REP MOVS is the fastest option for huge copies on Ivybridge and later by a large margin, as it uses special cache semantics that aren't available via any other

REP MOVS is the fastest option for huge copies on Ivybridge and later by a large margin, as it uses special cache semantics that aren't available via any other instructions. For small copies with buffers that aren't aligned or cacheline-multiple length and are resident in L1 cache, it's possible to be significantly faster than REP MOVS using a software sequence of AVX instructions. This is because the branches in these sequences are usually perfectly predictable in the real world, but the "microcode sequence"[1] used by REP MOVS does not benefit from the branch predictor. This imposes a static startup cost (a few tens of cycles) that exceeds function-call overhead, which keeps software implementations of memcpy in business. [1] Not exactly microcode as that term is classically understood, but there isn't really a better term for it that's widely used.