Comment by saagarjha

I agree with your sentiment, but the way I square that with what I mentioned is that the compiler can undo side effects. As far as I am aware there is nothing special about fflush in the standard where you can't go back to where the program was before it happened.

(I have never actually seen a compiler act on this, but I maintain that this is just because they're either not willing to optimize on this or unable to do so. But there's a lot of UB that compilers do not exploit, so this isn't particularly concerning to me.)