ref: 7fcae265252c0a29cf2a4b0c1474b35e7ad39619 dir: /plan9_builtins_amd64.s/
TEXT __builtin_clz(SB),1,$0 BYTE $0x0F; BYTE $0xBD; BYTE $0xC5 /* BSRL RARG, AX */ XORL $31, AX RET