if (failed(spec->scale_fn(this, op, new_items, changed))) {
        return failure();
      }
      break;
    }
    case ScaleConstraintType::OutputInputSameScale: {
      auto params = GetQuantParamsForSameScaleConstraint(op);
      if (EmptyParams(params)) {
        *changed |= false;
        break;
      }
      // propagate this params to all the quantizable ports.

  //   - use the first one in the collection,
  // - use the single input if it is ready, or,
  // - use the single output if it is ready, or,
  // - use the first ready one in the collection.
  QuantParams GetQuantParamsForSameScaleConstraint(Operation *op);

  // Propagate `params` to all the quantizable port of the `op`. The adjacent
  // ops, which have the parameters propagated to, are collected by `new_items`,

// - use the single input if it is ready, or,
// - use the single output if it is ready, or,
// - use the first ready one in the collection.
QuantizedType QuantizationDriver::GetQuantParamsForSameScaleConstraint(
    Operation* op) {
  // Two vector to collect Non-empty operands and results states.
  std::vector<QuantState*> mutable_states, immutable_states;
  for (int i = 0; i < op->getNumOperands(); ++i) {

  // Returns the quantization parameter satisfies the same scale
  // constraints for the op. Returns an empty option if this quantization
  // parameter doesn't exist.
  QuantizedType GetQuantParamsForSameScaleConstraint(Operation* op);

  // Returns the state of the index-th operand of the op.
  QuantState& GetOperandQuantState(Operation* op, const int index) {
    return states_[operand_states_[{op, index}]];
  }