op_kernel.h

/* 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.
==============================================================================*/

#ifndef TENSORFLOW_FRAMEWORK_OP_KERNEL_H_
#define TENSORFLOW_FRAMEWORK_OP_KERNEL_H_

#include <functional>

#include <vector>
#include "tensorflow/core/framework/allocator.h"
#include "tensorflow/core/framework/cancellation.h"
#include "tensorflow/core/framework/control_flow.h"
#include "tensorflow/core/framework/device_base.h"
#include "tensorflow/core/framework/function.h"
#include "tensorflow/core/framework/graph.pb.h"
#include "tensorflow/core/framework/kernel_def.pb.h"
#include "tensorflow/core/framework/kernel_def_builder.h"
#include "tensorflow/core/framework/node_def_util.h"
#include "tensorflow/core/framework/op.h"
#include "tensorflow/core/framework/rendezvous.h"
#include "tensorflow/core/framework/selective_registration.h"
#include "tensorflow/core/framework/session_state.h"
#include "tensorflow/core/framework/step_stats.pb.h"
#include "tensorflow/core/framework/tensor.h"
#include "tensorflow/core/framework/tensor_shape.h"
#include "tensorflow/core/framework/tensor_shape.pb.h"
#include "tensorflow/core/framework/tracking_allocator.h"
#include "tensorflow/core/framework/types.h"
#include "tensorflow/core/framework/types.pb.h"
#include "tensorflow/core/framework/unique_tensor_references.h"
#include "tensorflow/core/lib/core/errors.h"
#include "tensorflow/core/lib/core/status.h"
#include "tensorflow/core/lib/gtl/array_slice.h"
#include "tensorflow/core/lib/gtl/manual_constructor.h"
#include "tensorflow/core/platform/env.h"
#include "tensorflow/core/platform/logging.h"
#include "tensorflow/core/platform/macros.h"
#include "tensorflow/core/platform/mutex.h"
#include "tensorflow/core/platform/thread_annotations.h"
#include "tensorflow/core/platform/types.h"

namespace Eigen {
struct ThreadPoolDevice;
struct GpuDevice;
struct SyclDevice;
}  // end namespace Eigen

namespace tensorflow {

namespace checkpoint {
class TensorSliceReaderCacheWrapper;
}  // namespace checkpoint

class AsyncOpKernel;
class OpKernelConstruction;  // declared below
class OpKernelContext;       // declared below
class ResourceMgr;

class OpKernel {
 public:
  // OpKernel won't be instantiated by the scheduler, so you may perform
  // expensive initialization in the descendant's constructor.
  explicit OpKernel(OpKernelConstruction* context);
  virtual ~OpKernel();

  // An OpKernel's computation can be either synchronous or
  // asynchronous. All OpKernel Compute() methods must be thread-safe as they
  // may be called concurrently (e.g. by multiple executions of the same graph
  // concurrently).
  //
  // Most OpKernels should compute synchronously.  They should
  // subclass OpKernel and override the Compute() method and have it
  // return after completing the supplied work.
  //
  // A few special kernels might need to be asynchronous to bound the
  // number of threads (e.g., network receive operations). These
  // kernels must subclass AsyncOpKernel and override
  // AsyncOpKernel::ComputeAsync().
  //
  // In both cases, implementations of Compute() and ComputeAsync()
  // get inputs and write outputs through the given OpKernelContext
  // and returns a status via context->SetStatus(). They must be
  // thread-safe.

  // Synchronous compute.
  //
  // "context" is guaranteed to be alive until Compute() returns.
  virtual void Compute(OpKernelContext* context) = 0;

  // Returns nullptr iff this op kernel is synchronous.
  virtual AsyncOpKernel* AsAsync() { return nullptr; }

  // Returns true iff this op kernel is considered "expensive". The
  // runtime may use this flag to optimize graph execution for example
  // to "inline" inexpensive kernels.
  virtual bool IsExpensive() { return expensive_; }

  // Accessors.
  const NodeDef& def() const { return def_; }
  const string& name() const { return def_.name(); }
  const string& type_string() const { return def_.op(); }
  bool is_internal() const { return is_internal_; }

  int num_inputs() const { return input_types_.size(); }
  DataType input_type(int i) const { return input_types_[i]; }
  const DataTypeVector& input_types() const { return input_types_; }
  const MemoryTypeVector& input_memory_types() const {
    return input_memory_types_;
  }

  int num_outputs() const { return output_types_.size(); }
  DataType output_type(int o) const { return output_types_[o]; }
  const DataTypeVector& output_types() const { return output_types_; }
  const MemoryTypeVector& output_memory_types() const {
    return output_memory_types_;
  }

  Status InputRange(StringPiece input_name, int* start, int* stop) const;
  Status OutputRange(StringPiece output_name, int* start, int* stop) const;

  // We allow legacy scalars within Google up until GraphDef version 6.
  // TODO(irving): Remove when we can drop support for GraphDef version 5.
  bool allow_legacy_scalars() const {
#if defined(PLATFORM_GOOGLE) || defined(PLATFORM_GOOGLE_ANDROID)
    return graph_def_version_ < 6;
#else
    return false;
#endif
  }

  // Allow either scalars or (if allowing legacy scalars) shape (1,).
  bool IsLegacyScalar(const TensorShape& shape) const {
    return shape.dims() == 0 || (allow_legacy_scalars() && shape.dims() == 1 &&
                                 shape.dim_size(0) == 1);
  }

  // Allow rank 1 or (if allowing legacy scalars) rank 0.
  bool IsLegacyVector(const TensorShape& shape) const {
    return shape.dims() == 1 || (allow_legacy_scalars() && shape.dims() == 0);
  }

 private:
  const NodeDef def_;
  const DataTypeVector input_types_;
  const MemoryTypeVector input_memory_types_;
  const DataTypeVector output_types_;
  const MemoryTypeVector output_memory_types_;
  const int graph_def_version_;
  const bool is_internal_;  // True if this is an internal operation
  NameRangeMap input_name_map_;
  NameRangeMap output_name_map_;
  bool expensive_;

  TF_DISALLOW_COPY_AND_ASSIGN(OpKernel);
};

class AsyncOpKernel : public OpKernel {
 public:
  using OpKernel::OpKernel;  // Lift OpKernel constructors.

