ref: 588c432ea9d25fb5b6820c10580f5364f8dbc9c1 dir: /plan9_builtins_amd64.s/
TEXT __builtin_clz(SB),1,$0 BYTE $0x0F; BYTE $0xBD; BYTE $0xC5 /* BSRL RARG, AX */ XORL $31, AX RET