described above, stack adjustments are made in the body of the function, for example to push/pop arguments around function calls. If the qualifier bounded is also present, the amount of these adjustments is bounded at compile time and the second field is an upper bound of the total amount of stack used by the function. If it is not present, the amount of these adjustments is not bounded at compile time and the second field only represents the bounded part.

-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 matches the source files more closely if you do not optimize.

-fdbg-cnt-list

Print the name and the counter upper bound for all debug counters.

-fdbg-cnt=counter-value-list

Set the internal debug counter upper bound. counter-value-list is a comma-separated list of name:value pairs which sets the upper bound of each debug counter name to value. All debug counters have the initial upper bound of UINT_MAX; thus dbg_cnt() returns true always unless the upper bound is set by this option. For example, with -fdbg-cnt=dce:10,tail_call:0, dbg_cnt(dce) returns true only for first 10 invocations.

-fenable-kind-pass

-fdisable-kind-pass=range-list

This is a set of options that are used to explicitly disable/enable optimization passes. These options are intended for use for debugging GCC. Compiler users should use regular options for enabling/disabling passes instead.

-fdisable-ipa-pass

Disable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.

-fdisable-rtl-pass

-fdisable-rtl-pass=range-list

Disable RTL pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1. range-list is a comma-separated list of function ranges or assembler names. Each range is a number pair separated by a colon. The range is inclusive in both ends. If the range is trivial, the number pair can be simplified as a single number. If the function’s call graph node’s uid falls within one of the specified ranges, the pass is disabled for that function. The uid is shown in the function header of a dump file, and the pass names can be dumped by using option -fdump-passes.

-fdisable-tree-pass

-fdisable-tree-pass=range-list

Disable tree pass pass. See -fdisable-rtl for the description of option arguments.

-fenable-ipa-pass

Enable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.

-fenable-rtl-pass

-fenable-rtl-pass=range-list

Enable RTL pass pass. See -fdisable-rtl for option argument description and examples.

-fenable-tree-pass

-fenable-tree-pass=range-list

Enable tree pass pass. See -fdisable-rtl for the description of option arguments.

Here are some examples showing uses of these options.

# disable ccp1 for all functions
   -fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
   -fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
   -fdisable-rtl-gcse2=foo,foo2
# disable early inlining
   -fdisable-tree-einline
# disable ipa inlining
   -fdisable-ipa-inline
# enable tree full unroll
   -fenable-tree-unroll

-dletters

-fdump-rtl-pass

-fdump-rtl-pass=filename

Says to make debugging dumps during compilation at times specified by letters. This is used for debugging the RTL-based passes of the compiler. The file names for most of the dumps are made by appending a pass number and a word to the dumpname, and the files are created in the directory of the output file. In case of =filename option, the dump is output on the given file instead of the pass numbered dump files. Note that the pass number is computed statically as passes get registered into the pass manager. Thus the numbering is not related to the dynamic order of execution of passes. In particular, a pass installed by a plugin could have a number over 200 even if it executed quite early. 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 subexpression elimination passes.

-fdump-rtl-dce

Dump after the standalone dead code elimination passes.

-fdump-rtl-dbr

Dump after delayed branch scheduling.

-fdump-rtl-dce1

-fdump-rtl-dce2

-fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store elimination passes.

-fdump-rtl-eh

Dump after finalization of EH handling code.

-fdump-rtl-eh_ranges

Dump after conversion of EH handling range regions.

-fdump-rtl-expand

Dump after RTL generation.

-fdump-rtl-fwprop1

-fdump-rtl-fwprop2

-fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward propagation passes.

-fdump-rtl-gcse1

-fdump-rtl-gcse2

-fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common subexpression elimination.

-fdump-rtl-init-regs

Dump after the initialization of the registers.

-fdump-rtl-initvals

Dump after the computation of the initial value sets.

-fdump-rtl-into_cfglayout

Dump after converting to cfglayout mode.

-fdump-rtl-ira

Dump after iterated register allocation.

-fdump-rtl-jump

Dump after the second jump optimization.

-fdump-rtl-loop2

-fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.

-fdump-rtl-mach

Dump after performing the machine dependent reorganization pass, if that pass exists.

-fdump-rtl-mode_sw

Dump after removing redundant mode switches.

-fdump-rtl-rnreg

Dump after register renumbering.

-fdump-rtl-outof_cfglayout

Dump after converting from cfglayout mode.

-fdump-rtl-peephole2

Dump after the peephole pass.

-fdump-rtl-postreload

Dump after post-reload optimizations.

-fdump-rtl-pro_and_epilogue

Dump after generating the function prologues and epilogues.

-fdump-rtl-sched1

-fdump-rtl-sched2

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

-fdump-rtl-ree

Dump after sign/zero extension elimination.

-fdump-rtl-seqabstr

Dump after common sequence discovery.

-fdump-rtl-shorten

Dump after shortening branches.

-fdump-rtl-sibling

Dump after sibling call optimizations.

-fdump-rtl-split1

-fdump-rtl-split2

-fdump-rtl-split3

-fdump-rtl-split4

-fdump-rtl-split5

-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.

-da

-fdump-rtl-all

Produce all the dumps listed above.

-dA

Annotate the assembler output with miscellaneous debugging information.

-dD

Dump all macro definitions, at the end of preprocessing, in addition to normal output.

-dH

Produce a core dump whenever an error occurs.

-dp

Annotate the assembler output with a comment indicating which pattern and alternative is used. The length 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.

-dx

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

-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-unnumbered-links

When doing debugging dumps (see -d option above), suppress instruction numbers for the links to the previous and next instructions in a sequence.

-fdump-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, and the file is created in the same directory as the output file. 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, and the file is created in the same directory as the output file. 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, and the file is created in the same directory as the output file. The following dumps are possible:

‘all’

Enables all inter-procedural analysis dumps.

‘cgraph’

Dumps information about call-graph optimization, unused function removal, and inlining decisions.

‘inline’

Dump after function inlining.

-fdump-passes

Dump the list of optimization passes that are turned on and off by the current command-line options.

-fdump-statistics-option

Enable and control dumping of pass statistics in a separate file. The file name is generated by appending a suffix ending in ‘.statistics’ to the source file name, and the file is created in the same directory as the output file. If the ‘-option’ form is used, ‘-stats’ causes counters to be summed over the whole compilation unit while ‘-details’ dumps every event as the passes generate them. The default with no option is to sum counters for each function compiled.

-fdump-tree-switch

-fdump-tree-switch-options

-fdump-tree-switch-options=filename

Control the dumping at various stages of processing the intermediate language tree to a file. The file name is generated by appending a switch-specific suffix to the source file name, and the file is created in the same directory as the output file. In case of =filename option, the dump is output on the given file instead of the auto named dump files. If the ‘-options’ form is used, options is a list of ‘-’ separated options which control the details of the dump. Not all options are applicable to all dumps; those that are not meaningful are ignored. The following options are available

‘address’

Print the address of each node. Usually this is not meaningful as it changes according to the environment and source file. Its primary use is for tying up a dump file with a debug environment.

