[hmry]

-O3 also makes build times longer (sometimes significantly), and occasionally the resulting program is actually slightly slower than -O2.

IME -O3 should only be used if you have benchmarks that show -O3 actually produces a speedup for your specific codebase.

[fyrn_]

This various a lot between compilers. Clang for example treats O3 perf regressions a bugs In many cases at least) and is a bit more reasonable with O3 on. GCC goes full mad max and you don't know what it's going to do.