The only thing that has changed is that the compiler has got better at identifying code that is recalculating the same value over and over, and gets rid of that

The only thing that has changed is that the compiler has got better at identifying code that is recalculating the same value over and over, and gets rid of that code. i.e. the compiler identifies pure functions and tries to move them out of any loops. On the other hand, you can't really restructure impure code that much. So basically, you are confirming what your parent said - the compiler has to first analyze the AST and hack its way around any IO and side effects before turning the optimized intermediate program representation into machine code. Btw, the actual hardware could as well be a lambda calculus reducer instead of a von Neumann machine - in that case a compiler wouldn't emit anything imperative at all.