GCC

Section: GNU Tools (1)
Updated:

NAME

gcc, g++ - GNU 工程的 C 和 C++ 編譯器 (egcs-1.1.2)

總覽 (SYNOPSIS)

gcc [ option | filename ]...
g++ [ option | filename ]...

警告 (WARNING)

本手冊頁內容摘自 GNU C 編譯器的完整文件, 僅限於解釋選項的含義.

除非有人自願維護, 否則本手冊頁不再更新. 如果發現手冊頁和軟體之間有所矛盾, 請查對 Info 檔案, Info 檔案是權威文件.

如果我們發覺本手冊頁的內容由於過時而導致明顯的混亂和抱怨時, 我們就停止發佈它. 不可能有其他選擇, 像更新 Info 檔案同時更新 man 手冊, 因為其他維護 GNU CC 的工作沒有留給我們時間做這個. GNU 工程認為 man 手冊是過時產物, 應該把時間用到別的地方.

如果需要完整和最新的文件, 請查閱 Info 檔案 `gcc' 或 Using and Porting GNU CC (for version 2.0) (使用和移植 GNU CC 2.0) 手冊. 二者均來自 Texinfo 原檔案 gcc.texinfo.

描述 (DESCRIPTION)

C 和 C++ 編譯器是集成的. 他們都要用四個步驟中的一個或多個處理輸入檔案: 預處理(preprocessing), 編譯(compilation), 匯編(assembly) 和連接(linking). 源檔案延伸檔名名標識源檔案的語言, 但是對編譯器來說, 延伸檔名名控制著預設設定:

gcc: 認為預處理後的檔案 (.i) 是 C 檔案, 並且設定 C 形式的連接.
g++: 認為預處理後的檔案 (.ii) 是 C++ 檔案, 並且設定 C++ 形式的連接.

源檔案延伸檔名名指出語言種類以及後期的操作:

.c      C 源程式; 預處理, 編譯, 匯編
.C      C++ 源程式; 預處理, 編譯, 匯編
.cc     C++ 源程式; 預處理, 編譯, 匯編
.cxx    C++ 源程式; 預處理, 編譯, 匯編
.m      Objective-C 源程式; 預處理, 編譯, 匯編
.i      預處理後的 C 檔案; 編譯, 匯編
.ii     預處理後的 C++ 檔案; 編譯, 匯編
.s      匯編語言源程式; 匯編
.S      匯編語言源程式; 預處理, 匯編
.h      預處理器檔案; 通常 不出現在 命令行 上

其他延伸檔名名的檔案被傳遞給連接器(linker). 通常包括:

.o      目標檔案 (Object file)
.a      歸檔庫檔案 (Archive file)

除非使用了 -c, -S, 或 -E 選項 (或者編譯錯誤阻止了完整的過程), 否則連接總是最後的步驟. 在連接階段中, 所有對應於源程式的 .o 檔案, -l 庫檔案, 無法識別的檔名 (包括指定的 .o 目標檔案和 .a 庫檔案) 按命令行中的順序傳遞給連接器.

選項 (OPTIONS)

選項必須分立給出: `-dr' 完全不同於 `-d -r '.

大多數 `-f' 和 `-W' 選項有兩個相反的格式: -fname 和 -fno-name (或 -Wname 和 -Wno-name). 這裏只列舉不是預設選項的格式.

下面是所有選項的摘要, 按類型分組, 解釋放在後面的章節中.

總體選項 (Overall Option)

-c -S -E -o file -pipe -v -x language

語言選項 (Language Option)

-ansi -fall-virtual -fcond-mismatch -fdollars-in-identifiers -fenum-int-equiv -fexternal-templates -fno-asm -fno-builtin -fhosted -fno-hosted -ffreestanding -fno-freestanding -fno-strict-prototype -fsigned-bitfields -fsigned-char -fthis-is-variable -funsigned-bitfields -funsigned-char -fwritable-strings -traditional -traditional-cpp -trigraphs

警告選項 (Warning Option)

-fsyntax-only -pedantic -pedantic-errors -w -W -Wall -Waggregate-return -Wcast-align -Wcast-qual -Wchar-subscript -Wcomment -Wconversion -Wenum-clash -Werror -Wformat -Wid-clash-len -Wimplicit -Wimplicit-int -Wimplicit-function-declaration -Winline -Wlong-long -Wmain -Wmissing-prototypes -Wmissing-declarations -Wnested-externs -Wno-import -Wparentheses -Wpointer-arith -Wredundant-decls -Wreturn-type -Wshadow -Wstrict-prototypes -Wswitch -Wtemplate-debugging -Wtraditional -Wtrigraphs -Wuninitialized -Wunused -Wwrite-strings

調試選項 (Debugging Option)

-a -dletters -fpretend-float -g -glevel -gcoff -gxcoff -gxcoff+ -gdwarf -gdwarf+ -gstabs -gstabs+ -ggdb -p -pg -save-temps -print-file-name=library -print-libgcc-file-name -print-prog-name=program

優化選項 (Optimization Option)

-fcaller-saves -fcse-follow-jumps -fcse-skip-blocks -fdelayed-branch -felide-constructors -fexpensive-optimizations -ffast-math -ffloat-store -fforce-addr -fforce-mem -finline-functions -fkeep-inline-functions -fmemoize-lookups -fno-default-inline -fno-defer-pop -fno-function-cse -fno-inline -fno-peephole -fomit-frame-pointer -frerun-cse-after-loop -fschedule-insns -fschedule-insns2 -fstrength-reduce -fthread-jumps -funroll-all-loops -funroll-loops -O -O2 -O3

預處理器選項 (Preprocessor Option)

-Aassertion -C -dD -dM -dN -Dmacro[=defn] -E -H -idirafter dir -include file -imacros file -iprefix file -iwithprefix dir -M -MD -MM -MMD -nostdinc -P -Umacro -undef

匯編器選項 (Assembler Option)

-Wa,option

連接器選項 (Linker Option)

-llibrary -nostartfiles -nostdlib -static -shared -symbolic -Xlinker option -Wl,option -u symbol

目錄選項 (Directory Option)

-Bprefix -Idir -I- -Ldir

目標機選項 (Target Option)

-b machine -V version

配置相關選項 (Configuration Dependent Option)

M680x0 選項
-m68000 -m68020 -m68020-40 -m68030 -m68040 -m68881 -mbitfield -mc68000 -mc68020 -mfpa -mnobitfield -mrtd -mshort -msoft-float

VAX 選項
-mg -mgnu -munix

SPARC 選項
-mepilogue -mfpu -mhard-float -mno-fpu -mno-epilogue -msoft-float -msparclite -mv8 -msupersparc -mcypress

Convex 選項
-margcount -mc1 -mc2 -mnoargcount

AMD29K 選項
-m29000 -m29050 -mbw -mdw -mkernel-registers -mlarge -mnbw -mnodw -msmall -mstack-check -muser-registers

M88K 選項
-m88000 -m88100 -m88110 -mbig-pic -mcheck-zero-division -mhandle-large-shift -midentify-revision -mno-check-zero-division -mno-ocs-debug-info -mno-ocs-frame-position -mno-optimize-arg-area -mno-serialize-volatile -mno-underscores -mocs-debug-info -mocs-frame-position -moptimize-arg-area -mserialize-volatile -mshort-data-num -msvr3 -msvr4 -mtrap-large-shift -muse-div-instruction -mversion-03.00 -mwarn-passed-structs

RS6000 選項
-mfp-in-toc -mno-fop-in-toc

RT 選項
-mcall-lib-mul -mfp-arg-in-fpregs -mfp-arg-in-gregs -mfull-fp-blocks -mhc-struct-return -min-line-mul -mminimum-fp-blocks -mnohc-struct-return

MIPS 選項
-mcpu=cpu type -mips2 -mips3 -mint64 -mlong64 -mmips-as -mgas -mrnames -mno-rnames -mgpopt -mno-gpopt -mstats -mno-stats -mmemcpy -mno-memcpy -mno-mips-tfile -mmips-tfile -msoft-float -mhard-float -mabicalls -mno-abicalls -mhalf-pic -mno-half-pic -G num -nocpp

i386 選項
-m486 -mno-486 -msoft-float -mno-fp-ret-in-387

HPPA 選項
-mpa-risc-1-0 -mpa-risc-1-1 -mkernel -mshared-libs -mno-shared-libs -mlong-calls -mdisable-fpregs -mdisable-indexing -mtrailing-colon

i960 選項
-mcpu-type -mnumerics -msoft-float -mleaf-procedures -mno-leaf-procedures -mtail-call -mno-tail-call -mcomplex-addr -mno-complex-addr -mcode-align -mno-code-align -mic-compat -mic2.0-compat -mic3.0-compat -masm-compat -mintel-asm -mstrict-align -mno-strict-align -mold-align -mno-old-align

DEC Alpha 選項
-mfp-regs -mno-fp-regs -mno-soft-float -msoft-float

System V 選項
-G -Qy -Qn -YP,paths -Ym,dir

代碼生成選項 (Code Generation Option)

-fcall-saved-reg -fcall-used-reg -ffixed-reg -finhibit-size-directive -fnonnull-objects -fno-common -fno-ident -fno-gnu-linker -fpcc-struct-return -fpic -fPIC -freg-struct-return -fshared-data -fshort-enums -fshort-double -fvolatile -fvolatile-global -fverbose-asm

總體選項 (Overall Option)

-x language: 明確指出後面輸入檔案的語言為 language (而不是從檔名延伸檔名得到的預設選擇). 這個選項應用於後面所有的輸入檔案, 直到遇著下一個 `-x' 選項. language 的可選值有 `c', `objective-c', `c-header', `c++', `cpp-output', `assembler', 和 `assembler-with-cpp'.
-x none: 關閉任何對語種的明確說明, 因此依據檔名延伸檔名處理後面的檔案 (就像是從未使用過 `-x' 選項).

如果只操作四個階段 (預處理, 編譯, 匯編, 連接) 中的一部分, 可以使用 `-x' 選項 (或檔名延伸檔名) 告訴 gcc 從哪裏開始, 用 `-c', `-S', 或 `-E' 選項告訴gcc 到哪裏結束. 注意, 某些選項組合 (例如, `-x cpp-output -E') 使 gcc 不作任何事情.

-c

編譯或匯編源檔案, 但是不作連接. 編譯器輸出對應於源檔案的目標檔案.

預設情況下, GCC 通過用 `.o' 替換源檔名延伸檔名 `.c', `.i', `.s', 等等, 產生目標檔名. 可以使用 -o 選項選擇其他名字.

GCC 忽略 -c 選項後面任何無法識別的輸入檔案 (他們不需要編譯或匯編).

-S

編譯後即停止, 不進行匯編. 對於每個輸入的非匯編語言檔案, 輸出檔案是匯編語言檔案.

預設情況下, GCC 通過用 `.o' 替換源檔名延伸檔名 `.c', `.i', 等等, 產生目標檔名. 可以使用 -o 選項選擇其他名字.

GCC 忽略任何不需要編譯的輸入檔案.

-E

預處理後即停止, 不進行編譯. 預處理後的代碼送往標準輸出.

GCC 忽略任何不需要預處理的輸入檔案.

-o file

指定輸出檔案為 file. 該選項不在乎 GCC 產生什麼輸出, 無論是可執行檔案, 目標檔案, 匯編檔案還是預處理後的 C 代碼.

由於只能指定一個輸出檔案, 因此編譯多個輸入檔案時, 使用 `-o' 選項沒有意義, 除非輸出一個可執行檔案.

如果沒有使用 `-o' 選項, 預設的輸出結果是: 可執行檔案為 `a.out', `source.suffix ' 的目標檔案是`source.o', 匯編檔案是 `source.s', 而預處理後的 C 原始碼送往標準輸出.

-v

(在標準錯誤) 顯示執行編譯階段的命令. 同時顯示編譯器驅動程式, 預處理器, 編譯器的版本號.

-pipe

在編譯過程的不同階段間使用管道而非臨時檔案進行通信. 這個選項在某些系統上無法工作, 因為那些系統的匯編器不能從管道讀取數據. GNU 的匯編器沒有這個問題.

語言選項 (LANGUAGE OPTIONS)

下列選項控制編譯器能夠接受的 C "方言":

-ansi

支持符合 ANSI 標準的 C 程式.

這樣就會關閉 GNU C 中某些不相容 ANSI C 的特性, 例如 asm, inline 和 typeof 關鍵字, 以及諸如 unix 和 vax 這些表明當前系統類型的預定義宏. 同時開啟不受歡迎和極少使用的 ANSI trigraph 特性, 以及禁止 `$' 成為標識符的一部分.

儘管使用了 `-ansi' 選項, 下面這些可選的關鍵字, __asm__, __extension__, __inline__ 和 __typeof__ 仍然有效. 你當然不會把他們用在 ANSI C 程式中, 但可以把他們放在頭檔案裏, 因為編譯包含這些頭檔案的程式時, 可能會指定 `-ansi' 選項. 另外一些預定義宏, 如 __unix__ 和 __vax__, 無論有沒有使用 `-ansi' 選項, 始終有效.

使用 `-ansi' 選項不會自動拒絕編譯非ANSI程式, 除非增加 `-pedantic' 選項作為 `-ansi' 選項的補充.

使用 `-ansi' 選項的時候, 預處理器會預定義一個 __STRICT_ANSI__ 宏. 有些頭檔案關注此宏, 以避免聲明某些函數, 或者避免定義某些宏, 這些函數和宏不被 ANSI 標準調用; 這樣就不會幹擾在其他地方使用這些名字的程式了.

-fno-asm

不把 asm, inline 或 typeof 當作關鍵字, 因此這些詞可以用做標識符. 用 __asm__, __inline__ 和 __typeof__ 能夠替代他們. `-ansi' 隱含聲明了 `-fno-asm'.

-fno-builtin

不接受不是兩個下劃線開頭的內建函數 (built-in function). 目前受影響的函數有 _exit, abort, abs, alloca, cos, exit, fabs, labs, memcmp,memcpy, sin, sqrt, strcmp, strcpy, 和 strlen.

`-ansi' 選項能夠阻止 alloca 和 _exit 成為內建函數.

-fhosted

按宿主環境編譯; 他隱含聲明了 `-fbuiltin' 選項, 而且警告不正確的 main 函數聲明.

-ffreestanding

按獨立環境編譯; 他隱含聲明了 `-fno-builtin' 選項, 而且對 main 函數沒有特別要求.

(譯注: 宿主環境 (hosted environment) 下所有的標準庫可用, main 函數返回一個 int 值, 典型例子是除了核心以外幾乎所有的程式. 對應的獨立環境 (freestanding environment) 不存在標準庫, 程式入口也不一定是 main, 最明顯的例子就是作業系統核心. 詳情參考 gcc 網站最近的資料)

-fno-strict-prototype

對於沒有參數的函數聲明, 例如 `int foo ();', 按 C 風格處理---即不說明參數個數或類型. (僅針對 C++). 正常情況下, 這樣的函數 foo 在 C++ 中意味著參數為空.

-trigraphs

支持 ANSI C trigraphs. `-ansi' 選項隱含聲明了 `-trigraphs'.

-traditional

試圖支持傳統 C 編譯器的某些方面. 詳見 GNU C 手冊, 我們已經把細節清單從這裏刪除, 這樣當內容過時後, 人們也不會埋怨我們.

除了一件事: 對於 C++ 程式 (不是 C), `-traditional' 選項帶來一個附加效應, 允許對 this 賦值. 他和 `-fthis-is-variable' 選項的效果一樣.

-traditional-cpp

試圖支持傳統 C 預處理器的某些方面. 特別是上面提到有關預處理器的內容, 但是不包括 `-traditional' 選項的其他效應.

-fdollars-in-identifiers

允許在標識符(identifier) 中使用 `$' 字符 (僅針對 C++). 你可以指定 `-fno-dollars-in-identifiers' 選項顯明禁止使用 `$' 符. (GNU C++ 在某些目標系統預設允許 `$' 符, 但不是所有系統.)

-fenum-int-equiv

允許 int 類型到枚舉類型 (enumeration) 的隱式轉換 (僅限於 C++). 正常情況下 GNU C++ 允許從 enum 到 int 的轉換, 反之則不行.

-fexternal-templates

為模板聲明 (template declaration) 產生較小的代碼 (僅限於 C++), 方法是對於每個模板函數 (template function), 只在定義他們的地方生成一個副本. 想要成功使用這個選項, 你必須在所有使用模板的檔案中, 標記 `#pragma implementation' (定義) 或 `#pragma interface' (聲明).

當程式用 `-fexternal-templates' 編譯時, 模板實例 (template instantiation) 全部是外部類型. 你必須讓需要的實例在實現檔案中出現. 可以通過typedef 實現這一點, 他引用所需的每個實例. 相對應的, 如果編譯時使用預設選項 `-fno-external-templates', 所有模板實例明確的設為內置.

-fall-virtual

所有可能的成員函數預設為虛函數. 所有的成員函數 (除了構造子函數和 new 或 delete 成員操作符) 視為所在類的虛函數.

這不表明每次調用成員函數都將通過內部虛函數表. 有些情況下, 編譯器能夠判斷出可以直接調用某個虛函數; 這時就直接調用.

-fcond-mismatch

允許條件表達式的第二和第三個參數的類型不匹配. 這種表達式的值是 void.

-fthis-is-variable

允許對 this 賦值 (僅對 C++). 合並使用者自定義的自由存儲管理機制到 C++ 後, 使可賦值的 `this' 顯得不合時宜. 因此, 預設情況下, 類成員函數內部對 this 賦值是無效操作. 然而為了向後相容, 你可以通過 `-fthis-is-variable' 選項使這種操作有效.

-funsigned-char

把 char 定義為無符號類型, 如同 unsigned char.

各種機器都有自己預設的 char 類型. 既可能是 unsigned char 也可能是 signed char .

理想情況下, 當依賴於數據的符號性時, 一個可移植程式總是應該使用 signed char 或 unsigned char. 但是許多程式已經寫成只用簡單的 char, 並且期待這是有符號數 (或者無符號數, 具體情況取決於編寫程式的目標機器). 這個選項, 和它的反義選項, 使那樣的程式工作在對應的預設值上.

char 的類型始終應該明確定義為 signed char 或 unsigned char, 即使它表現的和其中之一完全一樣.

-fsigned-char

把 char 定義為有符號類型, 如同 signed char.

這個選項等同於 `-fno-unsigned-char', 他是 the negative form of `-funsigned-char' 的相反選項. 同樣, `-fno-signed-char' 等價於 `-funsigned-char'.

-fsigned-bitfields

-funsigned-bitfields

-fno-signed-bitfields

-fno-unsigned-bitfields

如果沒有明確聲明 `signed' 或 `unsigned' 修飾符, 這些選項用來定義有符號位域 (bitfield) 或無符號位域. 預設情況下, 位域是有符號的, 因為他們繼承的基本整數類型, 如 int, 是有符號數.

然而, 如果指定了 `-traditional' 選項, 位域永遠是無符號數.

-fwritable-strings

把字符串常量存儲到可寫數據段, 而且不做特別對待. 這是為了相容一些老程式, 他們假設字符串常量是可寫的. `-traditional' 選項也有相同效果.

篡改字符串常量是一個非常糟糕的想法; ``常量'' 就應該是常量.

預處理器選項 (Preprocessor Option)

下列選項針對 C 預處理器, 預處理器用在正式編譯以前, 對 C 源檔案進行某種處理.

如果指定了 `-E' 選項, GCC 只進行預處理工作. 下面的某些選項必須和 `-E' 選項一起才有意義, 因為他們的輸出結果不能用於編譯.

-include file

在處理常規輸入檔案之前, 首先處理檔案 file, 其結果是, 檔案 file 的內容先得到編譯. 命令行上任何 `-D' 和 `-U' 選項永遠在 `-include file' 之前處理, 無論他們在命令行上的順序如何. 然而 `-include' 和 `-imacros' 選項按書寫順序處理.

-imacros file

在處理常規輸入檔案之前, 首先處理檔案 file, 但是忽略輸出結果. 由於丟棄了檔案 file 的輸出內容, `-imacros file' 選項的唯一效果就是使檔案 file 中的宏定義生效, 可以用於其他輸入檔案. 在處理 `-imacrosfile' 選項之前, 預處理器首先處理 `-D' 和 `-U' 選項, 並不在乎他們在命令行上的順序. 然而 `-include' 和 `-imacros' 選項按書寫順序處理.

-idirafter dir

把目錄 dir 添加到第二包含路徑中. 如果某個頭檔案在主包含路徑 (用`-I' 添加的路徑) 中沒有找到, 預處理器就搜索第二包含路徑.

-iprefix prefix

指定 prefix 作為後續 `-iwithprefix' 選項的前綴.

-iwithprefix dir

把目錄添加到第二包含路徑中. 目錄名由 prefix 和 dir 合並而成, 這裏 prefix 被先前的 `-iprefix' 選項指定.

-nostdinc

不要在標準系統目錄中尋找頭檔案. 只搜索 `-I' 選項指定的目錄 (以及當前目錄, 如果合適).

結合使用 `-nostdinc' 和 `-I-' 選項, 你可以把包含檔案搜索限制在顯式指定的目錄.

-nostdinc++

不要在 C++ 專用標準目錄中尋找頭檔案, 但是仍然搜索其他標準目錄. (當建立 `libg++' 時使用這個選項.)

-undef

不要預定義任何非標準宏. (包括系統結構標誌).

-E

僅運行 C 預處理器. 預處理所有指定的 C 源檔案, 結果送往標準輸出或指定的輸出檔案.

-C

告訴預處理器不要丟棄注釋. 配合 `-E' 選項使用.

-P

告訴預處理器不要產生 `#line' 命令. 配合 `-E' 選項使用.

-M [ -MG ]

告訴預處理器輸出一個適合 make 的規則, 用於描述各目標檔案的依賴關系. 對於每個源檔案, 預處理器輸出一個 make 規則, 該規則的目標項 (target) 是源檔案對應的目標檔名, 依賴項 (dependency) 是源檔案中 `#include 引用的所有檔案. 生成的規則可以是單行, 但如果太長, 就用 `\'-換行符續成多行. 規則顯示在標準輸出, 不產生預處理過的 C 程式.

`-M' 隱含了 `-E' 選項.

`-MG' 要求把缺失的頭檔案按存在對待, 並且假定他們和源程式檔案在同一目錄下. 必須和 `-M' 選項一起用.

-MM [ -MG ]

和 `-M' 選項類似, 但是輸出結果僅涉及使用者頭檔案, 像這樣 `#include file"'. 忽略系統頭檔案如 `#include <file>'.

-MD

和 `-M' 選項類似, 但是把依賴信息輸出在檔案中, 檔名通過把輸出檔名末尾的 `.o' 替換為 `.d' 產生. 同時繼續指定的編譯工作 ---`-MD' 不像 `-M' 那樣阻止正常的編譯任務.

Mach 的實用工具 `md' 能夠合並 `.d' 檔案, 產生適用於 `make' 命令的單一的依賴檔案.

-MMD

和 `-MD' 選項類似, 但是輸出結果僅涉及使用者頭檔案, 忽略系統頭檔案.

-H

除了其他普通的操作, GCC 顯示引用過的頭檔案名.

-Aquestion(answer)

如果預處理器做條件測試, 如 `#if #question(answer)', 該選項可以斷言 (Assert) question 的答案是 answer. -A-' 關閉一般用於描述目標機的標準斷言.

-Dmacro

定義宏 macro, 宏的內容定義為字符串 `1'.

-Dmacro=defn

定義宏 macro 的內容為 defn. 命令行上所有的 `-D' 選項在 `-U' 選項之前處理.

-Umacro

取消宏 macro. `-U' 選項在所有的 `-D' 選項之後處理, 但是優先於任何 `-include' 或 `-imacros' 選項.

-dM

告訴預處理器輸出有效的宏定義列表 (預處理結束時仍然有效的宏定義). 該選項需結合 `-E' 選項使用.

-dD

告訴預處理器把所有的宏定義傳遞到輸出端, 按照出現的順序顯示.

-dN

和 `-dD'選項類似, 但是忽略宏的參量或內容. 只在輸出端顯示 `#define name.

匯編器選項 (ASSEMBLER OPTION)

-Wa,option: 把選項 option 傳遞給匯編器. 如果 option 含有逗號, 就在逗號處分割成多個選項.

連接器選項 (LINKER OPTION)

下面的選項用於編譯器連接目標檔案, 輸出可執行檔案的時候. 如果編譯器不進行連接, 他們就毫無意義.

object-file-name

如果某些檔案沒有特別明確的延伸檔名 a special recognized suffix, GCC 就認為他們是目標檔案或庫檔案. (根據檔案內容, 連接器能夠區分目標檔案和庫檔案). 如果 GCC 執行連接操作, 這些目標檔案將成為連接器的輸入檔案.

-llibrary

連接名為 library 的庫檔案.

連接器在標準搜索目錄中尋找這個庫檔案, 庫檔案的真正名字是 `liblibrary.a'. 連接器會當做檔名得到準確說明一樣引用這個檔案.

搜索目錄除了一些系統標準目錄外, 還包括使用者以 `-L' 選項指定的路徑.

一般說來用這個方法找到的檔案是庫檔案---即由目標檔案組成的歸檔檔案 (archive file). 連接器處理歸檔檔案的方法是: 掃描歸檔檔案, 尋找某些成員, 這些成員的符號目前已被引用, 不過還沒有被定義. 但是, 如果連接器找到普通的目標檔案, 而不是庫檔案, 就把這個目標檔案按平常方式連接進來. 指定 `-l' 選項和指定檔名的唯一區別是, `-l選項用 `lib' 和 `.a' 把 library 包裹起來, 而且搜索一些目錄.

-lobjc

這個 -l 選項的特殊形式用於連接 Objective C 程式.

-nostartfiles

不連接系統標準啟動檔案, 而標準庫檔案仍然正常使用.

-nostdlib

不連接系統標準啟動檔案和標準庫檔案. 只把指定的檔案傳遞給連接器.

-static

在支持動態連接 (dynamic linking) 的系統上, 阻止連接共享庫. 該選項在其他系統上無效.

-shared

生成一個共享目標檔案, 他可以和其他目標檔案連接產生可執行檔案. 只有部分系統支持該選項.

-symbolic

建立共享目標檔案的時候, 把引用綁定到全局符號上. 對所有無法解析的引用作出警告 (除非用連接編輯選項 `-Xlinker -z -Xlinker defs' 取代). 只有部分系統支持該選項.

-Xlinker option

把選項 option 傳遞給連接器. 可以用他傳遞系統特定的連接選項, GNU CC 無法識別這些選項.

如果需要傳遞攜帶參數的選項, 你必須使用兩次 `-Xlinker', 一次傳遞選項, 另一次傳遞他的參數. 例如, 如果傳遞 `-assert definitions', 你必須寫成 `-Xlinker -assert -Xlinker definitions', 而不能寫成 `-Xlinker "-assert definitions"', 因為這樣會把整個字符串當做一個參數傳遞, 顯然這不是連接器期待的.

-Wl,option

把選項 option 傳遞給連接器. 如果 option 中含有逗號, 就在逗號處分割成多個選項.

-u symbol

使連接器認為取消了 symbol 的符號定義, 從而連接庫模塊以取得定義. 你可以使用多個 `-u' 選項, 各自跟上不同的符號, 使得連接器調入附加的庫模塊.

目錄選項 (DIRECTORY OPTION)

下列選項指定搜索路徑, 用於查找頭檔案, 庫檔案, 或編譯器的某些成員:

-Idir

在頭檔案的搜索路徑列表中添加 dir 目錄.

-I-

任何在 `-I-' 前面用 `-I' 選項指定的搜索路徑只適用於 `#include "file"' 這種情況; 他們不能用來搜索 `#include <file>' 包含的頭檔案.

如果用 `-I' 選項指定的搜索路徑位於 `-I-' 選項後面, 就可以在這些路徑中搜索所有的 `#include' 指令. (一般說來 -I 選項就是這麼用的.)

還有, `-I-' 選項能夠阻止當前目錄 (存放當前輸入檔案的地方) 成為搜索 `#include "file"' 的第一選擇. 沒有辦法克服 `-I-' 選項的這個效應. 你可以指定 `-I.' 搜索那個目錄, 它在調用編譯器時是當前目錄. 這和預處理器的預設行為不完全一樣, 但是結果通常令人滿意.

`-I-' 不影響使用系統標準目錄, 因此, `-I-' 和 `-nostdinc' 是不同的選項.

-Ldir

在 `-l' 選項的搜索路徑列表中添加 dir 目錄.

-Bprefix

這個選項指出在何處尋找可執行檔案, 庫檔案, 以及編譯器自己的數據檔案.

編譯器驅動程式需要執行某些下面的子程式: `cpp', `cc1' (或 C++ 的 `cc1plus'), `as' 和 `ld'. 他把 prefix 當作欲執行的程式的前綴, 既可以包括也可以不包括 `machine/version/'.

對於要運行的子程式, 編譯器驅動程式首先試著加上 `-B' 前綴 (如果存在). 如果沒有找到檔案, 或沒有指定 `-B' 選項, 編譯器接著會試驗兩個標準前綴 `/usr/lib/gcc/' 和 `/usr/local/lib/gcc-lib/'. 如果仍然沒能夠找到所需檔案, 編譯器就在 `PATH' 環境變量指定的路徑中尋找沒加任何前綴的檔名.

如果有需要, 運行時 (run-time) 支持檔案 `libgcc.a' 也在 `-B' 前綴的搜索范圍之內. 如果這裏沒有找到, 就在上面提到的兩個標準前綴中尋找, 僅此而已. 如果上述方法沒有找到這個檔案, 就不連接他了. 多數情況的多數機器上, `libgcc.a' 並非必不可少.

你可以通過環境變量 GCC_EXEC_PREFIX 獲得近似的效果; 如果定義了這個變量, 其值就和上面說的一樣用做前綴. 如果同時指定了 `-B' 選項和GCC_EXEC_PREFIX 變量, 編譯器首先使用 `-B' 選項, 然後才嘗試環境變量值.

警告選項 (WARNING OPTION)

警告是針對程式結構的診斷信息, 程式不一定有錯誤, 而是存在風險, 或者可能存在錯誤.

下列選項控制 GNU CC 產生的警告的數量和類型:

-fsyntax-only

檢查程式中的語法錯誤, 但是不產生輸出信息.

-w

禁止所有警告訊息.

-Wno-import

禁止所有關於 #import 的警告訊息.

-pedantic

打開完全服從 ANSI C 標準所需的全部警告診斷; 拒絕接受採用了被禁止的語法擴展的程式.

無論有沒有這個選項, 符合 ANSI C 標準的程式應該能夠被正確編譯 (雖然極少數程式需要 `-ansi' 選項). 然而, 如果沒有這個選項, 某些 GNU 擴展和傳統 C 特性也得到支持. 使用這個選項可以拒絕這些程式. 沒有理由使用這個選項, 他存在只是為了滿足一些書呆子 (pedant).

對於替選關鍵字 (他們以 `__' 開始和結束) `-pedantic' 不會產生警告訊息. Pedantic 也不警告跟在 __extension__ 後面的表達式. 不過只應該在系統頭檔案中使用這種轉義措施, 應用程式最好避免.

-pedantic-errors

該選項和 `-pedantic' 類似, 但是顯示錯誤而不是警告.

-W

對下列事件顯示額外的警告訊息:

*

非易變自動變量 (nonvolatile automatic variable) 可能在調用 longjmp 時發生改變. 這些警告僅在優化編譯時發生.

編譯器只知道對 setjmp 的調用, 他不可能知道會在哪裏調用 longjmp, 事實上一個信號處理例程可以在程式的任何地點調用他. 其結果是, 即使程式沒有問題, 你也可能會得到警告, 因為無法在可能出現問題的地方調用 longjmp.

*

既可以返回值, 也可以不返回值的函數. (缺少結尾的函數體被看作不返回函數值) 例如, 下面的函數將導致這種警告:

foo (a)
{
  if (a > 0)
    return a;
}

由於 GNU CC 不知道某些函數永不返回 (含有 abort 和 longjmp), 因此有可能出現虛假警告.

*

表達式語句或逗號表達式的左側沒有產生作用 (side effect). 如果要防止這種警告, 應該把未使用的表達式強制轉換為 void 類型. 例如, 這樣的表達式 `x[i,j]' 會導致警告, 而 `x[(void)i,j]' 就不會.

*

無符號數用 `>' 或 `<=' 和零做比較.

-Wimplicit-int

警告沒有指定類型的聲明.

-Wimplicit-function-declaration

警告在聲明之前就使用的函數.

-Wimplicit

同 -Wimplicit-int 和 -Wimplicit-function-declaration.

-Wmain

如果把 main 函數聲明或定義成奇怪的類型, 編譯器就發出警告. 典型情況下, 這個函數用於外部連接, 返回 int 數值, 不需要參數, 或指定兩個參數.

-Wreturn-type

如果函數定義了返回類型, 而預設類型是 int 型, 編譯器就發出警告. 同時警告那些不帶返回值的 return 語句, 如果他們所屬的函數並非 void 類型.

-Wunused

如果某個局部變量除了聲明就沒再使用, 或者聲明了靜態函數但是沒有定義, 或者某條語句的運算結果顯然沒有使用, 編譯器就發出警告.

-Wswitch

如果某條 switch 語句的參數屬於枚舉類型, 但是沒有對應的 case 語句使用枚舉元素, 編譯器就發出警告. ( default 語句的出現能夠防止這個警告.) 超出枚舉范圍的 case 語句同樣會導致這個警告.

-Wcomment

如果注釋起始序列 `/*' 出現在注釋中, 編譯器就發出警告.

-Wtrigraphs

警告任何出現的 trigraph (假設允許使用他們).

-Wformat

檢查對 printf 和 scanf 等函數的調用, 確認各個參數類型和格式串中的一致.

-Wchar-subscripts

警告類型是 char 的數組下標. 這是常見錯誤, 程式員經常忘記在某些機器上 char 有符號.

-Wuninitialized

在初始化之前就使用自動變量.

這些警告只可能做優化編譯時出現, 因為他們需要數據流信息, 只有做優化的時候才估算數據流信息. 如果不指定 `-O' 選項, 就不會出現這些警告.

這些警告僅針對等候分配寄存器的變量. 因此不會發生在聲明為 volatile 的變量上面, 不會發生在已經取得地址的變量, 或長度不等於 1, 2, 4, 8 字節的變量. 同樣也不會發生在結構, 聯合或數組上面, 即使他們在寄存器中.

注意, 如果某個變量只計算了一個從未使用過的值, 這裏可能不會警告. 因為在顯示警告之前, 這樣的計算已經被數據流分析刪除了.

這些警告作為可選項是因為 GNU CC 還沒有智能到判別所有的情況, 知道有些看上去錯誤的代碼其實是正確的. 下面是一個這樣的例子:

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

如果 y 始終是 1, 2 或 3, 那麼 x 總會被初始化, 但是 GNU CC 不知道這一點. 下面是另一個普遍案例:

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

這裏沒有錯誤, 因為只有設置了 save_y 才使用他.

把所有不返回的函數定義為 volatile 可以避免某些似是而非的警告.

-Wparentheses

在某些情況下如果忽略了括號, 編譯器就發出警告.

-Wtemplate-debugging

當在 C++ 程式中使用 template 的時候, 如果調試 (debugging) 沒有完全生效, 編譯器就發出警告. (僅用於 C++).

-Wall

結合所有上述的 `-W' 選項. 通常我們建議避免這些被警告的用法，我們相信, 恰當結合宏的使用能夠輕易避免這些用法。

剩下的 `-W...' 選項不包括在 `-Wall' 中, 因為我們認為在必要情況下, 這些被編譯器警告的程式結構, 可以合理的用在 "幹凈的" 程式中.

-Wtraditional: 如果某些程式結構在傳統 C 中的表現和 ANSI C 不同, 編譯器就發出警告.
*: 宏參出現在宏體的字符串常量內部. 傳統 C 會替換宏參, 而 ANSI C 則視其為常量的一部分.
*: 某個函數在塊(block) 中聲明為外部, 但在塊結束後才調用.
*: switch 語句的操作數類型是 long.

