[heap] Implement simple concurrent marking deque.

src/heap/concurrent-marking-deque.h

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef V8_HEAP_CONCURRENT_MARKING_DEQUE_
+#define V8_HEAP_CONCURRENT_MARKING_DEQUE_
+
+#include <deque>
+
+#include "src/base/platform/mutex.h"
+
+namespace v8 {
+namespace internal {
+
+class Heap;
+class Isolate;
+class HeapObject;
+
+enum class MarkingThread { kMain, kConcurrent };
+
+enum class TargetDeque { kShared, kBailout };
+
+// The concurrent marking deque supports deque operations for two threads
+// (main and concurrent) and two deques (shared and bailout).

[Hannes Payer (out of office), 2017/05/02 15:53:20]

it is implemented using two deques.

[ulan, 2017/05/02 16:33:03]

Done.

+//
+// The concurrent thread can use the push and pop operations with the
+// MarkingThread::kConcurrent argument. All other operations are intended
+// to be used by the main thread only.
+//
+// The interface of the concurrent marking deque for the main thread matches
+// that of the sequential marking deque, so they can be easily switched
+// at compile time without updating the main thread call-sites.
+//
+// The shared deque is shared between the main thread and the concurrent
+// thread, so both threads can push to and pop from the shared deque.
+// The bailout deque stores objects that cannot be processed by the concurrent
+// thread. Only the concurrent thread can push to it and only the main thread
+// can pop from it.
+class ConcurrentMarkingDeque {
+ public:
+  // The heap parameter is needed to match the interface
+  // of the sequential marking deque.
+  explicit ConcurrentMarkingDeque(Heap* heap) {}
+
+  // Pushes the object into the specified deque assuming that the function is
+  // called on the specified thread. The main thread can push only to the shared
+  // deque. The concurrent thread can push to both deques.
+  bool Push(HeapObject* object, MarkingThread thread = MarkingThread::kMain,
+            TargetDeque target = TargetDeque::kShared) {
+    DCHECK_IMPLIES(thread == MarkingThread::kMain,
+                   target == TargetDeque::kShared);
+    switch (target) {
+      case TargetDeque::kShared:
+        shared_deque_.Push(object);
+        break;
+      case TargetDeque::kBailout:
+        bailout_deque_.Push(object);
+        break;
+    }
+    return true;
+  }
+
+  // Pops an object from the bailout or shared deque assuming that the function
+  // is called on the specified thread. The main thread first tries to pop the
+  // bailout deque. If the deque is empty then it tries the shared deque.
+  // If the shared deque is also empty, then the function returns nullptr.
+  // The concurrent thread pops only from the shared deque.
+  HeapObject* Pop(MarkingThread thread = MarkingThread::kMain) {
+    if (thread == MarkingThread::kMain) {
+      HeapObject* result = bailout_deque_.Pop();
+      if (result != nullptr) return result;
+    }
+    return shared_deque_.Pop();
+  }
+
+  // All the following operations can used only by the main thread.
+  void Clear() {
+    bailout_deque_.Clear();
+    shared_deque_.Clear();
+  }
+
+  bool IsFull() { return false; }
+
+  bool IsEmpty() { return bailout_deque_.IsEmpty() && shared_deque_.IsEmpty(); }
+
+  int Size() { return bailout_deque_.Size() + shared_deque_.Size(); }
+
+  // This is used for a large array with a progress bar.
+  // For simpicity, unshift to the bailout deque so that the concurrent thread
+  // does not see such objects.
+  bool Unshift(HeapObject* object) {
+    bailout_deque_.Unshift(object);
+    return true;
+  }
+
+  // Calls the specified callback on each element of the deques and replaces
+  // the element with the result of the callback. If the callback returns
+  // nullptr then the element is removed from the deque.
+  // The callback must accept HeapObject* and return HeapObject*.
+  template <typename Callback>
+  void Update(Callback callback) {
+    bailout_deque_.Update(callback);
+    shared_deque_.Update(callback);
+  }
+
+  // These empty functions are needed to match the interface
+  // of the sequential marking deque.
+  void SetUp() {}
+  void TearDown() {}
+  void StartUsing() {}
+  void StopUsing() {}
+  void ClearOverflowed() {}
+  void SetOverflowed() {}
+  bool overflowed() const { return false; }
+
+ private:
+  // Simple, slow, and thread-safe deque that forwards all operations to
+  // a lock-protected std::deque.
+  class Deque {
+   public:
+    Deque() { cache_padding_[0] = 0; }
+    void Clear() {
+      base::LockGuard<base::Mutex> guard(&mutex_);
+      return deque_.clear();
+    }
+    bool IsEmpty() {
+      base::LockGuard<base::Mutex> guard(&mutex_);
+      return deque_.empty();
+    }
+    int Size() {
+      base::LockGuard<base::Mutex> guard(&mutex_);
+      return static_cast<int>(deque_.size());
+    }
+    void Push(HeapObject* object) {
+      base::LockGuard<base::Mutex> guard(&mutex_);
+      deque_.push_back(object);
+    }
+    HeapObject* Pop() {
+      base::LockGuard<base::Mutex> guard(&mutex_);
+      if (deque_.empty()) return nullptr;
+      HeapObject* result = deque_.back();
+      deque_.pop_back();
+      return result;
+    }
+    void Unshift(HeapObject* object) {
+      base::LockGuard<base::Mutex> guard(&mutex_);
+      deque_.push_front(object);
+    }
+    template <typename Callback>
+    void Update(Callback callback) {
+      base::LockGuard<base::Mutex> guard(&mutex_);
+      std::deque<HeapObject*> new_deque;
+      for (auto object : deque_) {
+        HeapObject* new_object = callback(object);
+        if (new_object) {
+          new_deque.push_back(new_object);
+        }
+      }
+      deque_.swap(new_deque);
+    }
+
+   private:
+    base::Mutex mutex_;
+    std::deque<HeapObject*> deque_;
+    // Ensure that two deques do not share the same cache line.
+    static int const kCachePadding = 64;
+    char cache_padding_[kCachePadding];
+  };
+  Deque bailout_deque_;
+  Deque shared_deque_;
+  DISALLOW_COPY_AND_ASSIGN(ConcurrentMarkingDeque);
+};
+
+}  // namespace internal
+}  // namespace v8
+
+#endif  // V8_CONCURRENT_MARKING_DEQUE_