Cool, one version of a compiler will do the cool optimization that makes it go fast. But since this is undefined behavior, it's also unspecified behavior, and o

Cool, one version of a compiler will do the cool optimization that makes it go fast. But since this is undefined behavior, it's also unspecified behavior, and one can't expect it to be reliable. Maybe in practice this doesn't matter, but theoretically there's nothing (besides perf regression testing) stopping the next version of the same compiler from not doing this. Furthermore, you'll be bound to a specific compiler if you care about performance enough to rely on specific behavior for UB, one can't reasonably expect all compilers to do the same thing when they are allowed to do UB.