‘asmname’

If DECL_ASSEMBLER_NAME has been set for a given decl, use that in the dump instead of DECL_NAME. Its primary use is ease of use working backward from mangled names in the assembly file.

‘slim’

When dumping front-end intermediate representations, inhibit dumping of members of a scope or body of a function merely because that scope has been reached. Only dump such items when they are directly reachable by some other path.

When dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.

When dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation.

‘raw’

Print a raw representation of the tree. By default, trees are pretty-printed into a C-like representation.

‘details’

Enable more detailed dumps (not honored by every dump option). Also include information from the optimization passes.

‘stats’

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

‘blocks’

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

‘graph’

For each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with GraphViz to file.passid.pass.dot. Each function in the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot.

This option currently only works for RTL dumps, and the RTL is always dumped in slim form.

‘vops’

Enable showing virtual operands for every statement.

‘lineno’

Enable showing line numbers for statements.

‘uid’

Enable showing the unique ID (DECL_UID) for each variable.

‘verbose’

Enable showing the tree dump for each statement.

‘eh’

Enable showing the EH region number holding each statement.

‘scev’

Enable showing scalar evolution analysis details.

‘optimized’

Enable showing optimization information (only available in certain passes).

‘missed’

Enable showing missed optimization information (only available in certain passes).

‘notes’

Enable other detailed optimization information (only available in certain passes).

‘=filename’

Instead of an auto named dump file, output into the given file name. The file names stdout and stderr are treated specially and are considered already open standard streams. For example,

gcc -O2 -ftree-vectorize -fdump-tree-vect-blocks=foo.dump
     -fdump-tree-pre=stderr file.c

outputs vectorizer dump into foo.dump, while the PRE dump is output on to stderr. If two conflicting dump filenames are given for the same pass, then the latter option overrides the earlier one.

‘all’

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

‘optall’

Turn on all optimization options, i.e., optimized, missed, and note.

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.

‘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.

‘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.

‘slp’

Dump each function after applying vectorization of basic blocks. The file name is made by appending .slp 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.

-fopt-info

-fopt-info-options

-fopt-info-options=filename

Controls optimization dumps from various optimization passes. If the ‘-options’ form is used, options is a list of ‘-’ separated options to select the dump details and optimizations. If options is not specified, it defaults to optimized for details and optall for optimization groups. If the filename is not specified, it defaults to stderr. Note that the output filename will be overwritten in case of multiple translation units. If a combined output from multiple translation units is desired, stderr should be used instead.

The options can be divided into two groups, 1) options describing the verbosity of the dump, and 2) options describing which optimizations should be included. The options from both the groups can be freely mixed as they are non-overlapping. However, in case of any conflicts, the latter options override the earlier options on the command line. Though multiple -fopt-info options are accepted, only one of them can have =filename. If other filenames are provided then all but the first one are ignored.

The dump verbosity has the following options

‘optimized’

Print information when an optimization is successfully applied. It is up to a pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops which got successfully vectorized.

‘missed’

Print information about missed optimizations. Individual passes control which information to include in the output. For example,

gcc -O2 -ftree-vectorize -fopt-info-vec-missed

will print information about missed optimization opportunities from vectorization passes on stderr.

‘note’

Print verbose information about optimizations, such as certain transformations, more detailed messages about decisions etc.

‘all’

Print detailed optimization information. This includes optimized, missed, and note.

The second set of options describes a group of optimizations and may include one or more of the following.

‘ipa’

Enable dumps from all interprocedural optimizations.

‘loop’

Enable dumps from all loop optimizations.

‘inline’

Enable dumps from all inlining optimizations.

‘vec’

Enable dumps from all vectorization optimizations.

‘optall’

Enable dumps from all optimizations. This is a superset of the optimization groups listed above.

For example,

gcc -O3 -fopt-info-missed=missed.all

outputs missed optimization report from all the passes into missed.all.

As another example,

gcc -O3 -fopt-info-inline-optimized-missed=inline.txt

will output information about missed optimizations as well as optimized locations from all the inlining passes into inline.txt.

If the filename is provided, then the dumps from all the applicable optimizations are concatenated into the filename. Otherwise the dump is output onto stderr. If options is omitted, it defaults to all-optall, which means dump all available optimization info from all the passes. In the following example, all optimization info is output on to stderr.

gcc -O3 -fopt-info

Note that -fopt-info-vec-missed behaves the same as -fopt-info-missed-vec.

As another example, consider

gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt

Here the two output filenames vec.miss and loop.opt are in conflict since only one output file is allowed. In this case, only the first option takes effect and the subsequent options are ignored. Thus only the vec.miss is produced which contains dumps from the vectorizer about missed opportunities.

-frandom-seed=string

This option provides a seed that GCC uses in place of random numbers in generating certain symbol names that have to be different in every compiled file. It is also used to place unique stamps in coverage data files and the object files that produce them. You can use the -frandom-seed option to produce reproducibly identical object files.

The string 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, .sched1 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

-save-temps=cwd

Store the usual “temporary” intermediate files permanently; place them in the current directory and name them based on the source file. Thus, compiling foo.c with -c -save-temps produces files foo.i and foo.s, as well as foo.o. This creates a preprocessed foo.i output file even though the compiler now normally uses an integrated preprocessor.

When used in combination with the -x command-line option, -save-temps is sensible enough to avoid over writing an input source file with the same extension as an intermediate file. The corresponding intermediate file may be obtained by renaming the source file before using -save-temps.

If you invoke GCC in parallel, compiling several different source files that share a common base name in different subdirectories or the same source file compiled for multiple output destinations, it is likely that the different parallel compilers will interfere with each other, and overwrite the temporary files. For instance:

gcc -save-temps -o outdir1/foo.o indir1/foo.c&
gcc -save-temps -o outdir2/foo.o indir2/foo.c&

may result in foo.i and foo.o being written to simultaneously by both compilers.

-save-temps=obj

Store the usual “temporary” intermediate files permanently. If the -o option is used, the temporary files are based on the object file. If the -o option is not used, the -save-temps=obj switch behaves like -save-temps.

For example:

gcc -save-temps=obj -c foo.c
gcc -save-temps=obj -c bar.c -o dir/xbar.o
gcc -save-temps=obj foobar.c -o dir2/yfoobar

creates foo.i, foo.s, dir/xbar.i, dir/xbar.s, dir2/yfoobar.i, dir2/yfoobar.s, and dir2/yfoobar.o.

-time[=file]

Report the CPU time taken by each subprocess in the compilation sequence. For C source files, this is the compiler proper and assembler (plus the linker if linking is done).

Without the specification of an output file, the output looks like this:

# cc1 0.12 0.01
# as 0.00 0.01

The first number on each line is the “user time”, that is time spent executing the program itself. The second number is “system time”, time spent executing operating system routines on behalf of the program. Both numbers are in seconds.

