It does, though. UB and associated optimisations wouldn't be an issue if defined behaviour would not have an impact on performance. If the cost would be zero or negligible, the compiler wouldn't need to care and hence warnings like this wouldn't need to be explicitly stated.

And yet Swift doesn't rely on these optimizations, preferring to trap instead. Again, the guess above was about the CPU and we're conflating language-specific UB optimisations.