I don't get it. AFAIK the goal of optimizers is to generate faster code while preserving the initial visible behaviour. If the initial behaviour is undefined,

I don't get it. AFAIK the goal of optimizers is to generate faster code while preserving the initial visible behaviour. If the initial behaviour is undefined, what can an optimizer try to preserve? Could someone give an example on how it would be possible not to have "UB in the compiler", while still having optimizations?