Nope. That optimisations must not change the visible behavior (except for running time) of a program was the cardinal rule of optimisation. Then it got flushe

Nope. That optimisations must not change the visible behavior (except for running time) of a program was the cardinal rule of optimisation. Then it got flushed down the toilet. (With the trick of retroactively redefining the behavior to be undefined in the first place and thus all bets being off).