Hi Yue, You can get a list of toggle-able optimization flags like this: touch Empty.cpp gcc -O2 -fverbose-asm -S Empty.cpp -o O2.s gcc -O3 -fverbose-asm -S Empty.cpp -o O3.s However, that will not tell you what non-toggle-able optimizations are enabled, since -O2 enables many optimizations that are not controlled by command line flags. And -O3 enables a few more, as I understand it. Nor will that tell you what optimization(s) were applied to a particular code segment. Sincerely, --Eljay
