ref: 13d9192178df6923cccccb5964c2506f3e5b43ee dir: /builtins_plan9_amd64.s/
TEXT __builtin_clz(SB),1,$0 BYTE $0x0F; BYTE $0xBD; BYTE $0xC5 /* BSRL RARG, AX */ XORL $31, AX RET