ref: 6e6df4d86d4e7768097985a3b84bad3555ea370d 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