runtime/gc/accounting/mod_union_table.cc - platform/art - Git at Google

 /*
  * Copyright (C) 2012 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  *      http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */

 #include "mod_union_table.h"

 #include <memory>

 #include "base/stl_util.h"
 #include "card_table-inl.h"
 #include "heap_bitmap.h"
 #include "gc/accounting/space_bitmap-inl.h"
 #include "gc/collector/mark_sweep.h"
 #include "gc/collector/mark_sweep-inl.h"
 #include "gc/heap.h"
 #include "gc/space/space.h"
 #include "mirror/art_field-inl.h"
 #include "mirror/object-inl.h"
 #include "mirror/class-inl.h"
 #include "mirror/object_array-inl.h"
 #include "space_bitmap-inl.h"
 #include "thread.h"

 using ::art::mirror::Object;

 namespace art {
 namespace gc {
 namespace accounting {

 class ModUnionClearCardSetVisitor {
  public:
   explicit ModUnionClearCardSetVisitor(ModUnionTable::CardSet* const cleared_cards)
     : cleared_cards_(cleared_cards) {
   }

   inline void operator()(byte* card, byte expected_value, byte new_value) const {
     if (expected_value == CardTable::kCardDirty) {
       cleared_cards_->insert(card);
     }
   }

  private:
   ModUnionTable::CardSet* const cleared_cards_;
 };

 class ModUnionClearCardVisitor {
  public:
   explicit ModUnionClearCardVisitor(std::vector<byte*>* cleared_cards)
     : cleared_cards_(cleared_cards) {
   }

   void operator()(byte* card, byte expected_card, byte new_card) const {
     if (expected_card == CardTable::kCardDirty) {
       cleared_cards_->push_back(card);
     }
   }
  private:
   std::vector<byte*>* const cleared_cards_;
 };

 class ModUnionUpdateObjectReferencesVisitor {
  public:
   ModUnionUpdateObjectReferencesVisitor(MarkHeapReferenceCallback* callback, void* arg)
     : callback_(callback),
       arg_(arg) {
   }

   // Extra parameters are required since we use this same visitor signature for checking objects.
   void operator()(Object* obj, MemberOffset offset, bool /*is_static*/) const
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
     // Only add the reference if it is non null and fits our criteria.
     mirror::HeapReference<Object>* obj_ptr = obj->GetFieldObjectReferenceAddr(offset);
     if (obj_ptr->AsMirrorPtr() != nullptr) {
       callback_(obj_ptr, arg_);
     }
   }

  private:
   MarkHeapReferenceCallback* const callback_;
   void* arg_;
 };

 class ModUnionScanImageRootVisitor {
  public:
   ModUnionScanImageRootVisitor(MarkHeapReferenceCallback* callback, void* arg)
       : callback_(callback), arg_(arg) {}

   void operator()(Object* root) const
       EXCLUSIVE_LOCKS_REQUIRED(Locks::heap_bitmap_lock_)
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
     DCHECK(root != NULL);
     ModUnionUpdateObjectReferencesVisitor ref_visitor(callback_, arg_);
     root->VisitReferences<kMovingClasses>(ref_visitor, VoidFunctor());
   }

  private:
   MarkHeapReferenceCallback* const callback_;
   void* const arg_;
 };

 void ModUnionTableReferenceCache::ClearCards() {
   CardTable* card_table = GetHeap()->GetCardTable();
   ModUnionClearCardSetVisitor visitor(&cleared_cards_);
   // Clear dirty cards in the this space and update the corresponding mod-union bits.
   card_table->ModifyCardsAtomic(space_->Begin(), space_->End(), AgeCardVisitor(), visitor);
 }

 class AddToReferenceArrayVisitor {
  public:
   explicit AddToReferenceArrayVisitor(ModUnionTableReferenceCache* mod_union_table,
                                       std::vector<mirror::HeapReference<Object>*>* references)
     : mod_union_table_(mod_union_table), references_(references) {
   }