  // Asynchronous compute.
  //
  // Implementations of ComputeAsync() must run "done" to signal the
  // completion of the computation. "context" is guaranteed to be
  // alive until the "done" callback starts.
  typedef std::function<void()> DoneCallback;
  virtual void ComputeAsync(OpKernelContext* context, DoneCallback done) = 0;

  AsyncOpKernel* AsAsync() final { return this; }

  void Compute(OpKernelContext* context) final;

  bool IsExpensive() override { return true; }
};

// Wraps a tensor that is held by an Op across calls to Compute(). For
// memory safety when using asynchronous devices like GPUs, the system
// must be notified when a Tensor is used inside an Op execution. The
// wrapper ensures that all uses of the Tensor are tracked, because in
// order to retrieve the Tensor the caller must use AccessTensor which
// notifies the context.
class PersistentTensor {
 public:
  PersistentTensor() {}
  explicit PersistentTensor(const Tensor& tensor) : tensor_(tensor) {}

  // Caller does not own the returned Tensor*.
  Tensor* AccessTensor(OpKernelConstruction* context);
  // Caller does not own the returned Tensor*.
  Tensor* AccessTensor(OpKernelContext* context);

  // The check for initialization does not need to access the
  // underlying tensor buffer.
  bool IsInitialized() const { return tensor_.IsInitialized(); }

  int64 NumElements() const { return tensor_.NumElements(); }

  int64 AllocatedBytes() const { return tensor_.AllocatedBytes(); }

 private:
  Tensor tensor_;
};

class OpKernelConstruction {
 public:
  OpKernelConstruction(DeviceType device_type, DeviceBase* device,
                       Allocator* allocator, const NodeDef* node_def,
                       const OpDef* op_def, FunctionLibraryRuntime* flib,
                       const DataTypeSlice& input_types,
                       const MemoryTypeSlice& input_memory_types,
                       const DataTypeSlice& output_types,
                       const MemoryTypeSlice& output_memory_types,
                       int graph_def_version, Status* status)
      : device_type_(device_type),
        device_(device),
        allocator_(allocator),
        def_(node_def),
        op_def_(op_def),
        flib_(flib),
        input_types_(input_types),
        input_memory_types_(input_memory_types),
        output_types_(output_types),
        output_memory_types_(output_memory_types),
        graph_def_version_(graph_def_version),
        status_(status) {}

  Env* env() const { return device_->env(); }

  // Allocation of tensors during kernel construction:
  //
  // It is legal to temporarily allocate scratch tensor storage during
  // Op kernel construction. Scratch tensors should be allocated using
  // allocate_temp below. Some kernels need to keep tensors in between
  // invocations. If such a Tensor is allocated during kernel
  // construction this must be done using allocate_persistent, and the
  // Op may only store the returned PersistentTensor object. When the
  // Tensor is needed in a subsequent invocation, it can be retrieved
  // from the PersistentTensor using the AccessTensor method. This
  // ensures that the system is made aware of any use of the tensor's
  // allocated memory, which is needed for correctness on asynchronous
  // devices such as GPUs.

  // Allocates a temporary Tensor of the specified type and shape. The
  // Tensor must not be used after kernel construction is
  // complete. See comment above.
  Status allocate_temp(DataType type, const TensorShape& shape,
                       Tensor* out_temp);

  // Allocates a Tensor of the specified type and shape which the Op
  // plans to maintain as persistent state. out_persistent holds the
  // PersistentTensor which is the object the caller should store. For
  // convenience, if out_tensor is non-null then it will be filled in
  // with a Tensor* pointing to the newly-allocated tensor which the
  // caller can use instead of calling
  // out_persistent->AccessTensor. The caller does not own out_tensor
  // and should not keep a copy of it. See comment above.
  Status allocate_persistent(DataType type, const TensorShape& shape,
                             PersistentTensor* out_persistent,
                             Tensor** out_tensor);

  // User-supplied configuration of this operation.
  const NodeDef& def() const { return *def_; }

  // Op registered for this op type.
  const OpDef& op_def() const { return *op_def_; }

  // For inspecting the inputs to this operation.
  int num_inputs() const { return input_types_.size(); }
  DataType input_type(int i) const { return input_types_[i]; }
  const DataTypeSlice& input_types() const { return input_types_; }
  const MemoryTypeSlice& input_memory_types() const {
    return input_memory_types_;
  }

  // For inspecting the outputs expected from this operation.
  int num_outputs() const { return output_types_.size(); }
  DataType output_type(int i) const { return output_types_[i]; }
  const DataTypeSlice& output_types() const { return output_types_; }
  const MemoryTypeSlice& output_memory_types() const {
    return output_memory_types_;
  }

  // If expected_inputs == inputs() and expected_outputs == output_types(),
  // returns OK, else returns INVALID_ARGUMENT with an error message.
  // Recommended for Ops with dynamic signatures.
  Status MatchSignature(const DataTypeSlice expected_inputs,
                        const DataTypeSlice expected_outputs);

  // For recording configuration errors during construction.
  void SetStatus(const Status& status);
  const Status& status() const { return *status_; }

  // Look up the attr with name attr_name and set *value to its value.  If no
  // attr with attr_name is found in def(), or the attr does not have
  // a matching type, a non-ok status will be returned.
  template <class T>
  Status GetAttr(StringPiece attr_name, T* value) const;

  // Return the device type.
  const DeviceType& device_type() const { return device_type_; }

  // If not nullptr, the kernel can instantiate functions defined in
  // the library. E.g.,
  // CHECK_NOTNULL(function_library())->Instantiate("Foo", ...).
  FunctionLibraryRuntime* function_library() const { return flib_; }

  // The GraphDef version whose behavior we should follow.
  const int graph_def_version() const { return graph_def_version_; }

  // Helper routines for the OP_REQUIRES macros
  void CtxFailure(Status s);
  void CtxFailureWithWarning(Status s);

  // Unrecommended functions: these are functions that have some
  // current uses but are not recommended for use, and may go away at
  // some future major version release.

  // May be used, e.g., to get GPU handles, etc.
  //
  // Currently only used to call MakeTensorFromProto() for
  // implementing ConstantOp for every device.  See comments
  // on Device::MakeTensorFromProto for longer-term replacement
  // ideas.
  DeviceBase* device() const { return device_; }

 private:
  const DeviceType device_type_;
  DeviceBase* const device_;
  Allocator* allocator_;
  const NodeDef* def_;
  const OpDef* op_def_;
  FunctionLibraryRuntime* flib_;
  DataTypeSlice input_types_;
  MemoryTypeSlice input_memory_types_;
  DataTypeSlice output_types_;
  MemoryTypeSlice output_memory_types_;
  const int graph_def_version_;
  Status* status_;

  TF_DISALLOW_COPY_AND_ASSIGN(OpKernelConstruction);
};

// TODO(mrry): Consider converting to a random_access_iterator, and upgrading
// tensorflow::gtl::iterator_range to make the below container classes
// unnecessary.
template <typename ListType, typename ElementType>
class OpArgIterator {
 public:
  typedef OpArgIterator<ListType, ElementType> ME;
  OpArgIterator(const ListType* list, int i) : list_(list), i_(i) {}
  bool operator==(const ME& rhs) {
    DCHECK(list_ == rhs.list_);
    return i_ == rhs.i_;
  }
  bool operator!=(const ME& rhs) {
    DCHECK(list_ == rhs.list_);
    return i_ != rhs.i_;
  }
  void operator++() { ++i_; }
  ElementType& operator*() { return (*list_)[i_]; }

 private:
  const ListType* const list_;
  int i_;
};

// Utility class for representing a list of immutable input tensors
// that are passed to the op as a single named argument.
class OpInputList {
 public:
  typedef OpArgIterator<OpInputList, const Tensor&> Iterator;
  OpInputList() : ctx_(nullptr), start_(0), stop_(0) {}
  OpInputList(OpKernelContext* ctx, int start, int stop)
      : ctx_(ctx), start_(start), stop_(stop) {}
  OpInputList& operator=(const OpInputList& other) = default;
  const Tensor& operator[](int i) const;
  int size() const { return stop_ - start_; }
  Iterator begin() const { return Iterator(this, 0); }
  Iterator end() const { return Iterator(this, size()); }

 private:
  OpKernelContext* ctx_;  // not owned
  int start_;
  int stop_;
};

