ref: c539d64d22a0fdef1bd044a6e28e02a6bcfd0adf
dir: /sys/src/libc/amd64/memchr.s/
TEXT memchr(SB),$0 MOVQ n+16(FP), CX CMPQ CX, $0 JEQ none MOVQ RARG, DI MOVBLZX c+8(FP), AX CLD /* * SCASB is memchr instruction */ REPN; SCASB JEQ found none: MOVL $0, AX RET found: MOVQ DI, AX SUBQ $1, AX RET