   // Extra parameters are required since we use this same visitor signature for checking objects.
   void operator()(Object* obj, MemberOffset offset, bool /*is_static*/) const
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
     mirror::HeapReference<Object>* ref_ptr = obj->GetFieldObjectReferenceAddr(offset);
     mirror::Object* ref = ref_ptr->AsMirrorPtr();
     // Only add the reference if it is non null and fits our criteria.
     if (ref != nullptr && mod_union_table_->ShouldAddReference(ref)) {
       // Push the adddress of the reference.
       references_->push_back(ref_ptr);
     }
   }

  private:
   ModUnionTableReferenceCache* const mod_union_table_;
   std::vector<mirror::HeapReference<Object>*>* const references_;
 };

 class ModUnionReferenceVisitor {
  public:
   explicit ModUnionReferenceVisitor(ModUnionTableReferenceCache* const mod_union_table,
                                     std::vector<mirror::HeapReference<Object>*>* references)
     : mod_union_table_(mod_union_table),
       references_(references) {
   }

   void operator()(Object* obj) const
       SHARED_LOCKS_REQUIRED(Locks::heap_bitmap_lock_, Locks::mutator_lock_) {
     // We don't have an early exit since we use the visitor pattern, an early
     // exit should significantly speed this up.
     AddToReferenceArrayVisitor visitor(mod_union_table_, references_);
     obj->VisitReferences<kMovingClasses>(visitor, VoidFunctor());
   }
  private:
   ModUnionTableReferenceCache* const mod_union_table_;
   std::vector<mirror::HeapReference<Object>*>* const references_;
 };

 class CheckReferenceVisitor {
  public:
   explicit CheckReferenceVisitor(ModUnionTableReferenceCache* mod_union_table,
                                  const std::set<const Object*>& references)
     : mod_union_table_(mod_union_table),
       references_(references) {
   }

   // Extra parameters are required since we use this same visitor signature for checking objects.
   void operator()(Object* obj, MemberOffset offset, bool /*is_static*/) const
       SHARED_LOCKS_REQUIRED(Locks::heap_bitmap_lock_, Locks::mutator_lock_) {
     mirror::Object* ref = obj->GetFieldObject<mirror::Object>(offset);
     if (ref != nullptr && mod_union_table_->ShouldAddReference(ref) &&
         references_.find(ref) == references_.end()) {
       Heap* heap = mod_union_table_->GetHeap();
       space::ContinuousSpace* from_space = heap->FindContinuousSpaceFromObject(obj, false);
       space::ContinuousSpace* to_space = heap->FindContinuousSpaceFromObject(ref, false);
       LOG(INFO) << "Object " << reinterpret_cast<const void*>(obj) << "(" << PrettyTypeOf(obj)
           << ")" << "References " << reinterpret_cast<const void*>(ref) << "(" << PrettyTypeOf(ref)
           << ") without being in mod-union table";
       LOG(INFO) << "FromSpace " << from_space->GetName() << " type "
           << from_space->GetGcRetentionPolicy();
       LOG(INFO) << "ToSpace " << to_space->GetName() << " type "
           << to_space->GetGcRetentionPolicy();
       heap->DumpSpaces(LOG(INFO));
       LOG(FATAL) << "FATAL ERROR";
     }
   }

  private:
   ModUnionTableReferenceCache* const mod_union_table_;
   const std::set<const Object*>& references_;
 };

 class ModUnionCheckReferences {
  public:
   explicit ModUnionCheckReferences(ModUnionTableReferenceCache* mod_union_table,
                                    const std::set<const Object*>& references)
       EXCLUSIVE_LOCKS_REQUIRED(Locks::heap_bitmap_lock_)
       : mod_union_table_(mod_union_table), references_(references) {
   }

   void operator()(Object* obj) const NO_THREAD_SAFETY_ANALYSIS {
     Locks::heap_bitmap_lock_->AssertSharedHeld(Thread::Current());
     CheckReferenceVisitor visitor(mod_union_table_, references_);
     obj->VisitReferences<kMovingClasses>(visitor, VoidFunctor());
   }

  private:
   ModUnionTableReferenceCache* const mod_union_table_;
   const std::set<const Object*>& references_;
 };

 void ModUnionTableReferenceCache::Verify() {
   // Start by checking that everything in the mod union table is marked.
   for (const auto& ref_pair : references_) {
     for (mirror::HeapReference<Object>* ref : ref_pair.second) {
       CHECK(heap_->IsLiveObjectLocked(ref->AsMirrorPtr()));
     }
   }