// Utility class for representing a list of mutable ("ref") input tensors
// that are passed to the op as a single named argument.
class OpMutableInputList {
 public:
  typedef OpArgIterator<OpMutableInputList, Tensor*> Iterator;
  OpMutableInputList(OpKernelContext* ctx, int start, int stop)
      : ctx_(ctx), start_(start), stop_(stop) {}
  OpMutableInputList() : ctx_(nullptr), start_(0), stop_(0) {}
  OpMutableInputList& operator=(const OpMutableInputList& other) = default;
  Tensor at(int i, bool lock_held);
  mutex* ref_mutex(int i);
  int size() const { return stop_ - start_; }
  Iterator begin() const { return Iterator(this, 0); }
  Iterator end() const { return Iterator(this, size()); }

 private:
  OpKernelContext* ctx_;  // not owned
  int start_;
  int stop_;
};

// Utility class for representing a list of output tensors that are
// grouped as a single named output.
class OpOutputList {
 public:
  typedef OpArgIterator<OpOutputList, const Tensor*> Iterator;
  OpOutputList() : ctx_(nullptr), start_(0), stop_(0) {}
  OpOutputList(OpKernelContext* ctx, int start, int stop)
      : ctx_(ctx), start_(start), stop_(stop) {}
  OpOutputList& operator=(const OpOutputList& other) = default;
  Tensor* operator[](int i);
  bool required(int i) const;
  Status allocate(int i, const TensorShape& shape, Tensor** output);
  void set(int i, const Tensor& tensor);
  void set_ref(int i, mutex* mu, Tensor* tensor_for_ref);
  int size() const { return stop_ - start_; }
  Iterator begin() const { return Iterator(this, 0); }
  Iterator end() const { return Iterator(this, size()); }

 private:
  OpKernelContext* ctx_;  // not owned
  int start_;
  int stop_;
};

// Holds a tensor or tensor reference. For tensor references, we need
// a mutex to prevent concurrent access to the tensor.
struct TensorValue {
  TensorValue() : mutex_if_ref(nullptr), tensor(nullptr) {}
  TensorValue(Tensor* t)  // NOLINT(runtime/explicit)
      : mutex_if_ref(nullptr), tensor(t) {}
  TensorValue(mutex* mu, Tensor* t) : mutex_if_ref(mu), tensor(t) {}
  Tensor* operator->() const { return tensor; }
  bool is_ref() const { return mutex_if_ref != nullptr; }

  mutex* mutex_if_ref;  // nullptr if not a ref, != nullptr if a ref
  Tensor* tensor;
};

class OpKernelContext {
 public:
  // The first element of a WrappedAllocator is a "base" Allocator and
  // the second element is that Allocator wrapped by a
  // TrackingAllocator
  typedef std::pair<Allocator*, TrackingAllocator*> WrappedAllocator;

  // TODO(zhifengc): Do some cleanup of Params.
  // The Params struct is passed in to initialize an OpKernelContext,
  // and must outlive the OpKernelContext.
  struct Params {
    ~Params() { delete eigen_gpu_device; }

    // The step being executed.
    int64 step_id = 0;

    // The op kernel being computed.
    OpKernel* op_kernel = nullptr;

    // The device on which the kernel is running.
    DeviceBase* device = nullptr;

    // The Eigen GPU device wrapper, which may include a per-op
    // wrapped allocator. The concrete type of this object depends on
    // the type of this->device, so eigen_gpu_device can't be an
    // inline member and must be heap allocated. However, we don't
    // want to allocate a new eigen_gpu_device for every Op that is
    // executed. Instead this member is allocated on first use using
    // ensure_eigen_gpu_device, and then if the Params structure is
    // re-used for subsequent Ops, the eigen_gpu_device is
    // ReInitialized in the OpKernelContext constructor. Unlike the
    // other pointers in Params, this one is owned by Params.
    PerOpGpuDevice* eigen_gpu_device = nullptr;

    inline void ensure_eigen_gpu_device() {
      DCHECK(device);
      if (nullptr == eigen_gpu_device) {
        // Surprisingly, MakeGpuDevice will return nullptr if the
        // device is not a GPU device. This is ok, since those devices
        // will never use eigen_gpu_device. It seems better to have
        // ensure_eigen_gpu_device fall through and regenerate the
        // nullptr every time an OpKernelContext is instantiated, than
        // to do an unnecessary allocation of a dummy eigen GPU
        // device for CPU device Ops.
        eigen_gpu_device = device->MakeGpuDevice();
      }
    }

    bool track_allocations = false;
    bool log_memory = false;
    bool record_tensor_accesses = false;

    // Array indexed by output number for this node
    const AllocatorAttributes* output_attr_array = nullptr;

    // Shared resources accessible by this op kernel invocation.
    ResourceMgr* resource_manager = nullptr;

    // Per-step resources accessible by this op kernel invocation should be
    // stored in this container..
    ScopedStepContainer* step_container = nullptr;

    // Mechanism used by this op kernel invocation to communicate with
    // computations running on other devices.
    Rendezvous* rendezvous = nullptr;

    // The session state for this op.
    SessionState* session_state = nullptr;

    // The tensor store for this op.
    TensorStore* tensor_store = nullptr;

    // Mechanism used by this op kernel invocation to register a callback
    // for its cancellation.
    CancellationManager* cancellation_manager = nullptr;

    // Inputs to this op kernel.
    const gtl::InlinedVector<TensorValue, 4>* inputs = nullptr;
    bool is_input_dead = false;

    const gtl::InlinedVector<AllocatorAttributes, 4>* input_alloc_attrs =
        nullptr;

    // Device contexts.
    const gtl::InlinedVector<DeviceContext*, 4>* input_device_contexts =
        nullptr;
    DeviceContext* op_device_context = nullptr;

    // Control-flow op supports.
    FrameAndIter frame_iter;

    // Function call supports.
    FunctionCallFrame* call_frame = nullptr;
    FunctionLibraryRuntime* function_library = nullptr;
    std::function<void(std::function<void()>)>* runner = nullptr;

    // TensorSliceReaderCache support.
    checkpoint::TensorSliceReaderCacheWrapper* slice_reader_cache = nullptr;
  };

  // params must outlive the OpKernelContext.
  explicit OpKernelContext(Params* params);
  OpKernelContext(Params* params, int noutputs);
  ~OpKernelContext();

  Env* env() const { return params_->device->env(); }

  int64 step_id() const { return params_->step_id; }

  const OpKernel& op_kernel() const { return *params_->op_kernel; }

  // Input/output signature.

  int num_inputs() const { return params_->inputs->size(); }
  DataType input_dtype(int index) const;
  Status input_dtype(StringPiece name, DataType* dtype) const;
  MemoryType input_memory_type(int index) const;

  int num_outputs() const { return outputs_.size(); }
  DataType expected_output_dtype(int index) const;
  MemoryType output_memory_type(int index) const;

  // Input

  // Returns an immutable input tensor. May only be used for non-Ref
  // inputs. For Ref inputs use mutable_input below.
  // REQUIRES: !IsRefType(input_dtype(index))
  // TODO(mrry): Convert this to return Status.
  const Tensor& input(int index);

