ref: 5ff984ac2f22876c81ea54c0ce7dcd77ca04d451 dir: /builtins_plan9_amd64.s/
TEXT __builtin_clz(SB),1,$0 BYTE $0x0F; BYTE $0xBD; BYTE $0xC5 /* BSRL RARG, AX */ XORL $31, AX RET