From "A collection of compiler optimizations with brief descriptions and examples" https://news.ycombinator.com/item?id=40275850 :

> Also, only SO has what specific optimizations are applied for clang's -O1/-O2/-03 args? https://stackoverflow.com/questions/15548023/clang-optimizat...

Intel uses -o2 or -o3 for Clear Linux packages, depending on which produces faster binaries. Worth checking out, if you want more performance.