Hi John, On my system, the flag differences between -O2 and -O3 are that -O3 also enables: -fgcse-after-reload -finline-functions -funswitch-loops That may not be the same on your platform, however. You can do this: echo '' | g++ -O2 -fverbose-asm -S -x c++ - -o O2.txt echo '' | g++ -O3 -fverbose-asm -S -x c++ - -o O3.txt And then compare the flag differences on your platform. Note, I do not know if there are other optimizations enabled by -O3 that do not have associated flags. Most optimizations do *NOT* have a knob to enable/disable. But, perhaps, most of those non-twiddle-able optimizations are already enabled at -O2. HTH, --Eljay