   // Check the references of each clean card which is also in the mod union table.
   CardTable* card_table = heap_->GetCardTable();
   ContinuousSpaceBitmap* live_bitmap = space_->GetLiveBitmap();
   for (const auto& ref_pair : references_) {
     const byte* card = ref_pair.first;
     if (*card == CardTable::kCardClean) {
       std::set<const Object*> reference_set;
       for (mirror::HeapReference<Object>* obj_ptr : ref_pair.second) {
         reference_set.insert(obj_ptr->AsMirrorPtr());
       }
       ModUnionCheckReferences visitor(this, reference_set);
       uintptr_t start = reinterpret_cast<uintptr_t>(card_table->AddrFromCard(card));
       live_bitmap->VisitMarkedRange(start, start + CardTable::kCardSize, visitor);
     }
   }
 }

 void ModUnionTableReferenceCache::Dump(std::ostream& os) {
   CardTable* card_table = heap_->GetCardTable();
   os << "ModUnionTable cleared cards: [";
   for (byte* card_addr : cleared_cards_) {
     uintptr_t start = reinterpret_cast<uintptr_t>(card_table->AddrFromCard(card_addr));
     uintptr_t end = start + CardTable::kCardSize;
     os << reinterpret_cast<void*>(start) << "-" << reinterpret_cast<void*>(end) << ",";
   }
   os << "]\nModUnionTable references: [";
   for (const auto& ref_pair : references_) {
     const byte* card_addr = ref_pair.first;
     uintptr_t start = reinterpret_cast<uintptr_t>(card_table->AddrFromCard(card_addr));
     uintptr_t end = start + CardTable::kCardSize;
     os << reinterpret_cast<void*>(start) << "-" << reinterpret_cast<void*>(end) << "->{";
     for (mirror::HeapReference<Object>* ref : ref_pair.second) {
       os << reinterpret_cast<const void*>(ref->AsMirrorPtr()) << ",";
     }
     os << "},";
   }
 }

 void ModUnionTableReferenceCache::UpdateAndMarkReferences(MarkHeapReferenceCallback* callback,
                                                           void* arg) {
   CardTable* card_table = heap_->GetCardTable();

   std::vector<mirror::HeapReference<Object>*> cards_references;
   ModUnionReferenceVisitor add_visitor(this, &cards_references);

   for (const auto& card : cleared_cards_) {
     // Clear and re-compute alloc space references associated with this card.
     cards_references.clear();
     uintptr_t start = reinterpret_cast<uintptr_t>(card_table->AddrFromCard(card));
     uintptr_t end = start + CardTable::kCardSize;
     auto* space = heap_->FindContinuousSpaceFromObject(reinterpret_cast<Object*>(start), false);
     DCHECK(space != nullptr);
     ContinuousSpaceBitmap* live_bitmap = space->GetLiveBitmap();
     live_bitmap->VisitMarkedRange(start, end, add_visitor);

     // Update the corresponding references for the card.
     auto found = references_.find(card);
     if (found == references_.end()) {
       if (cards_references.empty()) {
         // No reason to add empty array.
         continue;
       }
       references_.Put(card, cards_references);
     } else {
       found->second = cards_references;
     }
   }
   cleared_cards_.clear();
   size_t count = 0;
   for (const auto& ref : references_) {
     for (mirror::HeapReference<Object>* obj_ptr : ref.second) {
       callback(obj_ptr, arg);
     }
     count += ref.second.size();
   }
   if (VLOG_IS_ON(heap)) {
     VLOG(gc) << "Marked " << count << " references in mod union table";
   }
 }

 void ModUnionTableCardCache::ClearCards() {
   CardTable* card_table = GetHeap()->GetCardTable();
   ModUnionClearCardSetVisitor visitor(&cleared_cards_);
   // Clear dirty cards in the this space and update the corresponding mod-union bits.
   card_table->ModifyCardsAtomic(space_->Begin(), space_->End(), AgeCardVisitor(), visitor);
 }

