Comment by somat
2 months ago
I am not so sure. Assuming the program does not do anything undefined is sort of the worst possible take on leaving the behavior undefined in the first place. I mean the behavior was left undefined so that "something" could be done. the language standard just does not know what that "something" is. Hell the compiler could do nothing and that would make more sense.
But to make optimizations pretending it is an invariant, it can't happen, when the specification clearly says it could happen. That's wild, and I would argue out of specification.
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?