GCC standard optimizations behavior

问题

Here I compile an input program with -O2 optimization level (with gcc 4.8.4) and measure the execution time:

gcc -O2 -c test.c -o obj.o
TIMEFORMAT='%3R' &&  time(./obj.o)
execution time = 1.825

and when I replace -O2 flag with the list of options that are turned on as defined in GCC manuel in the level -O2 https://gcc.gnu.org/onlinedocs/gcc-4.8.4/gcc/Optimize-Options.html#Optimize-Options like that:

gcc -fauto-inc-dec -fcompare-elim -fcprop-registers -fdce -fdefer-pop -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 -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 -c test.c -o obj.o
    TIMEFORMAT='%3R' &&  time(./obj.o)
execution time = 2.652

My question is why the execution time is different even so, I applied the same optimizations ?

UPDATE

if (according to GCC documentation):

Not all optimizations are controlled directly by a flag.

So how can researchers use to reproduce optimization sequences even faster than standard optimization sequences (using evolutionary algorithms they use to generate thousands of optimization sequences and gather those with highest impact in term of execution time)

as an example "Acovea" http://hg.ahs3.net/acovea/debian/html/acoveaga.html

and "Cole" http://users.elis.ugent.be/~leeckhou/papers/cgo08.pdf

回答1:

There is 2 good optimization that should be know:

• -O2: Optimize space and time (caution: does not initialize variable)
• -Os: Optimize space and time (caution: does not initialize variable)

来源：https://stackoverflow.com/questions/33832997/gcc-standard-optimizations-behavior