从输出反向裁剪不参与本次执行的计算节点

389  // Add the fetch nodes, also updating "name_index".390  std::vector<Node*> fetch_nodes;391  if (!fetch_rewrites.empty()) {392    TF_RETURN_IF_ERROR(FetchOutputs(g, fetch_rewrites, &name_index,393                                    &fetch_nodes, &out_metadata->fetch_types));394  }395396  // Prune the graph to only compute what is needed for the fetch nodes and the397  // target nodes.398  if (!fetch_nodes.empty() || !target_node_names.empty()) {399    TF_RETURN_IF_ERROR(400        PruneForTargets(g, name_index, fetch_nodes, target_node_names));401  }

199  std::unordered_set<const Node*> targets;200  for (Node* n : fetch_nodes) {201    if (!AddNodeToTargets(n->name(), name_index, &targets)) {202      absl::StrAppend(&not_found, n->name(), " ");203    }204  }205  for (const std::string& s : target_nodes) {206    if (!AddNodeToTargets(s, name_index, &targets)) {207      absl::StrAppend(&not_found, s, " ");208    }209  }210  if (!not_found.empty()) {211    return absl::NotFoundError(absl::StrCat(212        "PruneForTargets: Some target nodes not found: ", not_found));213  }214  PruneForReverseReachability(g, std::move(targets));215216  // Reconnect nodes with no outgoing edges to the sink node217  FixupSourceAndSinkEdges(g);

224  std::vector<bool> visited(g->num_node_ids());225  for (auto node : start) {226    visited[node->id()] = true;227  }228  std::deque<const Node*> queue(start.begin(), start.end());229  while (!queue.empty()) {230    const Node* n = queue.front();231    queue.pop_front();232    for (const Node* in : n->in_nodes()) {233      if (!visited[in->id()]) {234        visited[in->id()] = true;235        queue.push_back(in);236        VLOG(2) << "Reverse reach : " << n->name() << " from " << in->name();237      }238    }239  }

241  // Make a pass over the graph to remove nodes not in "visited".242  bool any_removed = false;243  for (int i = 0; i < visited.size(); ++i) {244    if (!visited[i]) {245      Node* n = g->FindNodeId(i);246      if (n != nullptr && !n->IsSource() && !n->IsSink()) {247        g->RemoveNode(n);248        any_removed = true;249      }250    }251  }252  return any_removed;253}