with out RTFA -O3 is where a lot of the weird compiler deciding it's smarter than you optimizations come in, and most of the time it is smarter than you and som

with out RTFA -O3 is where a lot of the weird compiler deciding it's smarter than you optimizations come in, and most of the time it is smarter than you and sometimes it introduces weird behavior. ALSO it generally produces a larger binary which means less of it fits in the cache. I know a while back it was often suggested to try O3 and OS because often OS ended up being faster.