pretenuring-handler.h

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// Copyright 2022 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_PRETENURING_HANDLER_H_
#define V8_HEAP_PRETENURING_HANDLER_H_
 
#include <memory>
 
#include "src/objects/allocation-site.h"
#include "src/objects/heap-object.h"
#include "src/objects/map.h"
 
namespace v8 {
namespace internal {
 
template <typename T>
class GlobalHandleVector;
class Heap;
 
class PretenuringHandler final {
 public:
  static constexpr int kInitialFeedbackCapacity = 256;
 
  using PretenuringFeedbackMap =
      std::unordered_map<Tagged<AllocationSite>, size_t, Object::Hasher>;
  enum FindMementoMode { kForRuntime, kForGC };
 
  explicit PretenuringHandler(Heap* heap);
  ~PretenuringHandler();
 
  void reset();
 
  // If an object has an AllocationMemento trailing it, return it, otherwise
  // return a null AllocationMemento.
  template <FindMementoMode mode>
  static inline Tagged<AllocationMemento> FindAllocationMemento(
      Heap* heap, Tagged<Map> map, Tagged<HeapObject> object);
  template <FindMementoMode mode>
  static inline Tagged<AllocationMemento> FindAllocationMemento(
      Heap* heap, Tagged<Map> map, Tagged<HeapObject> object, int object_size);
 
  // ===========================================================================
  // Allocation site tracking. =================================================
  // ===========================================================================
 
  // Updates the AllocationSite of a given {object}. The entry (including the
  // count) is cached on the local pretenuring feedback.
  static inline void UpdateAllocationSite(
      Heap* heap, Tagged<Map> map, Tagged<HeapObject> object, int object_size,
      PretenuringFeedbackMap* pretenuring_feedback);
 
  // Merges local pretenuring feedback into the global one. Note that this
  // method needs to be called after evacuation, as allocation sites may be
  // evacuated and this method resolves forward pointers accordingly.
  void MergeAllocationSitePretenuringFeedback(
      const PretenuringFeedbackMap& local_pretenuring_feedback);
 
  // Adds an allocation site to the list of sites to be pretenured during the
  // next collection. Added allocation sites are pretenured independent of
  // their feedback.
  V8_EXPORT_PRIVATE void PretenureAllocationSiteOnNextCollection(
      Tagged<AllocationSite> site);
 
  // ===========================================================================
  // Pretenuring. ==============================================================
  // ===========================================================================
 
  // Pretenuring decisions are made based on feedback collected during new space
  // evacuation. Note that between feedback collection and calling this method
  // object in old space must not move.
  void ProcessPretenuringFeedback(size_t new_space_capacity_before_gc);
 
  // Removes an entry from the global pretenuring storage.
  void RemoveAllocationSitePretenuringFeedback(Tagged<AllocationSite> site);
 
  bool HasPretenuringFeedback() const {
    return !global_pretenuring_feedback_.empty();
  }
 
  V8_EXPORT_PRIVATE static int GetMinMementoCountForTesting();
 
 private:
  Heap* const heap_;
 
  // The feedback storage is used to store allocation sites (keys) and how often
  // they have been visited (values) by finding a memento behind an object. The
  // storage is only alive temporary during a GC. The invariant is that all
  // pointers in this map are already fixed, i.e., they do not point to
  // forwarding pointers.
  PretenuringFeedbackMap global_pretenuring_feedback_;
 
  std::unique_ptr<GlobalHandleVector<AllocationSite>>
      allocation_sites_to_pretenure_;
};
 
}  // namespace internal
}  // namespace v8
 
#endif  // V8_HEAP_PRETENURING_HANDLER_H_