There are other aliasing rules that have big performance impacts on certain architectures. The Xbox 360 and PS3 Power cores for example had a severe load-hit-st

There are other aliasing rules that have big performance impacts on certain architectures. The Xbox 360 and PS3 Power cores for example had a severe load-hit-store performance penalty that tended to be triggered by code that moved data between floating point and integer registers via memory. Strict aliasing rules that allow the compiler to assume float and int pointers don't alias could make a huge performance difference but those rules are also the source of much troublesome undefined behavior for code that does intentional type punning. The ISA in this case requires going via memory to move data between fp and integer registers and certain implementations of that ISA had major performance impacts associated with that. In this case UB rules really did allow for valuable optimizations but really did cause trouble elsewhere.