With the specification of an output file, the output is appended to the named file, and it looks like this:

0.12 0.01 cc1 options
0.00 0.01 as options

The “user time” and the “system time” are moved before the program name, and the options passed to the program are displayed, so that one can later tell what file was being compiled, and with which options.

-fvar-tracking

Run variable tracking pass. It computes where variables are stored at each position in code. Better debugging information is then generated (if the debugging information format supports this information).

It is enabled by default when compiling with optimization (-Os, -O, -O2, …), debugging information (-g) and the debug info format supports it.

-fvar-tracking-assignments

Annotate assignments to user variables early in the compilation and attempt to carry the annotations over throughout the compilation all the way to the end, in an attempt to improve debug information while optimizing. Use of -gdwarf-4 is recommended along with it.

It can be enabled even if var-tracking is disabled, in which case annotations are created and maintained, but discarded at the end.

-fvar-tracking-assignments-toggle

Toggle -fvar-tracking-assignments, in the same way that -gtoggle toggles -g.

-print-file-name=library

Print the full absolute name of the library file library that would be used when linking—and don’t do anything else. With this option, GCC does not compile or link anything; it just prints the file name.

-print-multi-directory

Print the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist in GCC_EXEC_PREFIX.

-print-multi-lib

Print the mapping from multilib directory names to compiler switches that enable them. The directory name is separated from the switches by ‘;’, and each switch starts with an ‘@’ instead of the ‘-’, without spaces between multiple switches. This is supposed to ease shell processing.

-print-multi-os-directory

Print the path to OS libraries for the selected multilib, relative to some lib subdirectory. If OS libraries are present in the lib subdirectory and no multilibs are used, this is usually just ., if OS libraries are present in libsuffix sibling directories this prints e.g. ../lib64, ../lib or ../lib32, or if OS libraries are present in lib/subdir subdirectories it prints e.g. amd64, sparcv9 or ev6.

-print-multiarch

Print the path to OS libraries for the selected multiarch, relative to some lib subdirectory.

-print-prog-name=program

Like -print-file-name, but searches for a program such as ‘cpp’.

-print-libgcc-file-name

Same as -print-file-name=libgcc.a.

This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You can do:

gcc -nostdlib files… `gcc -print-libgcc-file-name`

-print-search-dirs

Print the name of the configured installation directory and a list of program and library directories gcc searches—and don’t do anything else.

This is useful when gcc prints the error message ‘installation problem, cannot exec cpp0: No such file or directory’. To resolve this you either need to put cpp0 and the other compiler components where gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed them. Don’t forget the trailing ‘/’. See Environment Variables.

-print-sysroot

Print the target sysroot directory that is used during compilation. This is the target sysroot specified either at configure time or using the --sysroot option, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing.

-print-sysroot-headers-suffix

Print the suffix added to the target sysroot when searching for headers, or give an error if the compiler is not configured with such a suffix—and don’t do anything else.

-dumpmachine

Print the compiler’s target machine (for example, ‘i686-pc-linux-gnu’)—and don’t do anything else.

-dumpversion

Print the compiler version (for example, ‘3.0’)—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.

-fno-eliminate-unused-debug-types

Normally, when producing DWARF 2 output, GCC avoids producing debug symbol output for types that are nowhere used in the source file being compiled. Sometimes it is useful to have GCC emit debugging information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit, for example if, in the debugger, you want to cast a value to a type that is not actually used in your program (but is declared). More often, however, this results in a significant amount of wasted space.

3.10 Options That Control Optimization

These options control various sorts of optimizations.

Without any optimization option, the compiler’s goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you expect from the source code.

Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program.

The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them.

Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section.

Most optimizations are only enabled if an -O level is set on the command line. Otherwise they are disabled, even if individual optimization flags are specified.

Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each -O level than those listed here. You can invoke GCC with -Q --help=optimizers to find out the exact set of optimizations that are enabled at each level. See Overall Options, for examples.

-O

-O1

Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.

With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.

-O turns on the following optimization flags:

-fauto-inc-dec 
-fcompare-elim 
-fcprop-registers 
-fdce 
-fdefer-pop 
-fdelayed-branch 
-fdse 
-fguess-branch-probability 
-fif-conversion2 
-fif-conversion 
-fipa-pure-const 
-fipa-profile 
-fipa-reference 
-fmerge-constants
-fsplit-wide-types 
-ftree-bit-ccp 
-ftree-builtin-call-dce 
-ftree-ccp 
-ftree-ch 
-ftree-copyrename 
-ftree-dce 
-ftree-dominator-opts 
-ftree-dse 
-ftree-forwprop 
-ftree-fre 
-ftree-phiprop 
-ftree-slsr 
-ftree-sra 
-ftree-pta 
-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 
-fdevirtualize -fdevirtualize-speculatively 
-fexpensive-optimizations 
-fgcse  -fgcse-lm  
-fhoist-adjacent-loads 
-finline-small-functions 
-findirect-inlining 
-fipa-sra 
-fisolate-erroneous-paths-dereference 
-foptimize-sibling-calls 
-fpartial-inlining 
-fpeephole2 
-freorder-blocks  -freorder-functions 
-frerun-cse-after-loop  
-fsched-interblock  -fsched-spec 
-fschedule-insns  -fschedule-insns2 
-fstrict-aliasing -fstrict-overflow 
-ftree-switch-conversion -ftree-tail-merge 
-ftree-pre 
-ftree-vrp

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

-O3

Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the -finline-functions, -funswitch-loops, -fpredictive-commoning, -fgcse-after-reload, -ftree-loop-vectorize, -ftree-slp-vectorize, -fvect-cost-model, -ftree-partial-pre and -fipa-cp-clone 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

-Ofast

Disregard strict standards compliance. -Ofast enables all -O3 optimizations. It also enables optimizations that are not valid for all standard-compliant programs. It turns on -ffast-math and the Fortran-specific -fno-protect-parens and -fstack-arrays.

-Og

Optimize debugging experience. -Og enables optimizations that do not interfere with debugging. It should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience.

If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.

Options of the form -fflag specify machine-independent flags. Most flags have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed—the one you typically use. You can figure out the other form by either removing ‘no-’ or adding it.

The following options control specific optimizations. They are either activated by -O options or are related to ones that are. You can use the following flags in the rare cases when “fine-tuning” of optimizations to be performed is desired.

-fno-defer-pop

Always pop the arguments to each function call as soon as that function returns. For machines that 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 -O, -O2, -O3, -Os.

-ffp-contract=style

-ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables floating-point expression contraction such as forming of fused multiply-add operations if the target has native support for them. -ffp-contract=on enables floating-point expression contraction if allowed by the language standard. This is currently not implemented and treated equal to -ffp-contract=off.

The default is -ffp-contract=fast.

-fomit-frame-pointer

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 in GNU Compiler Collection (GCC) Internals.

Starting with GCC version 4.6, the default setting (when not optimizing for size) for 32-bit GNU/Linux x86 and 32-bit Darwin x86 targets has been changed to -fomit-frame-pointer. The default can be reverted to -fno-omit-frame-pointer by configuring GCC with the --enable-frame-pointer configure option.

