[heap] Fine-grained JSArrayBuffer tracking

src/heap/array-buffer-tracker.h

 // Copyright 2015 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_ARRAY_BUFFER_TRACKER_H_
 #define V8_HEAP_ARRAY_BUFFER_TRACKER_H_
 
 #include <map>
 
 #include "src/base/platform/mutex.h"
 #include "src/globals.h"
 
 namespace v8 {
 namespace internal {
 
 // Forward declarations.
 class Heap;
+class Page;
 class JSArrayBuffer;
 
+// LocalArrayBufferTracker is tracker for live and dead JSArrayBuffer objects.
+//
+// It consists of two sets, a live, and a not yet discovered set of buffers.
+// Upon registration (in the ArrayBufferTracker) the buffers are added to both
+// sets. When a buffer is encountered as live (or added is live) it is removed
+// from the not yet discovered set. Finally, after each round (sometime during
+// GC) the left over not yet discovered buffers are cleaned up. Upon starting
+// a new round the not yet discovered buffers are initialized from the live set.
+//
+// Caveats:
+// - Between cleaning up the buffers using |Free| we always need a |Reset| and
+//   thus another marking phase.
+class LocalArrayBufferTracker {
+ public:
+  typedef std::pair<void*, size_t> Value;
+  typedef JSArrayBuffer* Key;
+
+  enum LivenessIndicator { kForwardingPointer, kMarkBit };
+  enum CallbackResult { kKeepEntry, kKeepAndUpdateEntry, kRemoveEntry };
+
+  explicit LocalArrayBufferTracker(Heap* heap) : heap_(heap), started_(false) {}
+  ~LocalArrayBufferTracker();
+
+  void Add(Key key, const Value& value);
+  void AddLive(Key key, const Value& value);
+  Value Remove(Key key);
+  void MarkLive(Key key);
+  bool IsEmpty();
+
+  // Resets the tracking set, i.e., not yet discovered buffers are initialized
+  // from the remaining live set of buffers.
+  void Reset();
+
+  // Frees up any dead backing stores of not yet discovered array buffers.
+  // Requires that the buffers have been properly marked using MarkLive.
+  void FreeDead();
+
+  // Scans the whole tracker and decides based on liveness_indicator whether
+  // a JSArrayBuffer is still considered live.
+  template <LivenessIndicator liveness_indicator>
+  inline void ScanAndFreeDead();
+
+  bool IsTracked(Key key) { return live_.find(key) != live_.end(); }
+
+ private:
+  // TODO(mlippautz): Switch to unordered_map once it is supported on all
+  // platforms.
+  typedef std::map<Key, Value> TrackingMap;
+
+  // Processes buffers one by one. The CallbackResult decides whether the buffer
+  // will be dropped or not.
+  //
+  // Callback should be of type:
+  //   CallbackResult fn(JSArrayBuffer*, JSArrayBuffer**);
+  template <typename Callback>
+  inline void Process(Callback callback);
+
+  Heap* heap_;
+
+  // |live_| maps tracked JSArrayBuffers to the internally allocated backing
+  // store and length. For each GC round |not_yet_discovered_| is initialized
+  // as a copy of |live_|. Upon finding a JSArrayBuffer during GC, the buffer
+  // is removed from |not_yet_discovered_|. At the end of a GC, we free up the
+  // remaining JSArrayBuffers in |not_yet_discovered_|.
+  TrackingMap live_;
+  TrackingMap not_yet_discovered_;
+
+  bool started_;
+};
+
 class ArrayBufferTracker {
  public:
   explicit ArrayBufferTracker(Heap* heap) : heap_(heap) {}
   ~ArrayBufferTracker();
 
-  inline Heap* heap() { return heap_; }
-
   // The following methods are used to track raw C++ pointers to externally
   // allocated memory used as backing store in live array buffers.
 
-  // A new ArrayBuffer was created with |data| as backing store.
-  void RegisterNew(JSArrayBuffer* buffer);
-
-  // The backing store |data| is no longer owned by V8.
+  // Register/unregister a new JSArrayBuffer |buffer| for tracking.
+  // |track_live| indicates whether marking will still visit the buffer and we
+  // can delay marking it as live.
+  void RegisterNew(JSArrayBuffer* buffer, bool track_live);
   void Unregister(JSArrayBuffer* buffer);
 
-  // A live ArrayBuffer was discovered during marking/scavenge.
+  // Frees all backing store pointers for dead JSArrayBuffers in new space.
+  void FreeDeadInNewSpace();
+
+  void ResetTrackersInOldSpace();
+
+  // A live JSArrayBuffer was discovered during marking.
   void MarkLive(JSArrayBuffer* buffer);
 
-  // Frees all backing store pointers that weren't discovered in the previous
-  // marking or scavenge phase.
-  void FreeDead(bool from_scavenge);
+ private:
+  Heap* heap_;
+};
 
-  // Prepare for a new scavenge phase. A new marking phase is implicitly
-  // prepared by finishing the previous one.
-  void PrepareDiscoveryInNewSpace();
-
-  // An ArrayBuffer moved from new space to old space.
-  void Promote(JSArrayBuffer* buffer);
-
- private:
-  base::Mutex mutex_;
-  Heap* heap_;
-
-  // |live_array_buffers_| maps externally allocated memory used as backing
-  // store for ArrayBuffers to the length of the respective memory blocks.
-  //
-  // At the beginning of mark/compact, |not_yet_discovered_array_buffers_| is
-  // a copy of |live_array_buffers_| and we remove pointers as we discover live
-  // ArrayBuffer objects during marking. At the end of mark/compact, the
-  // remaining memory blocks can be freed.
-  std::map<void*, size_t> live_array_buffers_;
-  std::map<void*, size_t> not_yet_discovered_array_buffers_;
-
-  // To be able to free memory held by ArrayBuffers during scavenge as well, we
-  // have a separate list of allocated memory held by ArrayBuffers in new space.
-  //
-  // Since mark/compact also evacuates the new space, all pointers in the
-  // |live_array_buffers_for_scavenge_| list are also in the
-  // |live_array_buffers_| list.
-  std::map<void*, size_t> live_array_buffers_for_scavenge_;
-  std::map<void*, size_t> not_yet_discovered_array_buffers_for_scavenge_;
-};
 }  // namespace internal
 }  // namespace v8
 #endif  // V8_HEAP_ARRAY_BUFFER_TRACKER_H_