/* Copyright 2015 The TensorFlow Authors. All Rights Reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
==============================================================================*/

#include "tensorflow/core/common_runtime/function.h"

#include <deque>
#include <vector>

#include "tensorflow/core/common_runtime/device.h"
#include "tensorflow/core/common_runtime/executor.h"
#include "tensorflow/core/common_runtime/graph_optimizer.h"
#include "tensorflow/core/common_runtime/memory_types.h"
#include "tensorflow/core/common_runtime/rendezvous_mgr.h"
#include "tensorflow/core/framework/function.h"
#include "tensorflow/core/framework/node_def.pb.h"
#include "tensorflow/core/framework/node_def_util.h"
#include "tensorflow/core/framework/op.h"
#include "tensorflow/core/framework/op_kernel.h"
#include "tensorflow/core/framework/versions.pb.h"
#include "tensorflow/core/graph/algorithm.h"
#include "tensorflow/core/graph/control_flow.h"
#include "tensorflow/core/graph/gradients.h"
#include "tensorflow/core/graph/graph_constructor.h"
#include "tensorflow/core/graph/optimizer_cse.h"
#include "tensorflow/core/lib/gtl/map_util.h"
#include "tensorflow/core/platform/macros.h"

// See core/kernels/function_ops.cc for related kernels.

namespace tensorflow {

// A few string constant used throughout this module.
//
// TODO(zhifengc): Dedup some of these constants into
// framework/function.h
static constexpr const char* const kArgOp = "_Arg";
static constexpr const char* const kRetOp = "_Retval";
static constexpr const char* const kGradientOp =
    FunctionLibraryDefinition::kGradientOp;
static constexpr const char* const kNodeLabel = "Func";
static constexpr const char* const kFuncAttr =
    FunctionLibraryDefinition::kFuncAttr;

// Represents the index-th output of a node.
struct Endpoint {
  Node* node;
  int index;

  // Returns the string name represents this endpoint.
  string name() const {
    if (index == 0) {
      return node->name();
    } else {
      return strings::StrCat(node->name(), ":", index);
    }
  }

  DataType dtype() const { return node->output_type(index); }
};

struct EndpointHash {
  uint64 operator()(const Endpoint& x) const {
    return Hash64(reinterpret_cast<const char*>(&x.node), sizeof(Node*),
                  x.index);
  }
};

struct EndpointEq {
  bool operator()(const Endpoint& x, const Endpoint& y) const {
    return (x.node == y.node) && (x.index == y.index);
  }
};

// The following Add* routines are used to add a few graph nodes while
// functions are transformed.
static Node* AddNoOp(Graph* g) {
  NodeDef ndef;
  ndef.set_name(g->NewName(kNodeLabel));
  ndef.set_op("NoOp");
  Status s;
  Node* ret = g->AddNode(ndef, &s);
  TF_CHECK_OK(s);
  return ret;
}

static Node* AddIdentity(Graph* g, Endpoint input) {
  DCHECK_LT(0, input.dtype());
  DCHECK_LT(input.dtype(), DT_FLOAT_REF);
  NodeDef ndef;
  ndef.set_name(g->NewName(kNodeLabel));
  ndef.set_op("Identity");
  ndef.add_input(input.name());
  AddNodeAttr("T", input.dtype(), &ndef);
  Status s;
  Node* ret = g->AddNode(ndef, &s);
  TF_CHECK_OK(s);
  g->AddEdge(input.node, input.index, ret, 0);
  return ret;
}

static Node* AddArg(Graph* g, DataType dtype, int index) {
  DCHECK_LT(0, dtype);
  DCHECK_LT(dtype, DT_FLOAT_REF);
  NodeDef ndef;
  ndef.set_name(g->NewName(kNodeLabel));
  ndef.set_op(kArgOp);
  AddNodeAttr("T", dtype, &ndef);
  AddNodeAttr("index", index, &ndef);
  Status s;
  Node* ret = g->AddNode(ndef, &s);
  TF_CHECK_OK(s);
  return ret;
}

static Node* AddRet(Graph* g, Endpoint input, int index) {
  DCHECK_LT(0, input.dtype());
  DCHECK_LT(input.dtype(), DT_FLOAT_REF);
  NodeDef ndef;
  ndef.set_name(g->NewName(kNodeLabel));
  ndef.set_op(kRetOp);
  ndef.add_input(input.name());
  AddNodeAttr("T", input.dtype(), &ndef);
  AddNodeAttr("index", index, &ndef);
  Status s;
  Node* ret = g->AddNode(ndef, &s);
  TF_CHECK_OK(s);
  g->AddEdge(input.node, input.index, ret, 0);
  return ret;
}

class FunctionLibraryRuntimeImpl : public FunctionLibraryRuntime {
 public:
  FunctionLibraryRuntimeImpl(const DeviceMgr* dmgr, Env* env, Device* device,
                             int graph_def_version,
                             const FunctionLibraryDefinition* lib_def,
                             const OptimizerOptions& optimizer_options,
                             CustomKernelCreator custom_kernel_creator,
                             ProcessFunctionLibraryRuntime* parent);

  ~FunctionLibraryRuntimeImpl() override;

  Status Instantiate(const string& function_name, AttrSlice attrs,
                     const InstantiateOptions& options,
                     Handle* handle) override;

  Status ReleaseHandle(Handle handle) override;

  const FunctionBody* GetFunctionBody(Handle handle) override;

  Status CreateKernel(const NodeDef& ndef, OpKernel** kernel) override;

  void Run(const Options& opts, Handle handle, gtl::ArraySlice<Tensor> args,
           std::vector<Tensor>* rets, DoneCallback done) override;
  // NOTE(mrry): This overload is currently only implemented for local function
  // execution.
  // TODO(b/70346412): Implement support for remote function execution when
  // passing a call frame.
  void Run(const Options& opts, Handle handle, CallFrameInterface* frame,
           DoneCallback done) override;

  bool IsStateful(const string& function) override;

  const FunctionLibraryDefinition* GetFunctionLibraryDefinition()
      const override {
    return base_lib_def_;
  }

  Device* device() override { return device_; }
  Env* env() override { return env_; }
  int graph_def_version() override { return graph_def_version_; }

  string DebugString(Handle h) override;

 private:
  typedef FunctionLibraryRuntimeImpl ME;

  const DeviceMgr* const device_mgr_;
  Device* const device_;
  Env* const env_;
  const int graph_def_version_;
  const FunctionLibraryDefinition* const base_lib_def_;
  GraphOptimizer optimizer_;
  const CustomKernelCreator custom_kernel_creator_;
  const string device_name_;

  std::function<Status(const string&, const OpDef**)> get_func_sig_;
  std::function<Status(const NodeDef&, OpKernel**)> create_kernel_;

  mutable mutex mu_;

  int next_handle_ GUARDED_BY(mu_);

  // The instantiated and transformed function is encoded as a Graph
  // object, and an executor is created for the graph.
  struct Item : public core::RefCounted {
    const Graph* graph = nullptr;                            // Owned by exec.
    const FunctionLibraryDefinition* overlay_lib = nullptr;  // Not owned.
    FunctionBody* func_graph = nullptr;
    Executor* exec = nullptr;

    ~Item() override {
      delete this->func_graph;
      delete this->exec;
    }
  };
  std::unordered_map<Handle, Item*> items_ GUARDED_BY(mu_);

  ProcessFunctionLibraryRuntime* parent_ = nullptr;  // not owned.