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

-foptimize-sibling-calls

Optimize sibling and tail recursive calls.

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

-fno-inline

Do not expand any functions inline apart from those marked with the always_inline attribute. This is the default when not optimizing.

Single functions can be exempted from inlining by marking them with the noinline attribute.

-finline-small-functions

Integrate functions into their callers when their body is smaller than expected function call code (so overall size of program gets smaller). The compiler heuristically decides which functions are simple enough to be worth integrating in this way. This inlining applies to all functions, even those not declared inline.

Enabled at 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

Consider all functions for inlining, even if they are not declared inline. The compiler heuristically decides which functions are worth integrating in this way.

If all calls to a given function are integrated, and the function is declared static, then the function is normally not output as assembler code in its own right.

Enabled at 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.

-fipa-sra

Perform interprocedural scalar replacement of aggregates, removal of unused parameters and replacement of parameters passed by reference by parameters passed by value.

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

-finline-limit=n

By default, GCC limits the size of functions that can be inlined. This flag allows coarse control of this limit. n is the size of functions that can be inlined in number of pseudo instructions.

Inlining is actually controlled by a number of parameters, which may be specified individually by using --param name=value. The -finline-limit=n option sets some of these parameters as follows:

max-inline-insns-single

is set to n/2.

max-inline-insns-auto

is set to n/2.

See below for a documentation of the individual parameters controlling inlining and for the defaults of these parameters.

Note: there may be no value to -finline-limit that results in default behavior.

Note: pseudo instruction represents, in this particular context, an abstract measurement of function’s size. In no way does it represent a count of assembly instructions and as such its exact meaning might change from one release to an another.

-fno-keep-inline-dllexport

This is a more fine-grained version of -fkeep-inline-functions, which applies only to functions that are declared using the dllexport attribute or declspec (See Declaring Attributes of Functions.)

-fkeep-inline-functions

In C, emit static functions that are declared inline into the object file, even if the function has been inlined into all of its callers. This switch does not affect functions using the extern inline extension in GNU C90. In C++, emit any and all inline functions into the object file.

-fkeep-static-consts

Emit variables declared static const when optimization isn’t turned on, even if the variables aren’t referenced.

GCC enables this option by default. If you want to force the compiler to check if a variable is referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option.

-fmerge-constants

Attempt to merge identical constants (string constants and floating-point constants) across compilation units.

This option is the default for optimized compilation if the assembler and linker support it. Use -fno-merge-constants to inhibit this behavior.

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

-fmerge-all-constants

Attempt to merge identical constants and identical variables.

This option implies -fmerge-constants. In addition to -fmerge-constants this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating-point types. Languages like C or C++ require each variable, including multiple instances of the same variable in recursive calls, to have distinct locations, so using this option results in non-conforming behavior.

-fmodulo-sched

Perform swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost loops and reorders their instructions by overlapping different iterations.

-fmodulo-sched-allow-regmoves

Perform more aggressive SMS-based modulo scheduling with register moves allowed. By setting this flag certain anti-dependences edges are deleted, which triggers the generation of reg-moves based on the life-range analysis. This option is effective only with -fmodulo-sched enabled.

-fno-branch-count-reg

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.

-fthread-jumps

Perform optimizations that check to see if a jump branches to a location where another comparison subsumed by the first is found. If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false.

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

-fsplit-wide-types

When using a type that occupies multiple registers, such as long long on a 32-bit system, split the registers apart and allocate them independently. This normally generates better code for those types, but may make debugging more difficult.

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

-fcse-follow-jumps

In common subexpression elimination (CSE), scan through jump instructions when the target of the jump is not reached by any other path. For example, when CSE encounters an if statement with an else clause, CSE follows the jump when the condition tested is false.

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

-fcse-skip-blocks

This is similar to -fcse-follow-jumps, but causes CSE to follow jumps that conditionally skip over blocks. When CSE encounters a simple if statement with no else clause, -fcse-skip-blocks causes CSE to follow the jump around the body of the if.

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

-frerun-cse-after-loop

Re-run common subexpression elimination after loop optimizations are performed.

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

-fgcse

Perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation.

Note: When compiling a program using computed gotos, a GCC extension, you may get better run-time performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.

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

-fgcse-lm

When -fgcse-lm is enabled, global common subexpression elimination attempts to move loads that are only killed by stores into themselves. This allows a loop containing a load/store sequence to be changed to a load outside the loop, and a copy/store within the loop.

Enabled by default when -fgcse is enabled.

-fgcse-sm

When -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination. This pass attempts to move stores out of loops. When used in conjunction with -fgcse-lm, loops containing a load/store sequence can be changed to a load before the loop and a store after the loop.

Not enabled at any optimization level.

-fgcse-las

When -fgcse-las is enabled, the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location (both partial and full redundancies).

Not enabled at any optimization level.

-fgcse-after-reload

When -fgcse-after-reload is enabled, a redundant load elimination pass is performed after reload. The purpose of this pass is to clean up redundant spilling.

-faggressive-loop-optimizations

This option tells the loop optimizer to use language constraints to derive bounds for the number of iterations of a loop. This assumes that loop code does not invoke undefined behavior by for example causing signed integer overflows or out-of-bound array accesses. The bounds for the number of iterations of a loop are used to guide loop unrolling and peeling and loop exit test optimizations. This option is enabled by default.

-funsafe-loop-optimizations

This option tells the loop optimizer to assume that loop indices do not overflow, and that 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. If you use -Wunsafe-loop-optimizations, the compiler warns you if it finds this kind of loop.

-fcrossjumping

Perform cross-jumping transformation. This transformation unifies equivalent code and saves code size. The resulting code may or may not perform better than without cross-jumping.

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

-fauto-inc-dec

Combine increments or decrements of addresses with memory accesses. This pass is always skipped on architectures that do not have instructions to support this. Enabled by default at -O and higher on architectures that support this.

-fdce

Perform dead code elimination (DCE) on RTL. Enabled by default at -O and higher.

-fdse

Perform dead store elimination (DSE) on RTL. Enabled by default at -O and higher.

-fif-conversion

Attempt to transform conditional jumps into branch-less equivalents. This includes use of conditional moves, min, max, set flags and abs instructions, and some tricks doable by standard arithmetics. The use of conditional execution on chips where it is available is controlled by 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.

-fdeclone-ctor-dtor

The C++ ABI requires multiple entry points for constructors and destructors: one for a base subobject, one for a complete object, and one for a virtual destructor that calls operator delete afterwards. For a hierarchy with virtual bases, the base and complete variants are clones, which means two copies of the function. With this option, the base and complete variants are changed to be thunks that call a common implementation.

Enabled by -Os.

-fdelete-null-pointer-checks

Assume that programs cannot safely dereference null pointers, and that no code or data element resides there. This enables simple constant folding optimizations at all optimization levels. In addition, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers; these assume that if a pointer is checked after it has already been dereferenced, it cannot be null.

