[lifthrasiir]

Because i) not all programs benefit tremendously from -O3 and ii) it adds to the compile time and binary size? It would be great to have some optimization level between -O2 and -O3 so that only portions that have a potential to be improved more than, say, 5% are compiled using -O3. In fact the existence of `#pragma GCC optimize` does suggest that this might be possible today with some heuristics... (Or use PGO, which will have the same effect. But PGO is still a novelty in 2023.)

[glandium]

Also, in practice, -O3 doesn't necessarily lead to faster code. https://people.cs.umass.edu/~emery/pubs/stabilizer-asplos13.... https://m.youtube.com/watch?v=r-TLSBdHe1A

[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.