func ARM64RegisterExtension(a *obj.Addr, ext string, reg, num int16, isAmount, isIndex bool) error {
	Rnum := (reg & 31) + int16(num<<5)
	if isAmount {
		if num < 0 || num > 7 {
			return errors.New("index shift amount is out of range")
		}
	}
	if reg <= arm64.REG_R31 && reg >= arm64.REG_R0 {
		if !isAmount {
			return errors.New("invalid register extension")
		}
		switch ext {
		case "UXTB":