  Status CreateKernel(const NodeDef& ndef,
                      const FunctionLibraryDefinition* lib_def,
                      OpKernel** kernel);
  Status FunctionDefToBody(const FunctionDef& fdef, AttrSlice attrs,
                           const FunctionLibraryDefinition* lib_def,
                           FunctionBody** fbody);
  Status CreateItem(Handle handle, Item** item);
  Status GetOrCreateItem(Handle handle, Item** item);
  Status InstantiateSymbolicGradient(const NameAttrList& func,
                                     const FunctionLibraryDefinition* lib_def,
                                     FunctionBody** g_body);
  bool IsLocalTarget(const InstantiateOptions& options);
  AttrValueMap FixAttrs(const AttrSlice& attrs);
  void RunRemote(const Options& opts, Handle handle,
                 gtl::ArraySlice<Tensor> args, std::vector<Tensor>* rets,
                 Executor::Args* exec_args, Item* item, DoneCallback done);

  TF_DISALLOW_COPY_AND_ASSIGN(FunctionLibraryRuntimeImpl);
};

FunctionLibraryRuntimeImpl::FunctionLibraryRuntimeImpl(
    const DeviceMgr* dmgr, Env* env, Device* device, int graph_def_version,
    const FunctionLibraryDefinition* lib_def,
    const OptimizerOptions& optimizer_options,
    CustomKernelCreator custom_kernel_creator,
    ProcessFunctionLibraryRuntime* parent)
    : device_mgr_(dmgr),
      device_(device),
      env_(env),
      graph_def_version_(graph_def_version),
      base_lib_def_(lib_def),
      optimizer_(optimizer_options),
      custom_kernel_creator_(std::move(custom_kernel_creator)),
      device_name_(device_ == nullptr
                       ? ProcessFunctionLibraryRuntime::kDefaultFLRDevice
                       : device_->name()),
      next_handle_(0),
      parent_(parent) {
  get_func_sig_ = [this](const string& op, const OpDef** sig) {
    return base_lib_def_->LookUpOpDef(op, sig);
  };
  create_kernel_ = [this](const NodeDef& ndef, OpKernel** kernel) {
    return CreateKernel(ndef, kernel);
  };
}

FunctionLibraryRuntimeImpl::~FunctionLibraryRuntimeImpl() {
  // The most common patterns of FLR usage don't require the caller to
  // explicitly release handles. As a result, we try to unref each item until
  // it's erased.
  for (auto item : items_) {
    if (item.second) {
      while (!item.second->Unref()) {
      }
    }
  }
}

// An asynchronous op kernel which executes an instantiated function
// defined in a library.
class CallOp : public AsyncOpKernel {
 public:
  CallOp(FunctionLibraryRuntime::Handle handle, OpKernelConstruction* ctx)
      : AsyncOpKernel(ctx), handle_(handle) {}

  ~CallOp() override {}

  void ComputeAsync(OpKernelContext* ctx, DoneCallback done) override {
    FunctionLibraryRuntime* lib = ctx->function_library();
    OP_REQUIRES_ASYNC(ctx, lib != nullptr,
                      errors::Internal("No function library is provided."),
                      done);
    FunctionLibraryRuntime::Options opts;
    opts.step_id = ctx->step_id();
    opts.rendezvous = ctx->rendezvous();
    opts.cancellation_manager = ctx->cancellation_manager();
    opts.step_container = ctx->step_container();
    opts.stats_collector = ctx->stats_collector();
    opts.runner = ctx->runner();
    std::vector<Tensor> args;
    args.reserve(ctx->num_inputs());
    for (int i = 0; i < ctx->num_inputs(); ++i) {
      args.push_back(ctx->input(i));
    }
    std::vector<Tensor>* rets = new std::vector<Tensor>;
    lib->Run(opts, handle_, args, rets,
             [ctx, done, rets](const Status& status) {
               if (!status.ok()) {
                 ctx->SetStatus(status);
               } else {
                 const int ret_size = static_cast<int>(rets->size());
                 CHECK_EQ(ret_size, ctx->num_outputs());
                 for (int i = 0; i < ret_size; ++i) {
                   ctx->set_output(i, (*rets)[i]);
                 }
               }
               delete rets;
               done();
             });
  }

 private:
  FunctionLibraryRuntime::Handle handle_;

  TF_DISALLOW_COPY_AND_ASSIGN(CallOp);
};

const FunctionBody* FunctionLibraryRuntimeImpl::GetFunctionBody(Handle h) {
  LocalHandle local_handle = parent_->GetHandleOnDevice(device_name_, h);
  if (local_handle == kInvalidLocalHandle) {
    LOG(ERROR) << "Could not find Handle: " << h
               << " on device: " << device_name_;
    return nullptr;
  }

  mutex_lock l(mu_);
  CHECK_EQ(1, items_.count(local_handle));
  return items_[local_handle]->func_graph;
}

Status FunctionLibraryRuntimeImpl::CreateKernel(const NodeDef& ndef,
                                                OpKernel** kernel) {
  return CreateKernel(ndef, base_lib_def_, kernel);
}

Status FunctionLibraryRuntimeImpl::CreateKernel(
    const NodeDef& ndef, const FunctionLibraryDefinition* lib_def,
    OpKernel** kernel) {
  // If a custom kernel creator is given, try that.
  Status s;
  if (custom_kernel_creator_) {
    std::unique_ptr<OpKernel> ret;
    s = custom_kernel_creator_(this, ndef, &ret);
    if (s.ok()) {
      *kernel = ret.release();
      return s;
    } else {
      VLOG(2) << "Custom creator error: " << s;
      // Falls through.
      s = Status::OK();
    }
  }

  if (lib_def->Find(ndef.op()) == nullptr) {
    // A primitive operation. Creates the registered kernel.
    return CreateNonCachedKernel(device_, this, ndef, graph_def_version_,
                                 kernel);
  }

  // Try to instantiate this function for the func/attr. Maybe it's
  // cached already.
  InstantiateOptions options;
  if (lib_def != base_lib_def_) {
    options.overlay_lib = lib_def;
  }
  Handle handle;
  TF_RETURN_IF_ERROR(
      Instantiate(ndef.op(), AttrSlice(&ndef.attr()), options, &handle));

  const FunctionBody* fbody = GetFunctionBody(handle);
  CHECK_NOTNULL(fbody);

  // TODO(zhifengc): For now, we assume int32 and resources are always on host
  // memory and other types are always on device memory. We should do type
  // inference over function body to derive the correct input/output memory
  // types.
  MemoryTypeVector input_memory_types;
  for (const auto& t : fbody->arg_types) {
    input_memory_types.push_back(
        (t == DT_INT32 || t == DT_RESOURCE) ? HOST_MEMORY : DEVICE_MEMORY);
  }
  MemoryTypeVector output_memory_types;
  for (const auto& t : fbody->ret_types) {
    output_memory_types.push_back(t == DT_INT32 ? HOST_MEMORY : DEVICE_MEMORY);
  }

