Function ReachableFunctions

tensorflow/core/framework/function.cc:1468–1557

constexpr char kApiImplements[] = "api_implements";

std::set<string> ReachableFunctions(
    const FunctionLibraryDefinition& flib,
    const protobuf::RepeatedPtrField<NodeDef>& nodes) {
  // Functions that are reachable from the graph.
  std::set<string> reachable_funcs;

  // For any functions, if it has attribute "api_implements" =
  // "some_interface" and it is reachable, then it means any other
  // function with same attribute name and value could also be potentially
  // reachable, eg via implementation_selector swapping the nodedef.
  absl::flat_hash_set<string> reachable_api_interface;

  // Functions might be reachable from the nested function calls, so we keep a
  // queue of functions that we have to check.
  gtl::InlinedVector<const FunctionDef*, 4> func_queue;

  // Add reachable and not already processed functions to the functions queue.
  const auto add_to_func_queue = [&](const string& func_name) {
    const FunctionDef* func = flib.Find(func_name);
    if (func && reachable_funcs.find(func_name) == reachable_funcs.end()) {
      func_queue.push_back(func);
    }
  };

  // If any function with certain API name is reachable, all the other functions
  // with same API name should also be checked.
  const auto add_function_with_api_interface = [&](const string& api_name) {
    if (!reachable_api_interface.contains(api_name)) {
      reachable_api_interface.insert(api_name);
      for (const auto& func_name : flib.ListFunctionNames()) {
        const auto& func_def = flib.Find(func_name);
        const auto attr_it = func_def->attr().find(kApiImplements);
        if (attr_it != func_def->attr().end() &&
            attr_it->second.s() == api_name) {
          add_to_func_queue(func_name);
        }
      }
    }
  };

  // Add all the functions that are reachable from the given node to the queue.
  const auto process_node = [&](const NodeDef& node) {
    // Node itself can be a call to the function.
    add_to_func_queue(node.op());

    // Or node can have an attribute referencing a function.
    for (const auto& attr : node.attr()) {
      const auto& attr_value = attr.second;

      // 1. AttrValue.func
      if (attr_value.has_func()) {
        add_to_func_queue(attr_value.func().name());
      }

      // 2. AttrValue.ListValue.func
      if (attr_value.has_list()) {
        for (const auto& func : attr_value.list().func()) {
          add_to_func_queue(func.name());