Note however that in some environments this assumption is not true. Use -fno-delete-null-pointer-checks to disable this optimization for programs that depend on that behavior.

Some targets, especially embedded ones, disable this option at all levels. Otherwise it is enabled at all levels: -O0, -O1, -O2, -O3, -Os. Passes that use the information are enabled independently at different optimization levels.

-fdevirtualize

Attempt to convert calls to virtual functions to direct calls. This is done both within a procedure and interprocedurally as part of indirect inlining (-findirect-inlining) and interprocedural constant propagation (-fipa-cp). Enabled at levels -O2, -O3, -Os.

-fdevirtualize-speculatively

Attempt to convert calls to virtual functions to speculative direct calls. Based on the analysis of the type inheritance graph, determine for a given call the set of likely targets. If the set is small, preferably of size 1, change the call into an conditional deciding on direct and indirect call. The speculative calls enable more optimizations, such as inlining. When they seem useless after further optimization, they are converted back into original form.

-fexpensive-optimizations

Perform a number of minor optimizations that are relatively expensive.

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

-free

Attempt to remove redundant extension instructions. This is especially helpful for the x86-64 architecture, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half.

Enabled for Alpha, AArch64 and x86 at levels -O2, -O3, -Os.

-flive-range-shrinkage

Attempt to decrease register pressure through register live range shrinkage. This is helpful for fast processors with small or moderate size register sets.

-fira-algorithm=algorithm

Use the specified coloring algorithm for the integrated register allocator. The algorithm argument can be ‘priority’, which specifies Chow’s priority coloring, or ‘CB’, which specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring is not implemented for all architectures, but for those targets that do support it, it is the default because it generates better code.

-fira-region=region

Use specified regions for the integrated register allocator. The region argument should be one of the following:

‘all’

Use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set.

‘mixed’

Use all loops except for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed (-O, -O2, …).

‘one’

Use all functions as a single region. This typically results in the smallest code size, and is enabled by default for -Os or -O0.

-fira-hoist-pressure

Use IRA to evaluate register pressure in the code hoisting pass for decisions to hoist expressions. This option usually results in smaller code, but it can slow the compiler down.

This option is enabled at level -Os for all targets.

-fira-loop-pressure

Use IRA to evaluate register pressure in loops for decisions to move loop invariants. This option usually results in generation of faster and smaller code on machines with large register files (>= 32 registers), but it can slow the compiler down.

This option is enabled at level -O3 for some targets.

-fno-ira-share-save-slots

Disable sharing of stack slots used for saving call-used hard registers living through a call. Each hard register gets a separate stack slot, and as a result function stack frames are larger.

-fno-ira-share-spill-slots

Disable sharing of stack slots allocated for pseudo-registers. Each pseudo-register that does not get a hard register gets a separate stack slot, and as a result function stack frames are larger.

-fira-verbose=n

Control the verbosity of the dump file for the integrated register allocator. The default value is 5. If the value n is greater or equal to 10, the dump output is sent to stderr using the same format as n minus 10.

-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.

-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-pressure

Enable register pressure sensitive insn scheduling before register allocation. This only makes sense when scheduling before register allocation is enabled, i.e. with -fschedule-insns or at -O2 or higher. Usage of this option can improve the generated code and decrease its size by preventing register pressure increase above the number of available hard registers and subsequent spills in register allocation.

-fsched-spec-load

Allow speculative motion of some load instructions. This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-fsched-spec-load-dangerous

Allow speculative motion of more load instructions. This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-fsched-stalled-insns

-fsched-stalled-insns=n

Define how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready list during the second scheduling pass. -fno-sched-stalled-insns means that no insns are moved prematurely, -fsched-stalled-insns=0 means there is no limit on how many queued insns can be moved prematurely. -fsched-stalled-insns without a value is equivalent to -fsched-stalled-insns=1.

-fsched-stalled-insns-dep

-fsched-stalled-insns-dep=n

Define how many insn groups (cycles) are examined for a dependency on a stalled insn that is a candidate for premature removal from the queue of stalled insns. This has an effect only during the second scheduling pass, and only if -fsched-stalled-insns is used. -fno-sched-stalled-insns-dep is equivalent to -fsched-stalled-insns-dep=0. -fsched-stalled-insns-dep without a value is equivalent to -fsched-stalled-insns-dep=1.

-fsched2-use-superblocks

When scheduling after register allocation, use superblock scheduling. This allows motion across basic block boundaries, resulting in faster schedules. This option is experimental, as not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm.

This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.

-fsched-group-heuristic

Enable the group heuristic in the scheduler. This heuristic favors the instruction that belongs to a schedule group. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.

-fsched-critical-path-heuristic

Enable the critical-path heuristic in the scheduler. This heuristic favors instructions on the critical path. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.

-fsched-spec-insn-heuristic

Enable the speculative instruction heuristic in the scheduler. This heuristic favors speculative instructions with greater dependency weakness. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.

-fsched-rank-heuristic

Enable the rank heuristic in the scheduler. This heuristic favors the instruction belonging to a basic block with greater size or frequency. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.

-fsched-last-insn-heuristic

Enable the last-instruction heuristic in the scheduler. This heuristic favors the instruction that is less dependent on the last instruction scheduled. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.

-fsched-dep-count-heuristic

Enable the dependent-count heuristic in the scheduler. This heuristic favors the instruction that has more instructions depending on it. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.

-freschedule-modulo-scheduled-loops

Modulo scheduling is performed before traditional scheduling. If a loop is modulo scheduled, later scheduling passes may change its schedule. Use this option to control that behavior.

-fselective-scheduling

Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass.

-fselective-scheduling2

Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the second scheduler pass.

-fsel-sched-pipelining

Enable software pipelining of innermost loops during selective scheduling. This option has no effect unless one of -fselective-scheduling or -fselective-scheduling2 is turned on.

-fsel-sched-pipelining-outer-loops

When pipelining loops during selective scheduling, also pipeline outer loops. This option has no effect unless -fsel-sched-pipelining is turned on.

-fshrink-wrap

Emit function prologues only before parts of the function that need it, rather than at the top of the function. This flag is enabled by default at -O and higher.

-fcaller-saves

Enable allocation of values to registers that are clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls. Such allocation is done only when it seems to result in better code.

This option is always enabled by default on certain machines, usually those which have no call-preserved registers to use instead.

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

-fcombine-stack-adjustments

Tracks stack adjustments (pushes and pops) and stack memory references and then tries to find ways to combine them.

Enabled by default at -O1 and higher.

-fconserve-stack

Attempt to minimize stack usage. The compiler attempts to use less stack space, even if that makes the program slower. This option implies setting the large-stack-frame parameter to 100 and the large-stack-frame-growth parameter to 400.

-ftree-reassoc

Perform reassociation on trees. This flag is enabled by default at -O and higher.

-ftree-pre

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

-ftree-partial-pre

Make partial redundancy elimination (PRE) more aggressive. This flag is enabled by default at -O3.

-ftree-forwprop

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

-ftree-fre

Perform full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE only considers expressions that are computed on all paths leading to the redundant computation. This analysis is faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at -O and higher.

