To take an example from the post: in some cases a value was computed that could overflow, but it was not used because of a later overflow check. I think the optimizer would be fully within its rights to delete the code inside the overflow check, because the computation implicitly asserts that it won't overflow (since overflow is undefined). I think this is a more or less useful way of thinking around UB: any operation you put in your program implicitly asserts that the values are such that UB won't happen. For example, dereferencing a pointer implicitly means it cannot be NULL, because derefing NULL is UB, and anything downstream of that deref which checks for NULL can be deleted.