  // Constructs a CallOp kernel for running the instantiated function.
  auto device_type = DeviceType(device_->attributes().device_type());
  OpKernelConstruction construction(
      device_type, device_, device_->GetAllocator(AllocatorAttributes()), &ndef,
      &fbody->fdef.signature(), this, fbody->arg_types, input_memory_types,
      fbody->ret_types, output_memory_types, graph_def_version_, &s);
  *kernel = new CallOp(handle, &construction);
  if (!s.ok()) {
    delete *kernel;
  }
  return s;
}

Status FunctionLibraryRuntimeImpl::FunctionDefToBody(
    const FunctionDef& fdef, AttrSlice attrs,
    const FunctionLibraryDefinition* lib_def, FunctionBody** fbody) {
  if (lib_def == base_lib_def_) {
    return FunctionDefToBodyHelper(fdef, attrs, lib_def, get_func_sig_, fbody);
  } else {
    auto get_func_sig = [lib_def](const string& op, const OpDef** sig) {
      return lib_def->LookUpOpDef(op, sig);
    };
    return FunctionDefToBodyHelper(fdef, attrs, lib_def, get_func_sig, fbody);
  }
}

Status FunctionLibraryRuntimeImpl::InstantiateSymbolicGradient(
    const NameAttrList& func, const FunctionLibraryDefinition* lib_def,
    FunctionBody** g_body) {
  const FunctionDef* fdef = lib_def->Find(func.name());
  if (fdef == nullptr) {
    // f is a primitive op.
    gradient::Creator creator;
    TF_RETURN_IF_ERROR(gradient::GetOpGradientCreator(func.name(), &creator));
    if (creator == nullptr) {
      return errors::InvalidArgument("No gradient is defined for ",
                                     func.name());
    }
    FunctionDef grad_fdef;
    // TODO(josh11b): Should filter out the attrs from func that aren't used
    // by the gradient function.
    TF_RETURN_IF_ERROR(creator(AttrSlice(&func.attr()), &grad_fdef));
    TF_RETURN_IF_ERROR(
        FunctionDefToBody(grad_fdef, AttrSlice(&func.attr()), lib_def, g_body));
  } else {
    // f is a user-defined function.
    InstantiateOptions options;
    if (lib_def != base_lib_def_) {
      options.overlay_lib = lib_def;
    }
    Handle f_handle;
    TF_RETURN_IF_ERROR(
        Instantiate(func.name(), AttrSlice(&func.attr()), options, &f_handle));
    const FunctionBody* f_body = GetFunctionBody(f_handle);
    CHECK_NOTNULL(f_body);
    *g_body = SymbolicGradient(*f_body);
  }
  return Status::OK();
}

bool FunctionLibraryRuntimeImpl::IsLocalTarget(
    const InstantiateOptions& options) {
  if (device_ == nullptr) return true;
  if (options.target.empty()) return true;
  Device* target_device;
  if (!device_mgr_->LookupDevice(options.target, &target_device).ok()) {
    return false;
  }
  return target_device == device_;
}

Status FunctionLibraryRuntimeImpl::Instantiate(
    const string& function_name, AttrSlice attrs,
    const InstantiateOptions& options, Handle* handle) {
  if (!IsLocalTarget(options)) {
    return parent_->Instantiate(function_name, attrs, options, handle);
  }

  // Since this is a local target, ensure that the local `device_name_` appears
  // in the canonical key.
  InstantiateOptions options_copy(options);
  options_copy.target = device_name_;
  const string key = Canonicalize(function_name, attrs, options_copy);
  *handle = parent_->GetHandle(key);
  if (*handle != kInvalidHandle) {
    mutex_lock l(mu_);
    items_[parent_->GetHandleOnDevice(device_name_, *handle)]->Ref();
    return Status::OK();
  }

  Status s;
  const FunctionLibraryDefinition* lib_def =
      options.overlay_lib ? options.overlay_lib : base_lib_def_;
  FunctionBody* fbody = nullptr;
  if (function_name == kGradientOp) {
    const AttrValue* f = attrs.Find(kFuncAttr);
    if (f == nullptr) {
      return errors::InvalidArgument("SymbolicGradient is missing attr: f");
    }
    const auto& func = f->func();
    if (func.name() == kGradientOp) {
      return errors::InvalidArgument("Can't take gradient of SymbolicGradient");
    }
    const string grad = lib_def->FindGradient(func.name());
    if (!grad.empty()) {
      return Instantiate(grad, AttrSlice(&func.attr()), options, handle);
    }
    TF_RETURN_IF_ERROR(InstantiateSymbolicGradient(func, lib_def, &fbody));
  } else {
    const FunctionDef* fdef = lib_def->Find(function_name);
    if (fdef == nullptr) {
      return errors::NotFound("Function ", function_name, " is not defined.");
    }
    TF_RETURN_IF_ERROR(FunctionDefToBody(*fdef, attrs, lib_def, &fbody));
  }

  {
    mutex_lock l(mu_);
    *handle = parent_->GetHandle(key);
    if (*handle != kInvalidHandle) {
      delete fbody;
      items_[parent_->GetHandleOnDevice(device_name_, *handle)]->Ref();
    } else {
      *handle = parent_->AddHandle(key, device_name_, next_handle_);
      Item* item = new Item;
      item->func_graph = fbody;
      item->overlay_lib = options.overlay_lib;
      items_.insert({next_handle_, item});
      next_handle_++;
    }
  }
  return Status::OK();
}

Status FunctionLibraryRuntimeImpl::ReleaseHandle(Handle handle) {
  if (!parent_->IsInstantiatedOnDevice(device_name_, handle)) {
    return parent_->ReleaseHandle(handle);
  }

  LocalHandle h = parent_->GetHandleOnDevice(device_name_, handle);
  mutex_lock l(mu_);
  CHECK_EQ(1, items_.count(h));
  Item* item = items_[h];
  if (item->Unref()) {
    items_.erase(h);
    TF_RETURN_IF_ERROR(parent_->RemoveHandle(handle));
  }
  return Status::OK();
}

void DumpGraph(StringPiece label, const Graph* g) {
  // TODO(zhifengc): Change Graph to record #nodes.
  VLOG(1) << "Graph " << label << " #nodes " << g->num_nodes() << " #edges "
          << g->num_edges();
  if (VLOG_IS_ON(2)) {
    for (const auto& line : str_util::Split(DebugString(g), '\n')) {
      VLOG(2) << "|| " << line;
    }
  }
}

void OptimizeGraph(FunctionLibraryRuntime* lib, std::unique_ptr<Graph>* g) {
  OptimizerOptions opts;
  opts.set_do_common_subexpression_elimination(true);
  opts.set_do_function_inlining(true);
  opts.set_do_constant_folding(true);
  GraphOptimizer optimizer(opts);
  optimizer.Optimize(lib, lib->env(), lib->device(), g, /*shape_map=*/nullptr);
}

namespace {
// Removes all stateless nodes that do not contribute to a return
// value from the function body.  Unlike `RemoveDeadNodes()`, which is
// triggered by `OptimizerOptions.do_function_inlining`, this pass
// ignores the SINK node, from which (by definition) all nodes are
// reverse reachable.
void PruneFunctionBody(Graph* g) {
  VLOG(2) << "Pruning function body";
  std::unordered_set<const Node*> nodes;
  for (auto n : g->nodes()) {
    // NOTE(mrry): "_Retval" nodes are stateful, and so will be added
    // to the seed set of `nodes`.
    // TODO(mrry): Investigate whether the `n->IsControlFlow()` test is
    // still needed. It would be preferable to prune entire loops and/or
    // conditionals if they are not used in the graph.
    if (n->IsControlFlow() || n->op_def().is_stateful()) {
      nodes.insert(n);
    }
  }
  bool changed = PruneForReverseReachability(g, std::move(nodes));
  if (changed) {
    FixupSourceAndSinkEdges(g);
  }
}
}  // namespace

