-O3 doesn't cause undefined behavior.

No, but it can often expose undefined behavior that doesn't stop a program from "working" on lower optimization levels.

Which is why I'm in support of always building your own software with -O3.

Are you sure about that? I’m not aware of any additional UB exploitation enabled by O3 vs O2 or Os.

Nope, I'm not sure about it. I remember when I was using Gentoo about 10 years ago, this was the common reason given for using -O2 instead of -O3 in your build flags, and I'm just speaking from that memory.

Yeah there’s lots of cargo culting FUD like that that happens (see other posters in this thread that claim that glibc is a more stable allocator )

Yes, -O3 is seldom an improvement for most C compilers.