 // Mark all references to the alloc space(s).
 void ModUnionTableCardCache::UpdateAndMarkReferences(MarkHeapReferenceCallback* callback,
                                                      void* arg) {
   CardTable* card_table = heap_->GetCardTable();
   ModUnionScanImageRootVisitor scan_visitor(callback, arg);
   ContinuousSpaceBitmap* bitmap = space_->GetLiveBitmap();
   for (const byte* card_addr : cleared_cards_) {
     uintptr_t start = reinterpret_cast<uintptr_t>(card_table->AddrFromCard(card_addr));
     DCHECK(space_->HasAddress(reinterpret_cast<Object*>(start)));
     bitmap->VisitMarkedRange(start, start + CardTable::kCardSize, scan_visitor);
   }
 }

 void ModUnionTableCardCache::Dump(std::ostream& os) {
   CardTable* card_table = heap_->GetCardTable();
   os << "ModUnionTable dirty cards: [";
   for (const byte* card_addr : cleared_cards_) {
     auto start = reinterpret_cast<uintptr_t>(card_table->AddrFromCard(card_addr));
     auto end = start + CardTable::kCardSize;
     os << reinterpret_cast<void*>(start) << "-" << reinterpret_cast<void*>(end) << "\n";
   }
   os << "]";
 }

 }  // namespace accounting
 }  // namespace gc
 }  // namespace art
	/*
	* Copyright (C) 2012 The Android Open Source Project
	*
	* Licensed under the Apache License, Version 2.0 (the "License");
	* you may not use this file except in compliance with the License.
	* You may obtain a copy of the License at
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/

	#include "mod_union_table.h"

	#include <memory>

	#include "base/stl_util.h"
	#include "card_table-inl.h"
	#include "heap_bitmap.h"
	#include "gc/accounting/space_bitmap-inl.h"
	#include "gc/collector/mark_sweep.h"
	#include "gc/collector/mark_sweep-inl.h"
	#include "gc/heap.h"
	#include "gc/space/space.h"
	#include "mirror/art_field-inl.h"
	#include "mirror/object-inl.h"
	#include "mirror/class-inl.h"
	#include "mirror/object_array-inl.h"
	#include "space_bitmap-inl.h"
	#include "thread.h"

	using ::art::mirror::Object;

	namespace art {
	namespace gc {
	namespace accounting {

	class ModUnionClearCardSetVisitor {
	public:
	explicit ModUnionClearCardSetVisitor(ModUnionTable::CardSet* const cleared_cards)
	: cleared_cards_(cleared_cards) {
	}

	inline void operator()(byte* card, byte expected_value, byte new_value) const {
	if (expected_value == CardTable::kCardDirty) {
	cleared_cards_->insert(card);
	}
	}

	private:
	ModUnionTable::CardSet* const cleared_cards_;
	};

	class ModUnionClearCardVisitor {
	public:
	explicit ModUnionClearCardVisitor(std::vector<byte> cleared_cards)
	: cleared_cards_(cleared_cards) {
	}

	void operator()(byte* card, byte expected_card, byte new_card) const {
	if (expected_card == CardTable::kCardDirty) {
	cleared_cards_->push_back(card);
	}
	}
	private:
	std::vector<byte> const cleared_cards_;
	};

	class ModUnionUpdateObjectReferencesVisitor {
	public:
	ModUnionUpdateObjectReferencesVisitor(MarkHeapReferenceCallback* callback, void* arg)
	: callback_(callback),
	arg_(arg) {
	}

	// Extra parameters are required since we use this same visitor signature for checking objects.
	void operator()(Object* obj, MemberOffset offset, bool /is_static/) const
	SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
	// Only add the reference if it is non null and fits our criteria.
	mirror::HeapReference<Object>* obj_ptr = obj->GetFieldObjectReferenceAddr(offset);
	if (obj_ptr->AsMirrorPtr() != nullptr) {
	callback_(obj_ptr, arg_);
	}
	}

	private:
	MarkHeapReferenceCallback* const callback_;
	void* arg_;
	};

	class ModUnionScanImageRootVisitor {
	public:
	ModUnionScanImageRootVisitor(MarkHeapReferenceCallback* callback, void* arg)
	: callback_(callback), arg_(arg) {}

	void operator()(Object* root) const
	EXCLUSIVE_LOCKS_REQUIRED(Locks::heap_bitmap_lock_)
	SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
	DCHECK(root != NULL);
	ModUnionUpdateObjectReferencesVisitor ref_visitor(callback_, arg_);
	root->VisitReferences<kMovingClasses>(ref_visitor, VoidFunctor());
	}

	private:
	MarkHeapReferenceCallback* const callback_;
	void* const arg_;
	};

	void ModUnionTableReferenceCache::ClearCards() {
	CardTable* card_table = GetHeap()->GetCardTable();
	ModUnionClearCardSetVisitor visitor(&cleared_cards_);
	// Clear dirty cards in the this space and update the corresponding mod-union bits.
	card_table->ModifyCardsAtomic(space_->Begin(), space_->End(), AgeCardVisitor(), visitor);
	}

	class AddToReferenceArrayVisitor {
	public:
	explicit AddToReferenceArrayVisitor(ModUnionTableReferenceCache* mod_union_table,
	std::vector<mirror::HeapReference<Object>> references)
	: mod_union_table_(mod_union_table), references_(references) {
	}