Status FunctionLibraryRuntimeImpl::CreateItem(Handle handle, Item** item) {
  const FunctionBody* fbody;
  const FunctionLibraryDefinition* lib_def;
  {
    mutex_lock l(mu_);
    fbody = (*item)->func_graph;
    lib_def = (*item)->overlay_lib;
  }
  if (!lib_def) {
    lib_def = base_lib_def_;
  }
  std::unique_ptr<Graph> g(new Graph(lib_def));
  CopyGraph(*fbody->graph, g.get());

  PruneFunctionBody(g.get());
  optimizer_.Optimize(this, env(), device(), &g, /*shape_map=*/nullptr);
  TF_RETURN_IF_ERROR(EnsureMemoryTypes(DeviceType(device()->device_type()),
                                       device()->name(), g.get()));

  // Creates an executor based on the g.  This must be done without
  // holding mu_ because create_kernel_ calls back into the library.
  LocalExecutorParams params;
  params.device = device_;
  params.function_library = this;
  if (lib_def == base_lib_def_) {
    params.create_kernel = create_kernel_;
  } else {
    params.create_kernel = [this, lib_def](const NodeDef& ndef,
                                           OpKernel** kernel) {
      return CreateKernel(ndef, lib_def, kernel);
    };
  }
  params.delete_kernel = [](OpKernel* kernel) {
    DeleteNonCachedKernel(kernel);
  };
  Graph* graph = g.get();
  Executor* exec;
  TF_RETURN_IF_ERROR(NewLocalExecutor(params, g.release(), &exec));

  {
    // Guard item since it is already inserted in items_.
    mutex_lock l(mu_);
    if ((*item)->exec) {
      delete exec;
    } else {
      (*item)->graph = graph;
      (*item)->exec = exec;
    }
  }
  return Status::OK();
}

Status FunctionLibraryRuntimeImpl::GetOrCreateItem(Handle handle, Item** item) {
  LocalHandle local_handle = parent_->GetHandleOnDevice(device_name_, handle);
  {
    mutex_lock l(mu_);
    if (items_.count(local_handle) == 0) {
      return errors::NotFound("Function handle ", handle,
                              " is not valid. Likely an internal error.");
    }
    *item = items_[local_handle];
    if ((*item)->exec != nullptr) {
      return Status::OK();
    }
  }
  // NOTE: We need to call CreateItem out of mu_ because creating an
  // executor needs to call CreateKernel.
  return CreateItem(handle, item);
}

void FunctionLibraryRuntimeImpl::RunRemote(const Options& opts, Handle handle,
                                           gtl::ArraySlice<Tensor> args,
                                           std::vector<Tensor>* rets,
                                           Executor::Args* exec_args,
                                           Item* item, DoneCallback done) {
  DCHECK(exec_args->call_frame == nullptr);
  string target_device = parent_->GetDeviceName(handle);
  string source_device = opts.source_device;
  Rendezvous* rendezvous = opts.rendezvous;
  DeviceContext* device_context;
  Status s = parent_->GetDeviceContext(target_device, &device_context);
  if (!s.ok()) {
    delete exec_args;
    done(s);
    return;
  }
  int64 src_incarnation, target_incarnation;
  s = parent_->GetDeviceIncarnation(source_device, &src_incarnation);
  s.Update(parent_->GetDeviceIncarnation(target_device, &target_incarnation));
  if (!s.ok()) {
    delete exec_args;
    done(s);
    return;
  }

  const FunctionBody* fbody = GetFunctionBody(handle);
  FunctionCallFrame* frame =
      new FunctionCallFrame(fbody->arg_types, fbody->ret_types);
  exec_args->call_frame = frame;
  if (!s.ok()) {
    delete frame;
    delete exec_args;
    done(s);
    return;
  }

  // The ProcFLR sends the arguments to the function from the source_device to
  // the target_device. So here we receive those arguments. Similarly, when the
  // computation is done and stored in *rets, we send the return values back
  // to the source_device (caller) so that the ProcFLR can receive them later.
  std::vector<Tensor>* remote_args = new std::vector<Tensor>;
  ProcessFunctionLibraryRuntime::ReceiveTensorsAsync(
      source_device, target_device, "arg_", src_incarnation, args.size(),
      device_context, {}, rendezvous, remote_args,
      [frame, remote_args, item, source_device, target_device,
       target_incarnation, rendezvous, device_context, rets, done,
       exec_args](const Status& status) {
        Status s = status;
        if (s.ok()) {
          s = frame->SetArgs(*remote_args);
        }
        if (!s.ok()) {
          delete frame;
          delete remote_args;
          delete exec_args;
          done(s);
          return;
        }
        item->exec->RunAsync(
            *exec_args, [item, frame, rets, done, source_device, target_device,
                         target_incarnation, rendezvous, device_context,
                         remote_args, exec_args](const Status& status) {
              Status s = status;
              if (s.ok()) {
                s = frame->ConsumeRetvals(rets);
              }
              delete frame;
              if (!s.ok()) {
                delete remote_args;
                delete exec_args;
                done(s);
                return;
              }
              s = ProcessFunctionLibraryRuntime::SendTensors(
                  target_device, source_device, "ret_", target_incarnation,
                  *rets, device_context, {}, rendezvous);
              delete remote_args;
              delete exec_args;
              done(s);
            });
      });
}

void FunctionLibraryRuntimeImpl::Run(const Options& opts, Handle handle,
                                     gtl::ArraySlice<Tensor> args,
                                     std::vector<Tensor>* rets,
                                     DoneCallback done) {
  if (opts.cancellation_manager && opts.cancellation_manager->IsCancelled()) {
    done(errors::Cancelled(""));
    return;
  }
  Options run_opts = opts;
  if (opts.create_rendezvous) {
    Rendezvous* rendezvous = new IntraProcessRendezvous(device_mgr_);
    run_opts.rendezvous = rendezvous;
    run_opts.create_rendezvous = false;
    done = [done, rendezvous](const Status& status) {
      rendezvous->Unref();
      done(status);
    };
  }
  if (!parent_->IsInstantiatedOnDevice(device_name_, handle)) {
    parent_->Run(run_opts, handle, args, rets, done);
    return;
  }

  DCHECK(run_opts.runner != nullptr);

  Executor::Args* exec_args = new Executor::Args;
  // Inherit the step_id from the caller.
  exec_args->step_id = run_opts.step_id;
  exec_args->rendezvous = run_opts.rendezvous;
  exec_args->stats_collector = run_opts.stats_collector;
  exec_args->cancellation_manager = run_opts.cancellation_manager;
  exec_args->step_container = run_opts.step_container;
  exec_args->runner = *run_opts.runner;

