Comment by Quekid5

Undefined Behavior has a very specific meaning in the C and (respectively) C++ standards. And it is this.

I think you may be misunderstand how optimization works. It's not "AHA! Undefined Behavior, I'll poke at that!".

It's more that compilers are allowed to assume that some things just "don't happen"... and can optimize based on that. And it does make sense... if you were told to optimize a thing given certain rules, how would you do it?