Method MergeConcat

tensorflow/core/grappler/optimizers/constant_folding.cc:3435–3492

}

bool ConstantFolding::MergeConcat(bool use_shape_info,
                                  GraphDef* optimized_graph, NodeDef* node) {
  // We only optimize for ConcatV2.
  int axis;
  if (!use_shape_info || !GetConcatAxis(*node, &axis) ||
      nodes_to_preserve_.find(node->name()) != nodes_to_preserve_.end() ||
      node_map_->GetOutputs(node->name()).size() != 1) {
    return false;
  }

  // If all inputs are constant, don't merge and let folding take case of it.
  const int num_regular_inputs = NumNonControlInputs(*node);
  bool all_inputs_are_const = true;
  for (int i = 0; i < num_regular_inputs - 1; ++i) {
    const NodeDef* input_node = node_map_->GetNode(node->input(i));
    if (!IsReallyConstant(*input_node)) {
      all_inputs_are_const = false;
    }
  }
  if (all_inputs_are_const) return false;

  NodeDef* parent = *node_map_->GetOutputs(node->name()).begin();
  int parent_axis;
  if (!GetConcatAxis(*parent, &parent_axis) || axis != parent_axis) {
    return false;
  }

  protobuf::RepeatedPtrField<string> parent_inputs;
  parent_inputs.Swap(parent->mutable_input());
  std::vector<string> ctrl_output;
  // TODO(rmlarsen): IF the child occurs more than once, is it beneficial to
  // collapse it into the parent multiple times? Probably not.
  for (const auto& input : parent_inputs) {
    if (IsSameInput(input, node->name())) {
      for (int j = 0; j < num_regular_inputs - 1; ++j) {
        // Add tensor inputs to first child concat tensors (except the final
        // axis input) to the parent's inputs.
        parent->add_input(node->input(j));
        node_map_->UpdateInput(parent->name(), node->name(), node->input(j));
      }
    } else {
      parent->add_input(input);
    }
  }
  // Forward Add control inputs
  for (int i = num_regular_inputs; i < node->input_size(); ++i) {
    parent->add_input(node->input(i));
    node_map_->UpdateInput(parent->name(), node->name(), node->input(i));
  }
  node->clear_input();
  node->set_op("NoOp");
  node->clear_attr();
  node_map_->RemoveNode(node->name());
  (*parent->mutable_attr())["N"].set_i(NumNonControlInputs(*parent) - 1);
  DedupControlInputs(parent);

  return true;
}