How might we eliminate undefined behaviour?

I think you're mixing two different things. 1 is the warning or something similar that could be emitted in a clear bug like the one in the original code(and some static checkers should really find that kind of bug), 2 is the gcc's optimization, that has nothing to do with the bug. Sure, the optimization allows the exploitation of the bug, but the warning about the bug doesn't have anything to do with it and, even without the optimization, the bug is still there so the warning makes sense. I'm not saying that gcc should warn about it at all costs, but it's clearly something that a static checker should be able to do. For example it seems like cppcheck has accepted my suggestion to include it among its checks.