Comment by Dylan16807

This comment seems to mostly be describing avoiding undefined behavior. You can learn the rules to do that, though it's very hard to completely avoid UB mistakes.

But I'm talking about code that has undefined behavior. If there is any at all, you can't reliably learn what optimizations will happen or not, what will break your code or not. And you can't check for incorrect optimization in any meaningful way, because the result can change at any point in the future for the tiniest of reasons. You can try to avoid this situation, but again it's very hard to write code that has exactly zero undefined behavior.

When you talked about doing "a deep dive in your CPU architecture and the gcc docs and codebase", that is only necessary if you do have undefined behavior and you're trying to figure out what actually happens. But it's a waste of effort here. If you don't have UB, you don't need to do that. If you do have UB it's not enough, not nearly enough. It's useful for debugging but it won't predict whether your code is safe into the future.

To put it another way, if we're looking at optimizations listing gotchas, when there's UB it's like half the optimizations in the entire compiler are annotated with "this could break badly if there's UB". You can't predict it.