makefile中的 -O -O2选项含义

-O:optimization(优化的意思)
例如:CFLAGS = -I../lib -g -O2 -D_REENTRANT -Wall

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

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

-O
-O1
Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.

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

-O turns on the following optimization flags:

          -fauto-inc-dec 
          -fbranch-count-reg 
          -fcombine-stack-adjustments 
          -fcompare-elim 
          -fcprop-registers 
          -fdce 
          -fdefer-pop 
          -fdelayed-branch 
          -fdse 
          -fforward-propagate 
          -fguess-branch-probability 
          -fif-conversion2 
          -fif-conversion 
          -finline-functions-called-once 
          -fipa-pure-const 
          -fipa-profile 
          -fipa-reference 
          -fmerge-constants 
          -fmove-loop-invariants 
          -freorder-blocks 
          -fshrink-wrap 
          -fsplit-wide-types 
          -fssa-backprop 
          -fssa-phiopt 
          -ftree-bit-ccp 
          -ftree-ccp 
          -ftree-ch 
          -ftree-coalesce-vars 
          -ftree-copy-prop 
          -ftree-dce 
          -ftree-dominator-opts 
          -ftree-dse 
          -ftree-forwprop 
          -ftree-fre 
          -ftree-phiprop 
          -ftree-sink 
          -ftree-slsr 
          -ftree-sra 
          -ftree-pta 
          -ftree-ter 
          -funit-at-a-time

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

-O2
Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. As compared to -O, this option increases both compilation time and the performance of the generated code.

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

          -fthread-jumps 
          -falign-functions  -falign-jumps 
          -falign-loops  -falign-labels 
          -fcaller-saves 
          -fcrossjumping 
          -fcse-follow-jumps  -fcse-skip-blocks 
          -fdelete-null-pointer-checks 
          -fdevirtualize -fdevirtualize-speculatively 
          -fexpensive-optimizations 
          -fgcse  -fgcse-lm  
          -fhoist-adjacent-loads 
          -finline-small-functions 
          -findirect-inlining 
          -fipa-cp 
          -fipa-cp-alignment 
          -fipa-sra 
          -fipa-icf 
          -fisolate-erroneous-paths-dereference 
          -flra-remat 
          -foptimize-sibling-calls 
          -foptimize-strlen 
          -fpartial-inlining 
          -fpeephole2 
          -freorder-blocks-algorithm=stc 
          -freorder-blocks-and-partition -freorder-functions 
          -frerun-cse-after-loop  
          -fsched-interblock  -fsched-spec 
          -fschedule-insns  -fschedule-insns2 
          -fstrict-aliasing -fstrict-overflow 
          -ftree-builtin-call-dce 
          -ftree-switch-conversion -ftree-tail-merge 
          -fcode-hoisting 
          -ftree-pre 
          -ftree-vrp 
          -fipa-ra

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

-O3
Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the -finline-functions, -funswitch-loops, -fpredictive-commoning, -fgcse-after-reload, -ftree-loop-vectorize, -ftree-loop-distribute-patterns, -fsplit-paths -ftree-slp-vectorize, -fvect-cost-model, -ftree-partial-pre, -fpeel-loops and -fipa-cp-clone options.
-O0
Reduce compilation time and make debugging produce the expected results. This is the default.
-Os
Optimize for size. -Os enables all -O2 optimizations that do not typically increase code size. It also performs further optimizations designed to reduce code size.

-Os disables the following optimization flags:

          -falign-functions  -falign-jumps  -falign-loops 
          -falign-labels  -freorder-blocks  -freorder-blocks-algorithm=stc 
          -freorder-blocks-and-partition  -fprefetch-loop-arrays

-Ofast
Disregard strict standards compliance. -Ofast enables all -O3 optimizations. It also enables optimizations that are not valid for all standard-compliant programs. It turns on -ffast-math and the Fortran-specific -fno-protect-parens and -fstack-arrays.
-Og
Optimize debugging experience. -Og enables optimizations that do not interfere with debugging. It should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience.

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

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

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

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

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

-fforward-propagate
Perform a forward propagation pass on RTL. The pass tries to combine two instructions and checks if the result can be simplified. If loop unrolling is active, two passes are performed and the second is scheduled after loop unrolling.

This option is enabled by default at optimization levels -O, -O2, -O3, -Os.