> Nobody's talking about taking away all optimisations, just not trying to do extreme optimisations that exploit undefined behavior (or rather, assume it can ne

> Nobody's talking about taking away all optimisations, just not trying to do extreme optimisations that exploit undefined behavior (or rather, assume it can never occur). Those "extreme" optimizations are usually just surprising behavior that emerges from perfectly reasonable optimizations. For example, assuming that code that follows null dereference is dead is important. > Plenty of C compilers worked that way in the 90s and performance was perfectly acceptable (on hardware with a fraction of the speed and memory of today's computers and phones). You can get that experience with GCC -O0. Do you want to run code like that? I don't, and neither do customers. People who don't work on modern compilers often think that there is a subset of "simple" optimizations that catches "obvious" things, and optimizations beyond that are weird esoteric things nobody cares about. That isn't how it works. Tons of "obvious" optimizations require assumptions about undefined behavior.