  // Returns the named immutable input tensor in "tensor", as defined
  // in the OpDef. May only be used for non-Ref inputs. For Ref inputs
  // use mutable_input below.
  // REQUIRES: !IsRefType(input_dtype(index))
  // REQUIRES: the named input must not be a list.
  Status input(StringPiece name, const Tensor** tensor);

  // Returns the named list-valued immutable input in "list", as
  // defined in the OpDef.  If the named output is not list-valued,
  // returns a one-element list. May only be used for non-Ref
  // inputs. For Ref inputs use mutable_input below.
  // REQUIRES: !IsRefType(input_dtype(index))
  Status input_list(StringPiece name, OpInputList* list);

  // For mutable inputs, use the following together to make sure there
  // is no concurrent access to mutable_input(), e.g.:
  // {
  //   Tensor& t = context->mutable_input(index);
  //   mutex_lock lock(*context->input_ref_mutex(index));
  //   // modify the values in t
  // }
  // REQUIRES: IsRefType(input_dtype(index))
  Status input_ref_mutex(StringPiece name, mutex** out_mutex);

  // Returns a mutable input tensor. Must be used to access Ref
  // inputs.  REQUIRES: IsRefType(input_dtype(index)). The caller may
  // modify the values stored in the Tensor buffer, and modifications
  // will be visible to other Ops reading the same ref tensor. If
  // !lock_held the input mutex will be acquired before returning the
  // Tensor.
  // TODO(mrry): Convert this to return Status.
  Tensor mutable_input(int index, bool lock_held);

  // Returns the named mutable input tensor in "tensor", as defined in
  // the OpDef. Must be used to access Ref inputs. The values stored
  // in the Tensor buffer may be modified, and modifications will be
  // visible to other Ops reading the same ref tensor. If !lock_held
  // the input mutex will be acquired before returning the Tensor.
  // REQUIRES: the named input must not be a list.
  // REQUIRES: the named input must be a ref tensor.
  Status mutable_input(StringPiece name, Tensor* tensor, bool lock_held);

  // Returns the named list-valued mutable input in "list", as defined
  // in the OpDef.  If the named input is not list-valued, returns a
  // one-element list. Must be used to access Ref inputs. The values
  // stored in the Tensor buffer may be modified, and modifications
  // will be visible to other Ops reading the same ref tensor.
  // REQUIRES: the named input must be a ref tensor.
  Status mutable_input_list(StringPiece name, OpMutableInputList* list);

  // Replace the corresponding Ref Input to use the storage buffer
  // used by tensor. If !lock_held the input mutex will be acquired
  // before returning the Tensor.
  // REQUIRES: IsRefType(input_dtype(index)).
  void replace_ref_input(int index, const Tensor& tensor, bool lock_held);

  // Replace the corresponding named Ref Input to use the storage
  // buffer used by tensor. If !lock_held the input mutex will be
  // acquired before returning the Tensor.
  // REQUIRES: IsRefType(input_dtype(index)).
  Status replace_ref_input(StringPiece name, const Tensor& tensor,
                           bool lock_held);

  // Deletes the Tensor object used as the Ref Input at
  // input_index. This is not usually necessary and should be used
  // with caution. If !lock_held the input mutex will be acquired
  // before returning the Tensor.
  // REQUIRES: IsRefType(input_dtype(input_index)).
  void delete_ref_input(int input_index, bool lock_held);

  // Return true if there is input at the given index. An operator has no
  // input at index if its tensor is null. This is primarily used by the
  // merge operator.
  // TODO(mrry): Convert this to return Status.
  bool has_input(int index) const;

  // Returns true if all inputs are the same shape, otherwise sets the
  // status to a non-OK value and returns false.
  // Usage: if (!context->ValidateInputsAreSameShape(this)) return;
  bool ValidateInputsAreSameShape(OpKernel* op);

  // Input to output forwarding.

  // Set the output Ref Tensor at output_index to be an alias of the
  // input Ref Tensor at input_index.
  // REQUIRES: IsRefType(input_dtype(input_index)).
  // REQUIRES: IsRefType(output_dtype(output_index)).
  void forward_ref_input_to_ref_output(int input_index, int output_index);

  // Returns true when an alias to input[input_index], reshaped to output_shape,
  // which is is safe to use for in-place computation was written to *output.
  // Returns false if input[input_index] has a refcount greater than one, or if
  // its type does not match the expected output type of output[output_index],
  // or the number of elements in input[input_index] does not equal the number
  // of elements in output_shape.
  bool forward_input_to_output_with_shape(int input_index, int output_index,
                                          const TensorShape& output_shape,
                                          Tensor** output) TF_MUST_USE_RESULT;
  Status forward_input_to_output_with_shape(StringPiece input_name,
                                            StringPiece output_name,
                                            const TensorShape& output_shape,
                                            Tensor** output) TF_MUST_USE_RESULT;

  // Returns a pointer to a Tensor aliasing the underlying buffer backing
  // input[input_index] iff
  //   * input[input_index] is not a ref,
  //   * the data type, shape, memory type, and allocator attributes of
  //     input[input_index] are compatible with those given in dtype, shape,
  //     memory_type, and attr,
  //   * refcount on the underlying buffer is one.
  // Otherwise returns nullptr.
  // NOTE: For Cuda kernels that read inputs using the __ldg() intrinsic,
  // forwarding is only safe if there are no reads via __ldg() after writes
  // to the same address.
  std::unique_ptr<Tensor> forward_input(
      int input_index, DataType dtype, const TensorShape& shape,
      MemoryType memory_type,
      const AllocatorAttributes& attr) TF_MUST_USE_RESULT;

  // Tries to forward one of the inputs given in input_indices to
  // output[output_index]. If none of the given inputs can be forwarded, calls
  // allocate_output() to allocate a new output buffer.
  Status forward_input_or_allocate_output(
      gtl::ArraySlice<int> candidate_input_indices, int output_index,
      const TensorShape& output_shape, Tensor** output) TF_MUST_USE_RESULT;
  Status forward_input_or_allocate_output(
      gtl::ArraySlice<StringPiece> candidate_input_names,
      StringPiece output_name, const TensorShape& output_shape,
      Tensor** output) TF_MUST_USE_RESULT;

  // Tries to reuse one of of the inputs given in input_indices as a temporary.
  // If none of the given inputs can be forwarded, calls
  // allocate_temp() to allocate a new temporary buffer.
  Status forward_input_or_allocate_temp(
      gtl::ArraySlice<int> candidate_input_indices, DataType type,
      const TensorShape& shape, const AllocatorAttributes& allocator_attr,
      Tensor* out_temp) TF_MUST_USE_RESULT;

  Status forward_input_or_allocate_temp(
      gtl::ArraySlice<int> candidate_input_indices, DataType type,
      const TensorShape& shape, Tensor* out_temp) TF_MUST_USE_RESULT {
    return forward_input_or_allocate_temp(candidate_input_indices, type, shape,
                                          AllocatorAttributes(), out_temp);
  }

  // Output

  // Returns the named list-valued output in "list", as defined in the OpDef.
  // If the named output is not list-valued, returns a one-element list.
  Status output_list(StringPiece name, OpOutputList* list);

  // If output_required(index) returns true, the OpKernel's Compute() method
  // should call allocate_output(index, ...), set_output(index, ...),
  // set_output_ref(index, ...), or set the status to a non-ok value.
  // If it returns false, it may output, but is not required to do so.
  // TODO(mrry): Convert this to return Status, and implement a string
  // name version.
  bool output_required(int index) const {
    return true;  // TODO(josh11b): implement
  }