-Wshadow: 一旦某個局部變量屏蔽了另一個局部變量, 編譯器就發出警告.
-Wid-clash-len: 一旦兩個確定的標識符具有相同的前 len 個字符, 編譯器就發出警告. 他可以協助你開發一些將要在某些過時的, 危害大腦的編譯器上編譯的程式.
-Wpointer-arith: 任何語句如果依賴於函數類型的大小(size) 或者 void 類型的大小, 編譯器就發出警告. GNU C 為了便於計算 void * 指針和函數指針, 就把這些類型的大小定義為 1.
-Wcast-qual: 一旦某個指針強制類型轉換以便移除類型修飾符時, 編譯器就發出警告. 例如, 如果把 const char * 強制轉換為普通的 char * 時, 警告就會出現.
-Wcast-align: 一旦某個指針類型強制轉換時, 導致目標所需的地址對齊 (alignment) 增加, 編譯器就發出警告. 例如, 某些機器上只能在 2 或 4 字節邊界上訪問整數, 如果在這種機型上把 char * 強制轉換成 int * 類型, 編譯器就發出警告.
-Wwrite-strings: 規定字符串常量的類型是 const char[length], 因此, 把這樣的地址複製給 non-const char * 指針將產生警告. 這些警告能夠幫助你在編譯期間發現企圖寫入字符串常量的代碼, 但是你必須非常仔細的在聲明和原形中使用 const, 否則他們只能帶來麻煩; 所以我們沒有讓 `-Wall' 提供這些警告.
-Wconversion: 如果某函數原形導致的類型轉換和無函數原形時的類型轉換不同, 編譯器就發出警告. 這裏包括定點數和浮點數的互相轉換, 改變定點數的寬度或符號, 除非他們和預設聲明 (default promotion) 相同.
-Waggregate-return: 如果定義或調用了返回結構或聯合的函數, 編譯器就發出警告. (從語言角度你可以返回一個數組, 然而同樣會導致警告.)
-Wstrict-prototypes: 如果函數的聲明或定義沒有指出參數類型, 編譯器就發出警告. (如果函數的前向引用說明指出了參數類型, 則允許後面使用舊式風格的函數定義, 而不會產生警告.)
-Wmissing-prototypes: 如果沒有預先聲明函數原形就定義了全局函數, 編譯器就發出警告. 即使函數定義自身提供了函數原形也會產生這個警告. 他的目的是檢查沒有在頭檔案中聲明的全局函數.
-Wmissing-declarations: 如果沒有預先聲明就定義了全局函數, 編譯器就發出警告. 即使函數定義自身提供了函數原形也會產生這個警告. 這個選項的目的是檢查沒有在頭檔案中聲明的全局函數.
-Wredundant-decls: 如果在同一個可見域某定義多次聲明, 編譯器就發出警告, 即使這些重復聲明有效並且毫無差別.
-Wnested-externs: 如果某 extern 聲明出現在函數內部, 編譯器就發出警告.
-Wenum-clash: 對於不同枚舉類型之間的轉換發出警告 (僅適用於 C++).
-Wlong-long: 如果使用了 long long 類型就發出警告. 該警告是預設項. 使用 `-Wno-long-long' 選項能夠防止這個警告. `-Wlong-long' 和 `-Wno-long-long' 僅在 `-pedantic' 之下才起作用.
-Woverloaded-virtual: (僅適用於 C++.) 在繼承類中, 虛函數的定義必須匹配虛函數在基類中聲明的類型特征 (type signature). 當繼承類聲明了某個函數, 它可能是個錯誤的嘗試企圖定義一個虛函數, 使用這個選項能夠產生警告: 就是說, 當某個函數和基類中的虛函數同名, 但是類型特征不符合基類的任何虛函數, 編譯器將發出警告.
-Winline: 如果某函數不能內嵌(inline), 無論是聲明為 inline 或者是指定了 -finline-functions 選項, 編譯器都將發出警告.
-Werror: 視警告為錯誤; 出現任何警告即放棄編譯.

調試選項 (DEBUGGING OPTION)

GNU CC 擁有許多特別選項, 既可以調試使用者的程式, 也可以對 GCC 排錯:

-g

以作業系統的本地格式 (stabs, COFF, XCOFF, 或 DWARF). 產生調試信息. GDB 能夠使用這些調試信息.

在大多數使用 stabs 格式的系統上, `-g' 選項啟動只有 GDB 才使用的額外調試信息; 這些信息使 GDB 調試效果更好, 但是有可能導致其他調試器崩潰, 或拒絕讀入程式. 如果你確定要控制是否生成額外的信息, 使用`-gstabs+', `-gstabs', `-gxcoff+', `-gxcoff', `-gdwarf+', 或 `-gdwarf' (見下文).

和大多數 C 編譯器不同, GNU CC 允許結合使用 `-g' 和 `-O' 選項. 優化的代碼偶爾製造一些驚異的結果: 某些聲明過的變量根本不存在; 控制流程直接跑到沒有預料到的地方; 某些語句因為計算結果是常量或已經確定而沒有執行; 某些語句在其他地方執行, 因為他們被移到循環外面了.

然而它証明了調試優化的輸出是可能的. 對可能含有錯誤的程式使用優化器是合理的.

如果 GNU CC 支持輸出多種調試信息, 下面的選項則非常有用.

-ggdb: 以本地格式 (如果支持) 輸出調試信息, 盡可能包括 GDB 擴展.
-gstabs: 以 stabs 格式 (如果支持) 輸出調試信息, 不包括 GDB 擴展. 這是大多數 BSD 系統上 DBX 使用的格式.
-gstabs+: 以 stabs 格式 (如果支持) 輸出調試信息, 使用只有 GNU 調試器 (GDB) 理解的 GNU 擴展. 使用這些擴展有可能導致其他調試器崩潰或拒絕讀入程式.
-gcoff: 以 COFF 格式 (如果支持) 輸出調試信息. 這是在 System V 第四版以前的大多數 System V 系統上 SDB 使用的格式.
-gxcoff: 以 XCOFF 格式 (如果支持) 輸出調試信息. 這是 IBM RS/6000 系統上 DBX 調試器使用的格式.
-gxcoff+: 以 XCOFF 格式 (如果支持) 輸出調試信息, 使用只有 GNU 調試器 (GDB) 理解的 GNU 擴展. 使用這些擴展有可能導致其他調試器崩潰或拒絕讀入程式.
-gdwarf: 以 DWARF 格式 (如果支持) 輸出調試信息. 這是大多數 System V 第四版系統上 SDB 使用的格式.
-gdwarf+: 以 DWARF 格式 (如果支持) 輸出調試信息, 使用只有 GNU 調試器 (GDB) 理解的 GNU 擴展. 使用這些擴展有可能導致其他調試器崩潰或拒絕讀入程式.

-glevel
-ggdblevel
-gstabslevel
-gcofflevel -gxcofflevel

-gdwarflevel

請求生成調試信息, 同時用 level 指出需要多少信息. 預設的 level 值是 2.

Level 1 輸出最少量的信息, 僅夠在不打算調試的程式段內 backtrace. 包括函數和外部變量的描述, 但是沒有局部變量和行號信息.

Level 3 包含更多的信息, 如程式中出現的所有宏定義. 當使用 `-g3' 選項的時候, 某些調試器支持宏擴展.

-p

產生額外代碼, 用於輸出 profile 信息, 供分析程式 prof 使用.

-pg

產生額外代碼, 用於輸出 profile 信息, 供分析程式 gprof 使用.

-a

產生額外代碼, 用於輸出基本塊 (basic block) 的 profile 信息, 它記錄各個基本塊的執行次數, 供諸如 tcov 此類的程式分析. 但是注意, 這個數據格式並非 tcov 期待的. 最終 GNU gprof 將處理這些數據.

-ax

產生額外代碼, 用於從 'bb.in' 檔案讀取基本塊的 profile 參數, 把 profile 的結果寫到 'bb.out' 檔案. `bb.in' 包含一張函數列表. 一旦進入列表中的某個函數, profile 操作就開始, 離開最外層的函數後, profile 操作就結束. 以 `-' 為前綴名的函數排除在 profile 操作之外. 如果函數名不是唯一的, 它可以寫成 `/path/filename.d:functionname' 來澄清. `bb.out' 將列出一些有效的檔名. 這四個函數名具有特殊含義: `__bb_jumps__' 導致跳轉 (jump) 頻率寫進 `bb.out'. `__bb_trace__' 導致基本塊序列通過管道傳到 `gzip', 輸出 `bbtrace.gz' 檔案. `__bb_hidecall__' 導致從跟蹤 (trace) 中排除 call 指令. `__bb_showret__' 導致在跟蹤中包括返回指令.

-dletters

編譯的時候, 在 letters 指定的時刻做調試轉儲 (dump). 用於調試編譯器. 大多數轉儲的檔名通過源檔名添加字詞獲得 (例如 `foo.c.rtl' 或 `foo.c.jump').

-dM

預處理結束的時候轉儲所有的宏定義, 不輸出到檔案.

-dN

預處理結束的時候轉儲所有的宏名.

-dD

預處理結束的時候轉儲所有的宏定義, 同時進行正常輸出.

-dy

語法分析 (parse) 的時候在標準錯誤轉儲調試信息.

-dr

RTL 階段後轉儲到 `file.rtl'.

-dx

僅對函數生成 RTL, 而不是編譯. 通常和 `r' 聯用.

-dj

第一次跳轉優化後轉儲到 `file.jump'.

-ds

CSE (包括有時候跟在 CSE 後面的跳轉優化) 後轉儲到 `file.cse'.

-dL

循環優化後轉儲到 `file.loop'.

-dt

第二次 CSE 處理 (包括有時候跟在 CSE 後面的跳轉優化) 後轉儲到 `file.cse2'.

-df

流程分析 (flow analysis) 後轉儲到 `file.flow'.

-dc

指令組合 (instruction combination) 後轉儲到 `file.combine'.

-dS

第一次指令安排 (instruction schedule) 後轉儲到 `file.sched'.

-dl

局部寄存器分配後轉儲到 `file.lreg'.

-dg

全局寄存器分配後轉儲到 `file.greg'.

-dR

第二次指令安排 (instruction schedule) 後轉儲到 `file.sched2'.

-dJ

最後一次跳轉優化後轉儲到 `file.jump2'.

-dd

推遲分支調度 (delayed branch scheduling) 後轉儲到 `file.dbr'.

-dk

寄存器-堆棧轉換後轉儲到 `file.stack'.

-da

產生以上所有的轉儲.

-dm

運行結束後, 在標準錯誤顯示記憶體使用統計.

-dp

在匯編輸出加注指明使用了哪些模式 (pattern) 及其替代模式.

-fpretend-float

交叉編譯的時候, 假定目標機和宿主機使用同樣的浮點格式. 它導致輸出錯誤的浮點常數, 但是在目標機上運行的時候, 真實的指令序列有可能和 GNU CC 希望的一樣.

-save-temps

保存那些通常是 ``臨時'' 的中間檔案; 置於當前目錄下, 並且根據源檔案命名. 因此, 用 `-c -save-temps' 選項編譯 `foo.c ' 會生成 ` foo.cpp' 和 `foo.s' 以及 `foo.o' 檔案.

-print-file-name=library

顯示庫檔案 library 的全路徑名, 連接時會使用這個庫 --- 其他什麼事情都不作. 根據這個選項, GNU CC 既不編譯, 也不連接, 僅僅顯示檔名.

-print-libgcc-file-name

和 `-print-file-name=libgcc.a' 一樣.

-print-prog-name=program

類似於 `-print-file-name', 但是查找程式 program 如 `cpp'.

優化選項 (OPTIMIZATION OPTION)

這些選項控制多種優化措施:

-O

-O1

優化. 對於大函數, 優化編譯佔用稍微多的時間和相當大的記憶體.

不使用 `-O' 選項時, 編譯器的目標是減少編譯的開銷, 使編譯結果能夠調試. 語句是獨立的: 如果在兩條語句之間用斷點中止程式, 你可以對任何變量重新賦值, 或者在函數體內把程式計數器指到其他語句, 以及從源程式中精確地獲取你期待的結果.

不使用 `-O' 選項時, 只有聲明了 register 的變量才分配使用寄存器. 編譯結果比不用 `-O' 選項的 PCC 要略遜一籌.

使用了 `-O' 選項, 編譯器會試圖減少目標碼的大小和執行時間.

如果指定了 `-O' 選項, `-fthread-jumps' 和 `-fdefer-pop' 選項將被打開. 在有 delay slot 的機器上, `-fdelayed-branch' 選項將被打開. 在即使沒有幀指針 (frame pointer) 也支持調試的機器上, `-fomit-frame-pointer' 選項將被打開. 某些機器上還可能會打開其他選項.

-O2

多優化一些. 除了涉及空間和速度交換的優化選項, 執行幾乎所有的優化工作. 例如不進行循環展開 (loop unrolling) 和函數內嵌 (inlining). 和 -O 選項比較, 這個選項既增加了編譯時間, 也提高了生成代碼的運行效果.

-O3

優化的更多. 除了打開 -O2 所做的一切, 它還打開了 -finline-functions 選項.

-O0

不優化.

如果指定了多個 -O 選項, 不管帶不帶數字, 最後一個選項才是生效的選項.

諸如 `-fflag' 此類的選項描述一些機器無關的開關. 大多數開關具有肯定和否定兩種格式; `-ffoo' 開關選項的否定格式應該是 `-fno-foo'. 下面的列表只展示了一種格式 --- 那個不是預設選項的格式. 你可以通過去掉或添加 `no-' 構造出另一種格式.

-ffloat-store

不要在寄存器中存放浮點變量. 這樣可以防止某些機器上不希望的過高精度, 如 68000 的浮點寄存器 (來自 68881) 保存的精度超過了 double 應該具有的精度.

對於大多數程式, 過高精度只有好處. 但是有些程式嚴格依賴於 IEEE 浮點數的定義. 對這樣的程式可以使用 `-ffloat-store' 選項.

-fmemoize-lookups

-fsave-memoized

使用探索法 (heuristic) 進行更快的編譯 (僅對 C++). 預設情況下不使用探索法. 由於探索法只對某些輸入檔案有效, 其他程式的編譯速度會變得更慢.

第一次編譯器必須對成員函數 (或對成員數據的引用) 建立一個調用. 它必須 (1) 判斷出這個類是否實現了那個名字的成員函數; (2) 決定調用哪個成員函數 (涉及到推測需要做哪種類型轉換); (3) 檢查成員函數對調用者是否可見. 所有這些構成更慢的編譯. 一般情形, 第二次對成員函數 (或對成員數據的引用) 建立的調用, 必須再次經過相同長度的處理. 這意味著像這樣的代碼

cout << "This " << p << " has " << n << " legs.\n";

對整個三步驟要做六次遍歷. 通過使用軟體緩存, ``命中'' 能夠顯著地減少這種代價. 然而不幸的是, 使用這種緩存必須實現其他機制, 帶來了它自己的開銷. `-fmemoize-lookups' 選項打開軟體緩存.

因為函數的正文環境不同, 函數對成員和成員函數的訪問權 (可見性) 也可能不同, g++ 可能需要刷新緩存. 使用 `-fmemoize-lookups' 選項, 每編譯完一個函數就刷新緩存. 而 `-fsave-memoized' 選項也啟用同樣的緩存, 但是當編譯器發覺最後編譯的函數的正文環境產生的訪問權和下一個待編譯的函數相同, 編譯器就保留緩存內容. 這對某個類定義許多成員函數時非常有用: 除了某些其他類的友函數, 每個成員函數擁有和其他成員函數完全一樣的訪問權, 因而無需刷新緩存.

-fno-default-inline

預設為不要把成員函數內嵌, 因為它們定義在類的作用域內 (僅C++).

-fno-defer-pop

一旦函數返回, 參數就立即彈出. 對於那些調用函數後必須彈出參數的機器, 編譯器一般情況下讓幾次函數調用的參數堆積在棧上, 然後一次全部彈出.

-fforce-mem

做數學運算前把將要使用的記憶體操作數送入寄存器. 通過把記憶體訪問轉換成潛在的公共子表達式, 它可能產生較好的目標碼. 如果它們不是公共子表達式, 指令組合應該消除各自的寄存器載荷. 我樂意傾聽不同意見.

-fforce-addr

做數學運算前把將要使用的記憶體地址常數送入寄存器. 它可能和 `-fforce-mem' 一樣產生較好的目標碼. 我樂意傾聽不同意見.

-fomit-frame-pointer

對於不需要幀指針 (frame pointer) 的函數, 不要在寄存器中保存幀指針. 這樣能夠避免保存, 設置和恢復幀指針的指令; 同時對許多函數提供一個額外的寄存器. 但是在大多數機器上將無法調試.

某些機器上, 如 Vax, 這個選項無效, 因為標準調用序列自動處理幀指針, 通過假裝不存在而不保存任何東西. 機器描述宏 FRAME_POINTER_REQUIRED 控制目標機是否支持這個選項.

-finline-functions

把所有簡單的函數集成進調用者. 編譯器探索式地決定哪些函數足夠簡單, 值得這種集成.

如果集成了所有給定函數的調用, 而且函數聲明為 static, 那麼一般說來 GCC 有權不按匯編代碼輸出函數.

-fcaller-saves

允許在寄存器裏分配數值, 但是這個方案通常受到各個函數調用的沖擊, 因此 GCC 生成額外的代碼, 在函數調用的前後保存和復原寄存器內容. 僅當生成代碼看上去優於反之結果時才實現這樣的分配.

某些機器上該選項預設為允許, 通常這些機器沒有調用保護寄存器代替使用.

-fkeep-inline-functions

即使集成了某個函數的所有調用, 而且該函數聲明為 static, 仍然輸出這個函數一個獨立的, 運行時可調用的版本.

-fno-function-cse

不要把函數地址存入寄存器; 讓調用固定函數的指令顯式給出函數地址.

這個選項產生效率較低的目標碼, 但是如果不用這個選項, 某些不尋常的 hack, 改變匯編器的輸出, 可能因優化而帶來困惑.

-fno-peephole

禁止任何機器相關的 peephole 優化.

-ffast-math

這個選項出於速度優化, 允許 GCC 違反某些 ANSI 或 IEEE 規則/規格. 例如, 它允許編譯器假設 sqrt 函數的參數是非負數.

這個選項不被任何 `-O' 選項打開, 因為對於嚴格依靠 IEEE 或 ANSI 規則/規格實現的數學函數, 程式可能會產生錯誤的結果.

下列選項控制特定的優化. `-O2' 選項打開下面的大多數優化項, 除了 `-funroll-loops' 和 `-funroll-all-loops' 項.

而 `-O' 選項通常打開 `-fthread-jumps' 和 `-fdelayed-branch' 優化項, 但是特定的機器上的預設優化項有可能改變.

如果特別情況下非常需要 ``微調'' 優化, 你可以使用下面的選項.

-fstrength-reduce

執行循環強度縮小 (loop strength reduction) 優化, 並且消除重復變量.

-fthread-jumps

執行優化的地點是, 如果某個跳轉分支的目的地存在另一個條件比較, 而且該條件比較包含在前一個比較語句之內, 那麼執行優化. 根據條件是 true 或者 false, 前面那條分支重定向到第二條分支的目的地或者緊跟在第二條分支後面.

-funroll-loops

執行循環展開 (loop unrolling) 優化. 僅對循環次數能夠在編譯時或運行時確定的循環實行.

-funroll-all-loops

執行循環展開 (loop unrolling) 優化. 對所有循環實行. 通常使程式運行的更慢.

-fcse-follow-jumps

在公共子表達式消元 (common subexpression elimination) 的時候, 如果沒有其他路徑到達某個跳轉的目的地, 就掃過這條 jump 指令. 例如, 如果 CSE 遇到帶有 else從句的 if 語句, 當條件測試為 false 時, CSE 就跟在 jump 後面.

-fcse-skip-blocks

它類似於 `-fcse-follow-jumps' 選項, 但是 CSE 跟在條件跳轉後面, 條件跳轉跳過了語句塊(block). 如果 CSE 遇到一條簡單的 if 語句, 不帶 else 從句, `-fcse-skip-blocks' 選項將導致 CSE 跟在 if 產生的跳轉後面.

-frerun-cse-after-loop

執行循環優化後, 重新進行公共子表達式消元.

-felide-constructors

如果看上去合理就省略構造子 (僅C++). 根據這個選項, 對於下面的代碼, GNU C++ 直接從調用 foo 初始化 y, 而無需通過臨時變量:

A foo (); A y = foo ();

如果沒有這個選項, GNU C++ 首先通過調用類型 A 合適的構造子初始化 y; 然後把 foo 的結果賦給臨時變量; 最後, 用臨時變量替換 `y' 的初始值.

ANSI C++ 標準草案規定了預設行為 (`-fno-elide-constructors'). 如果程式的構造子存在副效應, `-felide-constructors' 選項能夠使程式有不同的表現, 因為可能忽略一些構造子的調用.

-fexpensive-optimizations

執行一些相對開銷較大的次要優化.

-fdelayed-branch

如果對目標機支持這個功能, 它試圖重新排列指令, 以便利用延遲分支 (delayed branch) 指令後面的指令空隙.

-fschedule-insns

如果對目標機支持這個功能, 它試圖重新排列指令, 以便消除因數據未緒造成的執行停頓. 這可以幫助浮點運算或記憶體訪問較慢的機器調取指令, 允許其他指令先執行, 直到調取指令或浮點運算完成.

-fschedule-insns2

類似於 `-fschedule-insns' 選項, 但是在寄存器分配完成後, 需要一個額外的指令調度過程. 對於寄存器數目相對較少, 而且取記憶體指令大於一個周期的機器, 這個選項特別有用.

目標機選項 (TARGET OPTION)

預設情況下, GNU CC 編譯出本機類型的目標碼. 然而也可以把他安裝成交叉編譯器, 為其他機型編譯程式. 事實上, 針對不同的目標機, 可以同時安裝 GNU CC 相應的配置. 然後用 `-b' 選項指定目標機種.

順便提一下, 新版本和舊版本的 GNU CC 可以共存. 其中一個版本 (可能是最新的那個) 為預設版本, 但是有時候你希望使用其他版本.

-b machine

參數 machine 指出編譯的目標機種. 這個選項用於安裝為交叉編譯器的 GNU CC.

參數 machine 的值和配置 GNU CC 交叉編譯器時設置的機器類型一樣. 例如, 如果交叉編譯器配置有 `configure i386v', 意思是編譯 80386 上的 System V 目標碼, 那麼你可以通過 `-b i386v' 運行交叉編譯器.

如果沒有指定 `-b' 選項, 通常指編譯本機目標碼.

-V version

參數 version 指出運行哪個版本的 GNU CC. 這個選項用於安裝了多個版本的 GCC. 例如, 如果 version 是 `2.0', 意味著運行 GNU CC 2.0 版.

如果沒有指定 `-V' 選項, 預設版本取決於 GNU CC 的安裝方式, 一般說來推薦使用通用版本.

機器相關選項 (MACHINE DEPENDENT OPTION)

每一種目標機型都有自己的特別選項, 這些選項用 `-m ' 開關引導, 選擇不同的硬體型號或配置 --- 例如, 68010 還是 68020, 有沒有浮點協處理器. 通過指定選項, 安裝編譯器的一個版本能夠為所有的型號或配置進行編譯.

此外, 編譯器的某些配置支持附加的特殊選項, 通常是為了在命令行上相容這個平台的其他編譯器.

下面是針對 68000 系列定義的 `-m' 選項:

-m68000

-mc68000

輸出 68000 的目標碼. 如果編譯器按基於 68000 的系統配置, 這個選項就是預設選項.

-m68020

-mc68020

輸出 68020 的目標碼 (而不是 68000). 如果編譯器按基於 68020 的系統配置, 這個選項就是預設選項.

-m68881

輸出包含 68881 浮點指令的目標碼. 對於大多數基於 68020 的系統這是預設選項, 除非設置編譯器時指定了 -nfp .

-m68030

輸出 68030 的目標碼. 如果編譯器按基於 68030 的系統配置, 這個選項就是預設選項.

-m68040

輸出 68040 的目標碼. 如果編譯器按基於 68040 的系統配置, 這個選項就是預設選項.

-m68020-40

輸出 68040 的目標碼, 但是不使用新指令. 生成的代碼可以在 68020/68881 上, 也可以在 68030 或 68040 上較有效地運行.

-mfpa

輸出包含 SUN FPA 浮點指令的目標碼.

-msoft-float

輸出包含浮點庫調用的目標碼. 警告: 所需的庫不是 GNU CC 的組成部分. 一般說來 GCC 使用該機型本地 C 編譯器的相應部件, 但是作交叉編譯時卻不能直接使用. 你必須自己管理提供合適的函數庫用於交叉編譯.

-mshort

認為 int 類型是 16 位寬, 相當於 short int.

-mnobitfield

不使用位域 (bit-field) 指令. `-m68000' 隱含指定了 `-mnobitfield'.

-mbitfield

使用位域指令. `-m68020' 隱含指定了 `-mbitfield'. 如果你使用未改裝的 gcc, 這就是預設選項.

-mrtd

採用另一種函數調用約定, 函數接受固定數目的參數, 用 rtd 指令返回, 該指令返回時彈出棧內的參數. 這個方法能夠使調用者節省一條指令, 因為他這裏不需要彈出參數.

這種調用約定不相容 UNIX 的正常調用. 因此如果你需要調用 UNIX 編譯器編譯的庫函數, 你就不能使用這個選項.

此外, 所有參數數量可變地函數必須提供函數原型 (包括 printf); 否則編譯器會生成錯誤的調用代碼.

另外, 如果調用函數時攜帶了過多的參數, 編譯器將生成嚴重錯誤的代碼. (正常情況下, 多餘的參數被安全無害的忽略.)

68010 和 68020 處理器支持 rtd 指令, 但是 68000 不支持.

下面是針對 VAX 定義的 `-m' 選項:

-munix: 禁止輸出某些跳轉指令 (aobleq 等等), VAX 的 UNIX 匯編器無法跨越長范圍 (long ranges) 進行處理.
-mgnu: 如果使用 GNU 匯編器, 則輸出那些跳轉指令,
-mg: 輸出 g-format 浮點數, 取代 d-format.

下面是 SPARC 支持的 `-m' 選項開關:

-mfpu

-mhard-float: 輸出包含浮點指令的目標碼. 這是預設選項.

-mno-fpu

-msoft-float

輸出包含浮點庫調用的目標碼. 警告: 沒有為 SPARC 提供 GNU 浮點庫. 一般說來使用該機型本地 C 編譯器的相應部件, 但是不能直接用於交叉編譯. 你必須自己安排, 提供用於交叉編譯的庫函數.

-msoft-float 改變了輸出檔案中的調用約定; 因此只有用這個選項編譯整個程式才有意義.

-mno-epilogue

-mepilogue

使用 -mepilogue (預設) 選項時, 編譯器總是把函數的退出代碼放在函數的尾部. 任何在函數中間的退出語句 (例如 C 中的 return 語句) 將產生出跳轉指令指向函數尾部.

使用 -mno-epilogue 選項時, 編譯器盡量在每個函數退出點嵌入退出代碼.

-mno-v8

-mv8

-msparclite

這三個選項選擇不同種類的 SPARC 系統.

預設情況下 (除非特別為 Fujitsu SPARClite 配置), GCC 生成 SPARC v7 目標碼.

-mv8 生成 SPARC v8 目標碼. 他和 v7 目標碼唯一的區別是, 編譯器生成整數乘法和整數除法指令, SPARC v8 支持該指令, 而 v7 體系不支持.

-msparclite 生成 SPARClite 目標碼. 增加了 SPARClite 支持的整數乘法, 整數除法單步掃描 (integer divide step and scan (ffs)) 指令. v7 體系不支持這些指令.

-mcypress

-msupersparc

這兩個選項選擇處理器型號, 針對處理器進行代碼優化.

-mcypress 選項 (預設項) 使編譯器對 Cypress CY7C602 芯片優化代碼, SparcStation/SparcServer 3xx 系列使用這種芯片. 該選項也適用於老式的 SparcStation 1, 2, IPX 等機型..

-msupersparc 選項使編譯器對 SuperSparc 處理器優化代碼, SparcStation 10, 1000 和 2000 系列使用這種芯片. 同時該選項啟用完整的 SPARC v8 指令集.

下面是針對 Convex 定義的 `-m' 選項:

-mc1: 輸出 C1 的目標碼. 當編譯器對 C1 配置時, 這是預設選項.
-mc2: 輸出 C2 的目標碼. 當編譯器對 C2 配置時, 這是預設選項.
-margcount: 在每個參數列表的前面放置一個參數計數字 (argument count word). 某些不可移植的 Convex 和 Vax 程式需要這個參數計數字. (調試器不需要他, 除非函數帶有變長參數列表; 這個信息存放在符號表中.)
-mnoargcount: 忽略參數計數字. 如果你使用未改裝的 gcc, 這是預設選項.

下面是針對 AMD Am29000 定義的 `-m' 選項:

-mdw

生成的目標碼認為 DW 置位, 就是說, 字節和半字操作由硬體直接支持. 該選項是預設選項.

-mnodw

生成的目標碼認為 DW 沒有置位.

-mbw

生成的目標碼認為系統支持字節和半字寫操作. 該選項是預設選項.

-mnbw

生成的目標碼認為系統不支持字節和半字寫操作. 該選項隱含開啟了 `-mnodw' 選項.

-msmall

使用小記憶體模式, 小記憶體模式假設所有函數的地址位於某個 256 KB 段內, 或者所有函數的絕對地址小於 256K. 這樣就可以用 call 指令代替const, consth, calli 指令序列.

-mlarge

假設不能使用 call 指令; 這是預設選項.

-m29050

輸出 Am29050 的目標碼.

-m29000

輸出 Am29000 的目標碼. 這是預設選項.

-mkernel-registers

生成的目標碼引用 gr64-gr95 寄存器而不是 gr96-gr127 寄存器. 該選項可以用於編譯核心代碼, 核心需要一組全局寄存器, 這些全局寄存器和使用者模式使用的寄存器完全無關.

注意, 使用這個選項時, `-f' 選項中的寄存器名字必須是 normal, user-mode, names.

-muser-registers

使用普通全局寄存器集 gr96-gr127. 這是預設選項.

-mstack-check

在每次堆棧調整後插入一條 __msp_check 調用. 這個選項常用於核心代碼.

下面是針對 Motorola 88K 體系定義的 `-m' 選項:

-m88000: 生成的目標碼可以在 m88100 和 m88110 上正常工作.
-m88100: 生成的目標碼在 m88100 上工作的最好, 但也可以在 m88110 上運行.
-m88110: 生成的目標碼在 m88110 上工作的最好, 可能不能在 m88100 上運行.
-midentify-revision: 在匯編器的輸出端包含一條 ident 指令, 記錄源檔名, 編譯器名字和版本, 時標, 以及使用的編譯選項,
-mno-underscores: 在匯編器的輸出端, 符號名字前面不添加下劃線. 預設情況是在每個名字前面增加下劃線前綴.
-mno-check-zero-division
-mcheck-zero-division: 早期型號的 88K 系統在除零操作上存在問題, 特定情況下許多機器無法自陷. 使用這些選項可以避免包含 (或可以顯明包含) 附加的代碼, 這些代碼能夠檢查除零錯, 發送例外信號. GCC 所有 88K 的配置預設使用 `-mcheck-zero-division' 選項.
-mocs-debug-info
-mno-ocs-debug-info: 包含 (或忽略) 附加的調試信息 (關於每個棧架結構中寄存器的使用), 88Open Object Compatibility Standard, ``OCS'', 對此信息做了說明. GDB 不需要這些額外信息. DG/UX, SVr4, 和 Delta 88 SVr3.2 的預設配置是包含調試信息, 其他 88k 機型的預設配置是忽略這個信息.
-mocs-frame-position
-mno-ocs-frame-position: 強制 (或不要求) 把寄存器值存儲到棧架結構中的指定位置 (按 OCS 的說明). DG/UX, Delta88 SVr3.2 和 BCS 的預設配置使用 `-mocs-frame-position' 選項; 其他 88k 機型的預設配置是 `-mno-ocs-frame-position'.
-moptimize-arg-area
-mno-optimize-arg-area: 控制如何在堆棧結構中存儲函數參數. `-moptimize-arg-area' 節省空間, 但是有可能宕掉某些調試器 (不是 GDB). `-mno-optimize-arg-area' 証實比標準選項好. 預設情況下 GCC 不優化參數域.
-mshort-data-: num 通過和 r0 關聯, 產生較小的數據引用 (data reference), 這樣就可以用單指令調入一個數值 (而不是平常的雙指令). 使用者通過選項中的 num 控制改變哪種數據引用. 例如, 如果你指定了 `-mshort-data-512', 那麼受影響的數據引用是小於 512 字節的數據移動. -mshort-data-num選項對大於 64K 的 num 無效.

-mserialize-volatile

-mno-serialize-volatile

產生, 或不產生代碼來保証對易變記憶體訪問的結果一致.

對於常用的處理器子型號, GNU CC 始終預設保証這種一致性. 如何實現結果一致取決於處理器子型號.

m88100 處理器不對記憶體引用重新安排, 因此訪問結果始終一致. 如果使用了 `-m88100' 選項, GNU CC 不產生任何針對結果一致的特別指令.

m88110 處理器的記憶體引用順序並不始終符合指令請求的引用順序. 特別是某條讀取指令可能在先前的存儲指令之前執行. 多處理器環境下, 亂序訪問擾亂了易變記憶體訪問的結果一致. 因此當使用 `-m88000' 或 `-m88110' 選項時, GNU CC 在適當的時候產生特別的指令迫使執行順序正確.

這些用於保証一致性的額外代碼有可能影響程式的性能. 如果你確認能夠安全地放棄這種保証, 你可以使用 `-mno-serialize-volatile' 選項.

如果你使用 `-m88100' 選項, 但是需要在 m88110 處理器上運行時的結果一致, 你應該加上 `-mserialize-volatile' 選項.

-msvr4

-msvr3: 打開 (`-msvr4') 或關閉 (`-msvr3') 和 System V 第四版 (SVr4) 相關的編譯器擴展. 效果如下:
*: 輸出哪種匯編語法 (你可以使用 `-mversion-03.00' 選項單獨選擇).
*: `-msvr4' 使 C 預處理器識別 `#pragma weak' 指令
*: `-msvr4' 使 GCC 輸出額外的聲明指令(declaration directive), 用於 SVr4.

除了 SVr4 配置, `-msvr3' 是所有 m88K 配置的預設選項.

-mtrap-large-shift
-mhandle-large-shift: 包含一些指令, 用於檢測大於 31 位的位移 (bit-shift); 根據相應的選項, 對這樣的位移發出自陷 (trap) 或執行適當的處理代碼. 預設情況下, GCC 對大位移不做特別處理.
-muse-div-instruction: 很早以前的 88K 型號沒有 (div) 除法指令, 因此預設情況下 GCC 避免產生這條指令. 而這個選項告訴 GCC 該指令是安全的.
-mversion-03.00: 在 DG/UX 配置中存在兩種風格的 SVr4. 這個選項修改 -msvr4 , 選擇 hybrid-COFF 或 real-ELF 風格. 其他配置均忽略該選項.
-mwarn-passed-structs: 如果某個函數把結構當做參數或結果傳遞, GCC 發出警告. 隨著 C 語言的發展, 人們已經改變了傳遞結構的約定, 它往往導致移植問題. 預設情況下, GCC 不會發出警告.

下面的選項用於 IBM RS6000:

-mfp-in-toc

-mno-fp-in-toc: 控制是否把浮點常量放到內容表 (TOC) 中, 內容表存放所有的全局變量和函數地址. 預設情況下, GCC 把浮點常量放到這裏; 如果 TOC 溢出, `-mno-fp-in-toc' 選項能夠減少 TOC 的大小, 這樣就可以避免溢出.

下面的 `-m' 選項用於 IBM RT PC:

-min-line-mul: 對於整數乘法使用嵌入代碼. 這是預設選項.
-mcall-lib-mul: 對於整數乘法使用 lmul$$ .
-mfull-fp-blocks: 生成全尺寸浮點數據塊, 包括 IBM 建議的最少數量的活動空間 (scratch space). 這是預設選項.
-mminimum-fp-blocks: 不要在浮點數據塊中包括額外的活動空間. 這樣就產生較小但是略慢的可執行程式, 因為活動空間必須動態分配.
-mfp-arg-in-fpregs: 採用不相容 IBM 調用約定的調用序列, 通過浮點寄存器傳送浮點參數. 注意, 如果指定了這個選項, varargs.h 和 stdargs.h 將無法支持浮點單元.
-mfp-arg-in-gregs: 使用正常的調用約定處理浮點參數. 這是預設選項.
-mhc-struct-return: 通過記憶體返回大於一個字的結構, 而不是通過寄存器. 用於相容 MetaWare HighC (hc) 編譯器. 使用 `-fpcc-struct-return' 選項可以相容 Portable C 編譯器 (pcc).
-mnohc-struct-return: 如果可以, 通過寄存器返回某些大於一個字的結構. 這是預設選項. 如果打算相容 IBM 提供的編譯器, 請使用 `-fpcc-struct-return' 或 `-mhc-struct-return' 選項.

下面的 `-m' 選項用於 MIPS 家族的電腦:

-mcpu=cpu-type

生成指令的時候, 假設預設的機器類型是 cpu-type . 預設情況下的 cpu-type 是 default, GCC 將選取任何機型上都是最長周期時間的指令, 這樣才能使代碼在所有的 MIPS 處理器上以合理的速度運行. cpu-type 的其他選擇是 r2000, r3000, r4000, 和 r6000. 雖然選定某個 cpu-type 後, GCC 將針對選定的芯片安排對應的工作, 但是如果不指定□□ -mips2 或 -mips3 選項, 編譯器不會輸出任何不符合 MIPS ISA (instruction set architecture) 一級的代碼.

-mips2

輸出 MIPS ISA 二級指令 (可能的擴展, 如平方根指令). -mcpu=r4000 或 -mcpu=r6000 選項必須和 -mips2 聯用.

-mips3

輸出 MIPS ISA 三級指令 (64位指令). -mcpu=r4000 選項必須和 -mips2 聯用. (譯注: 疑為 -mips3)

-mint64

-mlong64

-mlonglong128

這些選項目前不起作用.

-mmips-as

產生用於 MIPS 匯編器的代碼, 同時使用 mips-tfile 添加普通的調試信息. 對於大多數平台這是預設選項, 除了 OSF/1 參考平台, 它使用 OSF/rose 目標格式. 如果打開了任一個 -ggdb, -gstabs, 或 -gstabs+ 選項開關, mips-tfile 程式就把 stab 封裝在 MIPS ECOFF 裏面.

