UB was not introduced to facilitate optimization, period. At the time the ANSI standard was being written, such optimizations didn't even exist yet. The edge

UB was not introduced to facilitate optimization, period. At the time the ANSI standard was being written, such optimizations didn't even exist yet. The edge case trickery around "assume behavior is always defined" didn't start showing up until the late 90's, a full decade and a half later. UB was introduced to allow for variant/incompatible platform behavior (in your example, how the hardware treats a divide by zero condition) in a way that allowed pre-existing code to remain valid on the platform it was written, but to leave the core language semantics clear for future code.