Eljay, Let me try with the asm comparison, by the way, it's so huge and contains many source file :( On my platform, the -Os does not have following flag: -freorder-blocks. It has all -O2 flags. Here is the -Os flags GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 options passed: -v -iprefix -march=mips32 -msoft-float -auxbase -Os options enabled: -feliminate-unused-debug-types -fdefer-pop -fomit-frame-pointer -foptimize-sibling-calls -funit-at-a-time -fcse-follow-jumps -fcse-skip-blocks -fexpensive-optimizations -fthread-jumps -fstrength-reduce -fpeephole -fforce-mem -ffunction-cse -fkeep-static-consts -fcaller-saves -fpcc-struct-return -fgcse -fgcse-lm -fgcse-sm -fgcse-las -floop-optimize -fcrossjumping -fif-conversion -fif-conversion2 -frerun-cse-after-loop -frerun-loop-opt -fdelete-null-pointer-checks -fschedule-insns -fschedule-insns2 -fsched-interblock -fsched-spec -fsched-stalled-insns -fsched-stalled-insns-dep -fbranch-count-reg -fpic -freorder-functions -fcprop-registers -fcommon -fregmove -foptimize-register-move -fargument-alias -fstrict-aliasing -fmerge-constants -fzero-initialized-in-bss -fident -fpeephole2 -fguess-branch-probability -fmath-errno -ftrapping-math -mgas -msoft-float -mabicalls -meb -mexplicit-relocs -march=mips32 -mno-flush-func_flush_cache -mflush-func=_flush_cache And here is the -O2 flags GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 options passed: -v -iprefix -march=mips32 -msoft-float -auxbase -O2 options enabled: -feliminate-unused-debug-types -fdefer-pop -fomit-frame-pointer -foptimize-sibling-calls -funit-at-a-time -fcse-follow-jumps -fcse-skip-blocks -fexpensive-optimizations -fthread-jumps -fstrength-reduce -fpeephole -fforce-mem -ffunction-cse -fkeep-static-consts -fcaller-saves -fpcc-struct-return -fgcse -fgcse-lm -fgcse-sm -fgcse-las -floop-optimize -fcrossjumping -fif-conversion -fif-conversion2 -frerun-cse-after-loop -frerun-loop-opt -fdelete-null-pointer-checks -fschedule-insns -fschedule-insns2 -fsched-interblock -fsched-spec -fsched-stalled-insns -fsched-stalled-insns-dep -fbranch-count-reg -fpic -freorder-blocks -freorder-functions -fcprop-registers -fcommon -fregmove -foptimize-register-move -fargument-alias -fstrict-aliasing -fmerge-constants -fzero-initialized-in-bss -fident -fpeephole2 -fguess-branch-probability -fmath-errno -ftrapping-math -mgas -msoft-float -mabicalls -meb -mexplicit-relocs -march=mips32 -mno-flush-func_flush_cache -mflush-func=_flush_cache Regards, -Hieu -----Original Message----- From: gcc-help-owner@xxxxxxxxxxx [mailto:gcc-help-owner@xxxxxxxxxxx] On Behalf Of John (Eljay) Love-Jensen Sent: Monday, April 27, 2009 6:16 PM To: Hieu Le Trung; GCC-help Subject: Re: Difference between -Os and -O2 Hi Hieu Le Trung, To expand upon Georg-Johann Lay¹s recommendation, I would do this: echo '' | gcc -fverbose-asm -Os -S -x c - -o Os.s echo '' | gcc -fverbose-asm -O2 -S -x c - -o O2.s vi -d Os.s O2.s On my platform, -Os has these additional flags over -O2: -falign-jumps-max-skip -falign-loops-max-skip -finline-functions While -O2 has this flag that -Os does not have: -ftree-ch Those may not be the same differences on your platform, though. HTH, --Eljay