	// Extra parameters are required since we use this same visitor signature for checking objects.
	void operator()(Object* obj, MemberOffset offset, bool /is_static/) const
	SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
	mirror::HeapReference<Object>* ref_ptr = obj->GetFieldObjectReferenceAddr(offset);
	mirror::Object* ref = ref_ptr->AsMirrorPtr();
	// Only add the reference if it is non null and fits our criteria.
	if (ref != nullptr && mod_union_table_->ShouldAddReference(ref)) {
	// Push the adddress of the reference.
	references_->push_back(ref_ptr);
	}
	}

	private:
	ModUnionTableReferenceCache* const mod_union_table_;
	std::vector<mirror::HeapReference<Object>> const references_;
	};

	class ModUnionReferenceVisitor {
	public:
	explicit ModUnionReferenceVisitor(ModUnionTableReferenceCache* const mod_union_table,
	std::vector<mirror::HeapReference<Object>> references)
	: mod_union_table_(mod_union_table),
	references_(references) {
	}

	void operator()(Object* obj) const
	SHARED_LOCKS_REQUIRED(Locks::heap_bitmap_lock_, Locks::mutator_lock_) {
	// We don't have an early exit since we use the visitor pattern, an early
	// exit should significantly speed this up.
	AddToReferenceArrayVisitor visitor(mod_union_table_, references_);
	obj->VisitReferences<kMovingClasses>(visitor, VoidFunctor());
	}
	private:
	ModUnionTableReferenceCache* const mod_union_table_;
	std::vector<mirror::HeapReference<Object>> const references_;
	};

	class CheckReferenceVisitor {
	public:
	explicit CheckReferenceVisitor(ModUnionTableReferenceCache* mod_union_table,
	const std::set<const Object*>& references)
	: mod_union_table_(mod_union_table),
	references_(references) {
	}

	// Extra parameters are required since we use this same visitor signature for checking objects.
	void operator()(Object* obj, MemberOffset offset, bool /is_static/) const
	SHARED_LOCKS_REQUIRED(Locks::heap_bitmap_lock_, Locks::mutator_lock_) {
	mirror::Object* ref = obj->GetFieldObject<mirror::Object>(offset);
	if (ref != nullptr && mod_union_table_->ShouldAddReference(ref) &&
	references_.find(ref) == references_.end()) {
	Heap* heap = mod_union_table_->GetHeap();
	space::ContinuousSpace* from_space = heap->FindContinuousSpaceFromObject(obj, false);
	space::ContinuousSpace* to_space = heap->FindContinuousSpaceFromObject(ref, false);
	LOG(INFO) << "Object " << reinterpret_cast<const void*>(obj) << "(" << PrettyTypeOf(obj)
	<< ")" << "References " << reinterpret_cast<const void*>(ref) << "(" << PrettyTypeOf(ref)
	<< ") without being in mod-union table";
	LOG(INFO) << "FromSpace " << from_space->GetName() << " type "
	<< from_space->GetGcRetentionPolicy();
	LOG(INFO) << "ToSpace " << to_space->GetName() << " type "
	<< to_space->GetGcRetentionPolicy();
	heap->DumpSpaces(LOG(INFO));
	LOG(FATAL) << "FATAL ERROR";
	}
	}

	private:
	ModUnionTableReferenceCache* const mod_union_table_;
	const std::set<const Object*>& references_;
	};

	class ModUnionCheckReferences {
	public:
	explicit ModUnionCheckReferences(ModUnionTableReferenceCache* mod_union_table,
	const std::set<const Object*>& references)
	EXCLUSIVE_LOCKS_REQUIRED(Locks::heap_bitmap_lock_)
	: mod_union_table_(mod_union_table), references_(references) {
	}