-mgas

產生用於 GNU 匯編器的代碼. 在 OSF/1 參考平台上這是預設選項, 它使用 OSF/rose 目標格式.

-mrnames

-mno-rnames

-mrnames 開關選項告訴輸出代碼使用 MIPS 軟體名稱說明寄存器, 而不是硬體名稱 (就是說, 用 a0 代替 $4). GNU 匯編器不支持 -mrnames 選項, 而 MIPS 匯編器則運行 MIPS C 預處理器處理源檔案. -mno-rnames 是預設選項.

-mgpopt

-mno-gpopt

-mgpopt 開關選項要求在正文段中把所有的數據聲明寫到指令前面, 使各種 MIPS 匯編器對短類型全局或靜態數據項 (short global or static data items) 輸出單字記憶體訪問而不是雙字記憶體訪問. 當打開編譯優化時, 這是預設功能.

-mstats

-mno-stats

每次處理完非嵌入函數 (non-inline function) 後, -mstats 開關選項使編譯器向標準錯誤檔案輸出一行關於程式的統計資料 (保存的寄存器數目, 堆棧大小, 等等).

-mmemcpy

-mno-memcpy

-mmemcpy 開關選項使所有的塊移動操作調用適當的 string 函數 (memcpy 或 bcopy), 而不是生成嵌入代碼.

-mmips-tfile

-mno-mips-tfile

當 MIPS 匯編器生成 mips-tfile 檔案 (用於幫助調試) 後, -mno-mips-tfile 開關選項阻止編譯器使用 mips-tfile 後期處理 (postprocess) 目標檔案. 不運行 mips-tfile 就沒有調試器關注的局部變量. 另外, stage2 和 stage3 目標檔案將把臨時檔名傳遞給匯編器, 嵌在目標檔案中, 這意味著不比較目標檔案是否相同.

-msoft-float

輸出包含浮點庫調用. 警告: 所需庫不是 GNU CC 的一部分. 一般說來使用該機型本地 C 編譯器的相應部件, 但是不能直接用於交叉編譯, 你必須自己安排, 提供交叉編譯適用的庫函數.

-mhard-float

輸出包含浮點指令. 如果編譯器沒有被改動, 這就是預設選項.

-mfp64

編譯器認為狀態字的 FR 置位(on), 也就是說存在 32 64-bit 浮點寄存器, 而不是 32 32-bit 浮點寄存器. 同時必須打開 -mcpu=r4000 和 -mips3 開關.

-mfp32

認為存在 32 32-bit 浮點寄存器. 這是預設選項.

-mabicalls

-mno-abicalls: 輸出 (或不輸出) .abicalls, .cpload, 和 .cprestore 偽指令, 某些 System V.4 版本用於位置無關代碼.
-mhalf-pic
-mno-half-pic: -mhalf-pic 開關選項要求把外部引用的指針放到數據段, 並且載入記憶體, 而不放到正文段. 該選項目前不起作用.
-G num: 把小於等於 num 字節的全局或靜態數據放到小的數據段或 bss 段, 而不是普通的數據段或 bss 段. 這樣匯編器可以輸出基於全局指針 (gp 或 $28), 的單字記憶體訪問指令而非普通的雙字指令. 預設情況下, 用 MIPS 匯編器時 num 是 8, 而 GNU 匯編器則為 0. 另外, -Gnum 選項也被傳遞給匯編器和連接器. 所有的模塊必須在相同的 -Gnum 值下編譯.
-nocpp: 匯編使用者匯編檔案 (帶有 `.s' 延伸檔名) 時, 告訴 MIPS 匯編器不要運行預處理器.

下面的 `-m' 選項用於 Intel 80386 族電腦: -m486

-mno-486

控制是否生成對 486 優化的代碼.

-msoft-float

在函數把浮點返回值放在 80387 寄存器棧的機器上, 即使設置了 `-msoft-float' 選項, 也可能會發出一些浮點操作碼.

-mno-fp-ret-in-387

不用 FPU 寄存器返回函數值.

通常函數調用約定把 float 和 double 的返回值放在 FPU 寄存器中, 即使不存在 FPU. 這種作法的理念是作業系統應該仿真出 FPU.

而 `-mno-fp-ret-in-387' 選項使浮點值通過普通的 CPU 寄存器返回.

下面的 `-m' 選項用於 HPPA 族電腦:

-mpa-risc-1-0: 生成 PA 1.0 處理器的目標碼.
-mpa-risc-1-1: 生成 PA 1.1 處理器的目標碼.
-mkernel: 生成適用於核心的目標碼. 特別要避免 add 指令, 它有一個參數是 DP 寄存器; 用 addil 代替 add指令. 這樣可以避免 HP-UX 連接器的某個嚴重 bug.
-mshared-libs: 生成能夠連接 HP-UX 共享庫的目標碼. 該選項還沒有實現全部功能, 對 PA 目標預設為關閉. 使用這個選項會導致編譯器生成錯誤的目標碼.
-mno-shared-libs: 不生成連接 HP-UX 共享庫的目標碼. 這是 PA 目標的預設選項.
-mlong-calls: 生成的目標碼允許同一個源檔案中的函數調用, 調用點和被調函數的距離可以超過 256K 之遠. 不需要打開這個開關選項, 除非連接器給出 ``branch out of range errors`` 這樣的錯誤.
-mdisable-fpregs: 防止任何情況下使用浮點寄存器. 編譯核心需要這個選項, 核心切換浮點寄存器的執行環境速度非常緩慢. 如果打開了這個開關選項同時試圖浮點操作, 編譯將失敗.
-mdisable-indexing: 防止編譯器使用索引地址模式 (indexing address mode). 這樣在 MACH 上編譯 MIG 生成的代碼時, 可以避免一些非常晦澀的問題.
-mtrailing-colon: 在標記定義 (label definition) 的末尾添加一個冒號 (用於 ELF 匯編器).

下面的 `-m' 選項用於 Intel 80960 族電腦:

-mcpu-type: 預設機器類型為 cpu-type , 使編譯器產生對應的指令, 地址模式和記憶體對齊. 預設的 cpu-type 是 kb; 其他選擇有 ka, mc, ca, cf, sa, 和 sb.
-mnumerics
-msoft-float: -mnumerics 開關選項指出處理器不支持浮點指令. -msoft-float 開關選項指出不應該認為機器支持浮點操作.
-mleaf-procedures
-mno-leaf-procedures: 企圖 (或防止) 改變葉過程 (leaf procedure), 使其可被 bal 指令以及 call 指令調用. 對於直接函數調用, 如果 bal 指令能夠被匯編器或連接器替換, 這可以產生更有效的代碼, 但是其他情況下產生較低效的代碼, 例如通過函數指針調用函數, 或使用了不支持這種優化的連接器.
-mtail-call
-mno-tail-call: 執行 (或不執行) 更多的嘗試 (除過編譯器那些機器無關部分), 優化進入分支的尾遞歸 (tail-recursive) 調用. 你可能不需要這個, 因為檢測什麼地方無效沒有全部完成. 預設開關是 -mno-tail-call.
-mcomplex-addr
-mno-complex-addr: 認為 (或不認為) 在當前的 i960 設備上, 值得使用復合地址模式 (complex addressing mode). 復合地址模式可能不值得用到 K 系列, 但是一定值得用在 C 系列. 目前除了 CB 和 CC 處理器, 其他處理器上 -mcomplex-addr 是預設選項.
-mcode-align
-mno-code-align: 把目標碼對齊到 8 字節邊界上 (或者不必), 這樣讀取會快一些. 目前只對 C 系列預設打開.
-mic-compat
-mic2.0-compat
-mic3.0-compat: 相容 iC960 v2.0 或 v3.0.
-masm-compat
-mintel-asm: 相容 iC960 匯編器.
-mstrict-align
-mno-strict-align: 不允許 (或允許) 邊界不對齊的訪問.
-mold-align: 使結構對齊 (structure-alignment) 相容 Intel 的 gcc 發行版本 1.3 (基於 gcc 1.37). 目前這個選項有點問題, 因為 #pragma align 1 總是作同樣的設定, 而且無法關掉.

下面的 `-m' 選項用於 DEC Alpha 設備:

-mno-soft-float

-msoft-float

使用 (或不使用) 硬體浮點指令進行浮點運算. 打開 -msoft-float 時, 將使用 `libgcc1.c' 中的函數執行浮點運算. 除非它們被仿真浮點操作的例程替換, 或者類似, 它們被編譯為調用仿真例程, 這些例程將發出浮點操作. 如果你為不帶浮點操作的 Alpha 編譯程式, 你必須確保建立了這個庫, 以便不調用仿真例程.

注意, 不帶浮點操作的 Alpha 也要求擁有浮點寄存器.

-mfp-reg

-mno-fp-regs

生成使用 (或不使用) 浮點寄存器群的目標代碼. -mno-fp-regs 包含有 -msoft-float 開關選項. 如果不使用浮點寄存器, 浮點操作數就像整數一樣通過整數寄存器傳送, 浮點運算結果放到 $0 而不是 $f0. 這是非標準調用, 因此任何帶有浮點參數或返回值的函數, 如果被 -mno-fp-regs 開關編譯過的目標碼調用, 它也必須用這個選項編譯.

這個選項的典型用法是建立核心, 核心不使用任何浮點寄存器, 因此沒必要保存和恢復這些寄存器.

下面附加的選項出現在 System V 第四版中, 用於相容這些系統中的其他編譯器:

-G: 在 SVr4 系統中, gcc 出於相容接受了 `-G' 選項 (然後傳遞給連接器). 可是我們建議使用 `-symbolic' 或 `-shared'選項, 而不在 gcc 命令行上出現連接選項.
-Qy: 驗証編譯器用的工具的版本, 輸出到 .ident 匯編指令.
-Qn: 制止輸出端的 .ident 指令 (預設選項).
-YP,dirs: 對於 `-l' 指定的庫檔案, 只搜索 dirs. 你可以在 dirs 中用冒號隔開各個目錄項.
-Ym,dir: 在 dir 目錄中尋找 M4 預處理器. 匯編器使用這個選項.

代碼生成選項 (CODE GENERATION OPTION)

下面的選項和平台無關, 用於控制目標碼生成的接口約定.

大部分選項以 `-f' 開始. 這些選項擁有確定和否定兩種格式; `-ffoo' 的否定格式是 `-fno-foo'. 後面的描述將只列舉其中的一個格式 --- 非預設的格式. 你可以通過添加或去掉 `no-' 推測出另一個格式.

-fnonnull-objects

假設通過引用 (reference) 取得的對象不為 null (僅 C++).

一般說來, GNU C++ 對通過引用取得的對象作保守假設. 例如, 編譯器一定會檢查下似代碼中的 a 不為 null:

obj &a = g (); a.f (2);

檢查類似的引用需要額外的代碼, 然而對於很多程式是不必要的. 如果你的程式不要求這種檢查, 你可以用 `-fnonnull-objects' 選項忽略它.

-fpcc-struct-return

函數返回 struct 和 union 值時, 採用和本地編譯器相同的參數約定. 對於較小的結構, 這種約定的效率偏低, 而且很多機器上不能重入; 它的優點是允許 GCC 編譯的目標碼和 PCC 編譯的目標碼互相調用.

-freg-struct-return

一有可能就通過寄存器返回 struct 和 union 函數值. 對於較小的結構, 它比 -fpcc-struct-return 更有效率.

如果既沒有指定 -fpcc-struct-return , 也沒有指定 -freg-struct-return, GNU CC 預設使用目標機的標準約定. 如果沒有標準約定, GNU CC 預設採用 -fpcc-struct-return.

-fshort-enums

給 enum 類型只分配它聲明的值域范圍的字節數. 就是說, enum 類型等於大小足夠的最小整數類型.

-fshort-double

使 double 類型的大小和 float 一樣.

-fshared-data

要求編譯結果的數據和非 const 變量是共享數據, 而不是私有數據. 這種差別僅在某些作業系統上面有意義, 那裏的共享數據在同一個程式的若幹進程間共享, 而私有數據在每個進程內都有副件.

-fno-common

即使未初始化的全局變量也分配在目標檔案的 bss 段, 而不是把它們當做普通塊 (common block) 建立. 這樣的結果是, 如果在兩個不同的編譯結果中聲明了同一個變量 (沒使用 extern ), 連接它們時會產生錯誤. 這個選項可能有用的唯一情況是, 你希望確認程式能在其他系統上運行, 而其他系統總是這麼做.

-fno-ident

忽略 `#ident' 指令.

-fno-gnu-linker

不要把全局初始化部件 (如 C++ 的構造子和解構子) 輸出為 GNU 連接器使用的格式 (在 GNU 連接器是標準方法的系統上). 當你打算使用非 GNU 連接器的時候可以用這個選項, 非GNU連接器也需要 collect2 程式確保系統連接器放入構造子 (constructor) 和解構子 (destructor). (GNU CC 的發佈包中包含有 collect2 程式.) 對於必須使用 collect2 的系統, 編譯器驅動程式 gcc 自動配置為這麼做.

-finhibit-size-directive

不要輸出 .size 匯編指令, 或其他類似指令, 當某個函數一分為二, 兩部分在記憶體中距離很遠時會引起問題. 當編譯 `crtstuff.c' 時需要這個選項; 其他情況下都不應該使用.

-fverbose-asm

輸出匯編代碼時放些額外的注釋信息. 這個選項僅用於確實需要閱讀匯編輸出的時候 (可能調試編譯器自己的時候).

-fvolatile

使編譯器認為所有通過指針訪問的記憶體是易變記憶體 (volatile).

-fvolatile-global

使編譯器認為所有的外部和全局變量是易變記憶體.

-fpic

如果支持這種目標機, 編譯器就生成位置無關目標碼. 適用於共享庫 (shared library).

-fPIC

如果支持這種目標機, 編譯器就輸出位置無關目標碼. 適用於動態連接 (dynamic linking), 即使分支需要大范圍轉移.

-ffixed-reg

把名為 reg 的寄存器按固定寄存器看待 (fixed register); 生成的目標碼不應該引用它 (除了或許用作棧指針, 幀指針, 或其他固定的角色).

reg 必須是寄存器的名字. 寄存器名字取決於機器, 用機器描述宏檔案的 REGISTER_NAMES 宏定義.

這個選項沒有否定格式, 因為它列出三路選擇.

-fcall-used-reg

把名為 reg 的寄存器按可分配寄存器看待, 不能在函數調用間使用. 可以臨時使用或當做變量使用, 生存期不超過一個函數. 這樣編譯的函數無需保存和恢復 reg 寄存器.

如果在可執行模塊中, 把這個選項說明的寄存器用作固定角色將會產生災難性結果, 如棧指針或幀指針.

這個選項沒有否定格式, 因為它列出三路選擇.

-fcall-saved-reg

把名為 reg 的寄存器按函數保護的可分配寄存器看待. 可以臨時使用或當做變量使用, 它甚至能在函數間生存. 這樣編譯的函數會保存和恢復使用中的 reg 寄存器.

如果在可執行模塊中, 把這個選項說明的寄存器用作固定角色將會產生災難性結果, 如棧指針或幀指針.

另一種災難是用這個選項說明的寄存器返回函數值.

這個選項沒有否定格式, 因為它列出三路選擇.

PRAGMAS

GNU C++ 支持兩條 `#pragma' 指令使同一個頭檔案有兩個用途: 對象類的接口定義, 對象類完整的內容定義.

#pragma interface

(僅對 C++) 在定義對象類的頭檔案中, 使用這個指令可以節省大部分採用該類的目標檔案的大小. 一般說來, 某些信息 (內嵌成員函數的備份副件, 調試信息, 實現虛函數的內部表格等) 的本地副件必須保存在包含類定義的各個目標檔案中. 使用這個 pragma 指令能夠避免這樣的複製. 當編譯中引用包含 `#pragma interface' 指令的頭檔案時, 就不會產生這些輔助信息 (除非輸入的主檔案使用了 `#pragma implementation'指令). 作為替代, 目標檔案將包含可被連接時解析的引用 (reference).

#pragma implementation

#pragma implementation "objects.h"

(僅對 C++) 如果要求從頭檔案產生完整的輸出 (並且全局可見), 你應該在主輸入檔案中使用這條 pragma. 頭檔案中應該依次使用 `#pragma interface' 指令. 在 implementation 檔案中將產生全部內嵌成員函數的備份, 調試信息, 實現虛函數的內部表格等.

如果 `#pragma implementation' 不帶參數, 它指的是和源檔案有相同基本名的包含檔案; 例如, `allclass.cc' 中, `#pragma implementation' 等於 `#pragma implementation allclass.h'. 如果某個 implementation 檔案需要從多個頭檔案引入代碼, 就應該使用這個字符串參數.

不可能把一個頭檔案裏面的內容分割到多個 implementation 檔案中.

檔案 (FILE)

file.c             C 源檔案
file.h             C 頭檔案 (預處理檔案)
file.i             預處理後 的 C 源檔案
file.C             C++ 源檔案
file.cc            C++ 源檔案
file.cxx           C++ 源檔案
file.m             Objective-C 源檔案
file.s             匯編語言檔案
file.o             目標檔案
a.out              連接的輸出檔案
TMPDIR/cc*         臨時檔案
LIBDIR/cpp         預處理器
LIBDIR/cc1         C 編譯器
LIBDIR/cc1plus     C++ 編譯器
LIBDIR/collect     某些機器需要的連接器前端(front end)程式
LIBDIR/libgcc.a    GCC 子例程 (subroutine) 庫
/lib/crt[01n].o    啟動例程 (start-up)
LIBDIR/ccrt0       C++ 的附加啟動例程
/lib/libc.a        標準 C 庫, 另見 intro (3)
/usr/include       #include 檔案的標準目錄
LIBDIR/include     #include 檔案的標準 gcc 目錄
LIBDIR/g++-include #include 檔案的附加 g++ 目錄

LIBDIR 通常為 /usr/local/lib/machine/version.
TMPDIR 來自環境變量 TMPDIR (如果存在, 預設為 /usr/tmp , 否則為 /tmp).

另見 (SEE ALSO)

cpp(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1).
info中 `gcc', `cpp', `as', `ld', 和 `gdb' 的條目.
Using and Porting GNU CC (for version 2.0), Richard M. Stallman; The C Preprocessor, Richard M. Stallman; Debugging with GDB: the GNU Source-Level Debugger, Richard M. Stallman 和 Roland H. Pesch; Using as: the GNU Assembler, Dean Elsner, Jay Fenlason & friends; ld: the GNU linker, Steve Chamberlain 和 Roland Pesch.

BUGS

關於報告差錯的指導請查閱 GCC 手冊.

版權 (COPYING)

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be included in translations approved by the Free Software Foundation instead of in the original English.

作者 (AUTHORS)

關於 GNU CC 的奉獻者請查閱 GUN CC 手冊.

[中文版維護人]

徐明 <xuming@users.sourceforge.net>

[中文版最新更新]

2003/05/13 第一版

《中國Linux論壇man手冊頁翻譯計劃》

http://cmpp.linuxforum.net

#p#

NAME

gcc - GNU project C and C++ compiler

SYNOPSIS

gcc [-c|-S|-E] [-std=standard]
    [-g] [-pg] [-Olevel]
    [-Wwarn...] [-pedantic]
    [-Idir...] [-Ldir...]
    [-Dmacro[=defn]...] [-Umacro]
    [-foption...] [-mmachine-option...]
    [-o outfile] infile...

Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same options as gcc.

DESCRIPTION

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.

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: -dr is very different from -d -r.

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.

Many options have long names starting with -f or with -W---for example, -fforce-mem, -fstrength-reduce, -Wformat and so on. Most of these have both positive and negative forms; the negative form of -ffoo would be -fno-foo. This manual documents only one of these two forms, whichever one is not the default.

OPTIONS

Option Summary

Here is a summary of all the options, grouped by type. Explanations are in the following sections.

Overall Options

-c -S -E -o file -pipe -pass-exit-codes -x language -v -### --help --target-help --version

C Language Options

-ansi -std=standard -aux-info filename -fno-asm -fno-builtin -fno-builtin-function -fhosted -ffreestanding -fms-extensions -trigraphs -no-integrated-cpp -traditional -traditional-cpp -fallow-single-precision -fcond-mismatch -fsigned-bitfields -fsigned-char -funsigned-bitfields -funsigned-char -fwritable-strings

C++ Language Options

-fabi-version=n -fno-access-control -fcheck-new -fconserve-space -fno-const-strings -fno-elide-constructors -fno-enforce-eh-specs -ffor-scope -fno-for-scope -fno-gnu-keywords -fno-implicit-templates -fno-implicit-inline-templates -fno-implement-inlines -fms-extensions -fno-nonansi-builtins -fno-operator-names -fno-optional-diags -fpermissive -frepo -fno-rtti -fstats -ftemplate-depth-n -fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ -fno-default-inline -fvisibility-inlines-hidden -Wabi -Wctor-dtor-privacy -Wnon-virtual-dtor -Wreorder -Weffc++ -Wno-deprecated -Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions -Wsign-promo -Wsynth

Objective-C Language Options

-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime -fno-nil-receivers -fobjc-exceptions -freplace-objc-classes -fzero-link -gen-decls -Wno-protocol -Wselector -Wundeclared-selector

Language Independent Options

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

Warning Options

-fsyntax-only -pedantic -pedantic-errors -w -Wextra -Wall -Waggregate-return -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment -Wconversion -Wno-deprecated-declarations -Wdisabled-optimization -Wno-div-by-zero -Wendif-labels -Werror -Werror-implicit-function-declaration -Wfloat-equal -Wformat -Wformat=2 -Wno-format-extra-args -Wformat-nonliteral -Wformat-security -Wformat-y2k -Wimplicit -Wimplicit-function-declaration -Wimplicit-int -Wimport -Wno-import -Winit-self -Winline -Wno-invalid-offsetof -Winvalid-pch -Wlarger-than-len -Wlong-long -Wmain -Wmissing-braces -Wmissing-format-attribute -Wmissing-noreturn -Wno-multichar -Wnonnull -Wpacked -Wpadded -Wparentheses -Wpointer-arith -Wredundant-decls -Wreturn-type -Wsequence-point -Wshadow -Wsign-compare -Wstrict-aliasing -Wswitch -Wswitch-default -Wswitch-enum -Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized -Wunknown-pragmas -Wunreachable-code -Wunused -Wunused-function -Wunused-label -Wunused-parameter -Wunused-value -Wunused-variable -Wwrite-strings

C-only Warning Options

-Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes -Wnested-externs -Wold-style-definition -Wstrict-prototypes -Wtraditional -Wdeclaration-after-statement

Debugging Options

-dletters -dumpspecs -dumpmachine -dumpversion -fdump-unnumbered -fdump-translation-unit[-n] -fdump-class-hierarchy[-n] -fdump-tree-original[-n] -fdump-tree-optimized[-n] -fdump-tree-inlined[-n] -feliminate-dwarf2-dups -feliminate-unused-debug-types -feliminate-unused-debug-symbols -fmem-report -fprofile-arcs -frandom-seed=string -fsched-verbose=n -ftest-coverage -ftime-report -g -glevel -gcoff -gdwarf-2 -ggdb -gstabs -gstabs+ -gvms -gxcoff -gxcoff+ -p -pg -print-file-name=library -print-libgcc-file-name -print-multi-directory -print-multi-lib -print-prog-name=program -print-search-dirs -Q -save-temps -time

Optimization Options

-falign-functions=n -falign-jumps=n -falign-labels=n -falign-loops=n -fbranch-probabilities -fprofile-values -fvpt -fbranch-target-load-optimize -fbranch-target-load-optimize2 -fcaller-saves -fcprop-registers -fcse-follow-jumps -fcse-skip-blocks -fdata-sections -fdelayed-branch -fdelete-null-pointer-checks -fexpensive-optimizations -ffast-math -ffloat-store -fforce-addr -fforce-mem -ffunction-sections -fgcse -fgcse-lm -fgcse-sm -fgcse-las -floop-optimize -fcrossjumping -fif-conversion -fif-conversion2 -finline-functions -finline-limit=n -fkeep-inline-functions -fkeep-static-consts -fmerge-constants -fmerge-all-constants -fmove-all-movables -fnew-ra -fno-branch-count-reg -fno-default-inline -fno-defer-pop -fno-function-cse -fno-guess-branch-probability -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 -funsafe-math-optimizations -ffinite-math-only -fno-trapping-math -fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-register-move -foptimize-sibling-calls -fprefetch-loop-arrays -fprofile-generate -fprofile-use -freduce-all-givs -fregmove -frename-registers -freorder-blocks -freorder-functions -frerun-cse-after-loop -frerun-loop-opt -frounding-math -fschedule-insns -fschedule-insns2 -fno-sched-interblock -fno-sched-spec -fsched-spec-load -fsched-spec-load-dangerous -fsched-stalled-insns=n -sched-stalled-insns-dep=n -fsched2-use-superblocks -fsched2-use-traces -fsignaling-nans -fsingle-precision-constant -fstrength-reduce -fstrict-aliasing -ftracer -fthread-jumps -funroll-all-loops -funroll-loops -fpeel-loops -funswitch-loops -fold-unroll-loops -fold-unroll-all-loops --param name=value -O -O0 -O1 -O2 -O3 -Os

Preprocessor Options

-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 -M -MM -MF -MG -MP -MQ -MT -nostdinc -P -fworking-directory -remap -trigraphs -undef -Umacro -Wp,option -Xpreprocessor option

Assembler Option

-Wa,option -Xassembler option

Linker Options

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

Directory Options

-Bprefix -Idir -I- -Ldir -specs=file

Target Options

-V version -b machine

Machine Dependent Options

M680x0 Options -m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 -m68060 -mcpu32 -m5200 -m68881 -mbitfield -mc68000 -mc68020 -mnobitfield -mrtd -mshort -msoft-float -mpcrel -malign-int -mstrict-align -msep-data -mno-sep-data -mshared-library-id=n -mid-shared-library -mno-id-shared-library

M68hc1x Options -m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12 -mauto-incdec -minmax -mlong-calls -mshort -msoft-reg-count=count

VAX Options -mg -mgnu -munix

SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-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 -mimpure-text -mno-impure-text -mlittle-endian -mstack-bias -mno-stack-bias -munaligned-doubles -mno-unaligned-doubles -mv8plus -mno-v8plus -mvis -mno-vis -mcypress -mf930 -mf934 -msparclite -msupersparc -mv8 -threads -pthreads

ARM Options -mapcs-frame -mno-apcs-frame -mapcs-26 -mapcs-32 -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 -malignment-traps -mno-alignment-traps -msoft-float -mhard-float -mfpe -mthumb-interwork -mno-thumb-interwork -mcpu=name -march=name -mfpe=name -mstructure-size-boundary=n -mabort-on-noreturn -mlong-calls -mno-long-calls -msingle-pic-base -mno-single-pic-base -mpic-register=reg -mnop-fun-dllimport -mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns -mpoke-function-name -mthumb -marm -mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking -mcallee-super-interworking

MN10300 Options -mmult-bug -mno-mult-bug -mam33 -mno-am33 -mam33-2 -mno-am33-2 -mno-crt0 -mrelax

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

RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type -mpower -mno-power -mpower2 -mno-power2 -mpowerpc -mpowerpc64 -mno-powerpc -maltivec -mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt -mno-powerpc-gfxopt -mnew-mnemonics -mold-mnemonics -mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc -m64 -m32 -mxl-call -mno-xl-call -mpe -malign-power -malign-natural -msoft-float -mhard-float -mmultiple -mno-multiple -mstring -mno-string -mupdate -mno-update -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 -mprioritize-restricted-insns=priority -msched-costly-dep=dependence_type -minsert-sched-nops=scheme -mcall-sysv -mcall-netbsd -maix-struct-return -msvr4-struct-return -mabi=altivec -mabi=no-altivec -mabi=spe -mabi=no-spe -misel=yes -misel=no -mspe=yes -mspe=no -mfloat-gprs=yes -mfloat-gprs=no -mprototype -mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata -msdata=opt -mvxworks -mwindiss -G num -pthread

Darwin Options -all_load -allowable_client -arch -arch_errors_fatal -arch_only -bind_at_load -bundle -bundle_loader -client_name -compatibility_version -current_version -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 -image_base -init -install_name -keep_private_externs -multi_module -multiply_defined -multiply_defined_unused -noall_load -nofixprebinding -nomultidefs -noprebind -noseglinkedit -pagezero_size -prebind -prebind_all_twolevel_modules -private_bundle -read_only_relocs -sectalign -sectobjectsymbols -whyload -seg1addr -sectcreate -sectobjectsymbols -sectorder -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

MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips64 -mips16 -mno-mips16 -mabi=abi -mabicalls -mno-abicalls -mxgot -mno-xgot -membedded-pic -mno-embedded-pic -mgp32 -mgp64 -mfp32 -mfp64 -mhard-float -msoft-float -msingle-float -mdouble-float -mint64 -mlong64 -mlong32 -Gnum -membedded-data -mno-embedded-data -muninit-const-in-rodata -mno-uninit-const-in-rodata -msplit-addresses -mno-split-addresses -mexplicit-relocs -mno-explicit-relocs -mrnames -mno-rnames -mcheck-zero-division -mno-check-zero-division -mmemcpy -mno-memcpy -mlong-calls -mno-long-calls -mmad -mno-mad -mfused-madd -mno-fused-madd -nocpp -mfix-sb1 -mno-fix-sb1 -mflush-func=func -mno-flush-func -mbranch-likely -mno-branch-likely

i386 and x86-64 Options -mtune=cpu-type -march=cpu-type -mfpmath=unit -masm=dialect -mno-fancy-math-387 -mno-fp-ret-in-387 -msoft-float -msvr3-shlib -mno-wide-multiply -mrtd -malign-double -mpreferred-stack-boundary=num -mmmx -msse -msse2 -msse3 -m3dnow -mthreads -mno-align-stringops -minline-all-stringops -mpush-args -maccumulate-outgoing-args -m128bit-long-double -m96bit-long-double -mregparm=num -momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs -mcmodel=code-model -m32 -m64

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

Intel 960 Options -mcpu-type -masm-compat -mclean-linkage -mcode-align -mcomplex-addr -mleaf-procedures -mic-compat -mic2.0-compat -mic3.0-compat -mintel-asm -mno-clean-linkage -mno-code-align -mno-complex-addr -mno-leaf-procedures -mno-old-align -mno-strict-align -mno-tail-call -mnumerics -mold-align -msoft-float -mstrict-align -mtail-call

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

DEC Alpha/VMS Options -mvms-return-codes

H8/300 Options -mrelax -mh -ms -mn -mint32 -malign-300

SH Options -m1 -m2 -m2e -m3 -m3e -m4-nofpu -m4-single-only -m4-single -m4 -m5-64media -m5-64media-nofpu -m5-32media -m5-32media-nofpu -m5-compact -m5-compact-nofpu -mb -ml -mdalign -mrelax -mbigtable -mfmovd -mhitachi -mnomacsave -mieee -misize -mpadstruct -mspace -mprefergot -musermode

System V Options -Qy -Qn -YP,paths -Ym,dir

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

TMS320C3x/C4x Options -mcpu=cpu -mbig -msmall -mregparm -mmemparm -mfast-fix -mmpyi -mbk -mti -mdp-isr-reload -mrpts=count -mrptb -mdb -mloop-unsigned -mparallel-insns -mparallel-mpy -mpreserve-float

V850 Options -mlong-calls -mno-long-calls -mep -mno-ep -mprolog-function -mno-prolog-function -mspace -mtda=n -msda=n -mzda=n -mapp-regs -mno-app-regs -mdisable-callt -mno-disable-callt -mv850e1 -mv850e -mv850 -mbig-switch

NS32K Options -m32032 -m32332 -m32532 -m32081 -m32381 -mmult-add -mnomult-add -msoft-float -mrtd -mnortd -mregparam -mnoregparam -msb -mnosb -mbitfield -mnobitfield -mhimem -mnohimem

AVR Options -mmcu=mcu -msize -minit-stack=n -mno-interrupts -mcall-prologues -mno-tablejump -mtiny-stack

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

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

IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic -mvolatile-asm-stop -mb-step -mregister-names -mno-sdata -mconstant-gp -mauto-pic -minline-float-divide-min-latency -minline-float-divide-max-throughput -minline-int-divide-min-latency -minline-int-divide-max-throughput -mno-dwarf2-asm -mfixed-range=register-range

D30V Options -mextmem -mextmemory -monchip -mno-asm-optimize -masm-optimize -mbranch-cost=n -mcond-exec=n

S/390 and zSeries Options -mtune=cpu-type -march=cpu-type -mhard-float -msoft-float -mbackchain -mno-backchain -mkernel-backchain -msmall-exec -mno-small-exec -mmvcle -mno-mvcle -m64 -m31 -mdebug -mno-debug -mesa -mzarch -mfused-madd -mno-fused-madd -mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard

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

PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 -mbcopy -mbcopy-builtin -mint32 -mno-int16 -mint16 -mno-int32 -mfloat32 -mno-float64 -mfloat64 -mno-float32 -mabshi -mno-abshi -mbranch-expensive -mbranch-cheap -msplit -mno-split -munix-asm -mdec-asm

Xstormy16 Options -msim

Xtensa Options -mconst16 -mno-const16 -mfused-madd -mno-fused-madd -mtext-section-literals -mno-text-section-literals -mtarget-align -mno-target-align -mlongcalls -mno-longcalls

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 -mlibrary-pic -macc-4 -macc-8 -mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move -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 -mcpu=cpu

Code Generation Options

-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions -fnon-call-exceptions -funwind-tables -fasynchronous-unwind-tables -finhibit-size-directive -finstrument-functions -fno-common -fno-ident -fpcc-struct-return -fpic -fPIC -fpie -fPIE -freg-struct-return -fshared-data -fshort-enums -fshort-double -fshort-wchar -fverbose-asm -fpack-struct -fstack-check -fstack-limit-register=reg -fstack-limit-symbol=sym -fargument-alias -fargument-noalias -fargument-noalias-global -fleading-underscore -ftls-model=model -ftrapv -fwrapv -fbounds-check -fvisibility

Options Controlling the Kind of Output

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

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

file.c: C source code which must be preprocessed.
file.i: C source code which should not be preprocessed.
file.ii: C++ source code which should not be preprocessed.
file.m: Objective-C source code. Note that you must link with the library libobjc.a to make an Objective-C program work.
file.mi: Objective-C source code which should not be preprocessed.
file.h: C or C++ header file to be turned into a precompiled header.
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C: C++ source code which must be preprocessed. Note that in .cxx, the last two letters must both be literally x. Likewise, .Crefers to a literal capital C.
file.hh
file.H: C++ header file to be turned into a precompiled header.
file.f
file.for
file.FOR: Fortran source code which should not be preprocessed.
file.F
file.fpp
file.FPP: Fortran source code which must be preprocessed (with the traditional preprocessor).
file.r: Fortran source code which must be preprocessed with a RATFOR preprocessor (not included with GCC).
file.ads: Ada source code file which contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). Such files are also called specs.
file.adb: Ada source code file containing a library unit body (a subprogram or package body). Such files are also called bodies.
file.s: Assembler code.
file.S: Assembler code which must be preprocessed.
other: An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.

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

-x language

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

        c  c-header  cpp-output
        c++  c++-header  c++-cpp-output
        objective-c  objective-c-header  objc-cpp-output
        assembler  assembler-with-cpp
        ada
        f77  f77-cpp-input  ratfor
        java
        treelang

-x none

Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as they are if -x has not been used at all).

-pass-exit-codes

Normally the gcc program will exit with the code of 1 if any phase of the compiler returns a non-success return code. If you specify -pass-exit-codes, the gcc program will instead return with numerically highest error produced by any phase that returned an error indication.

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

-c

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

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

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

-S

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

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

Input files that don't require compilation are ignored.

-E

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

Input files which don't require preprocessing are ignored.

-o file

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

If you specify -o when compiling more than one input file, or you are producing an executable file as output, all the source files on the command line will be compiled at once.

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, and all preprocessed C source on standard output.

-v

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

-###

Like -v except the commands are not executed and all command arguments are quoted. This is useful for shell scripts to capture the driver-generated command lines.

-pipe

Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble.

--help

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

--target-help

Print (on the standard output) a description of target specific command line options for each tool.

--version

Display the version number and copyrights of the invoked GCC.

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 or.H; 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, C++ programs often require class libraries as well as a compiler that understands the C++ language---and under some circumstances, you might want to compile programs or header files from standard input, or otherwise without a suffix that flags them as C++ programs. You might also like to precompile a C header file with a .h extension to be used in C++ compilations.g++ is a program that calls GCC with the default language set to C++, and automatically specifies linking against the C++ library. 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.

Options Controlling C Dialect

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

-ansi

In C mode, support all ISO C90 programs. In C++ mode, remove GNU extensions that conflict with ISO C++.

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

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

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

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 which would normally be built in but do not have semantics defined by ISO C (such as "alloca" and "ffs") are not built-in functions with -ansi is used.