  Item* item = nullptr;
  Status s = GetOrCreateItem(handle, &item);
  if (!s.ok()) {
    delete exec_args;
    done(s);
    return;
  }

  if (run_opts.remote_execution) {
    // NOTE(mrry): `RunRemote()` will set `exec_args->call_frame` for us.
    RunRemote(run_opts, handle, args, rets, exec_args, item, done);
    return;
  }

  const FunctionBody* fbody = GetFunctionBody(handle);
  FunctionCallFrame* frame =
      new FunctionCallFrame(fbody->arg_types, fbody->ret_types);
  exec_args->call_frame = frame;
  s = frame->SetArgs(args);
  if (!s.ok()) {
    delete frame;
    delete exec_args;
    done(s);
    return;
  }

  item->exec->RunAsync(
      // Executor args
      *exec_args,
      // Done callback.
      [item, frame, rets, done, exec_args](const Status& status) {
        Status s = status;
        if (s.ok()) {
          s = frame->ConsumeRetvals(rets);
        }
        delete frame;
        delete exec_args;
        done(s);
      });
}

void FunctionLibraryRuntimeImpl::Run(const Options& opts, Handle handle,
                                     CallFrameInterface* frame,
                                     DoneCallback done) {
  if (opts.cancellation_manager && opts.cancellation_manager->IsCancelled()) {
    done(errors::Cancelled(""));
    return;
  }
  if (!parent_->IsInstantiatedOnDevice(device_name_, handle) ||
      opts.remote_execution) {
    done(errors::Unimplemented("Remote calling with CallFrameInterface"));
    return;
  }

  Options run_opts = opts;
  if (opts.create_rendezvous) {
    Rendezvous* rendezvous = new IntraProcessRendezvous(device_mgr_);
    run_opts.rendezvous = rendezvous;
    run_opts.create_rendezvous = false;
    done = std::bind(
        [rendezvous](DoneCallback done,
                     // Begin unbound arguments.
                     const Status& status) {
          rendezvous->Unref();
          done(status);
        },
        std::move(done), std::placeholders::_1);
  }

  Item* item = nullptr;
  Status s = GetOrCreateItem(handle, &item);
  if (!s.ok()) {
    done(s);
    return;
  }
  DCHECK(run_opts.runner != nullptr);

  Executor::Args* exec_args = new Executor::Args;
  // Inherit the step_id from the caller.
  exec_args->step_id = run_opts.step_id;
  exec_args->rendezvous = run_opts.rendezvous;
  exec_args->stats_collector = run_opts.stats_collector;
  exec_args->cancellation_manager = run_opts.cancellation_manager;
  exec_args->step_container = run_opts.step_container;
  exec_args->runner = *run_opts.runner;
  exec_args->call_frame = frame;

  item->exec->RunAsync(
      // Executor args
      *exec_args,
      // Done callback.
      std::bind(
          [item, frame, exec_args](DoneCallback done,
                                   // Start unbound arguments.
                                   const Status& status) {
            delete exec_args;
            done(status);
          },
          std::move(done), std::placeholders::_1));
}

bool FunctionLibraryRuntimeImpl::IsStateful(const string& func) {
  const OpDef* op_def;
  const Status s = base_lib_def_->LookUpOpDef(func, &op_def);
  return s.ok() && op_def->is_stateful();
}

string FunctionLibraryRuntimeImpl::DebugString(Handle handle) {
  Item* item = nullptr;
  Status s = GetOrCreateItem(handle, &item);
  if (s.ok()) {
    return tensorflow::DebugString(item->graph);
  } else {
    return s.ToString();
  }
}

namespace {

struct CustomCreatorSingleton {
  mutex mu;
  CustomKernelCreator custom_creator = nullptr;

  void Set(CustomKernelCreator cb) {
    mutex_lock l(mu);
    custom_creator = std::move(cb);
  }

  CustomKernelCreator Get() {
    mutex_lock l(mu);
    return custom_creator;
  }
};

CustomCreatorSingleton* GetCustomCreatorSingleton() {
  static CustomCreatorSingleton* ccs = new CustomCreatorSingleton;
  return ccs;
}

}  // namespace

void RegisterDefaultCustomKernelCreator(CustomKernelCreator cb) {
  GetCustomCreatorSingleton()->Set(std::move(cb));
}

std::unique_ptr<FunctionLibraryRuntime> NewFunctionLibraryRuntime(
    const DeviceMgr* device_mgr, Env* env, Device* device,
    int graph_def_version, const FunctionLibraryDefinition* lib_def,
    const OptimizerOptions& optimizer_options,
    CustomKernelCreator custom_kernel_creator,
    ProcessFunctionLibraryRuntime* parent) {
  return std::unique_ptr<FunctionLibraryRuntime>(new FunctionLibraryRuntimeImpl(
      device_mgr, env, device, graph_def_version, lib_def, optimizer_options,
      std::move(custom_kernel_creator), parent));
}

std::unique_ptr<FunctionLibraryRuntime> NewFunctionLibraryRuntime(
    const DeviceMgr* device_mgr, Env* env, Device* device,
    int graph_def_version, const FunctionLibraryDefinition* lib_def,
    const OptimizerOptions& optimizer_options,
    ProcessFunctionLibraryRuntime* parent) {
  return NewFunctionLibraryRuntime(device_mgr, env, device, graph_def_version,
                                   lib_def, optimizer_options,
                                   GetCustomCreatorSingleton()->Get(), parent);
}

bool RemoveDeadNodes(Graph* g) {
  VLOG(2) << "Removing dead nodes";
  std::unordered_set<const Node*> nodes;
  for (auto n : g->nodes()) {
    if (n->IsSource() || n->IsSink() || n->IsControlFlow() ||
        n->op_def().is_stateful()) {
      nodes.insert(n);
    }
  }
  return PruneForReverseReachability(g, std::move(nodes));
}

namespace {
// If 'edges' contains only 1 non-control edge, returns it. Otherwise,
// returns a nullptr.
const Edge* GetTheOnlyDataEdge(const EdgeSet& edges) {
  const Edge* ret = nullptr;
  for (const Edge* e : edges) {
    if (e->IsControlEdge() || ret) {
      // Don't touch it if there is a control edge.
      return nullptr;
    }
    if (IsRefType(e->src()->output_type(e->src_output()))) {
      // Don't touch it if the identity node is effectively de-reffing
      // a ref.
      return nullptr;
    }
    if (IsRecv(e->src()) || IsSwitch(e->src())) {
      // Don't touch it if the identity is introduced for control flow.
      // Recv disables all its successors if it receives a dead signal.
      // When Recv has an outgoing control edge, the current executor
      // would not disable the destination. The current solution (see
      // graph_partition.cc) is to add an identity after Recv and change
      // the control edge to be from this identity node. So the identity
      // can't be removed.
      return nullptr;
    }
    ret = e;
  }
  return ret;
}
}  // end namespace

bool RemoveIdentityNodes(Graph* g) {
  VLOG(2) << "Removing identity nodes";
  bool removed_any = false;
  gtl::InlinedVector<Node*, 8> matches;
  for (Node* n : g->nodes()) {
    if (!n->IsIdentity()) continue;
    if (!GetTheOnlyDataEdge(n->in_edges())) continue;

