Undefined behavior causing time travel

Question

One example of this article from a msdn blog made me ticker:

It says that this function:

void unwitting(bool door_is_open)
{
    if (door_is_open) {
         


        

Answer 1

That's what happens when people try to translate common sense to a spec and then interpret the spec without the common sense. In my personal opinion this is completely wrong, but this is what is being done in the course of language standardization.

In my personal opinion, a compiler should not optimize code with undefined behavior. But the current post-modern compilers just optimize it out. And the standard permits both.

The logic behind the particular misbehavior that you mentioned is that the compiler operates on branches: if something is undefined in a branch, it marks the whole branch as having undefined behavior; and if a branch has undefined behavior, it may be replaced by anything.

The worst thing about this all is that new versions of the compiler may break (and do break) existing code -- either by not compiling it or compiling it to nonsense. And "existing code" typically is a really large amount of code.