The compiler can elide the unconditional-UB branch and its side effects, and it can elide the check itself. But it cannot elide the input operation that produce

The compiler can elide the unconditional-UB branch and its side effects, and it can elide the check itself. But it cannot elide the input operation that produces the value which is checked, nor can it elide any side effects before that input operation, unless it can statically prove that no input values can possibly result in the non-UB branch.