> What's odd about that example is that the optimization is only valid if the loop in fact overflows the array every time. No, the optimization is valid if the

> What's odd about that example is that the optimization is only valid if the loop in fact overflows the array every time. No, the optimization is valid if the function is always called with "v" that is actually exists in the table; in this case the function should always return true so it's only proper for the compiler to throw out the extraneous code. And writing the loop in such way is programmer's promise/guarantee to the compiler that the function will indeed be called only in that manner. That's the essence of the UB: it's the programmer who promises to the compiler that she will perform all the necessary checks (or formal proofs of impossibilty) herself; the compiler may go forward and rely on the implied invariants and preconditions. And this is, of course, the main problem of the UB because 95% of the time the programmer does not actually intend to make such a gurantee: she simply is unaware (for whatever reason) that there is a pre-condition (checked by nobody!) that's required for the program's correctness to hold... or it's even just a typo she made.