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
