ref: 4b7065cc238365afc13bd3a92c500b4629ea629b dir: /src/clz.amd64.s/
TEXT h264bsdCountLeadingZeros(SB),$0 BYTE $0x0F; BYTE $0xBD; BYTE $0xC5 /* BSRL RARG, AX */ XORL $31, AX RET