Shouldn't suggest using -O3

[pixelherodev]

-O3 for GCC is explicitly unsafe, enabling - among other things - non-standards compliant floating point maths.

Given what's being discussed here, why is -O3 being suggested as a good idea? -O2 or -Os are usually much safer.

Compiling with -O3 is not a guaranteed way to improve performance, and in fact in many cases can slow down a system due to larger binaries and increased memory usage. -O3 is also known to break several packages. Therefore, using -O3 is not recommended.

A better, safer option for local builds is -O2 -march=native -mtune=native, possibly also with -flto.

[Ravenslofty]

I'm very late, but you appear to confuse -O3 (standards compliant) with -Ofast (explicitly non-standards compliant). -O3 is entirely safe providing your code is correct. Packages sometimes break with -O3 because they are not correct, and the analysis at -O3 can prove as such.

For modern, new programs there is very little reason to prefer -O2 over -O3 unless you're doing very hairy things with templates, especially because the more detailed flow analysis of -O3 can produce warnings (find bugs) that -O2 does not.

[pixelherodev]

...very late also, but I believe you are correct. I was remembering warnings not to use -O3 because a lot of code is not correct, and hasn't been fixed.