  // Allocation of tensors during kernel execution inside the Compute
  // method:
  //
  // There are three methods to allocate Tensors when an Op kernel
  // executes.
  //
  // 1) allocate_persistent. This is only needed for Tensors that will
  // be stored by the Op between invocations, and it *must* be used
  // for those Tensors. The call returns a PersistentTensor, and that
  // is the only object the Op is allowed to hold on to between
  // invocations. When the Tensor is needed in a subsequent
  // invocation, it can be retrieved from the PersistentTensor using
  // the AccessTensor method. This ensures that the system is made
  // aware of any use of the tensor's allocated memory, which is
  // needed for correctness on asynchronous devices such as GPUs.
  //
  // 2) allocate_output. This should be used to allocate any tensor
  // that is going to be used as an output from the Op at the end of
  // the current execution. The caller indicates which output the
  // Tensor will be assigned to, and the call returns the
  // newly-allocated Tensor. The Tensor can subsequently be assigned
  // to during kernel execution, and will be used as the designated
  // output when the kernel execution completes.
  //
  // 3) allocate_temp. This should be used to allocate any scratch
  // storage that is needed while the kernel is executing, and will
  // not be retained by the Op.
  //
  // In some cases a Tensor needs to be used as an output even though
  // it was previously allocated elsewhere. The Tensor may have been
  // passed as an input, or stored in a PersistentTensor during a
  // previous kernel execution, or allocated earlier in the kernel
  // execution at a time when it was not known which output it would
  // be assigned to. In this case the kernel can use set_output or
  // set_output_ref to indicate that the tensor should be used as the
  // designated output. It is legal to use any previously-allocated
  // Tensor as an argument to set_output or set_output_ref, including
  // Tensors allocated via allocate_temp. There may be a performance
  // penalty to using a Tensor that was not allocated using
  // allocate_output. This is because allocate_output uses the
  // AllocatorAttributes stored in output_attr_array for the
  // designated output. In some cases, using the wrong attributes may
  // cause an extra copy of the Tensor's buffer.

  // Allocates output for the specified output index with shape.
  // OpKernelContext retains ownership of the returned pointer. See
  // comment above.
  //
  // If memory allocation fails, returns an error status.
  //
  // REQUIRES: !IsRefType(expected_output_dtype(index))
  Status allocate_output(int index, const TensorShape& shape,
                         Tensor** tensor) TF_MUST_USE_RESULT;
  Status allocate_output(StringPiece name, const TensorShape& shape,
                         Tensor** tensor) TF_MUST_USE_RESULT;
  // The following methods use the supplied attributes instead of
  // those in output_attr_array. The caller is responsible for
  // ensuring that the attributes are "compatible" with the
  // output_attr_array, e.g. the tensor is allocated on the correct
  // device. See comment above.
  Status allocate_output(int index, const TensorShape& shape, Tensor** tensor,
                         AllocatorAttributes attr) TF_MUST_USE_RESULT;
  Status allocate_output(StringPiece name, const TensorShape& shape,
                         Tensor** tensor,
                         AllocatorAttributes attr) TF_MUST_USE_RESULT;

  // Allocates a temporary Tensor of the specified type and
  // shape. Devices such as GPUs that enqueue Ops for lazy execution
  // may retain references to the temporary tensors after the Op's
  // Compute method has run. See comment above.
  Status allocate_temp(DataType type, const TensorShape& shape,
                       Tensor* out_temp, AllocatorAttributes allocator_attr,
                       const AllocationAttributes& allocation_attr);
  Status allocate_temp(DataType type, const TensorShape& shape,
                       Tensor* out_temp, AllocatorAttributes allocator_attr) {
    return allocate_temp(type, shape, out_temp, allocator_attr,
                         AllocationAttributes());
  }
  Status allocate_temp(DataType type, const TensorShape& shape,
                       Tensor* out_temp) {
    return allocate_temp(type, shape, out_temp, AllocatorAttributes());
  }

  // Allocates a Tensor of the specified type and shape which the Op
  // plans to maintain as persistent state. out_persistent holds the
  // PersistentTensor which is the object the caller should store. For
  // convenience, if out_tensor is non-null then it will be filled in
  // with a Tensor* pointing to the newly-allocated tensor which the
  // caller can use instead of calling
  // out_persistent->AccessTensor. The caller does not own out_tensor
  // and should not keep a copy of it. See comment above.
  Status allocate_persistent(DataType type, const TensorShape& shape,
                             PersistentTensor* out_persistent,
                             Tensor** out_tensor, AllocatorAttributes attr);
  Status allocate_persistent(DataType type, const TensorShape& shape,
                             PersistentTensor* out_persistent,
                             Tensor** out_tensor) {
    return allocate_persistent(type, shape, out_persistent, out_tensor,
                               AllocatorAttributes());
  }

  // Copies a tensor (allocated by the caller) to the specified output
  // index.  REQUIRES: !IsRefType(expected_output_dtype(index))
  // REQUIRES: 'tensor' must have the same MemoryType as
  // output_memory_types[index]. See comment above.
  Status set_output(StringPiece name, const Tensor& tensor);

  // To output a reference.  Caller retains ownership of mu and tensor_for_ref,
  // and they must outlive all uses within the step. See comment above.
  // REQUIRES: IsRefType(expected_output_dtype(index))
  Status set_output_ref(StringPiece name, mutex* mu, Tensor* tensor_for_ref);

  // Returns nullptr if allocate_output() or set_output() have not been called.
  Status mutable_output(StringPiece name, Tensor** tensor);

  // Transfers ownership of an output tensor to the caller.
  // NOTE: For non-reference outputs, the caller takes responsibility
  // for deletion. For reference outputs, the caller does NOT take
  // responsibility for deletion.
  Status release_output(StringPiece name, TensorValue* value);

  // Records device specific state about how the input tensors were
  // computed.
  //
  // If using the templated function, the type must be a subclass
  // of DeviceContext.
  //
  // Get the DeviceContext used for the index input.  Returns nullptr
  // if no DeviceContext was provided.
  template <typename T>
  T* input_device_context(int index);
  DeviceContext* input_device_context(int index);

  // Return the DeviceContext that should be used for this Op.
  //
  // If using the templated function, the type must be a subclass
  // of DeviceContext.
  //
  // Returns nullptr if the device did not provide one.
  template <typename T>
  T* op_device_context();
  DeviceContext* op_device_context() {
    DeviceContext* ret = params_->op_device_context;
    if (ret == nullptr) {
      auto* dev_info = device()->tensorflow_gpu_device_info();
      if (dev_info) ret = dev_info->default_context;
    }
    return ret;
  }

  AllocatorAttributes input_alloc_attr(int index) const {
    DCHECK_GE(index, 0);
    DCHECK_LT(index, params_->input_alloc_attrs->size());
    return (*params_->input_alloc_attrs)[index];
  }

  AllocatorAttributes output_alloc_attr(int index) const {
    return params_->output_attr_array[index];
  }

  gtl::InlinedVector<WrappedAllocator, 4> wrapped_allocators() const {
    mutex_lock lock(mu_);
    gtl::InlinedVector<WrappedAllocator, 4> retrieved = wrapped_allocators_;
    return retrieved;
  }

