GCC不常见，但有用特别选项

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       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.  If dir begins

       with "=", then the "=" will be replaced by the sysroot prefix; see --sysroot and

       -isysroot.

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       Use dir as the logical root directory for headers and libraries.  For example, if the

       compiler normally searches for headers in /usr/include and libraries in /usr/lib, it

       instead searches dir/usr/include and dir/usr/lib.

       If you use both this option and the -isysroot option, then the --sysroot option

       applies to libraries, but the -isysroot option applies to header files.

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       On systems that support dynamic linking, this prevents linking with the shared libraries.

       On other systems, this option has no effect.

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      Instead of outputting the result of preprocessing, output a rule suitable for make describing the dependencies of the main source file.

      The preprocessor outputs one make rule containing the object file name for that source file, a colon, and the names of all the included files,

      including those coming from -include or -imacros command line options.

      Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any suffix replaced with object

      file suffix and with any leading directory parts  removed.  If there are many included files then the rule is split into several lines using \-newline.

      The rule has no commands.

      This option does not suppress the preprocessor's debug output, such as -dM.  To avoid mixing such debug output with the dependency

      rules you should explicitly specify the dependency output file

       with -MF, or use an environment variable like DEPENDENCIES_OUTPUT.  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.

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       a large function.

       With -O, the compiler tries to reduce code size and execution time, without performing

       any optimizations that take a great deal of compilation time.

       -O turns on the following optimization flags:

       -fauto-inc-dec -fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch

       -fdse -fguess-branch-probability -fif-conversion2 -fif-conversion -fipa-pure-const

       -fipa-profile -fipa-reference -fmerge-constants -fsplit-wide-types -ftree-bit-ccp

       -ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-copyrename -ftree-dce

       -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -ftree-phiprop -ftree-slsr

       -ftree-sra -ftree-pta -ftree-ter -funit-at-a-time

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

       with debugging.

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       involve a space-speed tradeoff.  As compared to -O, this option increases both

       compilation time and the performance of the generated code.

       -O2 turns on all optimization flags specified by -O.  It also turns on the following

       optimization flags: -fthread-jumps -falign-functions  -falign-jumps -falign-loops

       -falign-labels -fcaller-saves -fcrossjumping -fcse-follow-jumps  -fcse-skip-blocks

       -fdelete-null-pointer-checks -fdevirtualize -fexpensive-optimizations -fgcse

       -fgcse-lm -fhoist-adjacent-loads -finline-small-functions -findirect-inlining

       -fipa-sra -foptimize-sibling-calls -fpartial-inlining -fpeephole2 -fregmove

       -freorder-blocks  -freorder-functions -frerun-cse-after-loop -fsched-interblock

       -fsched-spec -fschedule-insns  -fschedule-insns2 -fstrict-aliasing -fstrict-overflow

       -ftree-switch-conversion -ftree-tail-merge -ftree-pre -ftree-vrp

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

       gotos.

       NOTE: In Ubuntu 8.10 and later versions, -D_FORTIFY_SOURCE=2 is set by default, and is

       activated when -O is set to 2 or higher.  This enables additional compile-time and

       run-time checks for several libc functions.  To disable, specify either

       -U_FORTIFY_SOURCE or -D_FORTIFY_SOURCE=0.

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       the -finline-functions, -funswitch-loops, -fpredictive-commoning, -fgcse-after-reload,

       -ftree-vectorize, -fvect-cost-model, -ftree-partial-pre and -fipa-cp-clone options.

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       default.

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       code size.  It also performs further optimizations designed to reduce code size.

       -Os disables the following optimization flags: -falign-functions  -falign-jumps

       -falign-loops -falign-labels  -freorder-blocks  -freorder-blocks-and-partition

       -fprefetch-loop-arrays  -ftree-vect-loop-version

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       Disregard strict standards compliance.  -Ofast enables all -O3 optimizations.  It also

       enables optimizations that are not valid for all standard-compliant programs.  It

       turns on -ffast-math and the Fortran-specific -fno-protect-parens and -fstack-arrays.

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       debugging. It should be the optimization level of choice for the standard edit-

       compile-debug cycle, offering a reasonable level of optimization while maintaining

       fast compilation and a good debugging experience.

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

       the one that is effective.

      优化等级

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       Emit extra code to check for buffer overflows, such as stack smashing attacks.  This

       is done by adding a guard variable to functions with vulnerable objects.  This

       includes functions that call "alloca", and functions with buffers larger than 8 bytes.

       The guards are initialized when a function is entered and then checked when the

       function exits.  If a guard check fails, an error message is printed and the program

       exits

       检查栈溢出，相反的      -fno-stack-protector

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       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.

       不使用标准C头文件

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       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.)

      不使用标准C++头文件

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       This enables all the warnings about constructions that some users consider

       questionable, and that are easy to avoid (or modify to prevent the warning), even in

       conjunction with macros.  This also enables some language-specific warnings described

       in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.

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       This enables some extra warning flags that are not enabled by -Wall. (This option used

       to be called -W.  The older name is still supported, but the newer name is more

       descriptive.)

       打开额外的警告，同-W（旧名）

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       Make all warnings into hard errors.  Source code which triggers warnings will be

       rejected.

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       Pass option as an option to the assembler.  If option contains commas, it is split

       into multiple options at the commas