Status GetFrameBasedTopologicalOrder(std::vector<Node*>* order);

  bool IsRootEnter(const Node* n) const {
    return IsEnter(n) && control_flow_info_[n->id()].parent_frame->IsSource();
  }

  bool IsRootExit(const Node* n) const {
    return IsExit(n) && control_flow_info_[n->id()].parent_frame->IsSource();
  }

  const Graph& graph_;
  absl::flat_hash_map<TensorId, Predicate*, TensorId::Hasher> predicate_map_;

    // integers starting from 0.
    CHECK_EQ(i, cycles->NewNode());
  }

  // Compute the loop structure of the graph.
  std::vector<ControlFlowInfo> control_flow_info;
  TF_RETURN_IF_ERROR(BuildControlFlowInfo(graph, &control_flow_info));

  // The clustering code must avoid adding cycles to the graph to prevent
  // deadlock. However, the graph may contain loops, which would trigger the

  // flow frame.
  while (!control_flow_info[src_node->id()].frame_name.empty()) {
    NodeDebugInfo debug_info(*src_node);
    NodeBuilder enter_builder(options.GetNameForOp("Enter"), "Enter",
                              options.op_registry(), &debug_info);
    enter_builder.Attr("frame_name",
                       control_flow_info[src_node->id()].frame_name);
    enter_builder.Attr("is_constant", true);