  // Communication.
  //
  // An op kernel communicates with outside environment through
  // Rendezvous Send() and Recv().
  Rendezvous* rendezvous() const { return params_->rendezvous; }

  // An op kernel can access the session state it belongs to.
  SessionState* session_state() const { return params_->session_state; }

  // An op kernel can access the tensor store of the run it belongs to.
  TensorStore* tensor_store() const { return params_->tensor_store; }

  // Function call support.
  //
  // If this kernel invocation is within a function execution,
  // call_frame() returns the call frame for the function call.
  FunctionCallFrame* call_frame() const { return params_->call_frame; }

  // If not nullptr, the kernel invoke functions defined in the
  // library. E.g., CHECK_NOTNULL(function_library())->Run("Foo", ...).
  FunctionLibraryRuntime* function_library() const {
    return params_->function_library;
  }

  std::function<void(std::function<void()>)>* runner() const {
    return params_->runner;
  }

  // Shared resources accessible to this kernel.
  ResourceMgr* resource_manager() const { return params_->resource_manager; }

  checkpoint::TensorSliceReaderCacheWrapper* slice_reader_cache() const {
    return params_->slice_reader_cache;
  }

  // Execution.
  //
  // OpKernels can use these eigen devices to carry out their
  // numerical computation.
  const Eigen::ThreadPoolDevice& eigen_cpu_device() const {
    return *device()->eigen_cpu_device();
  }
  const Eigen::GpuDevice& eigen_gpu_device() const {
    return params_->eigen_gpu_device->device();
  }
#ifdef TENSORFLOW_USE_SYCL
  const Eigen::SyclDevice& eigen_sycl_device() const {
    return *device()->eigen_sycl_device();
  }
#endif
  template <typename EigenDeviceType>
  const EigenDeviceType& eigen_device() const;

  // Error handling.

  // If expected_inputs == inputs() and expected_outputs == output_types(),
  // returns OK, else returns INVALID_ARGUMENT with an error message.
  // Recommended for Ops with dynamic signatures, where validation can only
  // be performed at runtime.
  Status MatchSignature(const DataTypeSlice expected_inputs,
                        const DataTypeSlice expected_outputs);

  // An OpKernel should call SetStatus() if Compute() encounters an
  // error.
  void SetStatus(const Status& status);
  const Status& status() const { return status_; }

  // Cancellation.
  //
  // EXPERIMENTAL. See the implementation in tensorflow::TensorQueue for an
  // example of how to use this API.
  CancellationManager* cancellation_manager() const {
    return params_->cancellation_manager;
  }

  // Other accessors.

  // For control flow.
  FrameAndIter frame_iter() const { return params_->frame_iter; }
  bool is_input_dead() const { return params_->is_input_dead; }
  bool* is_output_dead() { return &is_output_dead_; }

  // May be used, e.g., to get GPU handles, etc.
  // TODO(tucker): Add example usage.
  DeviceBase* device() const { return params_->device; }

  // Retrieve list of referenced tensors in out_vector. Once this is
  // called, it is not legal to reference any more tensors.  Should
  // not be called from Op kernels.
  void retrieve_accessed_tensors(TensorReferenceVector* out_vector);

  // Per-step container for use by white-listed internal ops.
  ScopedStepContainer* step_container() const {
    return params_->step_container;
  }

  // Helper routines for the OP_REQUIRES macros
  void CtxFailure(Status s);
  void CtxFailureWithWarning(Status s);

  // Unrecommended functions: these are functions that have some
  // current uses but are not recommended for use, and may go away at
  // some future major version release.
  //
  // The following functions all have versions that return Status
  // to capture error conditions, and are strongly preferred.
  Tensor* mutable_output(int index);
  void set_output(int index, const Tensor& tensor);
  mutex* input_ref_mutex(int index);
  void set_output_ref(int index, mutex* mu, Tensor* tensor_for_ref);
  TensorValue release_output(int index);

  bool track_allocations() const { return params_->track_allocations; }
  bool allocate_on_host(AllocatorAttributes alloc_attr) const;

  // Records temporary memory sizes.
  void record_host_temp_memory_size(int64 size) {
    host_temp_memory_size_ += size;
  }
  void record_device_temp_memory_size(int64 size) {
    device_temp_memory_size_ += size;
  }

  // Returns recorded size of temporary memory;
  int64 host_temp_memory_size() const { return host_temp_memory_size_; }
  int64 device_temp_memory_size() const { return device_temp_memory_size_; }

  // Records persistent memory allocation, size can be negative indicating
  // deallocation.
  void record_host_persistent_memory_allocation(int64 size,
                                                int64 alloc_id = -1);
  void record_device_persistent_memory_allocation(int64 size,
                                                  int64 alloc_id = -1);

  // Returns recorded size and ids of persistent memory.
  int64 host_persistent_memory_allocated() const {
    return host_persistent_memory_allocated_;
  }
  int64 device_persistent_memory_allocated() const {
    return device_persistent_memory_allocated_;
  }
  std::vector<int64> host_persistent_alloc_ids() const;
  std::vector<int64> device_persistent_alloc_ids() const;

 private:
  bool input_is_ref(int index) const;

  Allocator* get_allocator(AllocatorAttributes attr);

  // Internal method to add a tensor's buffer to the list of buffers
  // referenced during the execution of the Op, so that GPUs may
  // accurately track the memory that may not be reused until the Op
  // execution completes.
  void record_tensor_reference(const Tensor& tensor);
  void really_record_tensor_reference(const Tensor& tensor);

  // Internal common method used when allocating tensor memory
  Status allocate_tensor(DataType type, const TensorShape& shape,
                         Tensor* out_tensor,
                         AllocatorAttributes allocator_attr) {
    return allocate_tensor(type, shape, out_tensor, allocator_attr,
                           AllocationAttributes());
  }

  Status allocate_tensor(DataType type, const TensorShape& shape,
                         Tensor* out_tensor, AllocatorAttributes allocator_attr,
                         const AllocationAttributes& allocation_attr);

  // This is called by PersistentTensor::AccessTensor whenever the
  // wrapped tensor is retrieved, to ensure the runtime knows that the
  // Tensor is being accessed within an Op. This is necessary for
  // memory safety of devices like GPUs that queue Ops for
  // asynchronous execution after the Compute() method completes.
  friend class PersistentTensor;
  void NotifyUseOfPersistentTensor(const Tensor& tensor);

  Status status_;
  Params* params_;    // not owned
  mutable mutex mu_;  // mutable so const accessors can acquire the lock
  gtl::InlinedVector<WrappedAllocator, 4> wrapped_allocators_ GUARDED_BY(mu_);
  gtl::InlinedVector<TensorValue, 4> outputs_;

  // Constructed only if <params->record_tensor_accesses>.
  ManualConstructor<UniqueTensorReferences> referenced_tensors_ GUARDED_BY(mu_);

  bool is_output_dead_ = false;

  int64 host_temp_memory_size_;
  int64 device_temp_memory_size_;
  gtl::InlinedVector<int64, 2> host_persistent_alloc_ids_;
  gtl::InlinedVector<int64, 2> device_persistent_alloc_ids_;
  int64 host_persistent_memory_allocated_;
  int64 device_persistent_memory_allocated_;

