I was only saying that -O3 enables _even more_ optimizations, which can lead to UB code that works with -O2 but misbehaves on -O3. And because -O3 is seldom use

I was only saying that -O3 enables _even more_ optimizations, which can lead to UB code that works with -O2 but misbehaves on -O3. And because -O3 is seldom used (for reasons other than code breakage), the code patterns that are actually UB but don't get compiled to bad code at -O2 (by e.g. gcc) are lesser known. It all depends on how much a given compiler is willing to make assumptions based on UB and what's permitted by C spec to make optimizations. At -O3 it probably makes a lot more of those assumptions, along the lines of "the spec says this is undefined behavior so I will assume it can never happen and optimize it based on that". The more aggressive inlining in conjunction with that probably exposes more even more UB. Edit: found example where code works on -O2 but breaks on -O3, related to using abnormal amounts of stack space https://stackoverflow.com/questions/47058978/g-optimization-o2-flag-fixes-a-broken-code-where-o3-breaks-it-again https://stackoverflow.com/questions/47058978/g-optimization-...