ref: bf67cf62e629985306488cbe6495dffa117c3484 dir: /src/plan9/clz_amd64.s/
TEXT fl_clz(SB),1,$0 BYTE $0x0F; BYTE $0xBD; BYTE $0xC5 /* BSRL RARG, AX */ XORL $31, AX RET