  TF_DISALLOW_COPY_AND_ASSIGN(OpKernelContext);
};

// Register your OpKernel by specifying the Op's name, the device the
// kernel runs on, any type attr constraints for this kernel, any
// host-memory args, and the class to instantiate.  Examples:
//
//  // A kernel that supports all types.
//  REGISTER_KERNEL_BUILDER(Name("Save").Device(DEVICE_CPU), SaveOp);
//
//  // The following are equivalent ways of specifying that the kernel only
//  // works if the "T" type attr is set to DT_FLOAT.
//  REGISTER_KERNEL_BUILDER(
//      Name("Sub").Device(DEVICE_CPU).TypeConstraint<float>("T"),
//      SubOp<float>);
//  // (You would then repeat this for every type supported by "Sub".)
//
//  // This form allows you to specify a list of types as the constraint.
//  REGISTER_KERNEL_BUILDER(Name("Sub")
//                              .Device(DEVICE_CPU)
//                              .TypeConstraint("T", {DT_FLOAT}),
//                          SubOp<float>);
//
//  // A kernel that expects one of the input tensors in host memory.
//  REGISTER_KERNEL_BUILDER(
//      Name("Reshape").Device(DEVICE_GPU).HostMemory("shape"), ReshapeOp);
//
// See kernel_def_builder for details.

// Instantiate an OpKernel that has been registered.  Returns nullptr
// if no operation for that type of device / input signature combination
// (and a NOT_FOUND *status), or there is an error in construction (and
// an INVALID_ARGUMENT *status).  Otherwise, the caller takes ownership
// of the returned pointer.
// EXPECTED USAGE: unique_ptr<OpKernel> op = CreateOpKernel(...);
// REQUIRES: def has all attrs specified (e.g. using AddDefaultsToNodeDef()).
std::unique_ptr<OpKernel> CreateOpKernel(DeviceType device_type,
                                         DeviceBase* device,
                                         Allocator* allocator,
                                         const NodeDef& def,
                                         int graph_def_version, Status* status);
Status CreateOpKernel(DeviceType device_type, DeviceBase* device,
                      Allocator* allocator, FunctionLibraryRuntime* flib,
                      const NodeDef& def, int graph_def_version,
                      OpKernel** kernel);

// Returns into 'device_types' the subset of prioritized_types that this
// binary has registered for the given NodeDef.
//
// REQUIRES: * 'device_types' is not nullptr.
//           * def has all attrs specified (e.g. using AddDefaultsToNodeDef()).
Status SupportedDeviceTypesForNode(
    const std::vector<DeviceType>& prioritized_types, const NodeDef& def,
    DeviceTypeVector* device_types);

// Returns a message with a description of the kernels registered for op
// `op_name`.
string KernelsRegisteredForOp(StringPiece op_name);

// Call once after Op registration has completed.
Status ValidateKernelRegistrations(const OpRegistryInterface& op_registry);

// -----------------------------------------------------------------------------
// OpKernel registration implementation follows, please ignore.

// Allow the REGISTER_KERNEL_BUILDER(Name("op_name").Device(...)...) syntax.
namespace register_kernel {

class Name : public KernelDefBuilder {
 public:
  // With selective registration, kernels whose implementation class is not used
  // by any kernel are disabled with the SHOULD_REGISTER_OP_KERNEL call in
  // REGISTER_KERNEL_BUILDER_UNIQ. However, an unused kernel that shares an
  // implementation class with a used kernel would get through that mechanism.
  //
  // This mechanism stops that registration by changing the name of the kernel
  // for the unused op to one that is ignored by
  // OpKernelRegistrar::InitInternal.  Note that this method alone is
  // not sufficient - the compiler can't evaluate the entire KernelDefBuilder at
  // compilation time, so this method doesn't actually reduce code size.
  explicit Name(const char* op)
      : KernelDefBuilder(SHOULD_REGISTER_OP(op) ? op : "_no_register") {}
};

}  // namespace register_kernel

#define REGISTER_KERNEL_BUILDER(kernel_builder, ...) \
  REGISTER_KERNEL_BUILDER_UNIQ_HELPER(__COUNTER__, kernel_builder, __VA_ARGS__)

#define REGISTER_KERNEL_BUILDER_UNIQ_HELPER(ctr, kernel_builder, ...) \
  REGISTER_KERNEL_BUILDER_UNIQ(ctr, kernel_builder, __VA_ARGS__)

#define REGISTER_KERNEL_BUILDER_UNIQ(ctr, kernel_builder, ...)        \
  constexpr bool should_register_##ctr##__flag =                      \
      SHOULD_REGISTER_OP_KERNEL(#__VA_ARGS__);                        \
  static ::tensorflow::kernel_factory::OpKernelRegistrar              \
      registrar__body__##ctr##__object(                               \
          should_register_##ctr##__flag                               \
              ? ::tensorflow::register_kernel::kernel_builder.Build() \
              : nullptr,                                              \
          #__VA_ARGS__,                                               \
          [](::tensorflow::OpKernelConstruction* context)             \
              -> ::tensorflow::OpKernel* {                            \
            return new __VA_ARGS__(context);                          \
          });

void* GlobalKernelRegistry();

// If node_def has a corresponding kernel registered on device_type,
// returns OK and fill in the kernel def and kernel_class_name. <def> and
// <kernel_class_name> may be null.
Status FindKernelDef(DeviceType device_type, const NodeDef& node_def,
                     const KernelDef** def, string* kernel_class_name);

// Writes a list of all registered kernels to LOG(INFO), to help users debug
// missing kernel errors.
void LogAllRegisteredKernels();

namespace kernel_factory {

class OpKernelRegistrar {
 public:
  typedef OpKernel* (*Factory)(OpKernelConstruction*);

  OpKernelRegistrar(const KernelDef* kernel_def, StringPiece kernel_class_name,
                    Factory factory) {
    // Perform the check in the header to allow compile-time optimization
    // to a no-op, allowing the linker to remove the kernel symbols.
    if (kernel_def != nullptr) {
      InitInternal(kernel_def, kernel_class_name, factory);
    }
  }

 private:
  void InitInternal(const KernelDef* kernel_def, StringPiece kernel_class_name,
                    Factory factory);
};

}  // namespace kernel_factory

// -----------------------------------------------------------------------------
// Template and inline method implementations, please ignore

template <class T>
Status OpKernelConstruction::GetAttr(StringPiece attr_name, T* value) const {
  return GetNodeAttr(def(), attr_name, value);
}

inline DataType OpKernelContext::input_dtype(int index) const {
  DCHECK_GE(index, 0);
  DCHECK_LT(index, num_inputs());
  const TensorValue& value((*params_->inputs)[index]);
  if (value.is_ref()) {
    return MakeRefType(value->dtype());
  } else {
    return value->dtype();
  }
}

inline MemoryType OpKernelContext::input_memory_type(int index) const {
  DCHECK_GE(index, 0);
  DCHECK_LT(index, num_inputs());
  return op_kernel().input_memory_types()[index];
}

inline DataType OpKernelContext::expected_output_dtype(int index) const {
  DCHECK_GE(index, 0);
  DCHECK_LT(index, num_outputs());
  return params_->op_kernel->output_type(index);
}

inline MemoryType OpKernelContext::output_memory_type(int index) const {
  DCHECK_GE(index, 0);
  DCHECK_LT(index, num_outputs());
  return op_kernel().output_memory_types()[index];
}

inline bool OpKernelContext::input_is_ref(int index) const {
  return IsRefType(input_dtype(index));
}

inline void OpKernelContext::record_tensor_reference(const Tensor& tensor) {
  DCHECK_EQ(params_->device->RequiresRecordingAccessedTensors(),
            params_->record_tensor_accesses);
  if (params_->record_tensor_accesses) {
    really_record_tensor_reference(tensor);
  }
}

inline void OpKernelContext::retrieve_accessed_tensors(
    TensorReferenceVector* out_vector) {
  if (params_->record_tensor_accesses) {
    mutex_lock l(mu_);
    referenced_tensors_->FreezeAndReturnReferences(out_vector);
  }
}