-std=

Determine the language standard. This option is currently only supported when compiling C or C++. A value for this option must be provided; possible values are

c89
iso9899:1990: ISO C90 (same as -ansi).
iso9899:199409: ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x: ISO C99. Note that this standard is not yet fully supported; see <http://gcc.gnu.org/gcc-3.4/c99status.html> for more information. The names c9x and iso9899:199x are deprecated.
gnu89: Default, ISO C90 plus GNU extensions (including some C99 features).
gnu99
gnu9x: ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC, this will become the default. The name gnu9x is deprecated.
c++98: The 1998 ISO C++ standard plus amendments.
gnu++98: The same as -std=c++98 plus GNU extensions. This is the default for C++ code.

Even when this option is not specified, you can still use some of the features of newer standards in so far as they do not conflict with previous C standards. For example, you may use "__restrict__" even when -std=c99 is not specified.

The -std options specifying some version of ISO C have the same effects as -ansi, except that features that were not in ISO C90 but are in the specified version (for example, // comments and the "inline" keyword in ISO C99) are not disabled.

-aux-info filename

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

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

-fno-asm

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

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

-fno-builtin

-fno-builtin-function

Don't recognize built-in functions that do not begin with __builtin_ as prefix.

GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to "alloca"may become single instructions that adjust the stack directly, and calls to "memcpy" may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library.

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 this is not built-in in this version of GCC, this option is ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:

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

-fhosted

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

-ffreestanding

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

-fms-extensions

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

-trigraphs

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

-no-integrated-cpp

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

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

-traditional

-traditional-cpp

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

-fcond-mismatch

Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. This option is not supported for C++.

-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.

-fwritable-strings

Store string constants in the writable data segment and don't uniquize them. This is for compatibility with old programs which assume they can write into string constants.

Writing into string constants is a very bad idea; ``constants'' should be constant.

This option is deprecated.

Options Controlling C++ Dialect

This section describes the command-line options that are only meaningful for C++ programs; but you can also use most of theGNU compiler options regardless of what language your program is in. For example, you might compile a file "firstClass.C" like this:

        g++ -g -frepo -O -c firstClass.C

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

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

-fabi-version=n: Use version n of the C++ ABI. Version 2 is the version of the C++ ABI that first appeared in G++ 3.4. Version 1 is the version of the C++ ABI that first appeared in G++ 3.2. Version 0 will always be the version that conforms most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change as ABI bugs are fixed.
The default is version 2.
-fno-access-control: Turn off all access checking. This switch is mainly useful for working around bugs in the access control code.
-fcheck-new: Check that the pointer returned by "operator new" is non-null before attempting to modify the storage allocated. This check is normally unnecessary because the C++ standard specifies that "operator new" will only return 0 if it is declared throw(), in which case the compiler will always check the return value even without this option. In all other cases, when "operator new" has a non-empty exception specification, memory exhaustion is signalled by throwing "std::bad_alloc". See alsonew (nothrow).
-fconserve-space: Put uninitialized or runtime-initialized global variables into the common segment, as C does. This saves space in the executable at the cost of not diagnosing duplicate definitions. If you compile with this flag and your program mysteriously crashes after "main()" has completed, you may have an object that is being destroyed twice because two definitions were merged.
This option is no longer useful on most targets, now that support has been added for putting variables into BSS without making them common.
-fno-const-strings: Give string constants type "char *" instead of type "const char *". By default, G++ uses type "const char *" as required by the standard. Even if you use -fno-const-strings, you cannot actually modify the value of a string constant, unless you also use -fwritable-strings.
This option might be removed in a future release of G++. For maximum portability, you should structure your code so that it works with string constants that have type "const char *".
-fno-elide-constructors: The C++ standard allows an implementation to omit creating a temporary which is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases.
-fno-enforce-eh-specs: Don't check for violation of exception specifications at runtime. This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining NDEBUG. The compiler will still optimize based on the exception specifications.
-ffor-scope
-fno-for-scope: If -ffor-scope is specified, the scope of variables declared in a for-init-statement is limited to the for loop itself, as specified by the C++ standard. If -fno-for-scope is specified, the scope of variables declared in a for-init-statement extends to the end of the enclosing scope, as was the case in old versions of G++, and other (traditional) implementations of C++.
The default if neither flag is given to follow the standard, but to allow and give a warning for old-style code that would otherwise be invalid, or have different behavior.
-fno-gnu-keywords: Do not recognize "typeof" as a keyword, so that code can use this word as an identifier. You can use the keyword"__typeof__" instead. -ansi implies -fno-gnu-keywords.
-fno-implicit-templates: Never emit code for non-inline templates which are instantiated implicitly (i.e. by use); only emit code for explicit instantiations.
-fno-implicit-inline-templates: Don't emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles with and without optimization will need the same set of explicit instantiations.
-fno-implement-inlines: To save space, do not emit out-of-line copies of inline functions controlled by #pragma implementation. This will cause linker errors if these functions are not inlined everywhere they are called.
-fms-extensions: Disable pedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax.
-fno-nonansi-builtins: Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include "ffs", "alloca", "_exit","index", "bzero", "conjf", and other related functions.
-fno-operator-names: Do not treat the operator name keywords "and", "bitand", "bitor", "compl", "not", "or" and "xor" as synonyms as keywords.
-fno-optional-diags: Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class.
-fpermissive: Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using -fpermissive will allow some nonconforming code to compile.
-frepo: Enable automatic template instantiation at link time. This option also implies -fno-implicit-templates.
-fno-rtti: Disable generation of information about every class with virtual functions for use by the C++ runtime type identification features (dynamic_cast and typeid). If you don't use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but it will generate it as needed.
-fstats: Emit statistics about front-end processing at the end of the compilation. This information is generally only useful to the G++ development team.
-ftemplate-depth-n: Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17.
-fno-threadsafe-statics: Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn't need to be thread-safe.
-fuse-cxa-atexit: Register destructors for objects with static storage duration with the "__cxa_atexit" function rather than the "atexit"function. This option is required for fully standards-compliant handling of static destructors, but will only work if your C library supports "__cxa_atexit".
-fvisibility-inlines-hidden: Causes all inlined methods to be marked 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. While it can cause bloating through duplication of code within each DSO where it is used, often the wastage is less than the considerable space occupied by a long symbol name in the export table which is typical when using templates and namespaces. For even more savings, combine with the "-fvisibility=hidden" switch.
-fno-weak: Do not use weak symbol support, even if it is provided by the linker. By default, G++ will use weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it will result in inferior code and has no benefits. This option may be removed in a future release of G++.
-nostdinc++: Do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library.)

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

-fno-default-inline

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

-Wabi (C++ only)

Warn when G++ generates code that is probably not compatible with the vendor-neutral C++ ABI. Although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is generating incompatible code. There may also be cases where warnings are emitted even though the code that is generated will be compatible.

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

The known incompatibilities at this point include:

*

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

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

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

*

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

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

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

*

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

        union U { int i : 4096; };

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

*

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

        struct A {};

        struct B {
          A a;
          virtual void f ();
        };

        struct C : public B, public A {};

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

*

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

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

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

Instantiations of these templates may be mangled incorrectly.

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

Warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions.

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

Warn when a class appears to be polymorphic, thereby requiring a virtual destructor, yet it declares a non-virtual one. This warning is enabled by -Wall.

-Wreorder (C++ only)

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

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

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

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

-Weffc++ (C++ only)

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

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

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

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

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

-Wno-deprecated (C++ only)

Do not warn about usage of deprecated features.

-Wno-non-template-friend (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++ only)

Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts (static_cast,reinterpret_cast, and const_cast) are less vulnerable to unintended effects and much easier to search for.

-Woverloaded-virtual (C++ only)

Warn when a function declaration hides virtual functions from a base class. For example, in:

        struct A {
          virtual void f();
        };

        struct B: public A {
          void f(int);
        };

the "A" class version of "f" is hidden in "B", and code like:

        B* b;
        b->f();

will fail to compile.

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

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

-Wsign-promo (C++ only)

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

-Wsynth (C++ only)

Warn when G++'s synthesis behavior does not match that of cfront. For instance:

        struct A {
          operator int ();
          A& operator = (int);
        };

        main ()
        {
          A a,b;
          a = b;
        }

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

Options Controlling Objective-C Dialect

(NOTE: This manual does not describe the Objective-C language itself. See <http://gcc.gnu.org/readings.html> for references.)

This section describes the command-line options that are only meaningful for Objective-C programs, but you can also use most of the GNU compiler options regardless of what language your program is in. For example, you might compile a file "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 programs; you can use the other options with any language supported by GCC.

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

-fconstant-string-class=class-name

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

-fgnu-runtime

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

-fnext-runtime

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

-fno-nil-receivers

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

-fobjc-exceptions

Enable syntactic support for structured exception handling in Objective-C, similar to what is offered by C++ and Java. Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.

          @try {
            ...
               @throw expr;
            ...
          }
          @catch (AnObjCClass *exc) {
            ...
              @throw expr;
            ...
              @throw;
            ...
          }
          @catch (AnotherClass *exc) {
            ...
          }
          @catch (id allOthers) {
            ...
          }
          @finally {
            ...
              @throw expr;
            ...
          }

The @throw statement may appear anywhere in an Objective-C or Objective-C++ program; when used inside of a @catchblock, the @throw may appear without an argument (as shown above), in which case the object caught by the @catch will be rethrown.

Note that only (pointers to) Objective-C objects may be thrown and caught using this scheme. When an object is thrown, it will be caught by the nearest @catch clause capable of handling objects of that type, analogously to how "catch" blocks work in C++ and Java. A "@catch(id ...)" clause (as shown above) may also be provided to catch any and all Objective-C exceptions not caught by previous @catch clauses (if any).

The @finally clause, if present, will be executed upon exit from the immediately preceding "@try ... @catch" section. This will happen regardless of whether any exceptions are thrown, caught or rethrown inside the "@try ... @catch" section, analogously to the behavior of the "finally" clause in Java.

There are several caveats to using the new exception mechanism:

*: Although currently designed to be binary compatible with "NS_HANDLER"-style idioms provided by the"NSException" class, the new exceptions can only be used on Mac OS X 10.3 (Panther) and later systems, due to additional functionality needed in the (NeXT) Objective-C runtime.
*: As mentioned above, the new exceptions do not support handling types other than Objective-C objects. Furthermore, when used from Objective-C++, the Objective-C exception model does not interoperate with C++exceptions at this time. This means you cannot @throw an exception from Objective-C and "catch" it in C++, or vice versa (i.e., "throw ... @catch").

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

          @synchronized (ObjCClass *guard) {
            ...
          }

Upon entering the @synchronized block, a thread of execution shall first check whether a lock has been placed on the corresponding "guard" object by another thread. If it has, the current thread shall wait until the other thread relinquishes its lock. Once "guard" becomes available, the current thread will place its own lock on it, execute the code contained in the @synchronized block, and finally relinquish the lock (thereby making "guard" available to other threads).

Unlike Java, Objective-C does not allow for entire methods to be marked @synchronized. Note that throwing exceptions out of @synchronized blocks is allowed, and will cause the guarding object to be unlocked properly.

-freplace-objc-classes

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

-fzero-link

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

-gen-decls

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

-Wno-protocol

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

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.

-Wundeclared-selector

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.

Options to Control Diagnostic Messages Formatting

Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...). The options described below can be used to control the diagnostic messages formatting algorithm, e.g. how many characters per line, how often source location information should be reported. Right now, only the C++ front end can honor these options. However it is expected, in the near future, that the remaining front ends would be able to digest them correctly.

-fmessage-length=n: Try to format error messages so that they fit on lines of about n characters. The default is 72 characters for g++ and 0 for the rest of the front ends supported by GCC. If n is zero, then no line-wrapping will be done; each error message will appear on a single line.
-fdiagnostics-show-location=once: Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit once source location information; that is, in case the message is too long to fit on a single physical line and has to be wrapped, the source location won't be emitted (as prefix) again, over and over, in subsequent continuation lines. This is the default behavior.
-fdiagnostics-show-location=every-line: Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the same source location information (as prefix) for physical lines that result from the process of breaking a message which is too long to fit on a single line.

Options to Request or Suppress Warnings

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

You can request many specific warnings with options beginning -W, for example -Wimplicit to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning -Wno- to turn off warnings; for example, -Wno-implicit. This manual lists only one of the two forms, whichever is not the default.

The following options control the amount and kinds of warnings produced by GCC; for further, language-specific options also refer to C++ Dialect Options and Objective-C Dialect Options.

-fsyntax-only

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

-pedantic

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

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

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

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

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

Where the standard specified with -std represents a GNU extended dialect of C, such as gnu89 or gnu99, there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from -pedanticare given where they are required by the base standard. (It would not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.)

-pedantic-errors

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

-w

Inhibit all warning messages.

-Wno-import

Inhibit warning messages about the use of #import.

-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.

-Wcomment

Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a Backslash-Newline appears in a //comment.

-Wformat

Check calls to "printf" and "scanf", etc., to make sure that the arguments supplied have types appropriate to the format string specified, and that the conversions specified in the format string make sense. This includes standard functions, and others specified by format attributes, in the "printf", "scanf", "strftime" and "strfmon" (an X/Open extension, not in the C standard) families.

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

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

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

-Wformat-y2k

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

-Wno-format-extra-args

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

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

-Wno-format-zero-length

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

-Wformat-nonliteral

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

-Wformat-security

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

-Wformat=2

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

-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++, and Objective-C only)

Warn about uninitialized variables which are initialized with themselves. Note this option can only be used with the -Wuninitialized option, which in turn only works with -O1 and above.

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

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

-Wimplicit-int

Warn when a declaration does not specify a type.

-Wimplicit-function-declaration

-Werror-implicit-function-declaration

Give a warning (or error) whenever a function is used before being declared.

-Wimplicit

Same as -Wimplicit-int and -Wimplicit-function-declaration.

-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.

-Wmissing-braces

Warn if an aggregate or union initializer is not fully bracketed. In the following example, the initializer for a is not fully bracketed, but that for b is fully bracketed.

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

-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 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, every "else" branch belongs to the innermost possible "if" statement, which in this example is "if (b)". This is often not what the programmer expected, as illustrated in the above example by indentation the programmer chose. When there is the potential for this confusion, GCC will issue a warning when this flag is specified. To eliminate the warning, add explicit braces around the innermost "if" statement so there is no way the "else" could belong to the enclosing "if". The resulting code would look like this:

-Wsequence-point

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

The C standard defines the order in which expressions in a 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 standard specifies 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 present implementation of this option only works for C programs. A future implementation may also work for C++programs.

The C 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 GCCreadings page, at <http://gcc.gnu.org/readings.html>.

-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".

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.

-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.

-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.

-Wtrigraphs

Warn if any trigraphs are encountered that might change the meaning of the program (trigraphs within comments are not warned about).

-Wunused-function

Warn whenever a static function is declared but not defined or a non\-inline static function is unused.

-Wunused-label

Warn whenever a label is declared but not used.

To suppress this warning use the unused attribute.

-Wunused-parameter

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

To suppress this warning use the unused attribute.

-Wunused-variable

Warn whenever a local variable or non-constant static variable is unused aside from its declaration

To suppress this warning use the unused attribute.

-Wunused-value

Warn whenever a statement computes a result that is explicitly not used.

To suppress this warning cast the expression to void.

-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.

These warnings are possible only in optimizing compilation, because they require data flow information that is computed only when optimizing. If you don't specify -O, you simply won't get these warnings.

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

These warnings occur only for variables that are candidates for register allocation. Therefore, they do not occur for a variable that is declared "volatile", or whose address is taken, or whose size is other than 1, 2, 4 or 8 bytes. Also, they do not occur for structures, unions or arrays, even when they are in registers.

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

These warnings are made optional because GCC is not smart enough to see all the reasons why the code might be correct despite appearing to have an error. Here is one example of how this can happen:

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

If the value of "y" is always 1, 2 or 3, then "x" is always initialized, but GCC doesn't know this. Here is another common case:

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

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

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

The compiler sees only the calls to "setjmp". It cannot know where "longjmp" will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there is in fact no problem because"longjmp" cannot in fact be called at the place which would cause a problem.

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

-Wunknown-pragmas

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

-Wstrict-aliasing

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

-Wall

All of the above -W options combined. 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 Dialect Options.

The following -W... options 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.

-Wextra

(This option used to be called -W. The older name is still supported, but the newer name is more descriptive.) Print extra warning messages for these events:

*

A function can return either with or without a value. (Falling off the end of the function body is considered returning without a value.) For example, this function would evoke such a warning:

        foo (a)
        {
          if (a > 0)
            return a;
        }

*

An expression-statement or the left-hand side of a comma expression contains no side effects. To suppress the warning, cast the unused expression to void. For example, an expression such as x[i,j] will cause a warning, but x[(void)i,j] will not.

*

An unsigned value is compared against zero with < or >=.

*

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.

*

Storage-class specifiers like "static" are not the first things in a declaration. According to the C Standard, this usage is obsolescent.

*

The return type of a function has a type qualifier such as "const". Such a type qualifier has no effect, since the value returned by a function is not an lvalue. (But don't warn about the GNU extension of "volatile void"return types. That extension will be warned about if -pedantic is specified.)

*

If -Wall or -Wunused is also specified, warn about unused arguments.

*

A comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. (But don't warn if -Wno-sign-compare is also specified.)

*

An aggregate has an initializer which does not initialize all members. For example, the following code would cause such a warning, because "x.h" would be implicitly initialized to zero:

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

*

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

        void foo(bar) { }

*

An empty body occurs in an if or else statement.

*

A pointer is compared against integer zero with <, <=, >, or >=.

*

A variable might be changed by longjmp or vfork.

*

Any of several floating-point events that often indicate errors, such as overflow, underflow, loss of precision, etc.

*<(C++ only)>

An enumerator and a non-enumerator both appear in a conditional expression.

*<(C++ only)>

A non-static reference or non-static const member appears in a class without constructors.

*<(C++ only)>

Ambiguous virtual bases.

*<(C++ only)>

Subscripting an array which has been declared register.

*<(C++ only)>

Taking the address of a variable which has been declared register.

*<(C++ only)>

A base class is not initialized in a derived class' copy constructor.

-Wno-div-by-zero

Do not warn about compile-time integer division by zero. Floating point division by zero is not warned about, as it can be a legitimate way of obtaining infinities and NaNs.

-Wsystem-headers

Print warning messages for constructs found in system header files. Warnings from system headers are normally suppressed, on the assumption that they usually do not indicate real problems and would only make the compiler output harder to read. Using this command line option tells GCC to emit warnings from system headers as if they occurred in user code. However, note that using -Wall in conjunction with this option will not warn about unknown pragmas in system headers---for that, -Wunknown-pragmas must also be used.

-Wfloat-equal

Warn if floating point values are used in equality comparisons.

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

-Wtraditional (C only)

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

*: Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes place within string literals, but does not in ISO C.
*: In traditional C, some preprocessor directives did not exist. Traditional preprocessors would only consider a line to be a directive if the # appeared in column 1 on the line. Therefore -Wtraditional warns about directives that traditional C understands but would ignore because the # does not appear as the first character on the line. It also suggests you hide directives like #pragma not understood by traditional C by indenting them. Some traditional implementations would not recognize #elif, so it suggests avoiding it altogether.
*: A function-like macro that appears without arguments.
*: The unary plus operator.
*: The U integer constant suffix, or the F or L floating point constant suffixes. (Traditional C does support the Lsuffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in "<limits.h>". Use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases.
*: A function declared external in one block and then used after the end of the block.
*: A "switch" statement has an operand of type "long".
*: A non-"static" function declaration follows a "static" one. This construct is not accepted by some traditional C compilers.
*: The ISO type of an integer constant has a different width or signedness from its traditional type. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.
*: Usage of ISO string concatenation is detected.
*: Initialization of automatic aggregates.
*: Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
*: Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the assumption that the zero initializer in user code appears conditioned on e.g. "__STDC__" to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case.
*: Conversions by prototypes between fixed/floating point values and vice versa. The absence of these prototypes when compiling with traditional C would cause serious problems. This is a subset of the possible conversion warnings, for the full set use -Wconversion.
*: Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features will appear in your code when using libiberty's traditional C compatibility macros, "PARAMS" and "VPARAMS". This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility.

-Wdeclaration-after-statement (C only)

Warn when a declaration is found after a statement in a block. This construct, known from C++, was introduced with ISOC99 and is by default allowed in GCC. It is not supported by ISO C90 and was not supported by GCC versions before GCC3.0.

-Wundef

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

-Wendif-labels

Warn whenever an #else or an #endif are followed by text.

-Wshadow

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

-Wlarger-than-len

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

-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.

-Wbad-function-cast (C only)

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

-Wcast-qual

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

-Wcast-align

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

-Wwrite-strings

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

-Wconversion

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.

Also, warn if a negative integer constant expression is implicitly converted to an unsigned type. For example, warn about the assignment "x = -1" if "x" is unsigned. But do not warn about explicit casts like "(unsigned) -1".

-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.

-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.)

-Wstrict-prototypes (C only)

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

-Wold-style-definition (C only)

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

-Wmissing-prototypes (C only)

Warn if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. The aim is to detect global functions that fail to be declared in header files.

-Wmissing-declarations (C only)

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.

-Wmissing-noreturn

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

-Wmissing-format-attribute

If -Wformat is enabled, also warn about functions which might be candidates for "format" attributes. Note these are only possible candidates, not absolute ones. GCC will guess that "format" attributes might be appropriate for any function that calls a function like "vprintf" or "vscanf", but this might not always be the case, and some functions for which "format"attributes are appropriate may not be detected. This option has no effect unless -Wformat is enabled (possibly by -Wall).

-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.

-Wno-deprecated-declarations

Do not warn about uses of functions, variables, and types marked as deprecated by using the "deprecated" attribute. (@pxref{Function Attributes}, @pxref{Variable Attributes}, @pxref{Type Attributes}.)

-Wpacked

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

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

-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 only)

Warn if an "extern" declaration is encountered within a function.

-Wunreachable-code

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

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

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

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

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

-Winline

Warn if a function can not be inlined and it was declared as inline. Even with this option, the compiler will not warn about failures to inline functions declared in system headers.

The compiler uses a variety of heuristics to determine whether or not to inline a function. For example, the compiler takes into account the size of the function being inlined and the 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 -Winlineto appear or disappear.

-Wno-invalid-offsetof (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 PODtype 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.

-Winvalid-pch

Warn if a precompiled header is found in the search path but can't be used.

-Wlong-long

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

-Wdisabled-optimization

Warn if a requested optimization pass is disabled. This warning does not generally indicate that there is anything wrong with your code; it merely indicates that GCC's optimizers were unable to handle the code effectively. Often, the problem is that your code is too big or too complex; GCC will refuse to optimize programs when the optimization itself is likely to take inordinate amounts of time.

-Werror

Make all warnings into errors.

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). GDB can work with this debugging information.

On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but will probably make other debuggers crash or refuse to read the program. If you want to control for certain whether to generate the extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).

Unlike most other C compilers, GCC allows you to use -g with -O. The shortcuts taken by optimized code may occasionally produce surprising results: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values were already at hand; some statements may execute in different places because they were moved out of loops.

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

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

-ggdb

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

-gstabs

Produce debugging information in stabs format (if that is supported), without GDB extensions. This is the format used byDBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output which is not understood by DBX or SDB. On System V Release 4 systems this option requires the GNUassembler.

-feliminate-unused-debug-symbols

Produce debugging information in stabs format (if that is supported), for only symbols that are actually used.

-gstabs+

Produce debugging information in stabs format (if that is supported), using GNU extensions understood only by the GNUdebugger (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 IBMRS/6000 systems.

-gxcoff+

Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood only by the GNUdebugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail with an error.

-gdwarf-2

Produce debugging information in DWARF version 2 format (if that is supported). This is the format used by DBX on IRIX 6.

-gvms

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

-glevel

-ggdblevel

-gstabslevel

-gcofflevel

-gxcofflevel

-gvmslevel

Request debugging information and also use level to specify how much information. The default level is 2.

Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, but no information about local variables and no line numbers.

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

Note that in order to avoid confusion between DWARF1 debug level 2, and DWARF2 -gdwarf-2 does not accept a concatenated debug level. Instead use an additional -glevel option to change the debug level for DWARF2.

-feliminate-dwarf2-dups

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

-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.

-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).

@bullet: 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.
@cvmmfu: Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
@dwnngv: 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).
@exoohw: For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus -fbranch-probabilities.
@fyppix: 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 can use to show program coverage. Each source file's note file is called auxname.gcno. Refer to the -fprofile-arcs option above for a description of auxname and instructions on how to generate test coverage data. Coverage data will match the source files more closely, if you do not optimize.

-dletters

Says to make debugging dumps during compilation at times specified by letters. This is used for debugging the compiler. The file names for most of the dumps are made by appending a pass number and a word to the dumpname. dumpname is generated from the name of the output file, if explicitly specified and it is not an executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. foo.01.rtl or foo.02.sibling). Here are the possible letters for use in letters, and their meanings:

A: Annotate the assembler output with miscellaneous debugging information.
b: Dump after computing branch probabilities, to file.12.bp.
B: Dump after block reordering, to file.31.bbro.
c: Dump after instruction combination, to the file file.20.combine.
C: Dump after the first if conversion, to the file file.14.ce1. Also dump after the second if conversion, to the filefile.21.ce2.
d: Dump after branch target load optimization, to to file.32.btl. Also dump after delayed branch scheduling, tofile.36.dbr.
D: Dump all macro definitions, at the end of preprocessing, in addition to normal output.
E: Dump after the third if conversion, to file.30.ce3.
f: Dump after control and data flow analysis, to file.11.cfg. Also dump after life analysis, to file.19.life.
F: Dump after purging "ADDRESSOF" codes, to file.07.addressof.
g: Dump after global register allocation, to file.25.greg.
G: Dump after GCSE, to file.08.gcse. Also dump after jump bypassing and control flow optimizations, tofile.10.bypass.
h: Dump after finalization of EH handling code, to file.03.eh.
i: Dump after sibling call optimizations, to file.02.sibling.
j: Dump after the first jump optimization, to file.04.jump.
k: Dump after conversion from registers to stack, to file.34.stack.
l: Dump after local register allocation, to file.24.lreg.
L: Dump after loop optimization passes, to file.09.loop and file.16.loop2.
M: Dump after performing the machine dependent reorganization pass, to file.35.mach.
n: Dump after register renumbering, to file.29.rnreg.
N: Dump after the register move pass, to file.22.regmove.
o: Dump after post-reload optimizations, to file.26.postreload.
r: Dump after RTL generation, to file.01.rtl.
R: Dump after the second scheduling pass, to file.33.sched2.
s: Dump after CSE (including the jump optimization that sometimes follows CSE), to file.06.cse.
S: Dump after the first scheduling pass, to file.23.sched.
t: Dump after the second CSE pass (including the jump optimization that sometimes follows CSE), tofile.18.cse2.
T: Dump after running tracer, to file.15.tracer.
u: Dump after null pointer elimination pass to file.05.null.
U: Dump callgraph and unit-at-a-time optimization file.00.unit.
V: Dump after the value profile transformations, to file.13.vpt.
w: Dump after the second flow pass, to file.27.flow2.
z: Dump after the peephole pass, to file.28.peephole2.
Z: Dump after constructing the web, to file.17.web.
a: Produce all the dumps listed above.
H: Produce a core dump whenever an error occurs.
m: Print statistics on memory usage, at the end of the run, to standard error.
p: Annotate the assembler output with a comment indicating which pattern and alternative was used. The length of each instruction is also printed.
P: Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp annotation.
v: For each of the other indicated dump files (except for file.01.rtl), dump a representation of the control flow graph suitable for viewing with VCG to file.pass.vcg.
x: Just generate RTL for a function instead of compiling it. Usually used with r.
y: Dump debugging information during parsing, to standard error.

-fdump-unnumbered

When doing debugging dumps (see -d option above), suppress instruction numbers and line number note output. This makes it more feasible to use diff on debugging dumps for compiler invocations with different options, in particular with and without -g.

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

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

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

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

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

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

-fdump-tree-switch (C++ only)

-fdump-tree-switch-options (C++ only)

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

address: Print the address of each node. Usually this is not meaningful as it changes according to the environment and source file. Its primary use is for tying up a dump file with a debug environment.
slim: Inhibit dumping of members of a scope or body of a function merely because that scope has been reached. Only dump such items when they are directly reachable by some other path.
all: Turn on all options.

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.
inlined: Dump after function inlining, to file.inlined.

-frandom-seed=string

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

The string should be different for every file you compile.

-fsched-verbose=n

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

For n greater than zero, -fsched-verbose outputs the same information as -dRS. For n greater than one, it also output basic block probabilities, detailed ready list information and unit/insn info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n over four, -fsched-verbose also includes dependence info.

-save-temps

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

-time

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

        # cc1 0.12 0.01
        # as 0.00 0.01

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

-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 @samp{-, without spaces between multiple switches. This is supposed to ease shell-processing.

-print-prog-name=program

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

-print-libgcc-file-name

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

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

        gcc -nostdlib <files>... `gcc -print-libgcc-file-name`

-print-search-dirs

Print the name of the configured installation directory and a list of program and library directories gcc will search---and don't do anything else.

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

-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.)

-feliminate-unused-debug-types

Normally, when producing DWARF2 output, GCC will emit debugging information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit. Sometimes this is useful, such as if, in the debugger, you want to cast a value to a type that is not actually used in your program (but is declared). More often, however, this results in a significant amount of wasted space. With this option, GCC will avoid producing debug symbol output for types that are nowhere used in the source file being compiled.

Options That Control Optimization

These options control various sorts of optimizations.

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

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

The compiler performs optimization based on the knowledge it has of the program. Using the -funit-at-a-time flag will allow the compiler to consider information gained from later functions in the file when compiling a function. Compiling multiple files at once to a single output file (and using -funit-at-a-time) will allow the compiler to use information gained from all of the files when compiling each of them.

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

-O

-O1

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

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

-O turns on the following optimization flags: -fdefer-pop -fmerge-constants -fthread-jumps -floop-optimize -fif-conversion -fif-conversion2 -fdelayed-branch -fguess-branch-probability -fcprop-registers

-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. The compiler does not perform loop unrolling or function inlining when you specify -O2. 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: -fforce-mem -foptimize-sibling-calls -fstrength-reduce -fcse-follow-jumps -fcse-skip-blocks -frerun-cse-after-loop -frerun-loop-opt -fgcse -fgcse-lm -fgcse-sm -fgcse-las -fdelete-null-pointer-checks -fexpensive-optimizations -fregmove -fschedule-insns -fschedule-insns2 -fsched-interblock -fsched-spec -fcaller-saves -fpeephole2 -freorder-blocks -freorder-functions -fstrict-aliasing -funit-at-a-time -falign-functions -falign-jumps -falign-loops -falign-labels -fcrossjumping

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, -fweb and -frename-registers options.

-O0

Do not optimize. 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 -fprefetch-loop-arrays

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

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

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

-fno-default-inline

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

-fno-defer-pop

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

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

-fforce-mem

Force memory operands to be copied into registers before doing arithmetic on them. This produces better code by making all memory references potential common subexpressions. When they are not common subexpressions, instruction combination should eliminate the separate register-load.

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

-fforce-addr

Force memory address constants to be copied into registers before doing arithmetic on them. This may produce better code just as -fforce-mem may.

-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.

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

-foptimize-sibling-calls

Optimize sibling and tail recursive calls.

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

-fno-inline

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

-finline-functions

Integrate all simple functions into their callers. The compiler heuristically decides which functions are simple enough to be worth integrating in this way.

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

Enabled at level -O3.

-finline-limit=n

By default, GCC limits the size of functions that can be inlined. This flag allows the control of this limit for functions that are explicitly marked as inline (i.e., marked with the inline keyword or defined within the class definition in c++). n is the size of functions that can be inlined in number of pseudo instructions (not counting parameter handling). The default value of n is 600. Increasing this value can result in more inlined code at the cost of compilation time and memory consumption. Decreasing usually makes the compilation faster and less code will be inlined (which presumably means slower programs). This option is particularly useful for programs that use inlining heavily such as those based on recursive templates with C++.

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

 @item max-inline-insns-single
  is set to I<n>/2.
 @item max-inline-insns-auto
  is set to I<n>/2.
 @item min-inline-insns
  is set to 130 or I<n>/4, whichever is smaller.
 @item max-inline-insns-rtl
  is set to I<n>.

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

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

-fkeep-inline-functions

Even if all calls to a given function are integrated, and the function is declared "static", nevertheless output a separate run-time callable version of the function. This switch does not affect "extern inline" functions.

-fkeep-static-consts

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

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

-fmerge-constants

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

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

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

-fmerge-all-constants

Attempt to merge identical constants and identical variables.

This option implies -fmerge-constants. In addition to -fmerge-constants this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating point types. Languages like C or C++ require each non-automatic variable to have distinct location, so using this option will result in non-conforming behavior.

-fnew-ra

Use a graph coloring register allocator. Currently this option is meant only for testing. Users should not specify this option, since it is not yet ready for production use.

-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, enabled when -fstrength-reduce is enabled.

-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.

-fstrength-reduce

Perform the optimizations of loop strength reduction and elimination of iteration variables.

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

-fthread-jumps

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

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

-fcse-follow-jumps

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

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

-fcse-skip-blocks

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

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

-frerun-cse-after-loop

Re-run common subexpression elimination after loop optimizations has been performed.

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

-frerun-loop-opt

Run the loop optimizer twice.

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

-fgcse

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

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

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

-fgcse-lm

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

Enabled by default when gcse is enabled.

-fgcse-sm

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

Enabled by default when gcse is enabled.

-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).

Enabled by default when gcse is enabled.

-floop-optimize

Perform loop optimizations: move constant expressions out of loops, simplify exit test conditions and optionally do strength-reduction and loop unrolling as well.

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

-fcrossjumping

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

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

-fif-conversion

Attempt to transform conditional jumps into branch-less equivalents. This include use of conditional moves, min, max, set flags and abs instructions, and some tricks doable by standard arithmetics. The use of conditional execution on chips where it is available is controlled by "if-conversion2".

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

-fif-conversion2

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

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

-fdelete-null-pointer-checks

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

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

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

-fexpensive-optimizations

Perform a number of minor optimizations that are relatively expensive.

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

-foptimize-register-move

-fregmove

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

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

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

-fdelayed-branch

If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions.

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

-fschedule-insns

If supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to required data being unavailable. This helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating point instruction is required.

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

-fschedule-insns2

Similar to -fschedule-insns, but requests an additional pass of instruction scheduling after register allocation has been done. This is especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle.

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

-fno-sched-interblock

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

-fno-sched-spec

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

-fsched-spec-load

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

-fsched-spec-load-dangerous

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

-fsched-stalled-insns=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.

-fsched-stalled-insns-dep=n

Define how many insn groups (cycles) will be examined for a dependency on a stalled insn that is candidate for premature removal from the queue of stalled insns. Has an effect only during the second scheduling pass, and only if -fsched-stalled-insns is used and its value is not zero.

-fsched2-use-superblocks

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

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

-fsched2-use-traces

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

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

-fcaller-saves

Enable values to be allocated in registers that will be clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls. Such allocation is done only when it seems to result in better code than would otherwise be produced.

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

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

-fmove-all-movables

Forces all invariant computations in loops to be moved outside the loop.

-freduce-all-givs

Forces all general-induction variables in loops to be strength-reduced.

Note: When compiling programs written in Fortran, -fmove-all-movables and -freduce-all-givs are enabled by default when you use the optimizer.

These options may generate better or worse code; results are highly dependent on the structure of loops within the source code.

These two options are intended to be removed someday, once they have helped determine the efficacy of various approaches to improving loop optimizations.

Please contact <gcc@gcc.gnu.org>, and describe how use of these options affects the performance of your production code. Examples of code that runs slower when these options are enabled are very valuable.

-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 a randomized model.

Sometimes GCC will opt to use a randomized model to guess branch probabilities, when none are available from either profiling feedback (-fprofile-arcs) or __builtin_expect. This means that different runs of the compiler on the same program may produce different object code.

In a hard real-time system, people don't want different runs of the compiler to produce code that has different behavior; minimizing non-determinism is of paramount import. This switch allows users to reduce non-determinism, possibly at the expense of inferior optimization.

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-functions

Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code locality. This is implemented by using special subsections ".text.hot" for most frequently executed functions and ".text.unlikely" for unlikely executed functions. Reordering is done by the linker so object file format must support named sections and linker must place them in a reasonable way.

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

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

-fstrict-aliasing

Allows 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() {
          a_union t;
          t.d = 3.0;
          return t.i;
        }

The practice of reading from a different union member than the one most recently written to (called ``type-punning'') is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type. So, the code above will work as expected. However, this code might not:

        int f() {
          a_union t;
          int* ip;
          t.d = 3.0;
          ip = &t.i;
          return *ip;
        }

Every language that wishes to perform language-specific alias analysis should define a function that computes, given an"tree" node, an alias set for the node. Nodes in different alias sets are not allowed to alias. For an example, see the C front-end function "c_get_alias_set".

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

-falign-functions

-falign-functions=n

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

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

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

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

Enabled at levels -O2, -O3.

-falign-labels

-falign-labels=n

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

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

If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead.

If n is not specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no alignment.

Enabled at levels -O2, -O3.

-falign-loops

-falign-loops=n

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

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

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

Enabled at levels -O2, -O3.

-falign-jumps

-falign-jumps=n

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

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

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

Enabled at levels -O2, -O3.

-frename-registers

Attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation. This optimization will most benefit processors with lots of registers. It can, however, make debugging impossible, since variables will no longer stay in a ``home register''.

