-O3 isn't always strictly faster than -O2. There are different tradeoffs made about memory usage and binary size which interact with CPU caches in interesting

-O3 isn't always strictly faster than -O2. There are different tradeoffs made about memory usage and binary size which interact with CPU caches in interesting ways. I can't speak for the author but at my work we found -O2 to give better results on our OpenGL ES rendering engine.