I think you're reading me as saying something much stronger than what I actually meant. When we talk about C being "deceptive" we don't need to consider optimiz

I think you're reading me as saying something much stronger than what I actually meant. When we talk about C being "deceptive" we don't need to consider optimizations that respect the as-if rule. (Well, maybe when looking at the C->asm mapping or trying to benchmark, but that's not the level most people are working at.) The trend toward UB-based optimizations seems significant to me because they can and will take a bunch of C code which looks superficially reasonable, and which used to do X as intended, and suddenly (and perfectly legally) make it start doing Y instead. I assumed that's what barrkel was alluding to above. And yes, I'm sure there are old optimizations which will also do that, but basic things like hoisting and reordering and unrolling and dead-code elimination don't fall into that category.