// no input if tensor == nullptr.
inline bool OpKernelContext::has_input(int index) const {
  DCHECK_GE(index, 0);
  DCHECK_LT(index, num_inputs());
  return (*params_->inputs)[index].tensor != nullptr;
}

inline mutex* OpKernelContext::input_ref_mutex(int index) {
  DCHECK_GE(index, 0);
  DCHECK_LT(index, num_inputs());
  DCHECK(input_is_ref(index));
  return (*params_->inputs)[index].mutex_if_ref;
}

inline void OpKernelContext::NotifyUseOfPersistentTensor(const Tensor& t) {
  if (t.IsInitialized()) {
    record_tensor_reference(t);
  }
}

inline Tensor* OpKernelContext::mutable_output(int index) {
  DCHECK_GE(index, 0);
  DCHECK_LT(index, num_outputs());
  // No need to record_tensor_reference since the output must already
  // have been set by a call that did so.
  return outputs_[index].tensor;
}

inline TensorValue OpKernelContext::release_output(int index) {
  DCHECK_GE(index, 0);
  DCHECK_LT(index, num_outputs());
  TensorValue value = outputs_[index];
  outputs_[index] = TensorValue();
  return value;
}

inline Status OpKernelContext::forward_input_or_allocate_output(
    gtl::ArraySlice<int> candidate_input_indices, int output_index,
    const TensorShape& output_shape, Tensor** output) {
  for (int input_index : candidate_input_indices) {
    if (forward_input_to_output_with_shape(input_index, output_index,
                                           output_shape, output)) {
      return Status::OK();
    }
  }
  return allocate_output(output_index, output_shape, output);
}

inline Status OpKernelContext::forward_input_or_allocate_output(
    gtl::ArraySlice<StringPiece> candidate_input_names, StringPiece output_name,
    const TensorShape& output_shape, Tensor** output) {
  for (const StringPiece& input_name : candidate_input_names) {
    if (forward_input_to_output_with_shape(input_name, output_name,
                                           output_shape, output)
            .ok()) {
      return Status::OK();
    }
  }
  return allocate_output(output_name, output_shape, output);
}

template <typename T>
T* OpKernelContext::op_device_context() {
  static_assert(std::is_base_of<DeviceContext, T>::value,
                "T is not a subclass of DeviceContext");
  return static_cast<T*>(op_device_context());
}

template <typename T>
T* OpKernelContext::input_device_context(int index) {
  DCHECK_GE(index, 0);
  DCHECK_LT(index, params_->input_device_contexts->size());
  static_assert(std::is_base_of<DeviceContext, T>::value,
                "T is not a subclass of DeviceContext");
  return static_cast<T*>((*params_->input_device_contexts)[index]);
}

inline DeviceContext* OpKernelContext::input_device_context(int index) {
  DCHECK_GE(index, 0);
  DCHECK_LT(index, params_->input_device_contexts->size());
  return (*params_->input_device_contexts)[index];
}

inline const Tensor& OpInputList::operator[](int i) const {
  DCHECK_GE(i, 0);
  DCHECK_LT(i, stop_ - start_);
  return ctx_->input(start_ + i);
}

inline mutex* OpMutableInputList::ref_mutex(int i) {
  DCHECK_GE(i, 0);
  DCHECK_LT(i, stop_ - start_);
  return ctx_->input_ref_mutex(start_ + i);
}

inline Tensor OpMutableInputList::at(int i, bool lock_held) {
  DCHECK_GE(i, 0);
  DCHECK_LT(i, stop_ - start_);
  return ctx_->mutable_input(start_ + i, lock_held);
}

inline Tensor* OpOutputList::operator[](int i) {
  DCHECK_GE(i, 0);
  DCHECK_LT(i, stop_ - start_);
  return ctx_->mutable_output(start_ + i);
}

inline bool OpOutputList::required(int i) const {
  DCHECK_GE(i, 0);
  DCHECK_LT(i, stop_ - start_);
  return ctx_->output_required(start_ + i);
}

inline Status OpOutputList::allocate(int i, const TensorShape& shape,
                                     Tensor** output) {
  DCHECK_GE(i, 0);
  DCHECK_LT(i, stop_ - start_);
  return ctx_->allocate_output(start_ + i, shape, output);
}

inline void OpOutputList::set(int i, const Tensor& tensor) {
  DCHECK_GE(i, 0);
  DCHECK_LT(i, stop_ - start_);
  ctx_->set_output(start_ + i, tensor);
}

inline void OpOutputList::set_ref(int i, mutex* mu, Tensor* tensor_for_ref) {
  DCHECK_GE(i, 0);
  DCHECK_LT(i, stop_ - start_);
  ctx_->set_output_ref(i, mu, tensor_for_ref);
}

// Convenience macros for asserting and handling exceptional conditions.
// Analogous to the CHECK* macros provided by logging.h.
//
// Example use:
// void Compute(OperationContext* context) {
//   OP_REQUIRES(context, context->num_inputs() == 2,
//               errors::InvalidArgument("FooOp requires 2 arguments"));
//   ...
//   Status status = SomeUncertainMethod();
//   OP_REQUIRES_OK(context, status);
//   ...
// }

#define OP_REQUIRES(CTX, EXP, STATUS) \
  if (!TF_PREDICT_TRUE(EXP)) {        \
    (CTX)->CtxFailure((STATUS));      \
    return;                           \
  }

#define OP_REQUIRES_OK(CTX, STATUS)     \
  do {                                  \
    ::tensorflow::Status _s(STATUS);    \
    if (!TF_PREDICT_TRUE(_s.ok())) {    \
      (CTX)->CtxFailureWithWarning(_s); \
      return;                           \
    }                                   \
  } while (0)

#define OP_REQUIRES_ASYNC(CTX, EXP, STATUS, CALLBACK) \
  if (!TF_PREDICT_TRUE(EXP)) {                        \
    (CTX)->CtxFailure((STATUS));                      \
    (CALLBACK)();                                     \
    return;                                           \
  }

#define OP_REQUIRES_OK_ASYNC(CTX, STATUS, CALLBACK) \
  do {                                              \
    ::tensorflow::Status _s(STATUS);                \
    if (!TF_PREDICT_TRUE(_s.ok())) {                \
      (CTX)->CtxFailureWithWarning(_s);             \
      (CALLBACK)();                                 \
      return;                                       \
    }                                               \
  } while (0)

}  // namespace tensorflow

#endif  // TENSORFLOW_FRAMEWORK_OP_KERNEL_H_