ref: ce3af9e03b554081f6689a541438f32cdd2c44c5 dir: /builtins_plan9_amd64.s/
TEXT fl_clz(SB),1,$0 BYTE $0x0F; BYTE $0xBD; BYTE $0xC5 /* BSRL RARG, AX */ XORL $31, AX RET