ref: 440860780f767ae6af42ae2c9f49bcca46b6ceeb dir: /builtins.amd64.s/
TEXT memcpyf(SB),1,$0 MOVQ RARG, DI MOVQ DI, AX MOVQ p2+8(FP), SI MOVL n+16(FP), CX CLD; REP; MOVSB RET