return nil
			}
			a.Reg = arm64.REG_ELEM + (reg & 31) + ((arm64.ARNG_S & 15) << 5)
			a.Index = num
		case "D":
			if !isIndex {
				return nil
			}
			a.Reg = arm64.REG_ELEM + (reg & 31) + ((arm64.ARNG_D & 15) << 5)
			a.Index = num
		default:
			return errors.New("unsupported simd register extension type: " + ext)
		}
	} else {
		return errors.New("invalid register and extension combination")
	}