    // Some identity nodes are used as sink nodes to give names to output
    // tensors. These nodes are not going to be executed unless they are in the
    // fetch set. But if they are in the fetch set we don't want to remove them.
    if (n->out_edges().empty()) continue;

    matches.push_back(n);
  }
  if (!matches.empty()) {
    for (Node* n : matches) {
      const Edge* in = GetTheOnlyDataEdge(n->in_edges());
      for (const Edge* out : n->out_edges()) {
        if (out->IsControlEdge()) {
          g->AddControlEdge(in->src(), out->dst());
        } else {
          g->AddEdge(in->src(), in->src_output(), out->dst(), out->dst_input());
        }
      }
      VLOG(2) << "Remove Identity: " << n->DebugString();
      g->RemoveNode(n);
      removed_any = true;
    }
  }
  return removed_any;
}

bool RemoveListArrayConverter(Graph* g) {
  VLOG(2) << "Removing list array converter";
  gtl::InlinedVector<Node*, 8> matches;
  for (Node* n : g->nodes()) {
    if ((n->type_string() == "_ListToArray") ||
        (n->type_string() == "_ArrayToList")) {
      matches.push_back(n);
    }
  }
  bool removed_any = false;
  if (!matches.empty()) {
    for (Node* n : matches) {
      if (n->num_inputs() != n->num_outputs()) {
        continue;  // Not expected. Skip.
      }
      gtl::InlinedVector<Node*, 8> identity_nodes(n->num_inputs(), nullptr);

      // Process input edges first.
      Node* input_control_node = nullptr;
      for (const Edge* e : n->in_edges()) {
        if (e->IsControlEdge()) {
          if (input_control_node == nullptr) {
            // If node "n" has any control dependencies, adds a no-op
            // node (input_control_node) which the additional Identity
            // nodes depends on and the input_control_node depends on
            // the node "n"s control dependencies.
            input_control_node = AddNoOp(g);
          }
          g->AddControlEdge(e->src(), input_control_node);
        } else {
          const int index = e->dst_input();
          Node** id_node = &identity_nodes[index];
          if (*id_node != nullptr) {
            LOG(ERROR)
                << "RemoveListArrayConverter unexpected duplicated input: "
                << e->dst_input();
            return removed_any;
          }
          *id_node = AddIdentity(g, {e->src(), e->src_output()});
        }
      }

      // If node "n" has any control dependencies, the added identity
      // nodes should have control dependencies on input_control_node.
      if (input_control_node != nullptr) {
        for (Node* id : identity_nodes) {
          g->AddControlEdge(input_control_node, id);
        }
      }

      Node* output_control_node = nullptr;
      for (const Edge* e : n->out_edges()) {
        if (e->IsControlEdge()) {
          if (output_control_node == nullptr) {
            // If node "n" is control-depended upon by other nodes,
            // adds a no-op node (output_control_node) which those
            // nodes will depend on and output_control_node depends on
            // all Identity nodes.
            output_control_node = AddNoOp(g);
          }
          g->AddControlEdge(output_control_node, e->dst());
        } else {
          Node* id_node = identity_nodes[e->src_output()];
          if (id_node == nullptr) {
            LOG(ERROR) << "RemoveListArrayConverter unexpected missing input: "
                       << e->src_output();
            return removed_any;
          }
          CHECK(id_node);
          g->AddEdge(id_node, 0, e->dst(), e->dst_input());
        }
      }

      // If any nodes have control dependencies on node "n", those
      // nodes should have control dependencies on
      // output_control_node.
      if (output_control_node != nullptr) {
        for (Node* id : identity_nodes) {
          g->AddControlEdge(id, output_control_node);
        }
      }

      g->RemoveNode(n);
      removed_any = true;
    }
  }
  return removed_any;
}

// Returns true iff the function '*fbody' can be inlined at 'node'
// based on the type signature of 'node' and 'fbody'.
static bool ValidateInlining(const Node* node, const FunctionBody* fbody) {
  if (static_cast<size_t>(node->num_inputs()) != fbody->arg_types.size()) {
    return false;
  }
  if (static_cast<size_t>(node->num_inputs()) != fbody->arg_nodes.size()) {
    return false;
  }
  if (static_cast<size_t>(node->num_outputs()) != fbody->ret_types.size()) {
    return false;
  }
  if (static_cast<size_t>(node->num_outputs()) != fbody->ret_nodes.size()) {
    return false;
  }
  for (int i = 0; i < node->num_inputs(); ++i) {
    if (node->input_type(i) != fbody->arg_types[i]) return false;
  }
  for (int i = 0; i < node->num_outputs(); ++i) {
    if (node->output_type(i) != fbody->ret_types[i]) return false;
  }
  return true;
}

// Given a "caller" in "graph", which is a function call of a function
// to "fbody". Replaces the "caller" with fbody->graph and connects
// edges properly.
void InlineFunctionBody(const FunctionLibraryDefinition& flib_def, Graph* g,
                        Node* caller, const FunctionBody* fbody) {
  if (!ValidateInlining(caller, fbody)) {
    LOG(WARNING) << "Inlining mismatch: " << caller->DebugString() << " vs. "
                 << DebugString(fbody->graph);
    return;
  }

  // Input edges. For data edges coming into "caller", we first compute the
  // <src>:<src_output> for the i-th input in "inputs".
  // If "caller" has any input control dependencies, we add a NoOp
  // node "input_control_node", which depends on "caller"'s control inputs.
  std::vector<Endpoint> inputs(caller->num_inputs());
  Node* input_control_node = nullptr;
  for (const Edge* e : caller->in_edges()) {
    if (e->IsControlEdge()) {
      if (input_control_node == nullptr) {
        input_control_node = AddNoOp(g);
      }
      g->AddControlEdge(e->src(), input_control_node);
    } else {
      inputs[e->dst_input()] = {e->src(), e->src_output()};
    }
  }

  // Duplicate fbody->graph into 'g'.  First, we copy the nodes of
  // fbody->graph into 'g' except the source and sink nodes.  We copy
  // edges among nodes in 'fbody->graph'.
  //
  // If 'x' is a node in fbody->graph and its copy in 'g' is 'y', we
  // remember 'y' in node_map[x->id()].
  std::vector<Node*> node_map(fbody->graph->num_node_ids());
  Status s;
  for (Node* n : fbody->graph->op_nodes()) {
    NodeDef ndef = n->def();
    ndef.set_name(strings::StrCat(caller->name(), "/", ndef.name()));
    ndef.set_device(caller->def().device());
    Node* clone = g->AddNode(ndef, &s);
    TF_CHECK_OK(s);
    node_map[n->id()] = clone;

    // If there is an input control node, and one of:
    // a) the node has no data or control inputs, or
    // b) the node is a function call or SymbolicGradient,
    // then add a control edge from the input control node to the clone.
    //
    // We must not execute any nodes if the original function call would not
    // have executed. This is especially critical when the function call is
    // inside a control-flow construct like tf.cond(). Case (a) ensures that
    // such nodes do not run.
    //
    // The purpose of case (b) is to ensure that instances of case (a) created
    // by further inlining steps also receive the control dependency.
    if (input_control_node) {
      bool has_inputs = false;
      for (const Edge* e : n->in_edges()) {
        if (!e->src()->IsSource()) {
          has_inputs = true;
          break;
        }
      }
      if (!has_inputs || flib_def.Find(clone->type_string()) != nullptr ||
          clone->type_string() == "SymbolicGradient") {
        g->AddControlEdge(input_control_node, clone);
      }
    }
  }
  for (const Edge* e : fbody->graph->edges()) {
    if (e->src()->IsSource() || e->src()->IsSink() || e->dst()->IsSource() ||
        e->dst()->IsSink()) {
      continue;
    }
    Node* src_copy = node_map[e->src()->id()];
    Node* dst_copy = node_map[e->dst()->id()];
    g->AddEdge(src_copy, e->src_output(), dst_copy, e->dst_input());
  }

