g++代码编译用选项-O3前后对比，前后执行时间对比如下

g++代码编译用选项-O3前后对比，前后执行时间对比如下

g++ -std=c++11  main.cpp blur.cpp initialize_beliefs.cpp move.cpp normalize.cpp print.cpp sense.cpp zeros.cpp  

root@b8da9efe81e4:/home/workspace/optimized_code# ./a.out 
number of iterations: 10000 
duration milliseconds initialize beliefs 22.599
duration milliseconds sense 16.463
duration milliseconds blur 77.062
duration milliseconds normalize 20.721
duration milliseconds move 31.484
/////////////////////////今早2.13用该条编译器优化看看是否较上条提升了性能
g++ -std=c++11 -O3 main.cpp blur.cpp initialize_beliefs.cpp move.cpp normalize.cpp print.cpp sense.cpp zeros.cpp  
//编译优化后的
root@65c825f71a98:/home/workspace/optimized_code# ./a.out 
number of iterations: 10000 
duration milliseconds initialize beliefs 3.284
duration milliseconds sense 3.972
duration milliseconds blur 26.045
duration milliseconds normalize 2.177
duration milliseconds move 4.634

 

https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

 

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-conversion 
-fif-conversion2 
-finline-functions-called-once 
-fipa-profile 
-fipa-pure-const 
-fipa-reference 
-fipa-reference-addressable 
-fmerge-constants 
-fmove-loop-invariants 
-fomit-frame-pointer 
-freorder-blocks 
-fshrink-wrap 
-fshrink-wrap-separate 
-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-pta 
-ftree-scev-cprop 
-ftree-sink 
-ftree-slsr 
-ftree-sra 
-ftree-ter 
-funit-at-a-time

-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:

-falign-functions  -falign-jumps 
-falign-labels  -falign-loops 
-fcaller-saves 
-fcode-hoisting 
-fcrossjumping 
-fcse-follow-jumps  -fcse-skip-blocks 
-fdelete-null-pointer-checks 
-fdevirtualize  -fdevirtualize-speculatively 
-fexpensive-optimizations 
-ffinite-loops 
-fgcse  -fgcse-lm  
-fhoist-adjacent-loads 
-finline-functions 
-finline-small-functions 
-findirect-inlining 
-fipa-bit-cp  -fipa-cp  -fipa-icf 
-fipa-ra  -fipa-sra  -fipa-vrp 
-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  
-fschedule-insns  -fschedule-insns2 
-fsched-interblock  -fsched-spec 
-fstore-merging 
-fstrict-aliasing 
-fthread-jumps 
-ftree-builtin-call-dce 
-ftree-pre 
-ftree-switch-conversion  -ftree-tail-merge 
-ftree-vrp

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

-O3

Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the following optimization flags:

-fgcse-after-reload 
-fipa-cp-clone
-floop-interchange 
-floop-unroll-and-jam 
-fpeel-loops 
-fpredictive-commoning 
-fsplit-paths 
-ftree-loop-distribute-patterns 
-ftree-loop-distribution 
-ftree-loop-vectorize 
-ftree-partial-pre 
-ftree-slp-vectorize 
-funswitch-loops 
-fvect-cost-model 
-fversion-loops-for-strides

-O0

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

-Os

Optimize for size. -Os enables all -O2 optimizations except those that often increase code size:

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

It also enables -finline-functions, causes the compiler to tune for code size rather than execution speed, and performs further optimizations designed to reduce code size.