-fweb

Constructs webs as commonly used for register allocation purposes and assign each web individual pseudo register. This allows the register allocation pass to operate on pseudos directly, but also strengthens several other optimization passes, such as CSE, loop optimizer and trivial dead code remover. It can, however, make debugging impossible, since variables will no longer stay in a ``home register''.

Enabled at levels -O3.

-fno-cprop-registers

After register allocation and post-register allocation instruction splitting, we perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy.

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

-fprofile-generate

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".

-fprofile-use

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".

The following options control compiler behavior regarding floating point arithmetic. These options trade off between speed and correctness. All must be specifically enabled.

-ffloat-store

Do not store floating point variables in registers, and inhibit other options that might change whether a floating point value is taken from a register or memory.

This option prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a "double" is supposed to have. Similarly for the x86 architecture. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use -ffloat-store for such programs, after modifying them to store all pertinent intermediate computations into variables.

-ffast-math

Sets -fno-math-errno, -funsafe-math-optimizations, -fno-trapping-math, -ffinite-math-only, -fno-rounding-math and-fno-signaling-nans.

This option causes the preprocessor macro "__FAST_MATH__" to be defined.

This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

-fno-math-errno

Do not set ERRNO after calling math functions that are executed with a single instruction, e.g., sqrt. A program that relies on IEEE exceptions for math error handling may want to use this flag for speed while maintaining IEEE arithmetic compatibility.

The default is -fmath-errno.

-funsafe-math-optimizations

Allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and (b) may violateIEEE or ANSI standards. When used at link-time, it may include libraries or startup files that change the default FPUcontrol word or other similar optimizations.

The default is -fno-unsafe-math-optimizations.

-ffinite-math-only

Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs.

This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications.

The default is -fno-finite-math-only.

-fno-trapping-math

Compile code assuming that floating-point operations cannot generate user-visible traps. These traps include division by zero, overflow, underflow, inexact result and invalid operation. This option implies -fno-signaling-nans. Setting this option may allow faster code if one relies on ``non-stop'' IEEE arithmetic, for example.

The default is -ftrapping-math.

-frounding-math

Disable transformations and optimizations that assume default floating point rounding behavior. This is round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic truncations. This option should be specified for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode. This option disables constant folding of floating point expressions at compile-time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes.

The default is -fno-rounding-math.

This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode. Future versions of GCC may provide finer control of this setting using C99's "FENV_ACCESS" pragma. This command line option will be used to specify the default state for "FENV_ACCESS".

-fsignaling-nans

Compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations. Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs. This option implies -ftrapping-math.

This option causes the preprocessor macro "__SUPPORT_SNAN__" to be defined.

The default is -fno-signaling-nans.

This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.

-fsingle-precision-constant

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

The following options control optimizations that may improve performance, but are not enabled by any -O options. This section includes experimental options that may produce broken code.

-fbranch-probabilities

After running a program compiled with -fprofile-arcs, you can compile it a second time using -fbranch-probabilities, to improve optimizations based on the number of times each branch was taken. When the program compiled with -fprofile-arcs exits it saves arc execution counts to a file called sourcename.gcda for each source file The information in this data file is very dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations.

With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be used to improve optimization. Currently, they are only used in one place: in reorg.c, instead of guessing which path a branch is mostly to take, the REG_BR_PROB values are used to exactly determine which path is taken more often.

-fprofile-values

If combined with -fprofile-arcs, it adds code so that some data about values of expressions in the program is gathered.

With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions and addsREG_VALUE_PROFILE notes to instructions for their later usage in optimizations.

-fvpt

If combined with -fprofile-arcs, it instructs the compiler to add a code to gather information about values of expressions.

With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based on them. Currently the optimizations include specialization of division operation using the knowledge about the value of the denominator.

-fnew-ra

Use a graph coloring register allocator. Currently this option is meant for testing, so we are interested to hear about miscompilations with -fnew-ra.

-ftracer

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

-funit-at-a-time

Parse the whole compilation unit before starting to produce code. This allows some extra optimizations to take place but consumes more memory.

-funroll-loops

Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. -funroll-loopsimplies -frerun-cse-after-loop. It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations). 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.

-fpeel-loops

Peels the loops for that there is enough information that they do not roll much (from profile feedback). It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations).

-funswitch-loops

Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to result of the condition).

-fold-unroll-loops

Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop, using the old loop unroller whose loop recognition is based on notes from frontend. -fold-unroll-loops implies both -fstrength-reduce and -frerun-cse-after-loop. This option makes code larger, and may or may not make it run faster.

-fold-unroll-all-loops

Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This is done using the old loop unroller whose loop recognition is based on notes from frontend. This usually makes programs run more slowly. -fold-unroll-all-loops implies the same options as -fold-unroll-loops.

-funswitch-loops

Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to result of the condition).

-funswitch-loops

Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to result of the condition).

-fprefetch-loop-arrays

If supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays.

Disabled at level -Os.

-ffunction-sections

-fdata-sections

Place each function or data item into its own section in the output file if the target supports arbitrary sections. The name of the function or the name of the data item determines the section's name in the output file.

Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format and SPARC processors running Solaris 2 have linkers with such optimizations. AIX may have these optimizations in the future.

Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker will create larger object and executable files and will also be slower. You will not be able to use "gprof" on all systems if you specify this option and you may have problems with debugging if you specify both this option and -g.

-fbranch-target-load-optimize

Perform branch target register load optimization before prologue / epilogue threading. The use of target registers can typically be exposed only during reload, thus hoisting loads out of loops and doing inter-block scheduling needs a separate optimization pass.

-fbranch-target-load-optimize2

Perform branch target register load optimization after prologue / epilogue threading.

--param name=value

In some places, GCC uses various constants to control the amount of optimization that is done. For example, GCC will not inline functions that contain more that a certain number of instructions. You can control some of these constants on the command-line using the --param option.

The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases.

In each case, the value is an integer. The allowable choices for name are given in the following table:

max-crossjump-edges: The maximum number of incoming edges to consider for crossjumping. The algorithm used by -fcrossjumping is O(N^2) in the number of edges incoming to each block. Increasing values mean more aggressive optimization, making the compile time increase with probably small improvement in executable size.
max-delay-slot-insn-search: The maximum number of instructions to consider when looking for an instruction to fill a delay slot. If more than this arbitrary number of instructions is searched, the time savings from filling the delay slot will be minimal so stop searching. Increasing values mean more aggressive optimization, making the compile time increase with probably small improvement in executable run time.
max-delay-slot-live-search: When trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compile time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph.
max-gcse-memory: The approximate maximum amount of memory that will be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization will not be done.
max-gcse-passes: The maximum number of passes of GCSE to run.
max-pending-list-length: The maximum number of pending dependencies scheduling will allow before flushing the current state and starting over. Large functions with few branches or calls can create excessively large lists which needlessly consume memory and resources.
max-inline-insns-single: Several parameters control the tree inliner used in gcc. This number sets the maximum number of instructions (counted in GCC's internal representation) in a single function that the tree inliner will consider for inlining. This only affects functions declared inline and methods implemented in a class declaration (C++). The default value is 500.
max-inline-insns-auto: When you use -finline-functions (included in -O3), a lot of functions that would otherwise not be considered for inlining by the compiler will be investigated. To those functions, a different (more restrictive) limit compared to functions declared inline can be applied. The default value is 100.
large-function-insns: The limit specifying really large functions. For functions greater than this limit inlining is constrained by --param large-function-growth. This parameter is useful primarily to avoid extreme compilation time caused by non-linear algorithms used by the backend. This parameter is ignored when -funit-at-a-time is not used. The default value is 3000.
large-function-growth: Specifies maximal growth of large function caused by inlining in percents. This parameter is ignored when -funit-at-a-time is not used. The default value is 200.
inline-unit-growth: Specifies maximal overall growth of the compilation unit caused by inlining. This parameter is ignored when -funit-at-a-time is not used. The default value is 150.
max-inline-insns-rtl: For languages that use the RTL inliner (this happens at a later stage than tree inlining), you can set the maximum allowable size (counted in RTL instructions) for the RTL inliner with this parameter. The default value is 600.
max-unrolled-insns: The maximum number of instructions that a loop should have if that loop is unrolled, and if the loop is unrolled, it determines how many times the loop code is unrolled.
max-average-unrolled-insns: The maximum number of instructions biased by probabilities of their execution that a loop should have if that loop is unrolled, and if the loop is unrolled, it determines how many times the loop code is unrolled.
max-unroll-times: The maximum number of unrollings of a single loop.
max-peeled-insns: The maximum number of instructions that a loop should have if that loop is peeled, and if the loop is peeled, it determines how many times the loop code is peeled.
max-peel-times: The maximum number of peelings of a single loop.
max-completely-peeled-insns: The maximum number of insns of a completely peeled loop.
max-completely-peel-times: The maximum number of iterations of a loop to be suitable for complete peeling.
max-unswitch-insns: The maximum number of insns of an unswitched loop.
max-unswitch-level: The maximum number of branches unswitched in a single loop.
hot-bb-count-fraction: Select fraction of the maximal count of repetitions of basic block in program given basic block needs to have to be considered hot.
hot-bb-frequency-fraction: Select fraction of the maximal frequency of executions of basic block in function given basic block needs to have to be considered hot
tracer-dynamic-coverage
tracer-dynamic-coverage-feedback: This value is used to limit superblock formation once the given percentage of executed instructions is covered. This limits unnecessary code size expansion.
The tracer-dynamic-coverage-feedback is used only when profile feedback is available. The real profiles (as opposed to statically estimated ones) are much less balanced allowing the threshold to be larger value.
tracer-max-code-growth: Stop tail duplication once code growth has reached given percentage. This is rather hokey argument, as most of the duplicates will be eliminated later in cross jumping, so it may be set to much higher values than is the desired code growth.
tracer-min-branch-ratio: Stop reverse growth when the reverse probability of best edge is less than this threshold (in percent).
tracer-min-branch-ratio
tracer-min-branch-ratio-feedback: Stop forward growth if the best edge do have probability lower than this threshold.
Similarly to tracer-dynamic-coverage two values are present, one for compilation for profile feedback and one for compilation without. The value for compilation with profile feedback needs to be more conservative (higher) in order to make tracer effective.
max-cse-path-length: Maximum number of basic blocks on path that cse considers.
ggc-min-expand: GCC uses a garbage collector to manage its own memory allocation. This parameter specifies the minimum percentage by which the garbage collector's heap should be allowed to expand between collections. Tuning this may improve compilation speed; it has no effect on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If "getrlimit" is available, the notion of ``RAM'' is the smallest of actual RAM, RLIMIT_RSS, RLIMIT_DATA and RLIMIT_AS. IfGCC is not able to calculate RAM on a particular platform, the lower bound of 30% is used. Setting this parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity. This is extremely slow, but can be useful for debugging.
ggc-min-heapsize: Minimum size of the garbage collector's heap before it begins bothering to collect garbage. The first collection occurs after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. Again, tuning this may improve compilation speed, and has no effect on code generation.
The default is RAM/8, with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). If "getrlimit" is available, the notion of ``RAM'' is the smallest of actual RAM, RLIMIT_RSS,RLIMIT_DATA and RLIMIT_AS. If GCC is not able to calculate RAM on a particular platform, the lower bound is used. Setting this parameter very large effectively disables garbage collection. Setting this parameter andggc-min-expand to zero causes a full collection to occur at every opportunity.
max-reload-search-insns: The maximum number of instruction reload should look backward for equivalent register. Increasing values mean more aggressive optimization, making the compile time increase with probably slightly better performance. The default value is 100.
max-cselib-memory-location: The maximum number of memory locations cselib should take into acount. Increasing values mean more aggressive optimization, making the compile time increase with probably slightly better performance. The default value is 500.
min-pretend-dynamic-size: Force any automatic object whose size in bytes is equal to or greater than the specified value to be allocated dynamically, as if their size wasn't known to compile time. This enables their storage to be released at the end of the block containing them, reducing total stack usage if multiple functions with heavy stack use are inlined into a single function. It won't have any effect on objects that are suitable for allocation to registers (i.e., that are sufficiently small and that don't have their address taken), nor on objects allocated in the outermost block of a function. The default, zero, causes objects whose sizes are known at compile time to have storage allocated at function entry.
reorder-blocks-duplicate
reorder-blocks-duplicate-feedback: Used by basic block reordering pass to decide whether to use unconditional branch or duplicate the code on its destination. Code is duplicated when its estimated size is smaller than this value multiplied by the estimated size of unconditional jump in the hot spots of the program.
The reorder-block-duplicate-feedback is used only when profile feedback is available and may be set to higher values than reorder-block-duplicate since information about the hot spots is more accurate.

Options Controlling the Preprocessor

These options control the C preprocessor, which is run on each C source file before actual compilation.

If you use the -E option, nothing is done except preprocessing. Some of these options make sense only together with -E because they cause the preprocessor output to be unsuitable for actual compilation.

: You can use -Wp,option to bypass the compiler driver and pass option directly through to the preprocessor. If optioncontains commas, it is split into multiple options at the commas. However, many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses this phase. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wpand let the driver handle the options instead.

-Xpreprocessor option

Pass option as an option to the preprocessor. You can use this to supply system-specific preprocessor options whichGCC does not know how to recognize.

If you want to pass an option that takes an argument, you must use -Xpreprocessor twice, once for the option and once for the argument.

-D name

Predefine name as a macro, with definition 1.

-D name=definition

Predefine name as a macro, with definition definition. The contents of definition are tokenized and processed as if they appeared during translation phase three in a #define directive. In particular, the definition will be truncated by embedded newline characters.

If you are invoking the preprocessor from a shell or shell-like program you may need to use the shell's quoting syntax to protect characters such as spaces that have a meaning in the shell syntax.

If you wish to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you will need to quote the option. With sh and csh,-D'name(args...)=definition' works.

-D and -U options are processed in the order they are given on the command line. All -imacros file and -include fileoptions are processed after all -D and -U options.

-U name

Cancel any previous definition of name, either built in or provided with a -D option.

-undef

Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined.

-I dir

Add the directory dir to the list of directories to be searched for header files. Directories named by -I are searched before the standard system include directories. If the directory dir is a standard system include directory, the option is ignored to ensure that the default search order for system directories and the special treatment of system headers are not defeated .

-o file

Write output to file. This is the same as specifying file as the second non-option argument to cpp. gcc has a different interpretation of a second non-option argument, so you must use -o to specify the output file.

-Wall

Turns on all optional warnings which are desirable for normal code. At present this is -Wcomment, -Wtrigraphs, -Wmultichar and a warning about integer promotion causing a change of sign in "#if" expressions. Note that many of the preprocessor's warnings are on by default and have no options to control them.

-Wcomment

-Wcomments

Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline appears in a //comment. (Both forms have the same effect.)

-Wtrigraphs

@anchor{Wtrigraphs} Most trigraphs in comments cannot affect the meaning of the program. However, a trigraph that would form an escaped newline (??/ at the end of a line) can, by changing where the comment begins or ends. Therefore, only trigraphs that would form escaped newlines produce warnings inside a comment.

This option is implied by -Wall. If -Wall is not given, this option is still enabled unless trigraphs are enabled. To get trigraph conversion without warnings, but get the other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs.

-Wtraditional

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 problematic constructs which should be avoided.

-Wimport

Warn the first time #import is used.

-Wundef

Warn whenever an identifier which is not a macro is encountered in an #if directive, outside of defined. Such identifiers are replaced with zero.

-Wunused-macros

Warn about macros defined in the main file that are unused. A macro is used if it is expanded or tested for existence at least once. The preprocessor will also warn if the macro has not been used at the time it is redefined or undefined.

Built-in macros, macros defined on the command line, and macros defined in include files are not warned about.

Note: If a macro is actually used, but only used in skipped conditional blocks, then CPP will report it as unused. To avoid the warning in such a case, you might improve the scope of the macro's definition by, for example, moving it into the first skipped block. Alternatively, you could provide a dummy use with something like:

        #if defined the_macro_causing_the_warning
        #endif

-Wendif-labels

Warn whenever an #else or an #endif are followed by text. This usually happens in code of the form

        #if FOO
        ...
        #else FOO
        ...
        #endif FOO

The second and third "FOO" should be in comments, but often are not in older programs. This warning is on by default.

-Werror

Make all warnings into hard errors. Source code which triggers warnings will be rejected.

-Wsystem-headers

Issue warnings for code in system headers. These are normally unhelpful in finding bugs in your own code, therefore suppressed. If you are responsible for the system library, you may want to see them.

-w

Suppress all warnings, including those which GNU CPP issues by default.

-pedantic

Issue all the mandatory diagnostics listed in the C standard. Some of them are left out by default, since they trigger frequently on harmless code.

-pedantic-errors

Issue all the mandatory diagnostics, and make all mandatory diagnostics into errors. This includes mandatory diagnostics that GCC issues without -pedantic but treats as warnings.

-M

Instead of outputting the result of preprocessing, output a rule suitable for make describing the dependencies of the main source file. The preprocessor outputs one make rule containing the object file name for that source file, a colon, and the names of all the included files, including those coming from -include or -imacros command line options.

Unless specified explicitly (with -MT or -MQ), the object file name consists of the basename of the source file with any suffix replaced with object file suffix. If there are many included files then the rule is split into several lines using \-newline. The rule has no commands.

This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such debug output with the dependency rules you should explicitly specify the dependency output file with -MF, or use an environment variable likeDEPENDENCIES_OUTPUT. Debug output will still be sent to the regular output stream as normal.

Passing -M to the driver implies -E, and suppresses warnings with an implicit -w.

-MM

Like -M but do not mention header files that are found in system header directories, nor header files that are included, directly or indirectly, from such a header.

This implies that the choice of angle brackets or double quotes in an #include directive does not in itself determine whether that header will appear in -MM dependency output. This is a slight change in semantics from GCC versions 3.0 and earlier.

@anchor{dashMF}

-MF file

When used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given the preprocessor sends the rules to the same place it would have sent preprocessed output.

When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.

-MG

In conjunction with an option such as -M requesting dependency generation, -MG assumes missing header files are generated files and adds them to the dependency list without raising an error. The dependency filename is taken directly from the "#include" directive without prepending any path. -MG also suppresses preprocessed output, as a missing header file renders this useless.

This feature is used in automatic updating of makefiles.

-MP

This option instructs CPP to add a phony target for each dependency other than the main file, causing each to depend on nothing. These dummy rules work around errors make gives if you remove header files without updating the Makefile to match.

This is typical output:

        test.o: test.c test.h

        test.h:

-MT target

Change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input file, including any path, deletes any file suffix such as .c, and appends the platform's usual object suffix. The result is the target.

An -MT option will set the target to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to -MT, or use multiple -MT options.

For example, -MT '$(objpfx)foo.o' might give

        $(objpfx)foo.o: foo.c

-MQ target

Same as -MT, but it quotes any characters which are special to Make. -MQ '$(objpfx)foo.o' gives

        $$(objpfx)foo.o: foo.c

The default target is automatically quoted, as if it were given with -MQ.

-MD

-MD is equivalent to -M -MF file, except that -E is not implied. The driver determines file based on whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d, otherwise it take the basename of the input file and applies a .d suffix.

If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file (but@pxref{dashMF,,-MF}), but if used without -E, each -o is understood to specify a target object file.

Since -E is not implied, -MD can be used to generate a dependency output file as a side-effect of the compilation process.

-MMD

Like -MD except mention only user header files, not system -header files.

-fpch-deps

When using precompiled headers, this flag will cause the dependency-output flags to also list the files from the precompiled header's dependencies. If not specified only the precompiled header would be listed and not the files that were used to create it because those files are not consulted when a precompiled header is used.

-x c

-x c++

-x objective-c

-x assembler-with-cpp

Specify the source language: C, C++, Objective-C, or assembly. This has nothing to do with standards conformance or extensions; it merely selects which base syntax to expect. If you give none of these options, cpp will deduce the language from the extension of the source file: .c, .cc, .m, or .S. Some other common extensions for C++ and assembly are also recognized. If cpp does not recognize the extension, it will treat the file as C; this is the most generic mode.

Note: Previous versions of cpp accepted a -lang option which selected both the language and the standards conformance level. This option has been removed, because it conflicts with the -l option.

-std=standard

-ansi

Specify the standard to which the code should conform. Currently CPP knows about C and C++ standards; others may be added in the future.

standard may be one of:

iso9899:1990
c89: The ISO C standard from 1990. c89 is the customary shorthand for this version of the standard.
The -ansi option is equivalent to -std=c89.
iso9899:199409: The 1990 C standard, as amended in 1994.
iso9899:1999
c99
iso9899:199x
c9x: The revised ISO C standard, published in December 1999. Before publication, this was known as C9X.
gnu89: The 1990 C standard plus GNU extensions. This is the default.
gnu99
gnu9x: The 1999 C standard plus GNU extensions.
c++98: The 1998 ISO C++ standard plus amendments.
gnu++98: The same as -std=c++98 plus GNU extensions. This is the default for C++ code.

-I-

Split the include path. Any directories specified with -I options before -I- are searched only for headers requested with"#include "file""; they are not searched for "#include <file>". If additional directories are specified with -I options after the -I-, those directories are searched for all #include directives.

In addition, -I- inhibits the use of the directory of the current file directory as the first search directory for "#include "file"".

-nostdinc

Do not search the standard system directories for header files. Only the directories you have specified with -I options (and the directory of the current file, if appropriate) are searched.

-nostdinc++

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

-include file

Process file as if "#include "file"" appeared as the first line of the primary source file. However, the first directory searched for file is the preprocessor's working directory instead of the directory containing the main source file. If not found there, it is searched for in the remainder of the "#include "..."" search chain as normal.

If multiple -include options are given, the files are included in the order they appear on the command line.

-imacros file

Exactly like -include, except that any output produced by scanning file is thrown away. Macros it defines remain defined. This allows you to acquire all the macros from a header without also processing its declarations.

All files specified by -imacros are processed before all files specified by -include.

-idirafter dir

Search dir for header files, but do it after all directories specified with -I and the standard system directories have been exhausted. dir is treated as a system include directory.

-iprefix prefix

Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you should include the final /.

-iwithprefix dir

-iwithprefixbefore dir

Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. -iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it where -idirafter would.

-isystem dir

Search dir for header files, after all directories specified by -I but before the standard system directories. Mark it as a system directory, so that it gets the same special treatment as is applied to the standard system directories.

-fdollars-in-identifiers

@anchor{fdollars-in-identifiers} Accept $ in identifiers.

-fpreprocessed

Indicate to the preprocessor that the input file has already been preprocessed. This suppresses things like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes comments, so that you can pass a file preprocessed with -C to the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends.

-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. These are the extensions that GCC uses for preprocessed files created by -save-temps.

-ftabstop=width

Set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is ignored. The default is 8.

-fexec-charset=charset

Set the execution character set, used for string and character constants. The default is UTF-8. charset can be any encoding supported by the system's "iconv" library routine.

-fwide-exec-charset=charset

Set the wide execution character set, used for wide string and character constants. The default is UTF-32 or UTF-16, whichever corresponds to the width of "wchar_t". As with -ftarget-charset, charset can be any encoding supported by the system's "iconv" library routine; however, you will have problems with encodings that do not fit exactly in "wchar_t".

-finput-charset=charset

Set the input character set, used for translation from the character set of the input file to the source character set used byGCC. If the locale does not specify, or GCC cannot get this information from the locale, the default is UTF-8. This can be overridden by either the locale or this command line option. Currently the command line option takes precedence if there's a conflict. charset can be any encoding supported by the system's "iconv" library routine.

-fworking-directory

Enable generation of linemarkers in the preprocessor output that will let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preprocessor will emit, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC will use this directory, when it's present in the preprocessed input, as the directory emitted as the current working directory in some debugging information formats. This option is implicitly enabled if debugging information is enabled, but this can be inhibited with the negated form -fno-working-directory. If the -P flag is present in the command line, this option has no effect, since no "#line" directives are emitted whatsoever.

-fno-show-column

Do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that does not understand the column numbers, such as dejagnu.

-A predicate=answer

Make an assertion with the predicate predicate and answer answer. This form is preferred to the older form -Apredicate(answer), which is still supported, because it does not use shell special characters.

-A -predicate=answer

Cancel an assertion with the predicate predicate and answer answer.

-dCHARS

CHARS is a sequence of one or more of the following characters, and must not be preceded by a space. Other characters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify characters whose behavior conflicts, the result is undefined.

M

Instead of the normal output, generate a list of #define directives for all the macros defined during the execution of the preprocessor, including predefined macros. This gives you a way of finding out what is predefined in your version of the preprocessor. Assuming you have no file foo.h, the command

        touch foo.h; cpp -dM foo.h

will show all the predefined macros.

D

Like M except in two respects: it does not include the predefined macros, and it outputs both the #definedirectives and the result of preprocessing. Both kinds of output go to the standard output file.

N

Like D, but emit only the macro names, not their expansions.

I

Output #include directives in addition to the result of preprocessing.

-P

Inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the preprocessor on something that is not C code, and will be sent to a program which might be confused by the linemarkers.

-C

Do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive.

You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a #.

-CC

Do not discard comments, including during macro expansion. This is like -C, except that comments contained within macros are also passed through to the output file where the macro is expanded.

In addition to the side-effects of the -C option, the -CC option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line.

The -CC option is generally used to support lint comments.

-traditional-cpp

Try to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO C preprocessors.

-trigraphs

Process trigraph sequences. These are three-character sequences, all starting with ??, that are defined by ISO C to stand for single characters. For example, ??/ stands for \, so '??/n' is a character constant for a newline. By default, GCCignores trigraphs, but in standard-conforming modes it converts them. See the -std and -ansi options.

The nine trigraphs and their replacements are

        Trigraph:       ??(  ??)  ??<  ??>  ??=  ??/  ??'  ??!  ??-
        Replacement:      [    ]    {    }    #    \    ^    |    ~

-remap

Enable special code to work around file systems which only permit very short file names, such as MS-DOS.

--help

--target-help

Print text describing all the command line options instead of preprocessing anything.

-v

Verbose mode. Print out GNU CPP's version number at the beginning of execution, and report the final form of the include path.

-H

Print the name of each header file used, in addition to other normal activities. Each name is indented to show how deep in the #include stack it is. Precompiled header files are also printed, even if they are found to be invalid; an invalid precompiled header file is printed with ...x and a valid one with ...! .

-version

--version

Print out GNU CPP's version number. With one dash, proceed to preprocess as normal. With two dashes, exit immediately.

Passing Options to the Assembler

You can pass options to the assembler.

-Wa,option: Pass option as an option to the assembler. If option contains commas, it is split into multiple options at the commas.
-Xassembler option: Pass option as an option to the assembler. You can use this to supply system-specific assembler options which GCCdoes not know how to recognize.
If you want to pass an option that takes an argument, you must use -Xassembler twice, once for the option and once for the argument.

Options for Linking

These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step.

object-file-name

A file name that does not end in a special recognized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file contents.) If linking is done, these object files are used as input to the linker.

-c

-S

-E

If any of these options is used, then the linker is not run, and object file names should not be used as arguments.

-llibrary

-l library

Search the library named library when linking. (The second alternative with the library as a separate argument is only forPOSIX compliance and is not recommended.)

It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, foo.o -lz bar.o searches library z after file foo.o but before bar.o. If bar.o refers to functions in z, those functions may not be loaded.

The linker searches a standard list of directories for the library, which is actually a file named liblibrary.a. The linker then uses this file as if it had been specified precisely by name.

The directories searched include several standard system directories plus any that you specify with -L.

Normally the files found this way are library files---archive files whose members are object files. The linker handles an archive file by scanning through it for members which define symbols that have so far been referenced but not defined. But if the file that is found is an ordinary object file, it is linked in the usual fashion. The only difference between using an -loption and specifying a file name is that -l surrounds library with lib and .a and searches several directories.

-lobjc

You need this special case of the -l option in order to link an Objective-C program.

-nostartfiles

Do not use the standard system startup files when linking. The standard system libraries are used normally, unless -nostdlib or -nodefaultlibs is used.

-nodefaultlibs

Do not use the standard system libraries when linking. Only the libraries you specify will be passed to the linker. The standard startup files are used normally, unless -nostartfiles is used. The compiler may generate calls to memcmp, memset, and memcpy for System V (and ISO C) environments or to bcopy and bzero for BSD environments. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.

-nostdlib

Do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify will be passed to the linker. The compiler may generate calls to memcmp, memset, and memcpy for System V (and ISO C) environments or to bcopy and bzero for BSD environments. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.

One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines thatGCC uses to overcome shortcomings of particular machines, or special needs for some languages.

In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that you have no unresolved references to internal GCC library subroutines. (For example, __main, used to ensure C++ constructors will be called.)

-pie

Produce a position independent executable on targets which support it. For predictable results, you must also specify the same set of options that were used to generate code (-fpie, -fPIE, or model suboptions) when you specify this option.

-s

Remove all symbol table and relocation information from the executable.

-static

On systems that support dynamic linking, this prevents linking with the shared libraries. On other systems, this option has no effect.

-shared

Produce a shared object which can then be linked with other objects to form an executable. Not all systems support this option. For predictable results, you must also specify the same set of options that were used to generate code (-fpic, -fPIC, or model suboptions) when you specify this option.[1]

-shared-libgcc

-static-libgcc

On systems that provide libgcc as a shared library, these options force the use of either the shared or static version respectively. If no shared version of libgcc was built when the compiler was configured, these options have no effect.

There are several situations in which an application should use the shared libgcc instead of the static version. The most common of these is when the application wishes to throw and catch exceptions across different shared libraries. In that case, each of the libraries as well as the application itself should use the shared libgcc.

Therefore, the G++ and GCJ drivers automatically add -shared-libgcc whenever you build a shared library or a main executable, because C++ and Java programs typically use exceptions, so this is the right thing to do.

If, instead, you use the GCC driver to create shared libraries, you may find that they will not always be linked with the shared libgcc. If GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that does not support option --eh-frame-hdr, it will link the shared version of libgcc into shared libraries by default. Otherwise, it will take advantage of the linker and optimize away the linking with the shared version of libgcc, linking with the static version of libgcc by default. This allows exceptions to propagate through such shared libraries, without incurring relocation costs at library load time.

However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ or GCJdriver, as appropriate for the languages used in the program, or using the option -shared-libgcc, such that it is linked with the shared libgcc.

-symbolic

Bind references to global symbols when building a shared object. Warn about any unresolved references (unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this option.

-Xlinker option

Pass option as an option to the linker. You can use this to supply system-specific linker options which GCC does not know how to recognize.

If you want to pass an option that takes an argument, you must use -Xlinker twice, once for the option and once for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions. It does not work to write -Xlinker ``-assert definitions'', because this passes the entire string as a single argument, which is not what the linker expects.

-Wl,option

Pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas.

-u symbol

Pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use -u multiple times with different symbols to force loading of additional library modules.

Options for Directory Search

These options specify directories to search for header files, for libraries and for parts of the compiler:

-Idir

Add the directory dir to the head of the list of directories to be searched for header files. This can be used to override a system header file, substituting your own version, since these directories are searched before the system header file directories. However, you should not use this option to add directories that contain vendor-supplied system header files (use -isystem for that). If you use more than one -I option, the directories are scanned in left-to-right order; the standard system directories come after.

If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option will be ignored. The directory will still be searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the include_next directive are not inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options.

-I-

Any directories you specify with -I options before the -I- option are searched only for the case of #include "file"; they are not searched for #include <file>.

If additional directories are specified with -I options after the -I-, these directories are searched for all #include directives. (Ordinarily all -I directories are used this way.)

In addition, the -I- option inhibits the use of the current directory (where the current input file came from) as the first search directory for #include "file". There is no way to override this effect of -I-. With -I. you can specify searching the directory which was current when the compiler was invoked. That is not exactly the same as what the preprocessor does by default, but it is often satisfactory.

-I- does not inhibit the use of the standard system directories for header files. Thus, -I- and -nostdinc are independent.

-Ldir

Add directory dir to the list of directories to be searched for -l.

-Bprefix

This option specifies where to find the executables, libraries, include files, and data files of the compiler itself.

The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a prefix for each program it tries to run, both with and without machine/version/.

For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name is not found, or if -B was not specified, the driver tries two standard prefixes, which are /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your PATHenvironment variable.

The compiler will check to see if the path provided by the -B refers to a directory, and if necessary it will add a directory separator character at the end of the path.

-B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker. They also apply to includes files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor. In this case, the compiler appends include to the prefix.

The run-time support file libgcc.a can also be searched for using the -B prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means.

Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX.

As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it will be replaced by [dir/]include. This is to help with boot-strapping the compiler.

-specs=file

Process file after the compiler reads in the standard specs file, in order to override the defaults that the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc. More than one -specs=file can be specified on the command line, and they are processed in order, from left to right.

Specifying Target Machine and Compiler Version

The usual way to run GCC is to run the executable called gcc, or <machine>-gcc when cross-compiling, or <machine>-gcc-<version> to run a version other than the one that was installed last. Sometimes this is inconvenient, so GCC provides options that will switch to another cross-compiler or version.

-b machine: The argument machine specifies the target machine for compilation.
The value to use for machine is the same as was specified as the machine type when configuring GCC as a cross-compiler. For example, if a cross-compiler was configured with configure i386v, meaning to compile for an 80386 running System V, then you would specify -b i386v to run that cross compiler.
-V version: The argument version specifies which version of GCC to run. This is useful when multiple versions are installed. For example, version might be 2.0, meaning to run GCC version 2.0.

The -V and -b options work by running the <machine>-gcc-<version> executable, so there's no real reason to use them if you can just run that directly.

Hardware Models and Configurations

Earlier we discussed the standard option -b which chooses among different installed compilers for completely different target machines, such as VAX vs. 68000 vs. 80386.

In addition, each of these target machine types can have its own special options, starting with -m, to choose among various hardware models or configurations---for example, 68010 vs 68020, floating coprocessor or none. A single installed version of the compiler can compile for any model or configuration, according to the options specified.

Some configurations of the compiler also support additional special options, usually for compatibility with other compilers on the same platform.

These options are defined by the macro "TARGET_SWITCHES" in the machine description. The default for the options is also defined by that macro, which enables you to change the defaults.

M680x0 Options

These are the -m options defined for the 68000 series. The default values for these options depends on which style of 68000 was selected when the compiler was configured; the defaults for the most common choices are given below.

-m68000

-mc68000

Generate output for a 68000. This is the default when the compiler is configured for 68000-based systems.

Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.

-m68020

-mc68020

Generate output for a 68020. This is the default when the compiler is configured for 68020-based systems.

-m68881

Generate output containing 68881 instructions for floating point. This is the default for most 68020 systems unless --nfpwas specified when the compiler was configured.

-m68030

Generate output for a 68030. This is the default when the compiler is configured for 68030-based systems.

-m68040

Generate output for a 68040. This is the default when the compiler is configured for 68040-based systems.

This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. Use this option if your 68040 does not have code to emulate those instructions.

-m68060

Generate output for a 68060. This is the default when the compiler is configured for 68060-based systems.

This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use this option if your 68060 does not have code to emulate those instructions.

-mcpu32

Generate output for a CPU32. This is the default when the compiler is configured for CPU32-based systems.

Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360.

-m5200

Generate output for a 520X ``coldfire'' family cpu. This is the default when the compiler is configured for 520X-based systems.

Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5202.

-m68020-40

Generate output for a 68040, without using any of the new instructions. This results in code which can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68040.

-m68020-60

Generate output for a 68060, without using any of the new instructions. This results in code which can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68060.

-msoft-float

Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all m68k targets. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets m68k-*-aout and m68k-*-coff do provide software floating point support.

-mshort

Consider type "int" to be 16 bits wide, like "short int".

-mnobitfield

Do not use the bit-field instructions. The -m68000, -mcpu32 and -m5200 options imply -mnobitfield.

-mbitfield

Do use the bit-field instructions. The -m68020 option implies -mbitfield. This is the default if you use a configuration designed for a 68020.

-mrtd

Use a different function-calling convention, in which functions that take a fixed number of arguments return with the "rtd"instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there.

This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.

Also, you must provide function prototypes for all functions that take variable numbers of arguments (including "printf"); otherwise incorrect code will be generated for calls to those functions.

In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)

The "rtd" instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.

-malign-int

-mno-align-int

Control whether GCC aligns "int", "long", "long long", "float", "double", and "long double" variables on a 32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Aligning variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses at the expense of more memory.

Warning: if you use the -malign-int switch, GCC will align structures containing the above types differently than most published application binary interface specifications for the m68k.

-mpcrel

Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing. -fPIC is not presently supported with -mpcrel, though this could be supported for 68020 and higher processors.

-mno-strict-align

-mstrict-align

Do not (do) assume that unaligned memory references will be handled by the system.

-msep-data

Generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute in place in an environment without virtual memory management. This option implies -fPIC.

-mno-sep-data

Generate code that assumes that the data segment follows the text segment. This is the default.

-mid-shared-library

Generate code that supports shared libraries via the library ID method. This allows for execute in place and shared libraries in an environment without virtual memory management. This option implies -fPIC.

-mno-id-shared-library

Generate code that doesn't assume ID based shared libraries are being used. This is the default.

-mshared-library-id=n

Specified the identification number of the ID based shared library being compiled. Specifying a value of 0 will generate more compact code, specifying other values will force the allocation of that number to the current library but is no more space or time efficient than omitting this option.

