reference-summarizer.cc

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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.
 
#include "src/heap/reference-summarizer.h"
 
#include "src/heap/mark-compact-inl.h"
#include "src/heap/marking-visitor-inl.h"
#include "src/objects/embedder-data-array-inl.h"
#include "src/objects/js-array-buffer-inl.h"
 
namespace v8 {
namespace internal {
 
namespace {
 
// A class which acts as a MarkingState but does not actually update any marking
// bits. It reports all objects as white and all transitions as successful. It
// also tracks which objects are retained by the primary object according to the
// marking visitor.
class ReferenceSummarizerMarkingState final {
 public:
  explicit ReferenceSummarizerMarkingState(Tagged<HeapObject> object)
      : primary_object_(object),
        local_marking_worklists_(&marking_worklists_),
        local_weak_objects_(&weak_objects_) {}
 
  ~ReferenceSummarizerMarkingState() {
    // Clean up temporary state.
    local_weak_objects_.Publish();
    weak_objects_.Clear();
    local_marking_worklists_.Publish();
    marking_worklists_.Clear();
  }
 
  // Retrieves the references that were collected by this marker. This operation
  // transfers ownership of the set, so calling it again would yield an empty
  // result.
  ReferenceSummary DestructivelyRetrieveReferences() {
    ReferenceSummary tmp = std::move(references_);
    references_.Clear();
    return tmp;
  }
 
  // Standard marking visitor functions:
  bool TryMark(Tagged<HeapObject> obj) { return true; }
  bool IsUnmarked(Tagged<HeapObject> obj) const { return true; }
  bool IsMarked(Tagged<HeapObject> obj) const { return false; }
 
  // Adds a retaining relationship found by the marking visitor.
  void AddStrongReferenceForReferenceSummarizer(Tagged<HeapObject> host,
                                                Tagged<HeapObject> obj) {
    AddReference(host, obj, references_.strong_references());
  }
 
  // Adds a non-retaining weak reference found by the marking visitor. The value
  // in an ephemeron hash table entry is also included here, since it is not
  // known to be strong without further information about the key.
  void AddWeakReferenceForReferenceSummarizer(Tagged<HeapObject> host,
                                              Tagged<HeapObject> obj) {
    AddReference(host, obj, references_.weak_references());
  }
 
  // Other member functions, not part of the marking visitor contract:
 
  MarkingWorklists::Local* local_marking_worklists() {
    return &local_marking_worklists_;
  }
  WeakObjects::Local* local_weak_objects() { return &local_weak_objects_; }
 
 private:
  void AddReference(Tagged<HeapObject> host, Tagged<HeapObject> obj,
                    ReferenceSummary::UnorderedHeapObjectSet& references) {
    // It's possible that the marking visitor handles multiple objects at once,
    // such as a Map and its DescriptorArray, but we're only interested in
    // references from the primary object.
    if (host == primary_object_) {
      references.insert(obj);
    }
  }
 
  ReferenceSummary references_;
  Tagged<HeapObject> primary_object_;
  MarkingWorklists marking_worklists_;
  MarkingWorklists::Local local_marking_worklists_;
  WeakObjects weak_objects_;
  WeakObjects::Local local_weak_objects_;
};
 
class ReferenceSummarizerMarkingVisitor
    : public MarkingVisitorBase<ReferenceSummarizerMarkingVisitor> {
 public:
  ReferenceSummarizerMarkingVisitor(
      Heap* heap, ReferenceSummarizerMarkingState* marking_state)
      : MarkingVisitorBase(marking_state->local_marking_worklists(),
                           marking_state->local_weak_objects(), heap,
                           0 /*mark_compact_epoch*/, {} /*code_flush_mode*/,
                           true /*should_keep_ages_unchanged*/,
                           0 /*code_flushing_increase*/),
        marking_state_(marking_state) {}
 
  template <typename TSlot>
  void RecordSlot(Tagged<HeapObject> object, TSlot slot,
                  Tagged<HeapObject> target) {}
 
  void RecordRelocSlot(Tagged<InstructionStream> host, RelocInfo* rinfo,
                       Tagged<HeapObject> target) {}
 
  V8_INLINE void AddStrongReferenceForReferenceSummarizer(
      Tagged<HeapObject> host, Tagged<HeapObject> obj) {
    marking_state_->AddStrongReferenceForReferenceSummarizer(host, obj);
  }
 
  V8_INLINE void AddWeakReferenceForReferenceSummarizer(
      Tagged<HeapObject> host, Tagged<HeapObject> obj) {
    marking_state_->AddWeakReferenceForReferenceSummarizer(host, obj);
  }
 
  constexpr bool CanUpdateValuesInHeap() { return false; }
 
  ReferenceSummarizerMarkingState* marking_state() const {
    return marking_state_;
  }
 
  void MarkPointerTableEntry(Tagged<HeapObject> host,
                             IndirectPointerSlot slot) {}
 
  void VisitExternalPointer(Tagged<HeapObject> host,
                            ExternalPointerSlot slot) override {}
  void VisitCppHeapPointer(Tagged<HeapObject> host,
                           CppHeapPointerSlot slot) override {}
  void VisitJSDispatchTableEntry(Tagged<HeapObject> host,
                                 JSDispatchHandle handle) override {}
 
 private:
  ReferenceSummarizerMarkingState* marking_state_;
};
 
}  // namespace
 
ReferenceSummary ReferenceSummary::SummarizeReferencesFrom(
    Heap* heap, Tagged<HeapObject> obj) {
  ReferenceSummarizerMarkingState marking_state(obj);
 
  ReferenceSummarizerMarkingVisitor visitor(heap, &marking_state);
  visitor.Visit(obj->map(heap->isolate()), obj);
 
  return marking_state.DestructivelyRetrieveReferences();
}
 
}  // namespace internal
}  // namespace v8