  // Connect input edges.
  //
  // We create one Identity node for each input. Then, we connect inputs[i] to
  // the i-th identity node added. The nodes that previously connected
  // to the j-th output of i-th arg node are reconnected to the i-th
  // identity node.
  //
  // The added identity nodes depend on "input_control_node".
  for (std::size_t i = 0; i < fbody->arg_nodes.size(); ++i) {
    Node* arg = node_map[fbody->arg_nodes[i]->id()];
    Node* n = AddIdentity(g, inputs[i]);
    if (input_control_node) {
      g->AddControlEdge(input_control_node, n);
    }
    for (const Edge* e : arg->out_edges()) {
      if (e->IsControlEdge()) {
        g->AddControlEdge(n, e->dst());
      } else {
        g->AddEdge(n, 0, e->dst(), e->dst_input());
      }
    }
    node_map[fbody->arg_nodes[i]->id()] = n;
    g->RemoveNode(arg);  // 'arg' is disconnected.
  }

  // Connect output edges.
  //
  // For i-th return node in fbody->graph, we add in "g" an identity
  // node (outputs[i-th]). We then reconnect every incoming edge into
  // the i-th return node to the added identity node.
  //
  // For every data edge coming out of "callee"s i-th output, we
  // reconnect it to the i-th identity added above.
  //
  // If "callee" is control-depended upon by any other nodes, we add a
  // NoOp node "output_control_node". "output_control_node" depends on
  // all identity nodes added above. And nodes previously depend on
  // "callee" is changed to depend on "output_control_node".
  std::vector<Node*> outputs(caller->num_outputs());
  for (std::size_t i = 0; i < fbody->ret_nodes.size(); ++i) {
    Node* ret = node_map[fbody->ret_nodes[i]->id()];
    Endpoint data;  // Data input for the ret node.
    for (const Edge* e : ret->in_edges()) {
      if (!e->IsControlEdge()) {
        data = {e->src(), e->src_output()};
        break;
      }
    }
    CHECK(data.node != nullptr);
    Node* n = AddIdentity(g, data);
    outputs[i] = n;
    for (const Edge* e : ret->in_edges()) {
      if (e->IsControlEdge()) {
        g->AddControlEdge(e->src(), n);
      }
    }
    g->RemoveNode(ret);  // 'ret' is disconnected.
  }
  Node* output_control_node = nullptr;
  for (const Edge* e : caller->out_edges()) {
    if (e->IsControlEdge()) {
      if (output_control_node == nullptr) {
        output_control_node = AddNoOp(g);
        for (Node* n : outputs) {
          g->AddControlEdge(n, output_control_node);
        }
      }
      g->AddControlEdge(output_control_node, e->dst());
    } else {
      g->AddEdge(outputs[e->src_output()], 0, e->dst(), e->dst_input());
    }
  }
  g->RemoveNode(caller);  // 'caller' is replaced with inlined nodes.
}

bool ExpandInlineFunctions(FunctionLibraryRuntime* lib, Graph* graph) {
  std::vector<std::pair<Node*, const FunctionBody*>> candidates;
  const FunctionLibraryDefinition* fld = lib->GetFunctionLibraryDefinition();
  for (Node* node : graph->nodes()) {
    VLOG(3) << "Expanding " << node->DebugString();
    bool noinline;
    if (fld->GetAttr(*node, kNoInlineAttr, &noinline).ok() && noinline) {
      VLOG(3) << "noinline: " << node->DebugString();
      continue;
    }
    FunctionLibraryRuntime::Handle handle;
    Status s = lib->Instantiate(node->type_string(), node->attrs(), &handle);
    if (!s.ok()) {
      // Either "node" is a primitive op, or the instantiation failed.
      if (errors::IsNotFound(s)) {
        VLOG(3) << "ExpandInlineFunctions " << s;
      } else {
        LOG(ERROR) << "ExpandInlineFunctions " << s;
      }
      continue;
    }
    const FunctionBody* fbody = lib->GetFunctionBody(handle);
    CHECK_NOTNULL(fbody);
    candidates.push_back({node, fbody});
  }
  for (const auto& p : candidates) {
    InlineFunctionBody(*fld, graph, p.first, p.second);
  }
  return !candidates.empty();
}

string NewName(const Node* n, bool pretty) {
  if (pretty) {
    return strings::StrCat(n->type_string(), n->id());
  } else {
    return strings::StrCat("n", n->id());
  }
}

// TODO(zhifengc): Maybe this should be the default Graph::AsGraphDef.
// and stash the original NodeDef name as an attr for documentation
// purpose.
void ToGraphDef(const Graph* g, GraphDef* gdef, bool pretty) {
  // We visit nodes in forward topological sort order, which is a
  // possible execution order of the graph.
  gtl::InlinedVector<const Edge*, 4> inputs;
  gdef->Clear();
  gdef->mutable_versions()->CopyFrom(g->versions());

  std::vector<Node*> start_nodes;
  for (Node* n : g->nodes()) {
    if (n->out_edges().empty()) {
      start_nodes.push_back(n);
    }
  }

  ReverseDFSFrom(*g, start_nodes, nullptr, [gdef, pretty, &inputs](Node* n) {
    if (!n->IsOp()) return;
    NodeDef* ndef = gdef->add_node();
    ndef->set_name(NewName(n, pretty));
    ndef->set_op(n->type_string());
    for (const auto& attr : n->attrs()) {
      (*ndef->mutable_attr())[attr.first] = attr.second;
    }
    inputs.clear();
    inputs.resize(n->num_inputs());
    for (const Edge* e : n->in_edges()) {
      if (e->IsControlEdge()) {
        inputs.push_back(e);
      } else {
        if (inputs[e->dst_input()] == nullptr) {
          inputs[e->dst_input()] = e;
        } else {
          LOG(WARNING) << "Malformed graph node. multiple input edges: "
                       << n->DebugString();
        }
      }
    }
    // node->name() is merely NodeDef::name, which are not guaranteed
    // to be unique and stable after optimization rewrites. Therefore,
    // we use "n<node id>" instead.
    for (const Edge* e : inputs) {
      if (e == nullptr) {
        ndef->add_input("unknown");
        continue;
      }
      const string srcname = NewName(e->src(), pretty);
      if (!e->src()->IsOp()) {
      } else if (e->IsControlEdge()) {
        ndef->add_input(strings::StrCat("^", srcname));
      } else if (e->src_output() == 0) {
        ndef->add_input(srcname);
      } else {
        ndef->add_input(strings::StrCat(srcname, ":", e->src_output()));
      }
    }
  });
}

string DebugString(const Graph* g) {
  GraphDef gdef;
  ToGraphDef(g, &gdef);
  return DebugString(gdef);
}