M68hc1x Options

These are the -m options defined for the 68hc11 and 68hc12 microcontrollers. The default values for these options depends on which style of microcontroller was selected when the compiler was configured; the defaults for the most common choices are given below.

-m6811
-m68hc11: Generate output for a 68HC11. This is the default when the compiler is configured for 68HC11-based systems.
-m6812
-m68hc12: Generate output for a 68HC12. This is the default when the compiler is configured for 68HC12-based systems.
-m68S12
-m68hcs12: Generate output for a 68HCS12.
-mauto-incdec: Enable the use of 68HC12 pre and post auto-increment and auto-decrement addressing modes.
-minmax
-nominmax: Enable the use of 68HC12 min and max instructions.
-mlong-calls
-mno-long-calls: Treat all calls as being far away (near). If calls are assumed to be far away, the compiler will use the "call" instruction to call a function and the "rtc" instruction for returning.
-mshort: Consider type "int" to be 16 bits wide, like "short int".
-msoft-reg-count=count: Specify the number of pseudo-soft registers which are used for the code generation. The maximum number is 32. Using more pseudo-soft register may or may not result in better code depending on the program. The default is 4 for 68HC11 and 2 for 68HC12.

VAX Options

These -m options are defined for the VAX:

-munix: Do not output certain jump instructions ("aobleq" and so on) that the Unix assembler for the VAX cannot handle across long ranges.
-mgnu: Do output those jump instructions, on the assumption that you will assemble with the GNU assembler.
-mg: Output code for g-format floating point numbers instead of d-format.

SPARC Options

These -m options are supported on the SPARC:

-mno-app-regs

-mapp-regs

Specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications. This is the default.

To be fully SVR4 ABI compliant at the cost of some performance loss, specify -mno-app-regs. You should compile libraries and system software with this option.

-mfpu

-mhard-float

Generate output containing floating point instructions. This is the default.

-mno-fpu

-msoft-float

Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all SPARCtargets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets sparc-*-aout and sparclite-*-* do provide software floating point support.

-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.

-mhard-quad-float

Generate output containing quad-word (long double) floating point instructions.

-msoft-quad-float

Generate output containing library calls for quad-word (long double) floating point instructions. The functions called are those specified in the SPARC ABI. This is the default.

As of this writing, there are no SPARC implementations that have hardware support for the quad-word floating point instructions. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower than calling the ABI library routines. Thus the -msoft-quad-float option is the default.

-mno-flat

-mflat

With -mflat, the compiler does not generate save/restore instructions and will use a ``flat'' or single register window calling convention. This model uses %i7 as the frame pointer and is compatible with the normal register window model. Code from either may be intermixed. The local registers and the input registers (0--5) are still treated as ``call saved'' registers and will be saved on the stack as necessary.

With -mno-flat (the default), the compiler emits save/restore instructions (except for leaf functions) and is the normal mode of operation.

These options are deprecated and will be deleted in a future GCC release.

-mno-unaligned-doubles

-munaligned-doubles

Assume that doubles have 8 byte alignment. This is the default.

With -munaligned-doubles, GCC assumes that doubles have 8 byte alignment only if they are contained in another type, or if they have an absolute address. Otherwise, it assumes they have 4 byte alignment. Specifying this option avoids some rare compatibility problems with code generated by other compilers. It is not the default because it results in a performance loss, especially for floating point code.

-mno-faster-structs

-mfaster-structs

With -mfaster-structs, the compiler assumes that structures should have 8 byte alignment. This enables the use of pairs of "ldd" and "std" instructions for copies in structure assignment, in place of twice as many "ld" and "st" pairs. However, the use of this changed alignment directly violates the SPARC ABI. Thus, it's intended only for use on targets where the developer acknowledges that their resulting code will not be directly in line with the rules of the ABI.

-mimpure-text

-mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when linking a shared object. Using this option, you can link position-dependent code into a shared object.

-mimpure-text suppresses the ``relocations remain against allocatable but non-writable sections'' linker error message. However, the necessary relocations will trigger copy-on-write, and the shared object is not actually shared across processes. Instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC.

This option is only available on SunOS and Solaris.

-mv8

-msparclite

These two options select variations on the SPARC architecture. These options are deprecated and will be deleted in a future GCC release. They have been replaced with -mcpu=xxx.

-mcypress

-msupersparc

-mf930

-mf934

These four options select the processor for which the code is optimized. These options are deprecated and will be deleted in a future GCC release. They have been replaced with -mcpu=xxx.

-mcpu=cpu_type

Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are v7, cypress, v8, supersparc, sparclite, f930, f934, hypersparc, sparclite86x, sparclet, tsc701, v9,ultrasparc, and ultrasparc3.

Default instruction scheduling parameters are used for values that select an architecture and not an implementation. These are v7, v8, sparclite, sparclet, v9.

Here is a list of each supported architecture and their supported implementations.

            v7:             cypress
            v8:             supersparc, hypersparc
            sparclite:      f930, f934, sparclite86x
            sparclet:       tsc701
            v9:             ultrasparc, ultrasparc3

By default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture. With -mcpu=cypress, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer 3xx series. This is also appropriate for the older SPARCStation 1, 2, IPX etc.

With -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture. The only difference from V7 code is that the compiler emits the integer multiply and integer divide instructions which exist in SPARC-V8 but not in SPARC-V7. With -mcpu=supersparc, the compiler additionally optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series.

With -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC architecture. This adds the integer multiply, integer divide step and scan ("ffs") instructions which exist in SPARClite but not in SPARC-V7. With -mcpu=f930, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is the original SPARClite, with noFPU. With -mcpu=f934, the compiler additionally optimizes it for the Fujitsu MB86934 chip, which is the more recent SPARClite with FPU.

With -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC architecture. This adds the integer multiply, multiply/accumulate, integer divide step and scan ("ffs") instructions which exist in SPARClet but not in SPARC-V7. With -mcpu=tsc701, the compiler additionally optimizes it for the TEMIC SPARClet chip.

With -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture. This adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers and conditional move instructions. With -mcpu=ultrasparc, the compiler additionally optimizes it for the Sun UltraSPARC I/II chips. With -mcpu=ultrasparc3, the compiler additionally optimizes it for the Sun UltraSPARC III chip.

-mtune=cpu_type

Set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that the option -mcpu=cpu_type would.

The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are those that select a particular cpu implementation. Those are cypress, supersparc, hypersparc, f930, f934, sparclite86x, tsc701,ultrasparc, and ultrasparc3.

-mv8plus

-mno-v8plus

With -mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI is that the global and out registers are considered 64-bit wide. This is enabled by default on Solaris in 32-bit mode for all SPARC-V9 processors.

-mvis

-mno-vis

With -mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions. The default is -mno-vis.

These -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments:

-mlittle-endian: Generate code for a processor running in little-endian mode. It is only available for a few configurations and most notably not on Solaris.
-m32
-m64: Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
-mcmodel=medlow: Generate code for the Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. Programs can be statically or dynamically linked.
-mcmodel=medmid: Generate code for the Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
-mcmodel=medany: Generate code for the Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
-mcmodel=embmedany: Generate code for the Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data segments must be less than 2GB in size, both starting anywhere in memory (determined at link time). The global register%g4 points to the base of the data segment. Programs are statically linked and PIC is not supported.
-mstack-bias
-mno-stack-bias: With -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by -2047 which must be added back when making stack frame references. This is the default in 64-bit mode. Otherwise, assume no such offset is present.

These switches are supported in addition to the above on Solaris:

-threads: Add support for multithreading using the Solaris threads library. This option sets flags for both the preprocessor and linker. This option does not affect the thread safety of object code produced by the compiler or that of libraries supplied with it.
-pthreads: Add support for multithreading using the POSIX threads library. This option sets flags for both the preprocessor and linker. This option does not affect the thread safety of object code produced by the compiler or that of libraries supplied with it.

ARM Options

These -m options are defined for Advanced RISC Machines (ARM) architectures:

-mapcs-frame

Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this is not strictly necessary for correct execution of the code. Specifying -fomit-frame-pointer with this option will cause the stack frames not to be generated for leaf functions. The default is -mno-apcs-frame.

-mapcs

This is a synonym for -mapcs-frame.

-mapcs-26

Generate code for a processor running with a 26-bit program counter, and conforming to the function calling standards for the APCS 26-bit option.

This option is deprecated. Future releases of the GCC will only support generating code that runs in apcs-32 mode.

-mapcs-32

Generate code for a processor running with a 32-bit program counter, and conforming to the function calling standards for the APCS 32-bit option.

This flag is deprecated. Future releases of GCC will make this flag unconditional.

-mthumb-interwork

Generate code which supports calling between the ARM and Thumb instruction sets. Without this option the two instruction sets cannot be reliably used inside one program. The default is -mno-thumb-interwork, since slightly larger code is generated when -mthumb-interwork is specified.

-mno-sched-prolog

Prevent the reordering of instructions in the function prolog, or the merging of those instruction with the instructions in the function's body. This means that all functions will start with a recognizable set of instructions (or in fact one of a choice from a small set of different function prologues), and this information can be used to locate the start if functions inside an executable piece of code. The default is -msched-prolog.

-mhard-float

Generate output containing floating point instructions. This is the default.

-msoft-float

Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all ARMtargets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

-mlittle-endian

Generate code for a processor running in little-endian mode. This is the default for all standard configurations.

-mbig-endian

Generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor.

-mwords-little-endian

This option only applies when generating code for big-endian processors. Generate code for a little-endian word order but a big-endian byte order. That is, a byte order of the form 32107654. Note: this option should only be used if you require compatibility with code for big-endian ARM processors generated by versions of the compiler prior to 2.8.

-malignment-traps

Generate code that will not trap if the MMU has alignment traps enabled. On ARM architectures prior to ARMv4, there were no instructions to access half-word objects stored in memory. However, when reading from memory a feature of theARM architecture allows a word load to be used, even if the address is unaligned, and the processor core will rotate the data as it is being loaded. This option tells the compiler that such misaligned accesses will cause a MMU trap and that it should instead synthesize the access as a series of byte accesses. The compiler can still use word accesses to load half-word data if it knows that the address is aligned to a word boundary.

This option has no effect when compiling for ARM architecture 4 or later, since these processors have instructions to directly access half-word objects in memory.

-mno-alignment-traps

Generate code that assumes that the MMU will not trap unaligned accesses. This produces better code when the target instruction set does not have half-word memory operations (i.e. implementations prior to ARMv4).

Note that you cannot use this option to access unaligned word objects, since the processor will only fetch one 32-bit aligned object from memory.

The default setting is -malignment-traps, since this produces code that will also run on processors implementing ARMarchitecture version 6 or later.

This option is deprecated and will be removed in the next release of GCC.

-mcpu=name

This specifies the name of the target ARM processor. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. Permissible names are: arm2, arm250, arm3, arm6, arm60, arm600, arm610,arm620, arm7, arm7m, arm7d, arm7dm, arm7di, arm7dmi, arm70, arm700, arm700i, arm710, arm710c, arm7100,arm7500, arm7500fe, arm7tdmi, arm8, strongarm, strongarm110, strongarm1100, arm8, arm810, arm9, arm9e,arm920, arm920t, arm926ejs, arm940t, arm9tdmi, arm10tdmi, arm1020t, arm1026ejs, arm1136js, arm1136jfs,xscale, iwmmxt, ep9312.

-mtune=name

This option is very similar to the -mcpu= option, except that instead of specifying the actual target processor type, and hence restricting which instructions can be used, it specifies that GCC should tune the performance of the code as if the target were of the type specified in this option, but still choosing the instructions that it will generate based on the cpu specified by a -mcpu= option. For some ARM implementations better performance can be obtained by using this option.

-march=name

This specifies the name of the target ARM architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. This option can be used in conjunction with or instead of the -mcpu= option. Permissible names are: armv2, armv2a, armv3, armv3m, armv4, armv4t, armv5, armv5t, armv5te, armv6j,iwmmxt, ep9312.

-mfpe=number

-mfp=number

This specifies the version of the floating point emulation available on the target. Permissible values are 2 and 3. -mfp= is a synonym for -mfpe=, for compatibility with older versions of GCC.

-mstructure-size-boundary=n

The size of all structures and unions will be rounded up to a multiple of the number of bits set by this option. Permissible values are 8 and 32. The default value varies for different toolchains. For the COFF targeted toolchain the default value is 8. Specifying the larger number can produce faster, more efficient code, but can also increase the size of the program. The two values are potentially incompatible. Code compiled with one value cannot necessarily expect to work with code or libraries compiled with the other value, if they exchange information using structures or unions.

-mabort-on-noreturn

Generate a call to the function "abort" at the end of a "noreturn" function. It will be executed if the function tries to return.

-mlong-calls

-mno-long-calls

Tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function will lie outside of the 64 megabyte addressing range of the offset based version of subroutine call instruction.

Even if this switch is enabled, not all function calls will be turned into long calls. The heuristic is that static functions, functions which have the short-call attribute, functions that are inside the scope of a #pragma no_long_calls directive and functions whose definitions have already been compiled within the current compilation unit, will not be turned into long calls. The exception to this rule is that weak function definitions, functions with the long-call attribute or the sectionattribute, and functions that are within the scope of a #pragma long_calls directive, will always be turned into long calls.

This feature is not enabled by default. Specifying -mno-long-calls will restore the default behavior, as will placing the function calls within the scope of a #pragma long_calls_off directive. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers.

-mnop-fun-dllimport

Disable support for the "dllimport" attribute.

-msingle-pic-base

Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The run-time system is responsible for initializing this register with an appropriate value before execution begins.

-mpic-register=reg

Specify the register to be used for PIC addressing. The default is R10 unless stack-checking is enabled, when R9 is used.

-mcirrus-fix-invalid-insns

Insert NOPs into the instruction stream to in order to work around problems with invalid Maverick instruction combinations. This option is only valid if the -mcpu=ep9312 option has been used to enable generation of instructions for the Cirrus Maverick floating point co-processor. This option is not enabled by default, since the problem is only present in older Maverick implementations. The default can be re-enabled by use of the -mno-cirrus-fix-invalid-insns switch.

-mpoke-function-name

Write the name of each function into the text section, directly preceding the function prologue. The generated code is similar to this:

             t0
                 .ascii "arm_poke_function_name", 0
                 .align
             t1
                 .word 0xff000000 + (t1 - t0)
             arm_poke_function_name
                 mov     ip, sp
                 stmfd   sp!, {fp, ip, lr, pc}
                 sub     fp, ip, #4

When performing a stack backtrace, code can inspect the value of "pc" stored at "fp + 0". If the trace function then looks at location "pc - 12" and the top 8 bits are set, then we know that there is a function name embedded immediately preceding this location and has length "((pc[-3]) & 0xff000000)".

-mthumb

Generate code for the 16-bit Thumb instruction set. The default is to use the 32-bit ARM instruction set.

-mtpcs-frame

Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-tpcs-frame.

-mtpcs-leaf-frame

Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-apcs-leaf-frame.

-mcallee-super-interworking

Gives all externally visible functions in the file being compiled an ARM instruction set header which switches to Thumb mode before executing the rest of the function. This allows these functions to be called from non-interworking code.

-mcaller-super-interworking

Allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code has been compiled for interworking or not. There is a small overhead in the cost of executing a function pointer if this option is enabled.

MN10300 Options

These -m options are defined for Matsushita MN10300 architectures:

-mmult-bug: Generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default.
-mno-mult-bug: Do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.
-mam33: Generate code which uses features specific to the AM33 processor.
-mno-am33: Do not generate code which uses features specific to the AM33 processor. This is the default.
-mno-crt0: Do not link in the C run-time initialization object file.
-mrelax: Indicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This option only has an effect when used on the command line for the final link step.
This option makes symbolic debugging impossible.

M32R/D Options

These -m options are defined for Renesas M32R/D architectures:

-m32r2: Generate code for the M32R/2.
-m32rx: Generate code for the M32R/X.
-m32r: Generate code for the M32R. This is the default.
-mmodel=small: Assume all objects live in the lower 16MB of memory (so that their addresses can be loaded with the "ld24" instruction), and assume all subroutines are reachable with the "bl" instruction. This is the default.
The addressability of a particular object can be set with the "model" attribute.
-mmodel=medium: Assume objects may be anywhere in the 32-bit address space (the compiler will generate "seth/add3" instructions to load their addresses), and assume all subroutines are reachable with the "bl" instruction.
-mmodel=large: Assume objects may be anywhere in the 32-bit address space (the compiler will generate "seth/add3" instructions to load their addresses), and assume subroutines may not be reachable with the "bl" instruction (the compiler will generate the much slower "seth/add3/jl" instruction sequence).
-msdata=none: Disable use of the small data area. Variables will be put into one of .data, bss, or .rodata (unless the "section" attribute has been specified). This is the default.
The small data area consists of sections .sdata and .sbss. Objects may be explicitly put in the small data area with the"section" attribute using one of these sections.
-msdata=sdata: Put small global and static data in the small data area, but do not generate special code to reference them.
-msdata=use: Put small global and static data in the small data area, and generate special instructions to reference them.
-G num: Put global and static objects less than or equal to num bytes into the small data or bss sections instead of the normal data or bss sections. The default value of num is 8. The -msdata option must be set to one of sdata or use for this option to have any effect.
All modules should be compiled with the same -G num value. Compiling with different values of num may or may not work; if it doesn't the linker will give an error message---incorrect code will not be generated.
-mdebug: Makes the M32R specific code in the compiler display some statistics that might help in debugging programs.
-malign-loops: Align all loops to a 32-byte boundary.
-mno-align-loops: Do not enforce a 32-byte alignment for loops. This is the default.
-missue-rate=number: Issue number instructions per cycle. number can only be 1 or 2.
-mbranch-cost=number: number can only be 1 or 2. If it is 1 then branches will be preferred over conditional code, if it is 2, then the opposite will apply.
-mflush-trap=number: Specifies the trap number to use to flush the cache. The default is 12. Valid numbers are between 0 and 15 inclusive.
-mno-flush-trap: Specifies that the cache cannot be flushed by using a trap.
-mflush-func=name: Specifies the name of the operating system function to call to flush the cache. The default is _flush_cache, but a function call will only be used if a trap is not available.
-mno-flush-func: Indicates that there is no OS function for flushing the cache.

IBM RS/6000 and PowerPC Options

These -m options are defined for the IBM RS/6000 and PowerPC:

-mpower

-mno-power

-mpower2

-mno-power2

-mpowerpc

-mno-powerpc

-mpowerpc-gpopt

-mno-powerpc-gpopt

-mpowerpc-gfxopt

-mno-powerpc-gfxopt

-mpowerpc64

-mno-powerpc64

GCC supports two related instruction set architectures for the RS/6000 and PowerPC. The POWER instruction set are those instructions supported by the rios chip set used in the original RS/6000 systems and the PowerPC instruction set is the architecture of the Motorola MPC5xx, MPC6xx, MPC8xx microprocessors, and the IBM 4xx microprocessors.

Neither architecture is a subset of the other. However there is a large common subset of instructions supported by both. An MQ register is included in processors supporting the POWER architecture.

You use these options to specify which instructions are available on the processor you are using. The default value of these options is determined when configuring GCC. Specifying the -mcpu=cpu_type overrides the specification of these options. We recommend you use the -mcpu=cpu_type option rather than the options listed above.

The -mpower option allows GCC to generate instructions that are found only in the POWER architecture and to use theMQ register. Specifying -mpower2 implies -power and also allows GCC to generate instructions that are present in thePOWER2 architecture but not the original POWER architecture.

The -mpowerpc option allows GCC to generate instructions that are found only in the 32-bit subset of the PowerPC architecture. Specifying -mpowerpc-gpopt implies -mpowerpc and also allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying -mpowerpc-gfxoptimplies -mpowerpc and also allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select.

The -mpowerpc64 option allows GCC to generate the additional 64-bit instructions that are found in the full PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to -mno-powerpc64.

If you specify both -mno-power and -mno-powerpc, GCC will use only the instructions in the common subset of both architectures plus some special AIX common-mode calls, and will not use the MQ register. Specifying both -mpower and-mpowerpc permits GCC to use any instruction from either architecture and to allow use of the MQ register; specify this for the Motorola MPC601.

-mnew-mnemonics

-mold-mnemonics

Select which mnemonics to use in the generated assembler code. With -mnew-mnemonics, GCC uses the assembler mnemonics defined for the PowerPC architecture. With -mold-mnemonics it uses the assembler mnemonics defined for the POWER architecture. Instructions defined in only one architecture have only one mnemonic; GCC uses that mnemonic irrespective of which of these options is specified.

GCC defaults to the mnemonics appropriate for the architecture in use. Specifying -mcpu=cpu_type sometimes overrides the value of these option. Unless you are building a cross-compiler, you should normally not specify either -mnew-mnemonics or -mold-mnemonics, but should instead accept the default.

-mcpu=cpu_type

Set architecture type, register usage, choice of mnemonics, and instruction scheduling parameters for machine typecpu_type. Supported values for cpu_type are 401, 403, 405, 405fp, 440, 440fp, 505, 601, 602, 603, 603e, 604, 604e, 620,630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, common, ec603e, G3, G4, G5, power, power2, power3, power4,power5, powerpc, powerpc64, rios, rios1, rios2, rsc, and rs64a.

-mcpu=common selects a completely generic processor. Code generated under this option will run on any POWER or PowerPC processor. GCC will use only the instructions in the common subset of both architectures, and will not use theMQ register. GCC assumes a generic processor model for scheduling purposes.

-mcpu=power, -mcpu=power2, -mcpu=powerpc, and -mcpu=powerpc64 specify generic POWER, POWER2, pure 32-bit PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes.

The other options specify a specific processor. Code generated under those options will run best on that processor, and may not run at all on others.

The -mcpu options automatically enable or disable the following options: -maltivec, -mhard-float, -mmfcrf, -mmultiple,-mnew-mnemonics, -mpower, -mpower2, -mpowerpc64, -mpowerpc-gpopt, -mpowerpc-gfxopt, -mstring. The particular options set for any particular CPU will vary between compiler versions, depending on what setting seems to produce optimal code for that CPU; it doesn't necessarily reflect the actual hardware's capabilities. If you wish to set an individual option to a particular value, you may specify it after the -mcpu option, like -mcpu=970 -mno-altivec.

On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu option at present, since AIXdoes not have full support for these options. You may still enable or disable them individually if you're sure it'll work in your environment.

-mtune=cpu_type

Set the instruction scheduling parameters for machine type cpu_type, but do not set the architecture type, register usage, or choice of mnemonics, as -mcpu=cpu_type would. The same values for cpu_type are used for -mtune as for -mcpu. If both are specified, the code generated will use the architecture, registers, and mnemonics set by -mcpu, but the scheduling parameters set by -mtune.

-maltivec

-mno-altivec

These switches enable or disable the use of built-in functions that allow access to the AltiVec instruction set. You may also need to set -mabi=altivec to adjust the current ABI with AltiVec ABI enhancements.

-mabi=spe

Extend the current ABI with SPE ABI extensions. This does not change the default ABI, instead it adds the SPE ABIextensions to the current ABI.

-mabi=no-spe

Disable Booke SPE ABI extensions for the current ABI.

-misel=yes/no

-misel

This switch enables or disables the generation of ISEL instructions.

-mspe=yes/no

-mspe

This switch enables or disables the generation of SPE simd instructions.

-mfloat-gprs=yes/no

-mfloat-gprs

This switch enables or disables the generation of floating point operations on the general purpose registers for architectures that support it. This option is currently only available on the MPC8540.

-mfull-toc

-mno-fp-in-toc

-mno-sum-in-toc

-mminimal-toc

Modify generation of the TOC (Table Of Contents), which is created for every executable file. The -mfull-toc option is selected by default. In that case, GCC will allocate at least one TOC entry for each unique non-automatic variable reference in your program. GCC will also place floating-point constants in the TOC. However, only 16,384 entries are available in the TOC.

If you receive a linker error message that saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC and -mno-sum-in-toc forces GCC to generate code to calculate the sum of an address and a constant at run-time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space.

If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc instead. This option causes GCC to make only one TOC entry for every file. When you specify this option, GCC will produce code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently executed code.

-maix64

-maix32

Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit "long" type, and the infrastructure needed to support them. Specifying -maix64 implies -mpowerpc64 and -mpowerpc, while -maix32 disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32.

-mxl-call

-mno-xl-call

On AIX, pass floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs. The AIX calling convention was extended but not initially documented to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than declared. AIX XL compilers access floating point arguments which do not fit in the RSA from the stack when a subroutine is compiled without optimization. Because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default and only is necessary when calling subroutines compiled by AIX XL compilers without optimization.

-mpe

Support IBM RS/6000 SP Parallel Environment (PE). Link an application written to use message passing with special startup code to enable the application to run. The system must have PE installed in the standard location (/usr/lpp/ppe.poe/), or the specs file must be overridden with the -specs= option to specify the appropriate directory location. The Parallel Environment does not support threads, so the -mpe option and the -pthread option are incompatible.

-malign-natural

-malign-power

On AIX, Darwin, and 64-bit PowerPC GNU/Linux, the option -malign-natural overrides the ABI-defined alignment of larger types, such as floating-point doubles, on their natural size-based boundary. The option -malign-power instructs GCC to follow the ABI-specified alignment rules. GCC defaults to the standard alignment defined in the ABI.

-msoft-float

-mhard-float

Generate code that does not use (uses) the floating-point register set. Software floating point emulation is provided if you use the -msoft-float option, and pass the option to GCC when linking.

-mmultiple

-mno-multiple

Generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use -mmultiple on little endian PowerPC systems, since those instructions do not work when the processor is in little endian mode. The exceptions are PPC740 and PPC750 which permit the instructions usage in little endian mode.

-mstring

-mno-string

Generate code that uses (does not use) the load string instructions and the store string word instructions to save multiple registers and do small block moves. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use -mstring on little endian PowerPC systems, since those instructions do not work when the processor is in little endian mode. The exceptions are PPC740 and PPC750 which permit the instructions usage in little endian mode.

-mupdate

-mno-update

Generate code that uses (does not use) the load or store instructions that update the base register to the address of the calculated memory location. These instructions are generated by default. If you use -mno-update, there is a small window between the time that the stack pointer is updated and the address of the previous frame is stored, which means code that walks the stack frame across interrupts or signals may get corrupted data.

-mfused-madd

-mno-fused-madd

Generate code that uses (does not use) the floating point multiply and accumulate instructions. These instructions are generated by default if hardware floating is used.

-mno-bit-align

-mbit-align

On System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field.

For example, by default a structure containing nothing but 8 "unsigned" bit-fields of length 1 would be aligned to a 4 byte boundary and have a size of 4 bytes. By using -mno-bit-align, the structure would be aligned to a 1 byte boundary and be one byte in size.

-mno-strict-align

-mstrict-align

On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references will be handled by the system.

-mrelocatable

-mno-relocatable

On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a different address at runtime. If you use -mrelocatable on any module, all objects linked together must be compiled with -mrelocatable or -mrelocatable-lib.

-mrelocatable-lib

-mno-relocatable-lib

On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a different address at runtime. Modules compiled with -mrelocatable-lib can be linked with either modules compiled without -mrelocatable and -mrelocatable-lib or with modules compiled with the -mrelocatable options.

-mno-toc

-mtoc

On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program.

-mlittle

-mlittle-endian

On System V.4 and embedded PowerPC systems compile code for the processor in little endian mode. The -mlittle-endian option is the same as -mlittle.

-mbig

-mbig-endian

On System V.4 and embedded PowerPC systems compile code for the processor in big endian mode. The -mbig-endianoption is the same as -mbig.

-mdynamic-no-pic

On Darwin and Mac OS X systems, compile code so that it is not relocatable, but that its external references are relocatable. The resulting code is suitable for applications, but not shared libraries.

-mprioritize-restricted-insns=priority

This option controls the priority that is assigned to dispatch-slot restricted instructions during the second scheduling pass. The argument priority takes the value 0/1/2 to assign no/highest/second-highest priority to dispatch slot restricted instructions.

-msched-costly-dep=dependence_type

This option controls which dependences are considered costly by the target during instruction scheduling. The argumentdependence_type takes one of the following values: no: no dependence is costly, all: all dependences are costly,true_store_to_load: a true dependence from store to load is costly, store_to_load: any dependence from store to load is costly, number: any dependence which latency >= number is costly.

-minsert-sched-nops=scheme

This option controls which nop insertion scheme will be used during the second scheduling pass. The argument schemetakes one of the following values: no: Don't insert nops. pad: Pad with nops any dispatch group which has vacant issue slots, according to the scheduler's grouping. regroup_exact: Insert nops to force costly dependent insns into separate groups. Insert exactly as many nops as needed to force an insn to a new group, according to the estimated processor grouping. number: Insert nops to force costly dependent insns into separate groups. Insert number nops to force an insn to a new group.

-mcall-sysv

On System V.4 and embedded PowerPC systems compile code using calling conventions that adheres to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC using powerpc-*-eabiaix.

-mcall-sysv-eabi

Specify both -mcall-sysv and -meabi options.

-mcall-sysv-noeabi

Specify both -mcall-sysv and -mno-eabi options.

-mcall-solaris

On System V.4 and embedded PowerPC systems compile code for the Solaris operating system.

-mcall-linux

On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system.

-mcall-gnu

On System V.4 and embedded PowerPC systems compile code for the Hurd-based GNU system.

-mcall-netbsd

On System V.4 and embedded PowerPC systems compile code for the NetBSD operating system.

-maix-struct-return

Return all structures in memory (as specified by the AIX ABI).

-msvr4-struct-return

Return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI).

-mabi=altivec

Extend the current ABI with AltiVec ABI extensions. This does not change the default ABI, instead it adds the AltiVec ABIextensions to the current ABI.

-mabi=no-altivec

Disable AltiVec ABI extensions for the current ABI.

-mprototype

-mno-prototype

On System V.4 and embedded PowerPC systems assume that all calls to variable argument functions are properly prototyped. Otherwise, the compiler must insert an instruction before every non prototyped call to set or clear bit 6 of the condition code register (CR) to indicate whether floating point values were passed in the floating point registers in case the function takes a variable arguments. With -mprototype, only calls to prototyped variable argument functions will set or clear the bit.

-msim

On embedded PowerPC systems, assume that the startup module is called sim-crt0.o and that the standard C libraries are libsim.a and libc.a. This is the default for powerpc-*-eabisim. configurations.

-mmvme

On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries arelibmvme.a and libc.a.

-mads

On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries arelibads.a and libc.a.

-myellowknife

On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are libyk.aand libc.a.

-mvxworks

On System V.4 and embedded PowerPC systems, specify that you are compiling for a VxWorks system.

-mwindiss

Specify that you are compiling for the WindISS simulation environment.

-memb

On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags header to indicate that eabi extended relocations are used.

-meabi

-mno-eabi

On System V.4 and embedded PowerPC systems do (do not) adhere to the Embedded Applications Binary Interface (eabi) which is a set of modifications to the System V.4 specifications. Selecting -meabi means that the stack is aligned to an 8 byte boundary, a function "__eabi" is called to from "main" to set up the eabi environment, and the -msdata option can use both "r2" and "r13" to point to two separate small data areas. Selecting -mno-eabi means that the stack is aligned to a 16 byte boundary, do not call an initialization function from "main", and the -msdata option will only use "r13" to point to a single small data area. The -meabi option is on by default if you configured GCC using one of the powerpc*-*-eabi*options.

-msdata=eabi

On System V.4 and embedded PowerPC systems, put small initialized "const" global and static data in the .sdata2section, which is pointed to by register "r2". Put small initialized non-"const" global and static data in the .sdata section, which is pointed to by register "r13". Put small uninitialized global and static data in the .sbss section, which is adjacent to the .sdata section. The -msdata=eabi option is incompatible with the -mrelocatable option. The -msdata=eabi option also sets the -memb option.

-msdata=sysv

On System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section, which is pointed to by register "r13". Put small uninitialized global and static data in the .sbss section, which is adjacent to the .sdatasection. The -msdata=sysv option is incompatible with the -mrelocatable option.

-msdata=default

-msdata

On System V.4 and embedded PowerPC systems, if -meabi is used, compile code the same as -msdata=eabi, otherwise compile code the same as -msdata=sysv.

-msdata-data

On System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section. Put small uninitialized global and static data in the .sbss section. Do not use register "r13" to address small data however. This is the default behavior unless other -msdata options are used.

-msdata=none

-mno-sdata

On embedded PowerPC systems, put all initialized global and static data in the .data section, and all uninitialized data in the .bss section.

-G num

On embedded PowerPC systems, put global and static items less than or equal to num bytes into the small data or bss sections instead of the normal data or bss section. By default, num is 8. The -G num switch is also passed to the linker. All modules should be compiled with the same -G num value.

-mregnames

-mno-regnames

On System V.4 and embedded PowerPC systems do (do not) emit register names in the assembly language output using symbolic forms.

-mlongcall

-mno-longcall

Default to making all function calls via pointers, so that functions which reside further than 64 megabytes (67,108,864 bytes) from the current location can be called. This setting can be overridden by the "shortcall" function attribute, or by"#pragma longcall(0)".

Some linkers are capable of detecting out-of-range calls and generating glue code on the fly. On these systems, long calls are unnecessary and generate slower code. As of this writing, the AIX linker can do this, as can the GNU linker for PowerPC/64. It is planned to add this feature to the GNU linker for 32-bit PowerPC systems as well.

On Mach-O (Darwin) systems, this option directs the compiler emit to the glue for every direct call, and the Darwin linker decides whether to use or discard it.

In the future, we may cause GCC to ignore all longcall specifications when the linker is known to generate glue.

-pthread

Adds support for multithreading with the pthreads library. This option sets flags for both the preprocessor and linker.

Darwin Options

These options are defined for all architectures running the Darwin operating system. They are useful for compatibility with other Mac OS compilers.

-all_load: Loads all members of static archive libraries. See man ld(1) for more information.
-arch_errors_fatal: Cause the errors having to do with files that have the wrong architecture to be fatal.
-bind_at_load: Causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched.
-bundle: Produce a Mach-o bundle format file. See man ld(1) for more information.
-bundle_loader executable: This specifies the executable that will be loading the build output file being linked. See man ld(1) for more information.
-allowable_client client_name
-arch_only
-client_name
-compatibility_version
-current_version
-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
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-nofixprebinding
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-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: These options are available for Darwin linker. Darwin linker man page describes them in detail.

MIPS Options

-EB

Generate big-endian code.

-EL

Generate little-endian code. This is the default for mips*el-*-* configurations.

-march=arch

Generate code that will run on arch, which can be the name of a generic MIPS ISA, or the name of a particular processor. The ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, and mips64. The processor names are: 4kc, 4kp,5kc, 20kc, m4k, r2000, r3000, r3900, r4000, r4400, r4600, r4650, r6000, r8000, rm7000, rm9000, orion, sb1, vr4100,vr4111, vr4120, vr4300, vr5000, vr5400 and vr5500. The special value from-abi selects the most compatible architecture for the selected ABI (that is, mips1 for 32-bit ABIs and mips3 for 64-bit ABIs).

In processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are optional, and vr may be written r.

GCC defines two macros based on the value of this option. The first is _MIPS_ARCH, which gives the name of target architecture, as a string. The second has the form _MIPS_ARCH_foo, where foo is the capitalized value of _MIPS_ARCH. For example, -march=r2000 will set _MIPS_ARCH to ``r2000'' and define the macro _MIPS_ARCH_R2000.

Note that the _MIPS_ARCH macro uses the processor names given above. In other words, it will have the full prefix and will not abbreviate 000 as k. In the case of from-abi, the macro names the resolved architecture (either ``mips1'' or``mips3''). It names the default architecture when no -march option is given.

-mtune=arch

Optimize for arch. Among other things, this option controls the way instructions are scheduled, and the perceived cost of arithmetic operations. The list of arch values is the same as for -march.

When this option is not used, GCC will optimize for the processor specified by -march. By using -march and -mtunetogether, it is possible to generate code that will run on a family of processors, but optimize the code for one particular member of that family.

-mtune defines the macros _MIPS_TUNE and _MIPS_TUNE_foo, which work in the same way as the -march ones described above.

-mips1

Equivalent to -march=mips1.

-mips2

Equivalent to -march=mips2.

-mips3

Equivalent to -march=mips3.

-mips4

Equivalent to -march=mips4.

-mips32

Equivalent to -march=mips32.

-mips32r2

Equivalent to -march=mips32r2.

-mips64

Equivalent to -march=mips64.

-mips16

-mno-mips16

Use (do not use) the MIPS16 ISA.

-mabi=32

-mabi=o64

-mabi=n32

-mabi=64

-mabi=eabi

Generate code for the given ABI.

Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a 64-bit architecture, but you can use -mgp32 to get 32-bit code instead.

-mabicalls

-mno-abicalls

Generate (do not generate) SVR4-style position-independent code. -mabicalls is the default for SVR4-based systems.

-mxgot

-mno-xgot

Lift (do not lift) the usual restrictions on the size of the global offset table.

GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it will only work if theGOT is smaller than about 64k. Anything larger will cause the linker to report an error such as:

        relocation truncated to fit: R_MIPS_GOT16 foobar

If this happens, you should recompile your code with -mxgot. It should then work with very large GOTs, although it will also be less efficient, since it will take three instructions to fetch the value of a global symbol.

Note that some linkers can create multiple GOTs. If you have such a linker, you should only need to use -mxgot when a single object file accesses more than 64k's worth of GOT entries. Very few do.