-ftree-phiprop

Perform hoisting of loads from conditional pointers on trees. This pass is enabled by default at -O and higher.

-fhoist-adjacent-loads

Speculatively hoist loads from both branches of an if-then-else if the loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction. This flag is enabled by default at -O2 and higher.

-ftree-copy-prop

Perform copy propagation on trees. This pass eliminates unnecessary copy operations. This flag is enabled by default at -O and higher.

-fipa-pure-const

Discover which functions are pure or constant. Enabled by default at -O and higher.

-fipa-reference

Discover which static variables do not escape the compilation unit. Enabled by default at -O and higher.

-fipa-pta

Perform interprocedural pointer analysis and interprocedural modification and reference analysis. This option can cause excessive memory and compile-time usage on large compilation units. It is not enabled by default at any optimization level.

-fipa-profile

Perform interprocedural profile propagation. The functions called only from cold functions are marked as cold. Also functions executed once (such as cold, noreturn, static constructors or destructors) are identified. Cold functions and loop less parts of functions executed once are then optimized for size. Enabled by default at -O and higher.

-fipa-cp

Perform interprocedural constant propagation. This optimization analyzes the program to determine when values passed to functions are constants and then optimizes accordingly. This optimization can substantially increase performance if the application has constants passed to functions. This flag is enabled by default at -O2, -Os and -O3.

-fipa-cp-clone

Perform function cloning to make interprocedural constant propagation stronger. When enabled, interprocedural constant propagation performs function cloning when externally visible function can be called with constant arguments. Because this optimization can create multiple copies of functions, it may significantly increase code size (see --param ipcp-unit-growth=value). This flag is enabled by default at -O3.

-fisolate-erroneous-paths-dereference

Detect paths which trigger erroneous or undefined behaviour due to dereferencing a NULL pointer. Isolate those paths from the main control flow and turn the statement with erroneous or undefined behaviour into a trap.

-fisolate-erroneous-paths-attribute

Detect paths which trigger erroneous or undefined behaviour due a NULL value being used in a way which is forbidden by a returns_nonnull or nonnull attribute. Isolate those paths from the main control flow and turn the statement with erroneous or undefined behaviour into a trap. This is not currently enabled, but may be enabled by -O2 in the future.

-ftree-sink

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

-ftree-bit-ccp

Perform sparse conditional bit constant propagation on trees and propagate pointer alignment information. This pass only operates on local scalar variables and is enabled by default at -O and higher. It requires that -ftree-ccp is enabled.

-ftree-ccp

Perform sparse conditional constant propagation (CCP) on trees. This pass only operates on local scalar variables and is enabled by default at -O and higher.

-ftree-switch-conversion

Perform conversion of simple initializations in a switch to initializations from a scalar array. This flag is enabled by default at -O2 and higher.

-ftree-tail-merge

Look for identical code sequences. When found, replace one with a jump to the other. This optimization is known as tail merging or cross jumping. This flag is enabled by default at -O2 and higher. The compilation time in this pass can be limited using max-tail-merge-comparisons parameter and max-tail-merge-iterations parameter.

-ftree-dce

Perform dead code elimination (DCE) on trees. This flag is enabled by default at -O and higher.

-ftree-builtin-call-dce

Perform conditional dead code elimination (DCE) for calls to built-in functions that may set errno but are otherwise 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 that is later overwritten by another store without any intervening loads. In this case the earlier store can be deleted. This flag is enabled by default at -O and higher.

-ftree-ch

Perform loop header copying on trees. This is beneficial since it increases effectiveness of code motion optimizations. It also saves one jump. This flag is enabled by default at -O and higher. It is not enabled for -Os, since it usually increases code size.

-ftree-loop-optimize

Perform loop optimizations on trees. This flag is enabled by default at -O and higher.

-ftree-loop-linear

Perform loop interchange transformations on tree. Same as -floop-interchange. To use this code transformation, GCC has to be configured with --with-ppl and --with-cloog to enable the Graphite loop transformation infrastructure.

-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 transforms the loop as if it were 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. The strip length can be changed using the loop-block-tile-size parameter. For example, given a loop like:

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

loop strip mining transforms the loop as if it were written:

DO II = 1, N, 51
  DO I = II, min (II + 50, 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. The strip length can be changed using the loop-block-tile-size parameter. 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 transforms the loop as if it were written:

DO II = 1, N, 51
  DO JJ = 1, M, 51
    DO I = II, min (II + 50, N)
      DO J = JJ, min (JJ + 50, 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 iterates over a smaller amount of data which 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.

-fgraphite-identity

Enable the identity transformation for graphite. For every SCoP we generate the polyhedral representation and transform it back to gimple. Using -fgraphite-identity we can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations are also performed by the code generator CLooG, like index splitting and dead code elimination in loops.

-floop-nest-optimize

Enable the ISL based loop nest optimizer. This is a generic loop nest optimizer based on the Pluto optimization algorithms. It calculates a loop structure optimized for data-locality and parallelism. This option is experimental.

-floop-parallelize-all

Use the Graphite data dependence analysis to identify loops that can be parallelized. Parallelize all the loops that can be analyzed to not contain loop carried dependences without checking that it is profitable to parallelize the loops.

-fcheck-data-deps

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

-ftree-loop-if-convert

Attempt to transform conditional jumps in the innermost loops to branch-less equivalents. The intent is to remove control-flow from the innermost loops in order to improve the ability of the vectorization pass to handle these loops. This is enabled by default if vectorization is enabled.

-ftree-loop-if-convert-stores

Attempt to also if-convert conditional jumps containing memory writes. This transformation can be unsafe for multi-threaded programs as it transforms conditional memory writes into unconditional memory writes. For example,

for (i = 0; i < N; i++)
  if (cond)
    A[i] = expr;

is transformed to

for (i = 0; i < N; i++)
  A[i] = cond ? expr : A[i];

potentially producing data races.

-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-distribute-patterns

Perform loop distribution of patterns that can be code generated with calls to a library. This flag is enabled by default at -O3.

This pass distributes the initialization loops and generates a call to memset zero. For example, the loop

DO I = 1, N
  A(I) = 0
  B(I) = A(I) + I
ENDDO

is transformed to

DO I = 1, N
   A(I) = 0
ENDDO
DO I = 1, N
   B(I) = A(I) + I
ENDDO

and the initialization loop is transformed into a call to memset zero.

-ftree-loop-im

Perform loop invariant motion on trees. This pass moves only invariants that are hard to handle at RTL level (function calls, operations that expand to nontrivial sequences of insns). With -funswitch-loops it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion.

-ftree-loop-ivcanon

Create a canonical counter for number of iterations in loops for which determining number of iterations requires complicated analysis. Later optimizations then may determine the number easily. Useful especially in connection with unrolling.

-fivopts

Perform induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees.

-ftree-parallelize-loops=n

Parallelize loops, i.e., split their iteration space to run in n threads. This is only possible for loops whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g. by memory bandwidth. This option implies -pthread, and thus is only supported on targets that have support for -pthread.

