If you propagate "some condition," that could allow dead-code elimination later on, especially if an if-statement uses "some condition" as a predicate later. This seems redundant in hand-written code, but keep in mind the if-statement may actually be from an inlined function.
If "some condition" is actually testing a variable for equality, then you could do constant-folding.
- doesn't terminate, doesn't have side effects -> optimization doesn't matter
- doesn't terminate, has side effects -> optimization isn't valid
- does terminate, doesn't has side effects -> optimization would be valid, but why would such a loop exist? One example that was brought up was generic code, but I'm not entirely convinced yet.
Because of interaction with other undefined behaviour (in the body of your loop), you might think you are in a different case than what the compiler thinks.
Also the definition of 'side effect' here is a weird one: reading and writing to variables is not considered a side effect, even though a different thread could change them while your loop is running.
(And to make it closer to our situation, assume that the loop being executed was looking for something that actually has a counterexample that could be found after a long time; instead of Fermat's last theorem.)
So, generic code with a loop, which, depending on parameters, sometimes has side effects and sometimes doesn't, and in the case that it doesn't have side effects it is still not easy to recognize if the loop terminates?
I suppose that is possible, although I'm having a hard time coming up with any reasonable examples.
If "some condition" is actually testing a variable for equality, then you could do constant-folding.