These options have no effect unless GCC is generating position independent code.

-membedded-pic

-mno-embedded-pic

Generate (do not generate) position-independent code suitable for some embedded systems. All calls are made using PCrelative addresses, and all data is addressed using the $gp register. No more than 65536 bytes of global data may be used. This requires GNU as and GNU ld, which do most of the work.

-mgp32

Assume that general-purpose registers are 32 bits wide.

-mgp64

Assume that general-purpose registers are 64 bits wide.

-mfp32

Assume that floating-point registers are 32 bits wide.

-mfp64

Assume that floating-point registers are 64 bits wide.

-mhard-float

Use floating-point coprocessor instructions.

-msoft-float

Do not use floating-point coprocessor instructions. Implement floating-point calculations using library calls instead.

-msingle-float

Assume that the floating-point coprocessor only supports single-precision operations.

-mdouble-float

Assume that the floating-point coprocessor supports double-precision operations. This is the default.

-mint64

Force "int" and "long" types to be 64 bits wide. See -mlong32 for an explanation of the default and the way that the pointer size is determined.

-mlong64

Force "long" types to be 64 bits wide. See -mlong32 for an explanation of the default and the way that the pointer size is determined.

-mlong32

Force "long", "int", and pointer types to be 32 bits wide.

The default size of "int"s, "long"s and pointers depends on the ABI. All the supported ABIs use 32-bit "int"s. The n64ABI uses 64-bit "long"s, as does the 64-bit EABI; the others use 32-bit "long"s. Pointers are the same size as "long"s, or the same size as integer registers, whichever is smaller.

-G num

Put global and static items less than or equal to num bytes into the small data or bss section instead of the normal data or bss section. This allows the data to be accessed using a single instruction.

All modules should be compiled with the same -G num value.

-membedded-data

-mno-embedded-data

Allocate variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data. This gives slightly slower code than the default, but reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems.

-muninit-const-in-rodata

-mno-uninit-const-in-rodata

Put uninitialized "const" variables in the read-only data section. This option is only meaningful in conjunction with -membedded-data.

-msplit-addresses

-mno-split-addresses

Enable (disable) use of the "%hi()" and "%lo()" assembler relocation operators. This option has been superceded by -mexplicit-relocs but is retained for backwards compatibility.

-mexplicit-relocs

-mno-explicit-relocs

Use (do not use) assembler relocation operators when dealing with symbolic addresses. The alternative, selected by -mno-explicit-relocs, is to use assembler macros instead.

-mexplicit-relocs is usually the default if GCC was configured to use an assembler that supports relocation operators. However, there are two exceptions:

*: GCC is not yet able to generate explicit relocations for the combination of -mabi=64 and -mno-abicalls. This will be addressed in a future release.
*: The combination of -mabicalls and -fno-unit-at-a-time implies -mno-explicit-relocs unless explicitly overridden. This is because, when generating abicalls, the choice of relocation depends on whether a symbol is local or global. In some rare cases, GCC will not be able to decide this until the whole compilation unit has been read.

-mrnames

-mno-rnames

Generate (do not generate) code that refers to registers using their software names. The default is -mno-rnames, which tells GCC to use hardware names like $4 instead of software names like a0. The only assembler known to support -rnames is the Algorithmics assembler.

-mcheck-zero-division

-mno-check-zero-division

Trap (do not trap) on integer division by zero. The default is -mcheck-zero-division.

-mmemcpy

-mno-memcpy

Force (do not force) the use of "memcpy()" for non-trivial block moves. The default is -mno-memcpy, which allows GCC to inline most constant-sized copies.

-mlong-calls

-mno-long-calls

Disable (do not disable) use of the "jal" instruction. Calling functions using "jal" is more efficient but requires the caller and callee to be in the same 256 megabyte segment.

This option has no effect on abicalls code. The default is -mno-long-calls.

-mmad

-mno-mad

Enable (disable) use of the "mad", "madu" and "mul" instructions, as provided by the R4650 ISA.

-mfused-madd

-mno-fused-madd

Enable (disable) use of the floating point multiply-accumulate instructions, when they are available. The default is -mfused-madd.

When multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit. This may be undesirable in some circumstances.

-nocpp

Tell the MIPS assembler to not run its preprocessor over user assembler files (with a .s suffix) when assembling them.

-mfix-sb1

-mno-fix-sb1

Work around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2 ``F1'' and ``F2'' floating point errata.)

-mflush-func=func

-mno-flush-func

Specifies the function to call to flush the I and D caches, or to not call any such function. If called, the function must take the same arguments as the common "_flush_func()", that is, the address of the memory range for which the cache is being flushed, the size of the memory range, and the number 3 (to flush both caches). The default depends on the targetGCC was configured for, but commonly is either _flush_func or __cpu_flush.

-mbranch-likely

-mno-branch-likely

Enable or disable use of Branch Likely instructions, regardless of the default for the selected architecture. By default, Branch Likely instructions may be generated if they are supported by the selected architecture. An exception is for theMIPS32 and MIPS64 architectures and processors which implement those architectures; for those, Branch Likely instructions will not be generated by default because the MIPS32 and MIPS64 architectures specifically deprecate their use.

Intel 386 and AMD x86-64 Options

These -m options are defined for the i386 and x86-64 family of computers:

-mtune=cpu-type

Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. The choices for cpu-type are:

i386: Original Intel's i386 CPU.
i486: Intel's i486 CPU. (No scheduling is implemented for this chip.)
i586, pentium: Intel Pentium CPU with no MMX support.
pentium-mmx: Intel PentiumMMX CPU based on Pentium core with MMX instruction set support.
i686, pentiumpro: Intel PentiumPro CPU.
pentium2: Intel Pentium2 CPU based on PentiumPro core with MMX instruction set support.
pentium3, pentium3m: Intel Pentium3 CPU based on PentiumPro core with MMX and SSE instruction set support.
pentium-m: Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2 instruction set support. Used by Centrino notebooks.
pentium4, pentium4m: Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set support.
prescott: Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support.
nocona: Improved version of Intel Pentium4 CPU with 64-bit extensions, MMX, SSE, SSE2 and SSE3 instruction set support.
k6: AMD K6 CPU with MMX instruction set support.
k6-2, k6-3: Improved versions of AMD K6 CPU with MMX and 3dNOW! instruction set support.
athlon, athlon-tbird: AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and SSE prefetch instructions support.
athlon-4, athlon-xp, athlon-mp: Improved AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and full SSE instruction set support.
k8, opteron, athlon64, athlon-fx: AMD K8 core based CPUs with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, 3dNOW!, enhanced 3dNOW! and 64-bit instruction set extensions.)
winchip-c6: IDT Winchip C6 CPU, dealt in same way as i486 with additional MMX instruction set support.
winchip2: IDT Winchip2 CPU, dealt in same way as i486 with additional MMX and 3dNOW! instruction set support.
c3: Via C3 CPU with MMX and 3dNOW! instruction set support. (No scheduling is implemented for this chip.)
c3-2: Via C3-2 CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)

: While picking a specific cpu-type will schedule things appropriately for that particular chip, the compiler will not generate any code that does not run on the i386 without the -march=cpu-type option being used.

-march=cpu-type

Generate instructions for the machine type cpu-type. The choices for cpu-type are the same as for -mtune. Moreover, specifying -march=cpu-type implies -mtune=cpu-type.

-mcpu=cpu-type

A deprecated synonym for -mtune.

-m386

-m486

-mpentium

-mpentiumpro

These options are synonyms for -mtune=i386, -mtune=i486, -mtune=pentium, and -mtune=pentiumpro respectively. These synonyms are deprecated.

-mfpmath=unit

Generate floating point arithmetics for selected unit unit. The choices for unit are:

387

Use the standard 387 floating point coprocessor present majority of chips and emulated otherwise. Code compiled with this option will run almost everywhere. The temporary results are computed in 80bit precision instead of precision specified by the type resulting in slightly different results compared to most of other chips. See -ffloat-store for more detailed description.

This is the default choice for i386 compiler.

sse

Use scalar floating point instructions present in the SSE instruction set. This instruction set is supported by Pentium3 and newer chips, in the AMD line by Athlon-4, Athlon-xp and Athlon-mp chips. The earlier version of SSE instruction set supports only single precision arithmetics, thus the double and extended precision arithmetics is still done using 387. Later version, present only in Pentium4 and the future AMD x86-64 chips supports double precision arithmetics too.

For i387 you need to use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and make this option effective. For x86-64 compiler, these extensions are enabled by default.

The resulting code should be considerably faster in the majority of cases and avoid the numerical instability problems of 387 code, but may break some existing code that expects temporaries to be 80bit.

This is the default choice for the x86-64 compiler.

sse,387

Attempt to utilize both instruction sets at once. This effectively double the amount of available registers and on chips with separate execution units for 387 and SSE the execution resources too. Use this option with care, as it is still experimental, because the GCC register allocator does not model separate functional units well resulting in instable performance.

-masm=dialect

Output asm instructions using selected dialect. Supported choices are intel or att (the default one).

-mieee-fp

-mno-ieee-fp

Control whether or not the compiler uses IEEE floating point comparisons. These handle correctly the case where the result of a comparison is unordered.

-msoft-float

Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

On machines where a function returns floating point results in the 80387 register stack, some floating point opcodes may be emitted even if -msoft-float is used.

-mno-fp-ret-in-387

Do not use the FPU registers for return values of functions.

The usual calling convention has functions return values of types "float" and "double" in an FPU register, even if there is no FPU. The idea is that the operating system should emulate an FPU.

The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead.

-mno-fancy-math-387

Some 387 emulators do not support the "sin", "cos" and "sqrt" instructions for the 387. Specify this option to avoid generating those instructions. This option is the default on FreeBSD, OpenBSD and NetBSD. This option is overridden when -march indicates that the target cpu will always have an FPU and so the instruction will not need emulation. As of revision 2.6.1, these instructions are not generated unless you also use the -funsafe-math-optimizations switch.

-malign-double

-mno-align-double

Control whether GCC aligns "double", "long double", and "long long" variables on a two word boundary or a one word boundary. Aligning "double" variables on a two word boundary will produce code that runs somewhat faster on a Pentiumat the expense of more memory.

Warning: if you use the -malign-double switch, structures containing the above types will be aligned differently than the published application binary interface specifications for the 386 and will not be binary compatible with structures in code compiled without that switch.

-m96bit-long-double

-m128bit-long-double

These switches control the size of "long double" type. The i386 application binary interface specifies the size to be 96 bits, so -m96bit-long-double is the default in 32 bit mode.

Modern architectures (Pentium and newer) would prefer "long double" to be aligned to an 8 or 16 byte boundary. In arrays or structures conforming to the ABI, this would not be possible. So specifying a -m128bit-long-double will align "long double" to a 16 byte boundary by padding the "long double" with an additional 32 bit zero.

In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that "long double" is to be aligned on 16 byte boundary.

Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a "long double".

Warning: if you override the default value for your target ABI, the structures and arrays containing "long double" variables will change their size as well as function calling convention for function taking "long double" will be modified. Hence they will not be binary compatible with arrays or structures in code compiled without that switch.

-msvr3-shlib

-mno-svr3-shlib

Control whether GCC places uninitialized local variables into the "bss" or "data" segments. -msvr3-shlib places them into"bss". These options are meaningful only on System V Release 3.

-mrtd

Use a different function-calling convention, in which functions that take a fixed number of arguments return with the "ret"num instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there.

You can specify that an individual function is called with this calling sequence with the function attribute stdcall. You can also override the -mrtd option by using the function attribute cdecl.

Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.

Also, you must provide function prototypes for all functions that take variable numbers of arguments (including "printf"); otherwise incorrect code will be generated for calls to those functions.

In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)

-mregparm=num

Control how many registers are used to pass integer arguments. By default, no registers are used to pass arguments, and at most 3 registers can be used. You can control this behavior for a specific function by using the function attributeregparm.

Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules.

-mpreferred-stack-boundary=num

Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits), except when optimizing for code size (-Os), in which case the default is the minimum correct alignment (4 bytes for x86, and 8 bytes for x86-64).

On Pentium and PentiumPro, "double" and "long double" values should be aligned to an 8 byte boundary (see -malign-double) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type "__m128" suffers similar penalties if it is not 16 byte aligned.

To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary will most likely misalign the stack. It is recommended that libraries that use callbacks always use the default setting.

This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to -mpreferred-stack-boundary=2.

-mmmx

-mno-mmx

-msse

-mno-sse

-msse2

-mno-sse2

-msse3

-mno-sse3

-m3dnow

-mno-3dnow

These switches enable or disable the use of built-in functions that allow direct access to the MMX, SSE, SSE2, SSE3 and 3Dnow extensions of the instruction set.

To have SSE/SSE2 instructions generated automatically from floating-point code, see -mfpmath=sse.

-mpush-args

-mno-push-args

Use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as method usingSUB/MOV operations and is enabled by default. In some cases disabling it may improve performance because of improved scheduling and reduced dependencies.

-maccumulate-outgoing-args

If enabled, the maximum amount of space required for outgoing arguments will be computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when preferred stack boundary is not equal to 2. The drawback is a notable increase in code size. This switch implies -mno-push-args.

-mthreads

Support thread-safe exception handling on Mingw32. Code that relies on thread-safe exception handling must compile and link all code with the -mthreads option. When compiling, -mthreads defines -D_MT; when linking, it links in a special thread helper library -lmingwthrd which cleans up per thread exception handling data.

-mno-align-stringops

Do not align destination of inlined string operations. This switch reduces code size and improves performance in case the destination is already aligned, but GCC doesn't know about it.

-minline-all-stringops

By default GCC inlines string operations only when destination is known to be aligned at least to 4 byte boundary. This enables more inlining, increase code size, but may improve performance of code that depends on fast memcpy, strlen and memset for short lengths.

-momit-leaf-frame-pointer

Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers and makes an extra register available in leaf functions. The option -fomit-frame-pointer removes the frame pointer for all functions which might make debugging harder.

-mtls-direct-seg-refs

-mno-tls-direct-seg-refs

Controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for 32-bit, %fs for 64-bit), or whether the thread base pointer must be added. Whether or not this is legal depends on the operating system, and whether it maps the segment to cover the entire TLS area.

For systems that use GNU libc, the default is on.

These -m switches are supported in addition to the above on AMD x86-64 processors in 64-bit environments.

-m32
-m64: Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits and generates code that runs on any i386 system. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits and generates code for AMD's x86-64 architecture.
-mno-red-zone: Do not use a so called red zone for x86-64 code. The red zone is mandated by the x86-64 ABI, it is a 128-byte area beyond the location of the stack pointer that will not be modified by signal or interrupt handlers and therefore can be used for temporary data without adjusting the stack pointer. The flag -mno-red-zone disables this red zone.
-mcmodel=small: Generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the default code model.
-mcmodel=kernel: Generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space. This model has to be used for Linux kernel code.
-mcmodel=medium: Generate code for the medium model: The program is linked in the lower 2 GB of the address space but symbols can be located anywhere in the address space. Programs can be statically or dynamically linked, but building of shared libraries are not supported with the medium model.
-mcmodel=large: Generate code for the large model: This model makes no assumptions about addresses and sizes of sections. CurrentlyGCC does not implement this model.

HPPA Options

These -m options are defined for the HPPA family of computers:

-march=architecture-type

Generate code for the specified architecture. The choices for architecture-type are 1.0 for PA 1.0, 1.1 for PA 1.1, and 2.0for PA 2.0 processors. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper architecture option for your machine. Code compiled for lower numbered architectures will run on higher numbered architectures, but not the other way around.

PA 2.0 support currently requires gas snapshot 19990413 or later. The next release of binutils (current is 2.9.1) will probably contain PA 2.0 support.

-mpa-risc-1-0

-mpa-risc-1-1

-mpa-risc-2-0

Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.

-mbig-switch

Generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range branches within a switch table.

-mjump-in-delay

Fill delay slots of function calls with unconditional jump instructions by modifying the return pointer for the function call to be the target of the conditional jump.

-mdisable-fpregs

Prevent floating point registers from being used in any manner. This is necessary for compiling kernels which perform lazy context switching of floating point registers. If you use this option and attempt to perform floating point operations, the compiler will abort.

-mdisable-indexing

Prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIGgenerated code under MACH.

-mno-space-regs

Generate code that assumes the target has no space registers. This allows GCC to generate faster indirect calls and use unscaled index address modes.

Such code is suitable for level 0 PA systems and kernels.

-mfast-indirect-calls

Generate code that assumes calls never cross space boundaries. This allows GCC to emit code which performs faster indirect calls.

This option will not work in the presence of shared libraries or nested functions.

-mlong-load-store

Generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is equivalent to the+k option to the HP compilers.

-mportable-runtime

Use the portable calling conventions proposed by HP for ELF systems.

-mgas

Enable the use of assembler directives only GAS understands.

-mschedule=cpu-type

Schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type are 700 7100,7100LC, 7200, 7300 and 8000. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper scheduling option for your machine. The default scheduling is 8000.

-mlinker-opt

Enable the optimization pass in the HP-UX linker. Note this makes symbolic debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when linking some programs.

-msoft-float

Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all HPPAtargets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded target hppa1.1-*-pro does provide software floating point support.

-msio

Generate the predefine, "_SIO", for server IO. The default is -mwsio. This generates the predefines, "__hp9000s700","__hp9000s700__" and "_WSIO", for workstation IO. These options are available under HP-UX and HI-UX.

-mgnu-ld

Use GNU ld specific options. This passes -shared to ld when building a shared library. It is the default when GCC is configured, explicitly or implicitly, with the GNU linker. This option does not have any affect on which ld is called, it only changes what parameters are passed to that ld. The ld that is called is determined by the --with-ld configure option,GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which `gcc -print-prog-name=ld`.

-mhp-ld

Use HP ld specific options. This passes -b to ld when building a shared library and passes +Accept TypeMismatch to ld on all links. It is the default when GCC is configured, explicitly or implicitly, with the HP linker. This option does not have any affect on which ld is called, it only changes what parameters are passed to that ld. The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which `gcc -print-prog-name=ld`.

-mlong-calls

Generate code that uses long call sequences. This ensures that a call is always able to reach linker generated stubs. The default is to generate long calls only when the distance from the call site to the beginning of the function or translation unit, as the case may be, exceeds a predefined limit set by the branch type being used. The limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes.

Distances are measured from the beginning of functions when using the -ffunction-sections option, or when using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.

It is normally not desirable to use this option as it will degrade performance. However, it may be useful in large applications, particularly when partial linking is used to build the application.

The types of long calls used depends on the capabilities of the assembler and linker, and the type of code being generated. The impact on systems that support long absolute calls, and long pic symbol-difference or pc-relative calls should be relatively small. However, an indirect call is used on 32-bit ELF systems in pic code and it is quite long.

-nolibdld

Suppress the generation of link options to search libdld.sl when the -static option is specified on HP-UX 10 and later.

-static

The HP-UX implementation of setlocale in libc has a dependency on libdld.sl. There isn't an archive version of libdld.sl. Thus, when the -static option is specified, special link options are needed to resolve this dependency.

On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when the -static option is specified. This causes the resulting binary to be dynamic. On the 64-bit port, the linkers generate dynamic binaries by default in any case. The -nolibdld option can be used to prevent the GCC driver from adding these link options.

-threads

Add support for multithreading with the dce thread library under HP-UX. This option sets flags for both the preprocessor and linker.

Intel 960 Options

These -m options are defined for the Intel 960 implementations:

-mcpu-type: Assume the defaults for the machine type cpu-type for some of the other options, including instruction scheduling, floating point support, and addressing modes. The choices for cpu-type are ka, kb, mc, ca, cf, sa, and sb. The default is kb.
-mnumerics
-msoft-float: The -mnumerics option indicates that the processor does support floating-point instructions. The -msoft-float option indicates that floating-point support should not be assumed.
-mleaf-procedures
-mno-leaf-procedures: Do (or do not) attempt to alter leaf procedures to be callable with the "bal" instruction as well as "call". This will result in more efficient code for explicit calls when the "bal" instruction can be substituted by the assembler or linker, but less efficient code in other cases, such as calls via function pointers, or using a linker that doesn't support this optimization.
-mtail-call
-mno-tail-call: Do (or do not) make additional attempts (beyond those of the machine-independent portions of the compiler) to optimize tail-recursive calls into branches. You may not want to do this because the detection of cases where this is not valid is not totally complete. The default is -mno-tail-call.
-mcomplex-addr
-mno-complex-addr: Assume (or do not assume) that the use of a complex addressing mode is a win on this implementation of the i960. Complex addressing modes may not be worthwhile on the K-series, but they definitely are on the C-series. The default is currently -mcomplex-addr for all processors except the CB and CC.
-mcode-align
-mno-code-align: Align code to 8-byte boundaries for faster fetching (or don't bother). Currently turned on by default for C-series implementations only.
-mic-compat
-mic2.0-compat
-mic3.0-compat: Enable compatibility with iC960 v2.0 or v3.0.
-masm-compat
-mintel-asm: Enable compatibility with the iC960 assembler.
-mstrict-align
-mno-strict-align: Do not permit (do permit) unaligned accesses.
-mold-align: Enable structure-alignment compatibility with Intel's gcc release version 1.3 (based on gcc 1.37). This option implies -mstrict-align.
-mlong-double-64: Implement type long double as 64-bit floating point numbers. Without the option long double is implemented by 80-bit floating point numbers. The only reason we have it because there is no 128-bit long double support in fp-bit.c yet. So it is only useful for people using soft-float targets. Otherwise, we should recommend against use of it.

DEC Alpha Options

These -m options are defined for the DEC Alpha implementations:

-mno-soft-float

-msoft-float

Use (do not use) the hardware floating-point instructions for floating-point operations. When -msoft-float is specified, functions in libgcc.a will be used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulations routines, these routines will issue floating-point operations. If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them.

Note that Alpha implementations without floating-point operations are required to have floating-point registers.

-mfp-reg

-mno-fp-regs

Generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies -msoft-float. If the floating-point register set is not used, floating point operands are passed in integer registers as if they were integers and floating-point results are passed in $0 instead of $f0. This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with -mno-fp-regs must also be compiled with that option.

A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers.

-mieee

The Alpha architecture implements floating-point hardware optimized for maximum performance. It is mostly compliant with the IEEE floating point standard. However, for full compliance, software assistance is required. This option generates code fully IEEE compliant code except that the inexact-flag is not maintained (see below). If this option is turned on, the preprocessor macro "_IEEE_FP" is defined during compilation. The resulting code is less efficient but is able to correctly support denormalized numbers and exceptional IEEE values such as not-a-number and plus/minus infinity. Other Alpha compilers call this option -ieee_with_no_inexact.

-mieee-with-inexact

This is like -mieee except the generated code also maintains the IEEE inexact-flag. Turning on this option causes the generated code to implement fully-compliant IEEE math. In addition to "_IEEE_FP", "_IEEE_FP_EXACT" is defined as a preprocessor macro. On some Alpha implementations the resulting code may execute significantly slower than the code generated by default. Since there is very little code that depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call this option -ieee_with_inexact.

-mfp-trap-mode=trap-mode

This option controls what floating-point related traps are enabled. Other Alpha compilers call this option -fptm trap-mode. The trap mode can be set to one of four values:

n: This is the default (normal) setting. The only traps that are enabled are the ones that cannot be disabled in software (e.g., division by zero trap).
u: In addition to the traps enabled by n, underflow traps are enabled as well.
su: Like su, but the instructions are marked to be safe for software completion (see Alpha architecture manual for details).
sui: Like su, but inexact traps are enabled as well.

-mfp-rounding-mode=rounding-mode

Selects the IEEE rounding mode. Other Alpha compilers call this option -fprm rounding-mode. The rounding-mode can be one of:

n: Normal IEEE rounding mode. Floating point numbers are rounded towards the nearest machine number or towards the even machine number in case of a tie.
m: Round towards minus infinity.
c: Chopped rounding mode. Floating point numbers are rounded towards zero.
d: Dynamic rounding mode. A field in the floating point control register (fpcr, see Alpha architecture reference manual) controls the rounding mode in effect. The C library initializes this register for rounding towards plus infinity. Thus, unless your program modifies the fpcr, d corresponds to round towards plus infinity.

-mtrap-precision=trap-precision

In the Alpha architecture, floating point traps are imprecise. This means without software assistance it is impossible to recover from a floating trap and program execution normally needs to be terminated. GCC can generate code that can assist operating system trap handlers in determining the exact location that caused a floating point trap. Depending on the requirements of an application, different levels of precisions can be selected:

p: Program precision. This option is the default and means a trap handler can only identify which program caused a floating point exception.
f: Function precision. The trap handler can determine the function that caused a floating point exception.
i: Instruction precision. The trap handler can determine the exact instruction that caused a floating point exception.

: Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.

-mieee-conformant

This option marks the generated code as IEEE conformant. You must not use this option unless you also specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect is to emit the line .eflag 48 in the function prologue of the generated assembly file. Under DEC Unix, this has the effect that IEEE-conformant math library routines will be linked in.

-mbuild-constants

Normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions. If it cannot, it will output the constant as a literal and generate code to load it from the data segment at runtime.

Use this option to require GCC to construct all integer constants using code, even if it takes more instructions (the maximum is six).

You would typically use this option to build a shared library dynamic loader. Itself a shared library, it must relocate itself in memory before it can find the variables and constants in its own data segment.

-malpha-as

-mgas

Select whether to generate code to be assembled by the vendor-supplied assembler (-malpha-as) or by the GNUassembler -mgas.

-mbwx

-mno-bwx

-mcix

-mno-cix

-mfix

-mno-fix

-mmax

-mno-max

Indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets. The default is to use the instruction sets supported by the CPU type specified via -mcpu= option or that of the CPU on which GCC was built if none was specified.

-mfloat-vax

-mfloat-ieee

Generate code that uses (does not use) VAX F and G floating point arithmetic instead of IEEE single and double precision.

-mexplicit-relocs

-mno-explicit-relocs

Older Alpha assemblers provided no way to generate symbol relocations except via assembler macros. Use of these macros does not allow optimal instruction scheduling. GNU binutils as of version 2.12 supports a new syntax that allows the compiler to explicitly mark which relocations should apply to which instructions. This option is mostly useful for debugging, as GCC detects the capabilities of the assembler when it is built and sets the default accordingly.

-msmall-data

-mlarge-data

When -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations. When -msmall-data is used, objects 8 bytes long or smaller are placed in a small data area (the ".sdata" and ".sbss" sections) and are accessed via 16-bit relocations off of the $gp register. This limits the size of the small data area to 64KB, but allows the variables to be directly accessed via a single instruction.

The default is -mlarge-data. With this option the data area is limited to just below 2GB. Programs that require more than 2GB of data must use "malloc" or "mmap" to allocate the data in the heap instead of in the program's data segment.

When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.

-msmall-text

-mlarge-text

When -msmall-text is used, the compiler assumes that the code of the entire program (or shared library) fits in 4MB, and is thus reachable with a branch instruction. When -msmall-data is used, the compiler can assume that all local symbols share the same $gp value, and thus reduce the number of instructions required for a function call from 4 to 1.

The default is -mlarge-text.

-mcpu=cpu_type

Set the instruction set and instruction scheduling parameters for machine type cpu_type. You can specify either the EVstyle name or the corresponding chip number. GCC supports scheduling parameters for the EV4, EV5 and EV6 family of processors and will choose the default values for the instruction set from the processor you specify. If you do not specify a processor type, GCC will default to the processor on which the compiler was built.

Supported values for cpu_type are

ev4
ev45
21064: Schedules as an EV4 and has no instruction set extensions.
ev5
21164: Schedules as an EV5 and has no instruction set extensions.
ev56
21164a: Schedules as an EV5 and supports the BWX extension.
pca56
21164pc
21164PC: Schedules as an EV5 and supports the BWX and MAX extensions.
ev6
21264: Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
ev67
21264a: Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.

-mtune=cpu_type

Set only the instruction scheduling parameters for machine type cpu_type. The instruction set is not changed.

-mmemory-latency=time

Sets the latency the scheduler should assume for typical memory references as seen by the application. This number is highly dependent on the memory access patterns used by the application and the size of the external cache on the machine.

Valid options for time are

number: A decimal number representing clock cycles.
L1
L2
L3
main: The compiler contains estimates of the number of clock cycles for ``typical'' EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that L3 is only valid for EV5.

DEC Alpha/VMS Options

These -m options are defined for the DEC Alpha/VMS implementations:

-mvms-return-codes: Return VMS condition codes from main. The default is to return POSIX style condition (e.g. error) codes.

H8/300 Options

These -m options are defined for the H8/300 implementations:

-mrelax: Shorten some address references at link time, when possible; uses the linker option -relax.
-mh: Generate code for the H8/300H.
-ms: Generate code for the H8S.
-mn: Generate code for the H8S and H8/300H in the normal mode. This switch must be used either with -mh or -ms.
-ms2600: Generate code for the H8S/2600. This switch must be used with -ms.
-mint32: Make "int" data 32 bits by default.
-malign-300: On the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and H8S is to align longs and floats on 4 byte boundaries. -malign-300 causes them to be aligned on 2 byte boundaries. This option has no effect on the H8/300.

SH Options

These -m options are defined for the SH implementations:

-m1: Generate code for the SH1.
-m2: Generate code for the SH2.
-m2e: Generate code for the SH2e.
-m3: Generate code for the SH3.
-m3e: Generate code for the SH3e.
-m4-nofpu: Generate code for the SH4 without a floating-point unit.
-m4-single-only: Generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.
-m4-single: Generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
-m4: Generate code for the SH4.
-mb: Compile code for the processor in big endian mode.
-ml: Compile code for the processor in little endian mode.
-mdalign: Align doubles at 64-bit boundaries. Note that this changes the calling conventions, and thus some functions from the standard C library will not work unless you recompile it first with -mdalign.
-mrelax: Shorten some address references at link time, when possible; uses the linker option -relax.
-mbigtable: Use 32-bit offsets in "switch" tables. The default is to use 16-bit offsets.
-mfmovd: Enable the use of the instruction "fmovd".
-mhitachi: Comply with the calling conventions defined by Renesas.
-mnomacsave: Mark the "MAC" register as call-clobbered, even if -mhitachi is given.
-mieee: Increase IEEE-compliance of floating-point code.
-misize: Dump instruction size and location in the assembly code.
-mpadstruct: This option is deprecated. It pads structures to multiple of 4 bytes, which is incompatible with the SH ABI.
-mspace: Optimize for space instead of speed. Implied by -Os.
-mprefergot: When generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table.
-musermode: Generate a library function call to invalidate instruction cache entries, after fixing up a trampoline. This library function call doesn't assume it can write to the whole memory address space. This is the default when the target is "sh-*-linux*".

Options for System V

These additional options are available on System V Release 4 for compatibility with other compilers on those systems:

-G: Create a shared object. It is recommended that -symbolic or -shared be used instead.
-Qy: Identify the versions of each tool used by the compiler, in a ".ident" assembler directive in the output.
-Qn: Refrain from adding ".ident" directives to the output file (this is the default).
-YP,dirs: Search the directories dirs, and no others, for libraries specified with -l.
-Ym,dir: Look in the directory dir to find the M4 preprocessor. The assembler uses this option.

TMS320C3x/C4x Options

These -m options are defined for TMS320C3x/C4x implementations:

-mcpu=cpu_type: Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are c30, c31, c32, c40, and c44. The default is c40 to generate code for the TMS320C40.
-mbig-memory
-mbig
-msmall-memory
-msmall: Generates code for the big or small memory model. The small memory model assumed that all data fits into one 64K word page. At run-time the data page (DP) register must be set to point to the 64K page containing the .bss and .data program sections. The big memory model is the default and requires reloading of the DP register for every direct memory access.
-mbk
-mno-bk: Allow (disallow) allocation of general integer operands into the block count register BK.
-mdb
-mno-db: Enable (disable) generation of code using decrement and branch, DBcond(D), instructions. This is enabled by default for the C4x. To be on the safe side, this is disabled for the C3x, since the maximum iteration count on the C3x is 2^{23 + 1} (but who iterates loops more than 2^{23} times on the C3x?). Note that GCC will try to reverse a loop so that it can utilize the decrement and branch instruction, but will give up if there is more than one memory reference in the loop. Thus a loop where the loop counter is decremented can generate slightly more efficient code, in cases where the RPTB instruction cannot be utilized.
-mdp-isr-reload
-mparanoid: Force the DP register to be saved on entry to an interrupt service routine (ISR), reloaded to point to the data section, and restored on exit from the ISR. This should not be required unless someone has violated the small memory model by modifying the DP register, say within an object library.
-mmpyi
-mno-mpyi: For the C3x use the 24-bit MPYI instruction for integer multiplies instead of a library call to guarantee 32-bit results. Note that if one of the operands is a constant, then the multiplication will be performed using shifts and adds. If the -mmpyioption is not specified for the C3x, then squaring operations are performed inline instead of a library call.
-mfast-fix
-mno-fast-fix: The C3x/C4x FIX instruction to convert a floating point value to an integer value chooses the nearest integer less than or equal to the floating point value rather than to the nearest integer. Thus if the floating point number is negative, the result will be incorrectly truncated an additional code is necessary to detect and correct this case. This option can be used to disable generation of the additional code required to correct the result.
-mrptb
-mno-rptb: Enable (disable) generation of repeat block sequences using the RPTB instruction for zero overhead looping. The RPTBconstruct is only used for innermost loops that do not call functions or jump across the loop boundaries. There is no advantage having nested RPTB loops due to the overhead required to save and restore the RC, RS, and RE registers. This is enabled by default with -O2.
-mrpts=count
-mno-rpts: Enable (disable) the use of the single instruction repeat instruction RPTS. If a repeat block contains a single instruction, and the loop count can be guaranteed to be less than the value count, GCC will emit a RPTS instruction instead of aRPTB. If no value is specified, then a RPTS will be emitted even if the loop count cannot be determined at compile time. Note that the repeated instruction following RPTS does not have to be reloaded from memory each iteration, thus freeing up the CPU buses for operands. However, since interrupts are blocked by this instruction, it is disabled by default.
-mloop-unsigned
-mno-loop-unsigned: The maximum iteration count when using RPTS and RPTB (and DB on the C40) is 2^{31 + 1} since these instructions test if the iteration count is negative to terminate the loop. If the iteration count is unsigned there is a possibility than the 2^{31 + 1} maximum iteration count may be exceeded. This switch allows an unsigned iteration count.
-mti: Try to emit an assembler syntax that the TI assembler (asm30) is happy with. This also enforces compatibility with the APIemployed by the TI C3x C compiler. For example, long doubles are passed as structures rather than in floating point registers.
-mregparm
-mmemparm: Generate code that uses registers (stack) for passing arguments to functions. By default, arguments are passed in registers where possible rather than by pushing arguments on to the stack.
-mparallel-insns
-mno-parallel-insns: Allow the generation of parallel instructions. This is enabled by default with -O2.
-mparallel-mpy
-mno-parallel-mpy: Allow the generation of MPY||ADD and MPY||SUB parallel instructions, provided -mparallel-insns is also specified. These instructions have tight register constraints which can pessimize the code generation of large functions.

V850 Options

These -m options are defined for V850 implementations:

-mlong-calls

-mno-long-calls

Treat all calls as being far away (near). If calls are assumed to be far away, the compiler will always load the functions address up into a register, and call indirect through the pointer.

-mno-ep

-mep

Do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer into the "ep"register, and use the shorter "sld" and "sst" instructions. The -mep option is on by default if you optimize.

-mno-prolog-function

-mprolog-function

Do not use (do use) external functions to save and restore registers at the prologue and epilogue of a function. The external functions are slower, but use less code space if more than one function saves the same number of registers. The -mprolog-function option is on by default if you optimize.

-mspace

Try to make the code as small as possible. At present, this just turns on the -mep and -mprolog-function options.

-mtda=n

Put static or global variables whose size is n bytes or less into the tiny data area that register "ep" points to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte references).

-msda=n

Put static or global variables whose size is n bytes or less into the small data area that register "gp" points to. The small data area can hold up to 64 kilobytes.

-mzda=n

Put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory.

-mv850

Specify that the target processor is the V850.

-mbig-switch

Generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range branches within a switch table.

-mapp-regs

This option will cause r2 and r5 to be used in the code generated by the compiler. This setting is the default.

-mno-app-regs

This option will cause r2 and r5 to be treated as fixed registers.

-mv850e1

Specify that the target processor is the V850E1. The preprocessor constants __v850e1__ and __v850e__ will be defined if this option is used.

-mv850e

Specify that the target processor is the V850E. The preprocessor constant __v850e__ will be defined if this option is used.

If neither -mv850 nor -mv850e nor -mv850e1 are defined then a default target processor will be chosen and the relevant__v850*__ preprocessor constant will be defined.

The preprocessor constants __v850 and __v851__ are always defined, regardless of which processor variant is the target.

-mdisable-callt

This option will suppress generation of the CALLT instruction for the v850e and v850e1 flavors of the v850 architecture. The default is -mno-disable-callt which allows the CALLT instruction to be used.

