ref: 7e91f1538aa559da9f3b6a479c2d4227d6be9c38 dir: /builtins_plan9_amd64.s/
TEXT fl_clz(SB),1,$0 BYTE $0x0F; BYTE $0xBD; BYTE $0xC5 /* BSRL RARG, AX */ XORL $31, AX RET