Add COWPtr and its unittest

It will be used for LoD information in LoDTensor since LoD is a copy
on write field.

It is pretty slow for copying LoD information between operators. For
resnet it will cost roughly 10% time of whole time, including reading
data.

paddle/framework/CMakeLists.txt

@@ -78,3 +78,4 @@ cc_library(init SRCS init.cc DEPS gflags device_context place stringpiece)
 cc_test(init_test SRCS init_test.cc DEPS init)
 
 cc_test(op_kernel_type_test SRCS op_kernel_type_test.cc DEPS place device_context framework_proto)
+cc_test(cow_ptr_tests SRCS details/cow_ptr_test.cc)

paddle/framework/details/cow_ptr.h

+/* Copyright (c) 2016 PaddlePaddle 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. */
+
+#pragma once
+#include <memory>
+#include <thread>
+
+namespace paddle {
+namespace framework {
+namespace details {
+
+// Change it to thread safe flags if needed.
+class ThreadUnsafeOwnershipFlags {
+ public:
+  ThreadUnsafeOwnershipFlags(bool flag) : flag_(flag) {}
+
+  ThreadUnsafeOwnershipFlags(const ThreadUnsafeOwnershipFlags& o) = delete;
+  ThreadUnsafeOwnershipFlags& operator=(const ThreadUnsafeOwnershipFlags& o) =
+      delete;
+  ThreadUnsafeOwnershipFlags(ThreadUnsafeOwnershipFlags&& o) = default;
+
+  void SetOwnership(bool flag) { flag_ = flag; }
+
+  template <typename Callback>
+  void AcquireOwnershipOnce(Callback acquire) {
+    if (!flag_) {
+      acquire();
+      flag_ = true;
+    }
+  }
+
+ private:
+  bool flag_;
+};
+
+// Copy On Write pointer.
+// It will hold a T* pointer, and only copy once when `MutableData` is invoked.
+//
+// The template parameter OwnershipFlags should have:
+//   * a constructor takes a bool. True if own.
+//   * SetOwnership(bool flag).
+//   * AcquireOwnershipOnce(Callback). It will invoke the callback if it is not
+//     owned.
+template <typename T, typename OwnershipFlags = ThreadUnsafeOwnershipFlags>
+class COWPtr {
+ public:
+  // Ctor from raw pointer.
+  explicit COWPtr(T* ptr) : payload_(ptr), ownership_{true} {}
+
+  // Move methods. Steal ownership from origin
+  COWPtr(COWPtr&& o)
+      : payload_(o.payload_), ownership_{std::move(o.ownership_)} {}
+  COWPtr& operator=(COWPtr&& origin) = default;
+
+  // Copy methods. Not own payload
+  COWPtr(const COWPtr& o) : payload_(o.payload_), ownership_{false} {}
+  COWPtr& operator=(const COWPtr& o) {
+    payload_ = o.payload_;
+    ownership_.SetOwnership(false);
+    return *this;
+  }
+
+  const T& Data() const { return *payload_; }
+
+  T* MutableData() {
+    ownership_.AcquireOwnershipOnce(
+        [this] { payload_.reset(new T(*payload_)); });
+    return payload_.get();
+  }
+
+  void Reset() {
+    ownership_.AcquireOwnershipOnce([this] { payload_.reset(); });
+    payload_.reset(new T());
+  }
+
+ private:
+  std::shared_ptr<T> payload_;
+  OwnershipFlags ownership_;
+};
+
+}  // namespace details
+}  // namespace framework
+}  // namespace paddle

paddle/framework/details/cow_ptr_test.cc

+/* Copyright (c) 2016 PaddlePaddle 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 "paddle/framework/details/cow_ptr.h"
+#include "gtest/gtest.h"
+
+namespace paddle {
+namespace framework {
+namespace details {
+
+TEST(COWPtr, all) {
+  COWPtr<int> ptr(new int{0});
+  ASSERT_EQ(ptr.Data(), 0);
+  COWPtr<int> ptr2 = ptr;
+  ASSERT_EQ(ptr2.Data(), 0);
+  ASSERT_EQ(&ptr2.Data(), &ptr.Data());
+  *ptr2.MutableData() = 10;
+  ASSERT_EQ(ptr.Data(), 0);
+  ASSERT_EQ(ptr2.Data(), 10);
+
+  auto ptr_before = ptr2.MutableData();
+  ptr2.Reset();
+  ASSERT_NE(ptr2.MutableData(), ptr_before);
+}
+
+}  // namespace details
+}  // namespace framework
+}  // namespace paddle