[heap] Implement simple concurrent marking deque.

src/heap/sequential-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_SEQUENTIAL_MARKING_DEQUE_
+#define V8_HEAP_SEQUENTIAL_MARKING_DEQUE_
+
+#include <deque>
+
+#include "src/base/platform/mutex.h"
+#include "src/base/platform/platform.h"
+#include "src/cancelable-task.h"
+
+namespace v8 {
+namespace internal {
+
+class Heap;
+class Isolate;
+class HeapObject;
+
+// ----------------------------------------------------------------------------
+// Marking deque for tracing live objects.
+class SequentialMarkingDeque {
+ public:
+  explicit SequentialMarkingDeque(Heap* heap)
+      : backing_store_(nullptr),
+        backing_store_committed_size_(0),
+        array_(nullptr),
+        top_(0),
+        bottom_(0),
+        mask_(0),
+        overflowed_(false),
+        in_use_(false),
+        uncommit_task_pending_(false),
+        heap_(heap) {}
+
+  void SetUp();
+  void TearDown();
+
+  // Ensures that the marking deque is committed and will stay committed until
+  // StopUsing() is called.
+  void StartUsing();
+  void StopUsing();
+  void Clear();
+
+  inline bool IsFull() { return ((top_ + 1) & mask_) == bottom_; }
+
+  inline bool IsEmpty() { return top_ == bottom_; }
+
+  int Size() { return (top_ + mask_ + 1 - bottom_) & mask_; }
+
+  bool overflowed() const { return overflowed_; }
+
+  void ClearOverflowed() { overflowed_ = false; }
+
+  void SetOverflowed() { overflowed_ = true; }
+
+  // Push the object on the marking stack if there is room, otherwise mark the
+  // deque as overflowed and wait for a rescan of the heap.
+  INLINE(bool Push(HeapObject* object)) {
+    if (IsFull()) {
+      SetOverflowed();
+      return false;
+    } else {
+      array_[top_] = object;
+      top_ = ((top_ + 1) & mask_);
+      return true;
+    }
+  }
+
+  INLINE(HeapObject* Pop()) {
+    DCHECK(!IsEmpty());
+    top_ = ((top_ - 1) & mask_);
+    HeapObject* object = array_[top_];
+    return object;
+  }
+
+  // Unshift the object into the marking stack if there is room, otherwise mark
+  // the deque as overflowed and wait for a rescan of the heap.
+  INLINE(bool Unshift(HeapObject* object)) {
+    if (IsFull()) {
+      SetOverflowed();
+      return false;
+    } else {
+      bottom_ = ((bottom_ - 1) & mask_);
+      array_[bottom_] = object;
+      return true;
+    }
+  }
+
+  // Calls the specified callback on each element of the deque 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) {
+    int i = bottom_;
+    int new_top = bottom_;
+    while (i != top_) {
+      HeapObject* object = callback(array_[i]);
+      if (object) {
+        array_[new_top] = object;
+        new_top = (new_top + 1) & mask_;
+      }
+      i = (i + 1) & mask_;
+    }
+    top_ = new_top;
+  }
+
+ private:
+  // This task uncommits the marking_deque backing store if
+  // markin_deque->in_use_ is false.
+  class UncommitTask : public CancelableTask {
+   public:
+    explicit UncommitTask(Isolate* isolate,
+                          SequentialMarkingDeque* marking_deque)
+        : CancelableTask(isolate), marking_deque_(marking_deque) {}
+
+   private:
+    // CancelableTask override.
+    void RunInternal() override {
+      base::LockGuard<base::Mutex> guard(&marking_deque_->mutex_);
+      if (!marking_deque_->in_use_) {
+        marking_deque_->Uncommit();
+      }
+      marking_deque_->uncommit_task_pending_ = false;
+    }
+
+    SequentialMarkingDeque* marking_deque_;
+    DISALLOW_COPY_AND_ASSIGN(UncommitTask);
+  };
+
+  static const size_t kMaxSize = 4 * MB;
+  static const size_t kMinSize = 256 * KB;
+
+  // Must be called with mutex lock.
+  void EnsureCommitted();
+
+  // Must be called with mutex lock.
+  void Uncommit();
+
+  // Must be called with mutex lock.
+  void StartUncommitTask();
+
+  base::Mutex mutex_;
+
+  base::VirtualMemory* backing_store_;
+  size_t backing_store_committed_size_;
+  HeapObject** array_;
+  // array_[(top - 1) & mask_] is the top element in the deque.  The Deque is
+  // empty when top_ == bottom_.  It is full when top_ + 1 == bottom
+  // (mod mask + 1).
+  int top_;
+  int bottom_;
+  int mask_;
+  bool overflowed_;
+  // in_use_ == true after taking mutex lock implies that the marking deque is
+  // committed and will stay committed at least until in_use_ == false.
+  bool in_use_;
+  bool uncommit_task_pending_;
+  Heap* heap_;
+
+  DISALLOW_COPY_AND_ASSIGN(SequentialMarkingDeque);
+};
+
+}  // namespace internal
+}  // namespace v8
+
+#endif  // V8_SEQUENTIAL_MARKING_DEQUE_