ref: 6e44f3442f1aaf7ef3a201e365529d25c9cecc6c dir: /src/plan9/clz_amd64.s/
TEXT sl_clz(SB),1,$0 BYTE $0x0F; BYTE $0xBD; BYTE $0xC5 /* BSRL RARG, AX */ XORL $31, AX RET