-ftree-pta

Perform function-local points-to analysis on trees. This flag is enabled by default at -O and higher.

-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-coalesce-inlined-vars

Tell the copyrename pass (see -ftree-copyrename) to attempt to combine small user-defined variables too, but only if they were inlined from other functions. It is a more limited form of -ftree-coalesce-vars. This may harm debug information of such inlined variables, but it will keep variables of the inlined-into function apart from each other, such that they are more likely to contain the expected values in a debugging session. This was the default in GCC versions older than 4.7.

-ftree-coalesce-vars

Tell the copyrename pass (see -ftree-copyrename) to attempt to combine small user-defined variables too, instead of just compiler temporaries. This may severely limit the ability to debug an optimized program compiled with -fno-var-tracking-assignments. In the negated form, this flag prevents SSA coalescing of user variables, including inlined ones. This option is enabled by default.

-ftree-ter

Perform temporary expression replacement during the SSA->normal phase. Single use/single def temporaries are replaced at their use location with their defining expression. This results in non-GIMPLE code, but gives the expanders much more complex trees to work on resulting in better RTL generation. This is enabled by default at -O and higher.

-ftree-slsr

Perform straight-line strength reduction on trees. This recognizes related expressions involving multiplications and replaces them by less expensive calculations when possible. This is enabled by default at -O and higher.

-ftree-vectorize

Perform vectorization on trees. This flag enables -ftree-loop-vectorize and -ftree-slp-vectorize if not explicitly specified.

-ftree-loop-vectorize

Perform loop vectorization on trees. This flag is enabled by default at -O3 and when -ftree-vectorize is enabled.

-ftree-slp-vectorize

Perform basic block vectorization on trees. This flag is enabled by default at -O3 and when -ftree-vectorize is enabled.

-fvect-cost-model=model

Alter the cost model used for vectorization. The model argument should be one of unlimited, dynamic or cheap. With the unlimited model the vectorized code-path is assumed to be profitable while with the dynamic model a runtime check will guard the vectorized code-path to enable it only for iteration counts that will likely execute faster than when executing the original scalar loop. The cheap model will disable vectorization of loops where doing so would be cost prohibitive for example due to required runtime checks for data dependence or alignment but otherwise is equal to the dynamic model. The default cost model depends on other optimization flags and is either dynamic or cheap.

-fsimd-cost-model=model

Alter the cost model used for vectorization of loops marked with the OpenMP or Cilk Plus simd directive. The model argument should be one of unlimited, dynamic, cheap. All values of model have the same meaning as described in -fvect-cost-model and by default a cost model defined with -fvect-cost-model is used.

-ftree-vrp

Perform Value Range Propagation on trees. This is similar to the constant propagation pass, but instead of values, ranges of values are propagated. This allows the optimizers to remove unnecessary range checks like array bound checks and null pointer checks. This is enabled by default at -O2 and higher. Null pointer check elimination is only done if -fdelete-null-pointer-checks is enabled.

-ftracer

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

-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 expression of values of induction variables in later iterations of the unrolled loop using the value in the first iteration. This breaks long dependency chains, thus improving efficiency of the scheduling passes.

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

This optimization is enabled by default.

-fvariable-expansion-in-unroller

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

-fpartial-inlining

Inline parts of functions. This option has any effect only when inlining itself is turned on by the -finline-functions or -finline-small-functions options.

Enabled at level -O2.

-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 uses heuristics to guess branch probabilities if they are not provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are specified by ‘__builtin_expect’, then the heuristics are used to guess branch probabilities for the rest of the control flow graph, taking the ‘__builtin_expect’ info into account. The interactions between the heuristics and ‘__builtin_expect’ can be complex, and in some cases, it may be useful to disable the heuristics so that the effects of ‘__builtin_expect’ are easier to understand.

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.

Enabled for x86 at levels -O2, -O3.

-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 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 works as expected. See Structures unions enumerations and bit-fields implementation. However, this code might not:

int f() {
  union a_union t;
  int* ip;
  t.d = 3.0;
  ip = &t.i;
  return *ip;
}

Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior, even if the cast uses a union type, e.g.:

int f() {
  double d = 3.0;
  return ((union a_union *) &d)->i;
}

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

-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 does not happen. This permits various optimizations. For example, the compiler assumes that an expression like i + 10 > i is always 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 overflows 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 aligns 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 are not 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 are not aligned.

If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead.

If n is not specified or is zero, use a machine-dependent default which is very likely to be ‘1’, meaning no alignment.

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. If the loops are executed many times, this makes up for any execution of the dummy operations.

-fno-align-loops and -falign-loops=1 are equivalent and mean that loops are not 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 are not 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 are not removed. This option is intended to support existing code that relies on a particular ordering. For new code, it is better to use attributes when possible.

Enabled at level -O0. When disabled explicitly, it also implies -fno-section-anchors, which 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 no longer stay in a “home register”.

Enabled by default with -funroll-loops.

-fwhole-program

Assume that the current compilation unit represents the whole program being compiled. All public functions and variables with the exception of main and those merged by attribute externally_visible become static functions and in effect are optimized more aggressively by interprocedural optimizers.

This option should not be used in combination with -flto. Instead relying on a linker plugin should provide safer and more precise information.

-flto[=n]

This option runs the standard link-time optimizer. When invoked with source code, it generates GIMPLE (one of GCC’s internal representations) and writes it to special ELF sections in the object file. When the object files are linked together, all the function bodies are read from these ELF sections and instantiated as if they had been part of the same translation unit.

To use the link-time optimizer, -flto and optimization options should be specified at compile time and during the final link. For example:

gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o

The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o. The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two files into a single internal image, and compiles the result as usual. Since both foo.o and bar.o are merged into a single image, this causes all the interprocedural analyses and optimizations in GCC to work across the two files as if they were a single one. This means, for example, that the inliner is able to inline functions in bar.o into functions in foo.o and vice-versa.

Another (simpler) way to enable link-time optimization is:

gcc -o myprog -flto -O2 foo.c bar.c

The above generates bytecode for foo.c and bar.c, merges them together into a single GIMPLE representation and optimizes them as usual to produce myprog.

The only important thing to keep in mind is that to enable link-time optimizations you need to use the GCC driver to perform the link-step. GCC then automatically performs link-time optimization if any of the objects involved were compiled with the -flto. You generally should specify the optimization options to be used for link-time optimization though GCC will try to be clever at guessing an optimization level to use from the options used at compile-time if you fail to specify one at link-time. You can always override the automatic decision to do link-time optimization at link-time by passing -fno-lto to the link command.

To make whole program optimization effective, it is necessary to make certain whole program assumptions. The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit. When supported by the linker, the linker plugin (see -fuse-linker-plugin) passes information to the compiler about used and externally visible symbols. When the linker plugin is not available, -fwhole-program should be used to allow the compiler to make these assumptions, which leads to more aggressive optimization decisions.

