[saagarjha]

rep movsb might have been fast at one point but it definitely was not for a few decades in the middle, where vector stores were the fastest way to implement memcpy. Intel decided that they should probably make it fast again and they have slowly made it competitive with the extensions you’ve mentioned. But for processors that don’t support it, using rep movsb is going to be slow and probably not something you’d want to pick unless you have weird constraints (binary size?)