FunctionBody::FunctionBody(const FunctionDef& f, DataTypeSlice arg_t,
                           DataTypeSlice ret_t, Graph* g)
    : fdef(f),
      graph(g),
      arg_types(arg_t.begin(), arg_t.end()),
      ret_types(ret_t.begin(), ret_t.end()) {
  this->arg_nodes.resize(arg_types.size());
  this->ret_nodes.resize(ret_types.size());
  for (Node* n : this->graph->op_nodes()) {
    gtl::InlinedVector<Node*, 4>* node_vec;
    if (n->type_string() == kRetOp) {
      node_vec = &this->ret_nodes;
    } else if (n->type_string() == kArgOp) {
      node_vec = &this->arg_nodes;
    } else {
      continue;
    }
    int index;
    TF_CHECK_OK(GetNodeAttr(n->attrs(), "index", &index));
    CHECK_LE(0, index);
    CHECK_LT(index, node_vec->size());
    (*node_vec)[index] = n;
  }
}

FunctionBody::~FunctionBody() { delete this->graph; }

class SymbolicGradientHelper {
 public:
  explicit SymbolicGradientHelper(const FunctionBody& f) : fbody_(&f) {}

  ~SymbolicGradientHelper() { delete gbody_; }

  FunctionBody* Compute();

 private:
  const FunctionBody* fbody_;
  FunctionBody* gbody_ = nullptr;

  // Makes a copy of fbody_ in gbody_.
  void Copy();

  TF_DISALLOW_COPY_AND_ASSIGN(SymbolicGradientHelper);
};

void SymbolicGradientHelper::Copy() {
  const Graph& src = *(fbody_->graph);
  gbody_->graph = new Graph(src.op_registry());
  Graph* dst = gbody_->graph;

  std::vector<Node*> node_map(src.num_node_ids());

  // Copy the nodes.
  node_map[src.source_node()->id()] = dst->source_node();
  node_map[src.sink_node()->id()] = dst->sink_node();
  for (Node* n : src.op_nodes()) {
    node_map[n->id()] = dst->CopyNode(n);
  }

  // Copy the edges.
  for (const Edge* e : src.edges()) {
    Node* src_copy = node_map[e->src()->id()];
    Node* dst_copy = node_map[e->dst()->id()];
    dst->AddEdge(src_copy, e->src_output(), dst_copy, e->dst_input());
  }

  // Save inputs in copied graph.
  CHECK_EQ(fbody_->arg_types.size(), fbody_->arg_nodes.size());
  gbody_->arg_types = fbody_->arg_types;
  for (std::size_t i = 0; i < fbody_->arg_nodes.size(); ++i) {
    gbody_->arg_nodes.push_back(node_map[fbody_->arg_nodes[i]->id()]);
  }

  // Save outputs in copied graph.
  CHECK_EQ(fbody_->ret_types.size(), fbody_->ret_nodes.size());
  gbody_->ret_types = fbody_->ret_types;
  for (std::size_t i = 0; i < fbody_->ret_nodes.size(); ++i) {
    gbody_->ret_nodes.push_back(node_map[fbody_->ret_nodes[i]->id()]);
  }
}

FunctionBody* SymbolicGradientHelper::Compute() {
  CHECK(gbody_ == nullptr);
  gbody_ = new FunctionBody;

  // Copy fbody_ into gbody_.
  Copy();

  Graph* g = gbody_->graph;

  const int num_y = static_cast<int>(gbody_->ret_nodes.size());

  // Populate 'y_node_outputs_' with node function body outputs.
  // Populate 'y_grad_nodes' with initial gradient nodes for each return node of
  // the original function body (these will be 'arg' nodes in the function
  // gradient body).
  std::vector<NodeOut> y_node_outputs;
  y_node_outputs.reserve(num_y);
  std::vector<NodeOut> y_grad_node_outputs;
  y_grad_node_outputs.reserve(num_y);
  for (int i = 0; i < num_y; ++i) {
    Node* y = gbody_->ret_nodes[i];
    y_node_outputs.push_back({y, 0});
    DCHECK_EQ(y->type_string(), kRetOp);
    const DataType dtype = y->input_type(0);
    const int index = static_cast<int>(gbody_->arg_nodes.size());
    Node* dy = AddArg(g, dtype, index);
    gbody_->arg_types.push_back(dtype);
    gbody_->arg_nodes.push_back(dy);
    y_grad_node_outputs.push_back({dy, 0});
  }

  // Populate 'x_nodes' with function args (excluding 'y_grad_node_outputs').
  const size_t num_x = fbody_->arg_nodes.size();
  std::vector<NodeOut> x_node_outputs;
  x_node_outputs.reserve(num_x);
  for (size_t i = 0; i < fbody_->arg_nodes.size(); ++i) {
    x_node_outputs.push_back({gbody_->arg_nodes[i], 0});
  }

  // Call AddSymbolicGradients which will add nodes to graph 'g' that
  // compute the function gradient (adding an entry in 'x_grad_node_outputs' for
  // each node in 'x_node_outputs').
  std::vector<NodeOut> x_grad_node_outputs;
  TF_CHECK_OK(AddSymbolicGradients(y_node_outputs, x_node_outputs,
                                   y_grad_node_outputs, &x_grad_node_outputs,
                                   g));

  // Remove the old return nodes from the function body.
  for (Node* n : gbody_->ret_nodes) {
    g->RemoveNode(n);
  }
  gbody_->ret_types = fbody_->arg_types;
  gbody_->ret_nodes.clear();
  // Add new return nodes to the function gradient body for each node
  // in 'x_grad_nodes'.
  const int arg_types_size = static_cast<int>(fbody_->arg_types.size());
  for (int i = 0; i < arg_types_size; ++i) {
    Endpoint grad = {x_grad_node_outputs[i].node, x_grad_node_outputs[i].index};
    Node* ret = AddRet(g, grad, i);
    gbody_->ret_nodes.push_back(ret);
  }

  auto ret = gbody_;
  gbody_ = nullptr;
  return ret;
}

FunctionBody* SymbolicGradient(const FunctionBody& f) {
  return SymbolicGradientHelper(f).Compute();
}

Status FunctionDefToBodyHelper(
    const FunctionDef& fdef, const AttrSlice& attrs,
    const FunctionLibraryDefinition* const lib_def,
    const std::function<Status(const string&, const OpDef**)>& get_func_sig,
    FunctionBody** fbody) {
  // Instantiates the function template into a graph def.
  InstantiationResult result;
  TF_RETURN_IF_ERROR(InstantiateFunction(fdef, attrs, get_func_sig, &result));

  std::unique_ptr<Graph> graph(new Graph(lib_def));
  GraphConstructorOptions opts;
  opts.allow_internal_ops = true;
  opts.expect_device_spec = false;
  TF_RETURN_IF_ERROR(ConvertNodeDefsToGraph(opts, result.nodes, graph.get()));

  // Call BuildControlFlowInfo to validate that this function body has
  // well-formed control flow.
  // NOTE(skyewm): this is usually done in Partition(), but we don't partition
  // function bodies. This should be removed if function bodies ever go through
  // the Partition() path.
  std::vector<ControlFlowInfo> dummy;
  TF_RETURN_IF_ERROR(BuildControlFlowInfo(graph.get(), &dummy));

  *fbody = new FunctionBody(fdef, result.arg_types, result.ret_types,
                            graph.release());
  return Status::OK();
}

}  // end namespace tensorflow
