[KeplerBoy]

you can manually set the optimizations (and order of optimizations) you want. -O3 ist just a predefined set of optimizations.

[lifthrasiir]

But it is not portable among compilers (for example, both GCC and Clang support SLP-based autovectorization but with different flags). -O# is one of a few flags that have roughly same meaning across many compilers.