ARC Options

These options are defined for ARC implementations:

-EL: Compile code for little endian mode. This is the default.
-EB: Compile code for big endian mode.
-mmangle-cpu: Prepend the name of the cpu to all public symbol names. In multiple-processor systems, there are many ARC variants with different instruction and register set characteristics. This flag prevents code compiled for one cpu to be linked with code compiled for another. No facility exists for handling variants that are ``almost identical''. This is an all or nothing option.
-mcpu=cpu: Compile code for ARC variant cpu. Which variants are supported depend on the configuration. All variants support -mcpu=base, this is the default.
-mtext=text-section
-mdata=data-section
-mrodata=readonly-data-section: Put functions, data, and readonly data in text-section, data-section, and readonly-data-section respectively by default. This can be overridden with the "section" attribute.

NS32K Options

These are the -m options defined for the 32000 series. The default values for these options depends on which style of 32000 was selected when the compiler was configured; the defaults for the most common choices are given below.

-m32032

Generate output for a 32032. This is the default when the compiler is configured for 32032 and 32016 based systems.

-m32332

Generate output for a 32332. This is the default when the compiler is configured for 32332-based systems.

-m32532

Generate output for a 32532. This is the default when the compiler is configured for 32532-based systems.

-m32081

Generate output containing 32081 instructions for floating point. This is the default for all systems.

-m32381

Generate output containing 32381 instructions for floating point. This also implies -m32081. The 32381 is only compatible with the 32332 and 32532 cpus. This is the default for the pc532-netbsd configuration.

-mmulti-add

Try and generate multiply-add floating point instructions "polyF" and "dotF". This option is only available if the -m32381option is in effect. Using these instructions requires changes to register allocation which generally has a negative impact on performance. This option should only be enabled when compiling code particularly likely to make heavy use of multiply-add instructions.

-mnomulti-add

Do not try and generate multiply-add floating point instructions "polyF" and "dotF". This is the default on all platforms.

-msoft-float

Generate output containing library calls for floating point. Warning: the requisite libraries may not be available.

-mieee-compare

-mno-ieee-compare

Control whether or not the compiler uses IEEE floating point comparisons. These handle correctly the case where the result of a comparison is unordered. Warning: the requisite kernel support may not be available.

-mnobitfield

Do not use the bit-field instructions. On some machines it is faster to use shifting and masking operations. This is the default for the pc532.

-mbitfield

Do use the bit-field instructions. This is the default for all platforms except the pc532.

-mrtd

Use a different function-calling convention, in which functions that take a fixed number of arguments return pop their arguments on return with the "ret" instruction.

This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.

Also, you must provide function prototypes for all functions that take variable numbers of arguments (including "printf"); otherwise incorrect code will be generated for calls to those functions.

In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)

This option takes its name from the 680x0 "rtd" instruction.

-mregparam

Use a different function-calling convention where the first two arguments are passed in registers.

This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.

-mnoregparam

Do not pass any arguments in registers. This is the default for all targets.

-msb

It is OK to use the sb as an index register which is always loaded with zero. This is the default for the pc532-netbsd target.

-mnosb

The sb register is not available for use or has not been initialized to zero by the run time system. This is the default for all targets except the pc532-netbsd. It is also implied whenever -mhimem or -fpic is set.

-mhimem

Many ns32000 series addressing modes use displacements of up to 512MB. If an address is above 512MB then displacements from zero can not be used. This option causes code to be generated which can be loaded above 512MB. This may be useful for operating systems or ROM code.

-mnohimem

Assume code will be loaded in the first 512MB of virtual address space. This is the default for all platforms.

AVR Options

These options are defined for AVR implementations:

-mmcu=mcu

Specify ATMEL AVR instruction set or MCU type.

Instruction set avr1 is for the minimal AVR core, not supported by the C compiler, only for assembler programs (MCUtypes: at90s1200, attiny10, attiny11, attiny12, attiny15, attiny28).

Instruction set avr2 (default) is for the classic AVR core with up to 8K program memory space (MCU types: at90s2313, at90s2323, attiny22, at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515, at90c8534, at90s8535).

Instruction set avr3 is for the classic AVR core with up to 128K program memory space (MCU types: atmega103, atmega603, at43usb320, at76c711).

Instruction set avr4 is for the enhanced AVR core with up to 8K program memory space (MCU types: atmega8, atmega83, atmega85).

Instruction set avr5 is for the enhanced AVR core with up to 128K program memory space (MCU types: atmega16, atmega161, atmega163, atmega32, atmega323, atmega64, atmega128, at43usb355, at94k).

-msize

Output instruction sizes to the asm file.

-minit-stack=N

Specify the initial stack address, which may be a symbol or numeric value, __stack is the default.

-mno-interrupts

Generated code is not compatible with hardware interrupts. Code size will be smaller.

-mcall-prologues

Functions prologues/epilogues expanded as call to appropriate subroutines. Code size will be smaller.

-mno-tablejump

Do not generate tablejump insns which sometimes increase code size.

-mtiny-stack

Change only the low 8 bits of the stack pointer.

MCore Options

These are the -m options defined for the Motorola M*Core processors.

-mhardlit
-mno-hardlit: Inline constants into the code stream if it can be done in two instructions or less.
-mdiv
-mno-div: Use the divide instruction. (Enabled by default).
-mrelax-immediate
-mno-relax-immediate: Allow arbitrary sized immediates in bit operations.
-mwide-bitfields
-mno-wide-bitfields: Always treat bit-fields as int-sized.
-m4byte-functions
-mno-4byte-functions: Force all functions to be aligned to a four byte boundary.
-mcallgraph-data
-mno-callgraph-data: Emit callgraph information.
-mslow-bytes
-mno-slow-bytes: Prefer word access when reading byte quantities.
-mlittle-endian
-mbig-endian: Generate code for a little endian target.
-m210
-m340: Generate code for the 210 processor.

IA-64 Options

These are the -m options defined for the Intel IA-64 architecture.

-mbig-endian: Generate code for a big endian target. This is the default for HP-UX.
-mlittle-endian: Generate code for a little endian target. This is the default for AIX5 and GNU/Linux.
-mgnu-as
-mno-gnu-as: Generate (or don't) code for the GNU assembler. This is the default.
-mgnu-ld
-mno-gnu-ld: Generate (or don't) code for the GNU linker. This is the default.
-mno-pic: Generate code that does not use a global pointer register. The result is not position independent code, and violates the IA-64 ABI.
-mvolatile-asm-stop
-mno-volatile-asm-stop: Generate (or don't) a stop bit immediately before and after volatile asm statements.
-mb-step: Generate code that works around Itanium B step errata.
-mregister-names
-mno-register-names: Generate (or don't) in, loc, and out register names for the stacked registers. This may make assembler output more readable.
-mno-sdata
-msdata: Disable (or enable) optimizations that use the small data section. This may be useful for working around optimizer bugs.
-mconstant-gp: Generate code that uses a single constant global pointer value. This is useful when compiling kernel code.
-mauto-pic: Generate code that is self-relocatable. This implies -mconstant-gp. This is useful when compiling firmware code.
-minline-float-divide-min-latency: Generate code for inline divides of floating point values using the minimum latency algorithm.
-minline-float-divide-max-throughput: Generate code for inline divides of floating point values using the maximum throughput algorithm.
-minline-int-divide-min-latency: Generate code for inline divides of integer values using the minimum latency algorithm.
-minline-int-divide-max-throughput: Generate code for inline divides of integer values using the maximum throughput algorithm.
-mno-dwarf2-asm
-mdwarf2-asm: Don't (or do) generate assembler code for the DWARF2 line number debugging info. This may be useful when not using theGNU assembler.
-mfixed-range=register-range: Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator can not use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
-mearly-stop-bits
-mno-early-stop-bits: Allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit. This can improve instruction scheduling, but does not always do so.

D30V Options

These -m options are defined for D30V implementations:

-mextmem: Link the .text, .data, .bss, .strings, .rodata, .rodata1, .data1 sections into external memory, which starts at location0x80000000.
-mextmemory: Same as the -mextmem switch.
-monchip: Link the .text section into onchip text memory, which starts at location 0x0. Also link .data, .bss, .strings, .rodata,.rodata1, .data1 sections into onchip data memory, which starts at location 0x20000000.
-mno-asm-optimize
-masm-optimize: Disable (enable) passing -O to the assembler when optimizing. The assembler uses the -O option to automatically parallelize adjacent short instructions where possible.
-mbranch-cost=n: Increase the internal costs of branches to n. Higher costs means that the compiler will issue more instructions to avoid doing a branch. The default is 2.
-mcond-exec=n: Specify the maximum number of conditionally executed instructions that replace a branch. The default is 4.

S/390 and zSeries Options

These are the -m options defined for the S/390 and zSeries architecture.

-mhard-float
-msoft-float: Use (do not use) the hardware floating-point instructions and registers for floating-point operations. When -msoft-float is specified, functions in libgcc.a will be used to perform floating-point operations. When -mhard-float is specified, the compiler generates IEEE floating-point instructions. This is the default.
-mbackchain
-mno-backchain
-mkernel-backchain: In order to provide a backchain the address of the caller's frame is stored within the callee's stack frame. A backchain may be needed to allow debugging using tools that do not understand DWARF-2 call frame information. For -mno-backchainno backchain is maintained at all which is the default. If one of the other options is present the backchain pointer is placed either on top of the stack frame (-mkernel-backchain) or on the bottom (-mbackchain). Beside the different backchain location -mkernel-backchain also changes stack frame layout breaking the ABI. This option is intended to be used for code which internally needs a backchain but has to get by with a limited stack size e.g. the linux kernel. Internal unwinding code not using DWARF-2 info has to be able to locate the return address of a function. That will be eased be the fact that the return address of a function is placed two words below the backchain pointer.
-msmall-exec
-mno-small-exec: Generate (or do not generate) code using the "bras" instruction to do subroutine calls. This only works reliably if the total executable size does not exceed 64k. The default is to use the "basr" instruction instead, which does not have this limitation.
-m64
-m31: When -m31 is specified, generate code compliant to the GNU/Linux for S/390 ABI. When -m64 is specified, generate code compliant to the GNU/Linux for zSeries ABI. This allows GCC in particular to generate 64-bit instructions. For the s390targets, the default is -m31, while the s390x targets default to -m64.
-mzarch
-mesa: When -mzarch is specified, generate code using the instructions available on z/Architecture. When -mesa is specified, generate code using the instructions available on ESA/390. Note that -mesa is not possible with -m64. When generating code compliant to the GNU/Linux for S/390 ABI, the default is -mesa. When generating code compliant to the GNU/Linux for zSeries ABI, the default is -mzarch.
-mmvcle
-mno-mvcle: Generate (or do not generate) code using the "mvcle" instruction to perform block moves. When -mno-mvcle is specified, use a "mvc" loop instead. This is the default.
-mdebug
-mno-debug: Print (or do not print) additional debug information when compiling. The default is to not print debug information.
-march=cpu-type: Generate code that will run on cpu-type, which is the name of a system representing a certain processor type. Possible values for cpu-type are g5, g6, z900, and z990. When generating code using the instructions available on z/Architecture, the default is -march=z900. Otherwise, the default is -march=g5.
-mtune=cpu-type: Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. The list of cpu-type values is the same as for -march. The default is the value used for -march.
-mfused-madd
-mno-fused-madd: Generate code that uses (does not use) the floating point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used.
-mwarn-framesize=framesize: Emit a warning if the current function exceeds the given frame size. Because this is a compile time check it doesn't need to be a real problem when the program runs. It is intended to identify functions which most probably cause a stack overflow. It is useful to be used in an environment with limited stack size e.g. the linux kernel.
-mwarn-dynamicstack: Emit a warning if the function calls alloca or uses dynamically sized arrays. This is generally a bad idea with a limited stack size.
-mstack-guard=stack-guard
-mstack-size=stack-size: These arguments always have to be used in conjunction. If they are present the s390 back end emits additional instructions in the function prologue which trigger a trap if the stack size is stack-guard bytes above the stack-size(remember that the stack on s390 grows downward). These options are intended to be used to help debugging stack overflow problems. The addtionally emitted code cause only little overhead and hence can also be used in production like systems without greater performance degradation. The given values have to be exact powers of 2 and stack-size has to be greater than stack-guard. In order to be effecient the extra code makes the assumption that the stack starts at an address aligned to the value given by stack-size. So don't expect this to work correctly with a 8k stack size and an initial stack pointer like 0xffffefff.

CRIS Options

These options are defined specifically for the CRIS ports.

-march=architecture-type
-mcpu=architecture-type: Generate code for the specified architecture. The choices for architecture-type are v3, v8 and v10 for respectivelyETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0 except for cris-axis-linux-gnu, where the default is v10.
-mtune=architecture-type: Tune to architecture-type everything applicable about the generated code, except for the ABI and the set of available instructions. The choices for architecture-type are the same as for -march=architecture-type.
-mmax-stack-frame=n: Warn when the stack frame of a function exceeds n bytes.
-melinux-stacksize=n: Only available with the cris-axis-aout target. Arranges for indications in the program to the kernel loader that the stack of the program should be set to n bytes.
-metrax4
-metrax100: The options -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8 respectively.
-mmul-bug-workaround
-mno-mul-bug-workaround: Work around a bug in the "muls" and "mulu" instructions for CPU models where it applies. This option is active by default.
-mpdebug: Enable CRIS-specific verbose debug-related information in the assembly code. This option also has the effect to turn off the#NO_APP formatted-code indicator to the assembler at the beginning of the assembly file.
-mcc-init: Do not use condition-code results from previous instruction; always emit compare and test instructions before use of condition codes.
-mno-side-effects: Do not emit instructions with side-effects in addressing modes other than post-increment.
-mstack-align
-mno-stack-align
-mdata-align
-mno-data-align
-mconst-align
-mno-const-align: These options (no-options) arranges (eliminate arrangements) for the stack-frame, individual data and constants to be aligned for the maximum single data access size for the chosen CPU model. The default is to arrange for 32-bit alignment.ABI details such as structure layout are not affected by these options.
-m32-bit
-m16-bit
-m8-bit: Similar to the stack- data- and const-align options above, these options arrange for stack-frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment.
-mno-prologue-epilogue
-mprologue-epilogue: With -mno-prologue-epilogue, the normal function prologue and epilogue that sets up the stack-frame are omitted and no return instructions or return sequences are generated in the code. Use this option only together with visual inspection of the compiled code: no warnings or errors are generated when call-saved registers must be saved, or storage for local variable needs to be allocated.
-mno-gotplt
-mgotplt: With -fpic and -fPIC, don't generate (do generate) instruction sequences that load addresses for functions from the PLTpart of the GOT rather than (traditional on other architectures) calls to the PLT. The default is -mgotplt.
-maout: Legacy no-op option only recognized with the cris-axis-aout target.
-melf: Legacy no-op option only recognized with the cris-axis-elf and cris-axis-linux-gnu targets.
-melinux: Only recognized with the cris-axis-aout target, where it selects a GNU/linux-like multilib, include files and instruction set for -march=v8.
-mlinux: Legacy no-op option only recognized with the cris-axis-linux-gnu target.
-sim: This option, recognized for the cris-axis-aout and cris-axis-elf arranges to link with input-output functions from a simulator library. Code, initialized data and zero-initialized data are allocated consecutively.
-sim2: Like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data at 0x80000000.

MMIX Options

These options are defined for the MMIX:

-mlibfuncs
-mno-libfuncs: Specify that intrinsic library functions are being compiled, passing all values in registers, no matter the size.
-mepsilon
-mno-epsilon: Generate floating-point comparison instructions that compare with respect to the "rE" epsilon register.
-mabi=mmixware
-mabi=gnu: Generate code that passes function parameters and return values that (in the called function) are seen as registers $0 and up, as opposed to the GNU ABI which uses global registers $231 and up.
-mzero-extend
-mno-zero-extend: When reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load instructions by default, rather than sign-extending ones.
-mknuthdiv
-mno-knuthdiv: Make the result of a division yielding a remainder have the same sign as the divisor. With the default, -mno-knuthdiv, the sign of the remainder follows the sign of the dividend. Both methods are arithmetically valid, the latter being almost exclusively used.
-mtoplevel-symbols
-mno-toplevel-symbols: Prepend (do not prepend) a : to all global symbols, so the assembly code can be used with the "PREFIX" assembly directive.
-melf: Generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator.
-mbranch-predict
-mno-branch-predict: Use (do not use) the probable-branch instructions, when static branch prediction indicates a probable branch.
-mbase-addresses
-mno-base-addresses: Generate (do not generate) code that uses base addresses. Using a base address automatically generates a request (handled by the assembler and the linker) for a constant to be set up in a global register. The register is used for one or more base address requests within the range 0 to 255 from the value held in the register. The generally leads to short and fast code, but the number of different data items that can be addressed is limited. This means that a program that uses lots of static data may require -mno-base-addresses.
-msingle-exit
-mno-single-exit: Force (do not force) generated code to have a single exit point in each function.

PDP-11 Options

These options are defined for the PDP-11:

-mfpu: Use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported.)
-msoft-float: Do not use hardware floating point.
-mac0: Return floating-point results in ac0 (fr0 in Unix assembler syntax).
-mno-ac0: Return floating-point results in memory. This is the default.
-m40: Generate code for a PDP-11/40.
-m45: Generate code for a PDP-11/45. This is the default.
-m10: Generate code for a PDP-11/10.
-mbcopy-builtin: Use inline "movstrhi" patterns for copying memory. This is the default.
-mbcopy: Do not use inline "movstrhi" patterns for copying memory.
-mint16
-mno-int32: Use 16-bit "int". This is the default.
-mint32
-mno-int16: Use 32-bit "int".
-mfloat64
-mno-float32: Use 64-bit "float". This is the default.
-mfloat32
-mno-float64: Use 32-bit "float".
-mabshi: Use "abshi2" pattern. This is the default.
-mno-abshi: Do not use "abshi2" pattern.
-mbranch-expensive: Pretend that branches are expensive. This is for experimenting with code generation only.
-mbranch-cheap: Do not pretend that branches are expensive. This is the default.
-msplit: Generate code for a system with split I&D.
-mno-split: Generate code for a system without split I&D. This is the default.
-munix-asm: Use Unix assembler syntax. This is the default when configured for pdp11-*-bsd.
-mdec-asm: Use DEC assembler syntax. This is the default when configured for any PDP-11 target other than pdp11-*-bsd.

Xstormy16 Options

These options are defined for Xstormy16:

-msim: Choose startup files and linker script suitable for the simulator.

FRV Options

-mgpr-32: Only use the first 32 general purpose registers.
-mgpr-64: Use all 64 general purpose registers.
-mfpr-32: Use only the first 32 floating point registers.
-mfpr-64: Use all 64 floating point registers
-mhard-float: Use hardware instructions for floating point operations.
-msoft-float: Use library routines for floating point operations.
-malloc-cc: Dynamically allocate condition code registers.
-mfixed-cc: Do not try to dynamically allocate condition code registers, only use "icc0" and "fcc0".
-mdword: Change ABI to use double word insns.
-mno-dword: Do not use double word instructions.
-mdouble: Use floating point double instructions.
-mno-double: Do not use floating point double instructions.
-mmedia: Use media instructions.
-mno-media: Do not use media instructions.
-mmuladd: Use multiply and add/subtract instructions.
-mno-muladd: Do not use multiply and add/subtract instructions.
-mlibrary-pic: Enable PIC support for building libraries
-macc-4: Use only the first four media accumulator registers.
-macc-8: Use all eight media accumulator registers.
-mpack: Pack VLIW instructions.
-mno-pack: Do not pack VLIW instructions.
-mno-eflags: Do not mark ABI switches in e_flags.
-mcond-move: Enable the use of conditional-move instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-cond-move: Disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mscc: Enable the use of conditional set instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-scc: Disable the use of conditional set instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mcond-exec: Enable the use of conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-cond-exec: Disable the use of conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mvliw-branch: Run a pass to pack branches into VLIW instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-vliw-branch: Do not run a pass to pack branches into VLIW instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mmulti-cond-exec: Enable optimization of "&&" and "||" in conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-multi-cond-exec: Disable optimization of "&&" and "||" in conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mnested-cond-exec: Enable nested conditional execution optimizations (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-nested-cond-exec: Disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mtomcat-stats: Cause gas to print out tomcat statistics.
-mcpu=cpu: Select the processor type for which to generate code. Possible values are simple, tomcat, fr500, fr400, fr300, frv.

Xtensa Options

These options are supported for Xtensa targets:

-mconst16
-mno-const16: Enable or disable use of "CONST16" instructions for loading constant values. The "CONST16" instruction is currently not a standard option from Tensilica. When enabled, "CONST16" instructions are always used in place of the standard "L32R"instructions. The use of "CONST16" is enabled by default only if the "L32R" instruction is not available.
-mfused-madd
-mno-fused-madd: Enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option. This has no effect if the floating-point option is not also enabled. Disabling fused multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract operations. This may be desirable in some cases where strict IEEE 754-compliant results are required: the fused multiply add/subtract instructions do not round the intermediate result, thereby producing results with more bits of precision than specified by the IEEE standard. Disabling fused multiply add/subtract instructions also ensures that the program output is not sensitive to the compiler's ability to combine multiply and add/subtract operations.
-mtext-section-literals
-mno-text-section-literals: Control the treatment of literal pools. The default is -mno-text-section-literals, which places literals in a separate section in the output file. This allows the literal pool to be placed in a data RAM/ROM, and it also allows the linker to combine literal pools from separate object files to remove redundant literals and improve code size. With -mtext-section-literals, the literals are interspersed in the text section in order to keep them as close as possible to their references. This may be necessary for large assembly files.
-mtarget-align
-mno-target-align: When this option is enabled, GCC instructs the assembler to automatically align instructions to reduce branch penalties at the expense of some code density. The assembler attempts to widen density instructions to align branch targets and the instructions following call instructions. If there are not enough preceding safe density instructions to align a target, no widening will be performed. The default is -mtarget-align. These options do not affect the treatment of auto-aligned instructions like "LOOP", which the assembler will always align, either by widening density instructions or by inserting no-op instructions.
-mlongcalls
-mno-longcalls: When this option is enabled, GCC instructs the assembler to translate direct calls to indirect calls unless it can determine that the target of a direct call is in the range allowed by the call instruction. This translation typically occurs for calls to functions in other source files. Specifically, the assembler translates a direct "CALL" instruction into an "L32R" followed by a"CALLX" instruction. The default is -mno-longcalls. This option should be used in programs where the call target can potentially be out of range. This option is implemented in the assembler, not the compiler, so the assembly code generated by GCC will still show direct call instructions---look at the disassembled object code to see the actual instructions. Note that the assembler will use an indirect call for every cross-file call, not just those that really will be out of range.

Options for Code Generation Conventions

These machine-independent options control the interface conventions used in code generation.

Most of them have both positive and negative forms; the negative form of -ffoo would be -fno-foo. In the table below, only one of the forms is listed---the one which is not the default. You can figure out the other form by either removing no- or adding it.

-fbounds-check

For front-ends that support it, generate additional code to check that indices used to access arrays are within the declared range. This is currently only supported by the Java and Fortran 77 front-ends, where this option defaults to true and false respectively.

-ftrapv

This option generates traps for signed overflow on addition, subtraction, multiplication operations.

-fwrapv

This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation. This flag enables some optimizations and disables other. This option is enabled by default for the Java front-end, as required by the Java language specification.

-fexceptions

Enable exception handling. Generates extra code needed to propagate exceptions. For some targets, this implies GCCwill generate frame unwind information for all functions, which can produce significant data size overhead, although it does not affect execution. If you do not specify this option, GCC will enable it by default for languages like C++ which normally require exception handling, and disable it for languages like C that do not normally require it. However, you may need to enable this option when compiling C code that needs to interoperate properly with exception handlers written in C++. You may also wish to disable this option if you are compiling older C++ programs that don't use exception handling.

-fnon-call-exceptions

Generate code that allows trapping instructions to throw exceptions. Note that this requires platform-specific runtime support that does not exist everywhere. Moreover, it only allows trapping instructions to throw exceptions, i.e. memory references or floating point instructions. It does not allow exceptions to be thrown from arbitrary signal handlers such as"SIGALRM".

-funwind-tables

Similar to -fexceptions, except that it will just generate any needed static data, but will not affect the generated code in any other way. You will normally not enable this option; instead, a language processor that needs this handling would enable it on your behalf.

-fasynchronous-unwind-tables

Generate unwind table in dwarf2 format, if supported by target machine. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector).

-fpcc-struct-return

Return ``short'' "struct" and "union" values in memory like longer ones, rather than in registers. This convention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files compiled with other compilers, particularly the Portable C Compiler (pcc).

The precise convention for returning structures in memory depends on the target configuration macros.

Short structures and unions are those whose size and alignment match that of some integer type.

Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled with the -freg-struct-return switch. Use it to conform to a non-default application binary interface.

-freg-struct-return

Return "struct" and "union" values in registers when possible. This is more efficient for small structures than -fpcc-struct-return.

If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target. If there is no standard convention, GCC defaults to -fpcc-struct-return, except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative.

Warning: code compiled with the -freg-struct-return switch is not binary compatible with code compiled with the -fpcc-struct-return switch. Use it to conform to a non-default application binary interface.

-fshort-enums

Allocate to an "enum" type only as many bytes as it needs for the declared range of possible values. Specifically, the"enum" type will be equivalent to the smallest integer type which has enough room.

Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

-fshort-double

Use the same size for "double" as for "float".

Warning: the -fshort-double switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

-fshort-wchar

Override the underlying type for wchar_t to be short unsigned int instead of the default for the target. This option is useful for building programs to run under WINE.

Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

-fshared-data

Requests that the data and non-"const" variables of this compilation be shared data rather than private data. The distinction makes sense only on certain operating systems, where shared data is shared between processes running the same program, while private data exists in one copy per process.

-fno-common

In C, allocate even uninitialized global variables in the data section of the object file, rather than generating them as common blocks. This has the effect that if the same variable is declared (without "extern") in two different compilations, you will get an error when you link them. The only reason this might be useful is if you wish to verify that the program will work on other systems which always work this way.

-fno-ident

Ignore the #ident directive.

-finhibit-size-directive

Don't output a ".size" assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory. This option is used when compiling crtstuff.c; you should not need to use it for anything else.

-fverbose-asm

Put extra commentary information in the generated assembly code to make it more readable. This option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself).

-fno-verbose-asm, the default, causes the extra information to be omitted and is useful when comparing two assembler files.

-fpic

Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that -fpic does not work; in that case, recompile with -fPIC instead. (These maximums are 8k on the SPARC and 32k on the m68k and RS/6000. The 386 has no such limit.)

Position-independent code requires special support, and therefore works only on certain machines. For the 386, GCCsupports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent.

-fPIC

If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on the m68k and the SPARC.

Position-independent code requires special support, and therefore works only on certain machines.

-fpie

-fPIE

These options are similar to -fpic and -fPIC, but generated position independent code can be only linked into executables. Usually these options are used when -pie GCC option will be used during linking.

-ffixed-reg

Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role).

reg must be the name of a register. The register names accepted are machine-specific and are defined in the"REGISTER_NAMES" macro in the machine description macro file.

This flag does not have a negative form, because it specifies a three-way choice.

-fcall-used-reg

Treat the register named reg as an allocable register that is clobbered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way will not save and restore the register reg.

It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model will produce disastrous results.

This flag does not have a negative form, because it specifies a three-way choice.

-fcall-saved-reg

Treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way will save and restore the register reg if they use it.

A different sort of disaster will result from the use of this flag for a register in which function values may be returned.

This flag does not have a negative form, because it specifies a three-way choice.

-fpack-struct

Pack all structure members together without holes.

Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary interface.

-finstrument-functions

Generate instrumentation calls for entry and exit to functions. Just after function entry and just before function exit, the following profiling functions will be called with the address of the current function and its call site. (On some platforms,"__builtin_return_address" does not work beyond the current function, so the call site information may not be available to the profiling functions otherwise.)

        void __cyg_profile_func_enter (void *this_fn,
                                       void *call_site);
        void __cyg_profile_func_exit  (void *this_fn,
                                       void *call_site);

The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table.

This currently disables function inlining. This restriction is expected to be removed in future releases.

A function may be given the attribute "no_instrument_function", in which case this instrumentation will not be done. This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called (perhaps signal handlers, if the profiling routines generate output or allocate memory).

-fstack-check

Generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag if you are running in an environment with multiple threads, but only rarely need to specify it in a single-threaded environment since stack overflow is automatically detected on nearly all systems if there is only one stack.

Note that this switch does not actually cause checking to be done; the operating system must do that. The switch causes generation of code to ensure that the operating system sees the stack being extended.

-fstack-limit-register=reg

-fstack-limit-symbol=sym

-fno-stack-limit

Generate code to ensure that the stack does not grow beyond a certain value, either the value of a register or the address of a symbol. If the stack would grow beyond the value, a signal is raised. For most targets, the signal is raised before the stack overruns the boundary, so it is possible to catch the signal without taking special precautions.

For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Note that this may only work with the GNU linker.

-fargument-alias

-fargument-noalias

-fargument-noalias-global

Specify the possible relationships among parameters and between parameters and global data.

-fargument-alias specifies that arguments (parameters) may alias each other and may alias global storage.-fargument-noalias specifies that arguments do not alias each other, but may alias global storage.-fargument-noalias-globalspecifies that arguments do not alias each other and do not alias global storage.

Each language will automatically use whatever option is required by the language standard. You should not need to use these options yourself.

-fleading-underscore

This option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented in the object file. One use is to help link with legacy assembly code.

Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch.

-ftls-model=model

Alter the thread-local storage model to be used. The model argument should be one of "global-dynamic", "local-dynamic","initial-exec" or "local-exec".

The default without -fpic is "initial-exec"; with -fpic the default is "global-dynamic".

-fvisibility=default|internal|hidden|protected

Set the default ELF image symbol visibility to the specified option - all symbols will be marked with this unless overrided within the code. Using this feature can very substantially improve linking and load times of shared object libraries, produce more optimised code, provide near-perfect API export and prevent symbol clashes. It is strongly recommended that you use this in any shared objects you distribute.

Despite the nomenclature, "default" always means public ie; available to be linked against from outside the shared object."protected" and "internal" are pretty useless in real-world usage so the only other commonly used option will be "hidden". The default if -fvisibility isn't specified is "default" ie; make every symbol public - this causes the same behaviour as previous versions of GCC.

A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by ``How To Write Shared Libraries'' by Ulrich Drepper (which can be found at <http://people.redhat.com/~drepper/>) - however a superior solution made possible by this option to marking things hidden when the default is public is to make the default hidden and mark things public. This is the norm with DLL's on Windows and with -fvisibility=hidden and "__attribute__ ((visibility("default")))" instead of "__declspec(dllexport)" you get almost identical semantics with identical syntax. This is a great boon to those working with cross-platform projects.

For those adding visibility support to existing code, you may find #pragma GCC visibility of use. This works by you enclosing the declarations you wish to set visibility for with (for example) #pragma GCC visibility push(hidden) and#pragma GCC visibility pop. These can be nested up to sixteen times. Bear in mind that symbol visibility should be viewed as part of the API interface contract and thus all new code should always specify visibility when it is not the default ie; declarations only for use within the local DSO should always be marked explicitly as hidden as so to avoid PLTindirection overheads - making this abundantly clear also aids readability and self-documentation of the code. Note that due to ISO C++ specification requirements, operator new and operator delete must always be of default visibility.

An overview of these techniques, their benefits and how to use them is at <http://www.nedprod.com/programs/gccvisibility.html>.

ENVIRONMENT

This section describes several environment variables that affect how GCC operates. Some of them work by specifying directories or prefixes to use when searching for various kinds of files. Some are used to specify other aspects of the compilation environment.

Note that you can also specify places to search using options such as -B, -I and -L. These take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GCC.

LANG

LC_CTYPE

LC_MESSAGES

LC_ALL

These environment variables control the way that GCC uses localization information that allow GCC to work with different national conventions. GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it has been configured to do so. These locale categories can be set to any value supported by your installation. A typical value is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8.

The LC_CTYPE environment variable specifies character classification. GCC uses it to determine the character boundaries in a string; this is needed for some multibyte encodings that contain quote and escape characters that would otherwise be interpreted as a string end or escape.

The LC_MESSAGES environment variable specifies the language to use in diagnostic messages.

If the LC_ALL environment variable is set, it overrides the value of LC_CTYPE and LC_MESSAGES; otherwise,LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable. If none of these variables are set,GCC defaults to traditional C English behavior.

TMPDIR

If TMPDIR is set, it specifies the directory to use for temporary files. GCC uses temporary files to hold the output of one stage of compilation which is to be used as input to the next stage: for example, the output of the preprocessor, which is the input to the compiler proper.

GCC_EXEC_PREFIX

If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish.

If GCC_EXEC_PREFIX is not set, GCC will attempt to figure out an appropriate prefix to use based on the pathname it was invoked with.

If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram.

The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the value of "prefix" when you ran the configurescript.

Other prefixes specified with -B take precedence over this prefix.

This prefix is also used for finding files such as crt0.o that are used for linking.

In addition, the prefix is used in an unusual way in finding the directories to search for header files. For each of the standard directories whose name normally begins with /usr/local/lib/gcc (more precisely, with the value ofGCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name. Thus, with -Bfoo/, GCC will search foo/bar where it would normally search /usr/local/lib/bar. These alternate directories are searched first; the standard directories come next.

COMPILER_PATH

The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the directories thus specified when searching for subprograms, if it can't find the subprograms using GCC_EXEC_PREFIX.

LIBRARY_PATH

The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it can't find them usingGCC_EXEC_PREFIX. Linking using GCC also uses these directories when searching for ordinary libraries for the -l option (but directories specified with -L come first).

LANG

This variable is used to pass locale information to the compiler. One way in which this information is used is to determine the character set to be used when character literals, string literals and comments are parsed in C and C++. When the compiler is configured to allow multibyte characters, the following values for LANG are recognized:

C-JIS: Recognize JIS characters.
C-SJIS: Recognize SJIS characters.
C-EUCJP: Recognize EUCJP characters.

: If LANG is not defined, or if it has some other value, then the compiler will use mblen and mbtowc as defined by the default locale to recognize and translate multibyte characters.

Some additional environments variables affect the behavior of the preprocessor.

CPATH

C_INCLUDE_PATH

CPLUS_INCLUDE_PATH

OBJC_INCLUDE_PATH

Each variable's value is a list of directories separated by a special character, much like PATH, in which to look for header files. The special character, "PATH_SEPARATOR", is target-dependent and determined at GCC build time. For Microsoft Windows-based targets it is a semicolon, and for almost all other targets it is a colon.

CPATH specifies a list of directories to be searched as if specified with -I, but after any paths given with -I options on the command line. This environment variable is used regardless of which language is being preprocessed.

The remaining environment variables apply only when preprocessing the particular language indicated. Each specifies a list of directories to be searched as if specified with -isystem, but after any paths given with -isystem options on the command line.

In all these variables, an empty element instructs the compiler to search its current working directory. Empty elements can appear at the beginning or end of a path. For instance, if the value of CPATH is ":/special/include", that has the same effect as -I. -I/special/include.

DEPENDENCIES_OUTPUT

If this variable is set, its value specifies how to output dependencies for Make based on the non-system header files processed by the compiler. System header files are ignored in the dependency output.

The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that file, guessing the target name from the source file name. Or the value can have the form file target, in which case the rules are written to file file using target as the target name.

In other words, this environment variable is equivalent to combining the options -MM and -MF, with an optional -MT switch too.

SUNPRO_DEPENDENCIES

This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not ignored, so it implies -M rather than -MM. However, the dependence on the main input file is omitted.

BUGS

For instructions on reporting bugs, see <http://gcc.gnu.org/bugs.html>. Use of the gccbug script to report bugs is recommended.

FOOTNOTES

1.: On some systems, gcc -shared needs to build supplementary stub code for constructors to work. On multi-libbed systems, gcc -shared must select the correct support libraries to link against. Failing to supply the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is innocuous.

AUTHOR

See the Info entry for gcc, or <http://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC.

COPYRIGHT

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with the Invariant Sections being ``GNU General Public License'' and ``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 gfdl(7) man page.

(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.

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GCC

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描述 (DESCRIPTION)

選項 (OPTIONS)

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語言選項 (LANGUAGE OPTIONS)

預處理器選項 (Preprocessor Option)

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NAME

SYNOPSIS

DESCRIPTION

OPTIONS

Option Summary

Options Controlling the Kind of Output

Compiling C++ Programs

Options Controlling C Dialect

Options Controlling C++ Dialect

Options Controlling Objective-C Dialect

Options to Control Diagnostic Messages Formatting

Options to Request or Suppress Warnings

Options for Debugging Your Program or GCC

Options That Control Optimization

Options Controlling the Preprocessor

Passing Options to the Assembler

Options for Linking

Options for Directory Search

Specifying Target Machine and Compiler Version

Hardware Models and Configurations

M680x0 Options

M68hc1x Options

VAX Options

SPARC Options

ARM Options

MN10300 Options

M32R/D Options

IBM RS/6000 and PowerPC Options

Darwin Options

MIPS Options

Intel 386 and AMD x86-64 Options

HPPA Options

Intel 960 Options

DEC Alpha Options

DEC Alpha/VMS Options

H8/300 Options

SH Options

Options for System V

TMS320C3x/C4x Options

V850 Options

ARC Options

NS32K Options

AVR Options

MCore Options

IA-64 Options

D30V Options

S/390 and zSeries Options

CRIS Options

MMIX Options

PDP-11 Options

Xstormy16 Options

FRV Options