REP MOVS had a horrifyingly high setup penalty for some Intel CPUs, to the point where an ordinary function call to a simple unoptimized c copy implementation w

REP MOVS had a horrifyingly high setup penalty for some Intel CPUs, to the point where an ordinary function call to a simple unoptimized c copy implementation would beat it on sizes shorter than kilobytes. IIRC the first P4 cpus (Northwood) were especially bad, with the implementation steadily improving from there.