You would be surprised about what is happening in modern computers. I don't know about REP SCASB, but IIRC, REP MOVSB is now an insanely efficient way to memcpy

You would be surprised about what is happening in modern computers. I don't know about REP SCASB, but IIRC, REP MOVSB is now an insanely efficient way to memcpy on last Intel microarchs (not necessarily always the fastest, but really fast enough for tons of scenario, and very I cache friendly). But it might be less interesting on some other x86 processors. It makes sense to delegate some of the microoptims to the hardware. But regular scalar instructions are also optimized like crazy. Write a small loop, and your state of the art microarch might sort of unroll it by using register renaming and speculative execution, so sometimes basically multiple iterations are executed at the same time (and on top of that you sometimes get uOP cache locking, which then improves energy and hyperthreading efficiency).