[muldvarp]

Compilers already warn when they detect _unconditional_ undefined behavior. They just don't warn on _conditional_ undefined behavior because doing so would introduce far too many warnings.

Exploiting undefined behavior for optimization only requires local analysis, detecting whether that undefined behavior arises (either unconditionally or at all) requires global analysis. To put it differentially: The compiler often simply doesn't know whether the undefined behavior arises, it only knows that the optimization it introduces is valid anyway.