When -fuse-linker-plugin is not enabled then, when a file is compiled with -flto, the generated object file is larger than a regular object file because it contains GIMPLE bytecodes and the usual final code (see -ffat-lto-objects. This means that object files with LTO information can be linked as normal object files; if -fno-lto is passed to the linker, no interprocedural optimizations are applied. Note that when -fno-fat-lto-objects is enabled the compile-stage is faster but you cannot perform a regular, non-LTO link on them.

Additionally, the optimization flags used to compile individual files are not necessarily related to those used at link time. For instance,

gcc -c -O0 -ffat-lto-objects -flto foo.c
gcc -c -O0 -ffat-lto-objects -flto bar.c
gcc -o myprog -O3 foo.o bar.o

This produces individual object files with unoptimized assembler code, but the resulting binary myprog is optimized at -O3. If, instead, the final binary is generated with -fno-lto, then myprog is not optimized.

When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. Therefore, you can mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC automatically selects which files to optimize in LTO mode and which files to link without further processing.

There are some code generation flags preserved by GCC when generating bytecodes, as they need to be used during the final link stage. Generally options specified at link-time override those specified at compile-time.

If you do not specify an optimization level option -O at link-time then GCC will compute one based on the optimization levels used when compiling the object files. The highest optimization level will win here.

Currently, the following options and their setting are take from the first object file that explicitely specified it: -fPIC, -fpic, -fpie, -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags.

Certain ABI changing flags are required to match in all compilation-units and trying to override this at link-time with a conflicting value is ignored. This includes options such as -freg-struct-return and -fpcc-struct-return.

Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting translation units. Specifically -fno-strict-overflow, -fwrapv and -fno-trapv take precedence and for example -ffp-contract=off takes precedence over -ffp-contract=fast. You can override them at linke-time.

It is recommended that you compile all the files participating in the same link with the same options and also specify those options at link time.

If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The behavior is still undefined at run time. Similar diagnostics may be raised for other languages.

Another feature of LTO is that it is possible to apply interprocedural optimizations on files written in different languages:

gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran

Notice that the final link is done with g++ to get the C++ runtime libraries and -lgfortran is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation.

If object files containing GIMPLE bytecode are stored in a library archive, say libfoo.a, it is possible to extract and use them in an LTO link if you are using a linker with plugin support. To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; to show the symbols of object files with GIMPLE bytecode, use gcc-nm. Those commands require that ar, ranlib and nm have been compiled with plugin support. At link time, use the the flag -fuse-linker-plugin to ensure that the library participates in the LTO optimization process:

gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo

With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized.

If you are not using a linker with plugin support and/or do not enable the linker plugin, then the objects inside libfoo.a are extracted and linked as usual, but they do not participate in the LTO optimization process. In order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects.

Link-time optimizations do not require the presence of the whole program to operate. If the program does not require any symbols to be exported, it is possible to combine -flto and -fwhole-program to allow the interprocedural optimizers to use more aggressive assumptions which may lead to improved optimization opportunities. Use of -fwhole-program is not needed when linker plugin is active (see -fuse-linker-plugin).

The current implementation of LTO makes no attempt to generate bytecode that is portable between different types of hosts. The bytecode files are versioned and there is a strict version check, so bytecode files generated in one version of GCC will not work with an older or newer version of GCC.

Link-time optimization does not work well with generation of debugging information. Combining -flto with -g is currently experimental and expected to produce unexpected results.

If you specify the optional n, the optimization and code generation done at link time is executed in parallel using n parallel jobs by utilizing an installed make program. The environment variable MAKE may be used to override the program used. The default value for n is 1.

You can also specify -flto=jobserver to use GNU make’s job server mode to determine the number of parallel jobs. This is useful when the Makefile calling GCC is already executing in parallel. You must prepend a ‘+’ to the command recipe in the parent Makefile for this to work. This option likely only works if MAKE is GNU make.

-flto-partition=alg

Specify the partitioning algorithm used by the link-time optimizer. The value is either 1to1 to specify a partitioning mirroring the original source files or balanced to specify partitioning into equally sized chunks (whenever possible) or max to create new partition for every symbol where possible. Specifying none as an algorithm disables partitioning and streaming completely. The default value is balanced. While 1to1 can be used as an workaround for various code ordering issues, the max partitioning is intended for internal testing only.

-flto-compression-level=n

This option specifies the level of compression used for intermediate language written to LTO object files, and is only meaningful in conjunction with LTO mode (-flto). Valid values are 0 (no compression) to 9 (maximum compression). Values outside this range are clamped to either 0 or 9. If the option is not given, a default balanced compression setting is used.

-flto-report

Prints a report with internal details on the workings of the link-time optimizer. The contents of this report vary from version to version. It is meant to be useful to GCC developers when processing object files in LTO mode (via -flto).

Disabled by default.

-flto-report-wpa

Like -flto-report, but only print for the WPA phase of Link Time Optimization.

-fuse-linker-plugin

Enables the use of a linker plugin during link-time optimization. This option relies on plugin support in the linker, which is available in gold or in GNU ld 2.21 or newer.

This option enables the extraction of object files with GIMPLE bytecode out of library archives. This improves the quality of optimization by exposing more code to the link-time optimizer. This information specifies what symbols can be accessed externally (by non-LTO object or during dynamic linking). Resulting code quality improvements on binaries (and shared libraries that use hidden visibility) are similar to -fwhole-program. See -flto for a description of the effect of this flag and how to use it.

This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker supporting plugins (GNU ld 2.21 or newer or gold).

-ffat-lto-objects

Fat LTO objects are object files that contain both the intermediate language and the object code. This makes them usable for both LTO linking and normal linking. This option is effective only when compiling with -flto and is ignored at link time.

-fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete toolchain to be aware of LTO. It requires a linker with linker plugin support for basic functionality. Additionally, nm, ar and ranlib need to support linker plugins to allow a full-featured build environment (capable of building static libraries etc). GCC provides the gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the right options to these tools. With non fat LTO makefiles need to be modified to use them.

The default is -fno-fat-lto-objects on targets with linker plugin support.

-fcompare-elim

After register allocation and post-register allocation instruction splitting, identify arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic. If possible, eliminate the explicit comparison operation.

This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete.

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

-fuse-ld=bfd

Use the bfd linker instead of the default linker.

-fuse-ld=gold

Use the gold linker instead of the default linker.

-fcprop-registers

After register allocation and post-register allocation instruction splitting, perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy.

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

-fprofile-correction

Profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates. When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected.

-fprofile-dir=path

Set the directory to search for the profile data files in to path. This option affects only the profile data generated by -fprofile-generate, -ftest-coverage, -fprofile-arcs and used by -fprofile-use and -fbranch-probabilities and its related options. Both absolute and relative paths can be used. By default, GCC uses the current directory as path, thus the profile data file appears in the same directory as the object file.

-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 looks 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, -ftree-vectorize, ftree-loop-distribute-patterns

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 looks at the path to find the profile feedback data files. See -fprofile-dir.