	void operator()(Object* obj) const NO_THREAD_SAFETY_ANALYSIS {
	Locks::heap_bitmap_lock_->AssertSharedHeld(Thread::Current());
	CheckReferenceVisitor visitor(mod_union_table_, references_);
	obj->VisitReferences<kMovingClasses>(visitor, VoidFunctor());
	}

	private:
	ModUnionTableReferenceCache* const mod_union_table_;
	const std::set<const Object*>& references_;
	};

	void ModUnionTableReferenceCache::Verify() {
	// Start by checking that everything in the mod union table is marked.
	for (const auto& ref_pair : references_) {
	for (mirror::HeapReference<Object>* ref : ref_pair.second) {
	CHECK(heap_->IsLiveObjectLocked(ref->AsMirrorPtr()));
	}
	}

	// Check the references of each clean card which is also in the mod union table.
	CardTable* card_table = heap_->GetCardTable();
	ContinuousSpaceBitmap* live_bitmap = space_->GetLiveBitmap();
	for (const auto& ref_pair : references_) {
	const byte* card = ref_pair.first;
	if (*card == CardTable::kCardClean) {
	std::set<const Object*> reference_set;
	for (mirror::HeapReference<Object>* obj_ptr : ref_pair.second) {
	reference_set.insert(obj_ptr->AsMirrorPtr());
	}
	ModUnionCheckReferences visitor(this, reference_set);
	uintptr_t start = reinterpret_cast<uintptr_t>(card_table->AddrFromCard(card));
	live_bitmap->VisitMarkedRange(start, start + CardTable::kCardSize, visitor);
	}
	}
	}

	void ModUnionTableReferenceCache::Dump(std::ostream& os) {
	CardTable* card_table = heap_->GetCardTable();
	os << "ModUnionTable cleared cards: [";
	for (byte* card_addr : cleared_cards_) {
	uintptr_t start = reinterpret_cast<uintptr_t>(card_table->AddrFromCard(card_addr));
	uintptr_t end = start + CardTable::kCardSize;
	os << reinterpret_cast<void>(start) << "-" << reinterpret_cast<void>(end) << ",";
	}
	os << "]\nModUnionTable references: [";
	for (const auto& ref_pair : references_) {
	const byte* card_addr = ref_pair.first;
	uintptr_t start = reinterpret_cast<uintptr_t>(card_table->AddrFromCard(card_addr));
	uintptr_t end = start + CardTable::kCardSize;
	os << reinterpret_cast<void>(start) << "-" << reinterpret_cast<void>(end) << "->{";
	for (mirror::HeapReference<Object>* ref : ref_pair.second) {
	os << reinterpret_cast<const void*>(ref->AsMirrorPtr()) << ",";
	}
	os << "},";
	}
	}

	void ModUnionTableReferenceCache::UpdateAndMarkReferences(MarkHeapReferenceCallback* callback,
	void* arg) {
	CardTable* card_table = heap_->GetCardTable();

	std::vector<mirror::HeapReference<Object>*> cards_references;
	ModUnionReferenceVisitor add_visitor(this, &cards_references);

	for (const auto& card : cleared_cards_) {
	// Clear and re-compute alloc space references associated with this card.
	cards_references.clear();
	uintptr_t start = reinterpret_cast<uintptr_t>(card_table->AddrFromCard(card));
	uintptr_t end = start + CardTable::kCardSize;
	auto* space = heap_->FindContinuousSpaceFromObject(reinterpret_cast<Object*>(start), false);
	DCHECK(space != nullptr);
	ContinuousSpaceBitmap* live_bitmap = space->GetLiveBitmap();
	live_bitmap->VisitMarkedRange(start, end, add_visitor);

	// Update the corresponding references for the card.
	auto found = references_.find(card);
	if (found == references_.end()) {
	if (cards_references.empty()) {
	// No reason to add empty array.
	continue;
	}
	references_.Put(card, cards_references);
	} else {
	found->second = cards_references;
	}
	}
	cleared_cards_.clear();
	size_t count = 0;
	for (const auto& ref : references_) {
	for (mirror::HeapReference<Object>* obj_ptr : ref.second) {
	callback(obj_ptr, arg);
	}
	count += ref.second.size();
	}
	if (VLOG_IS_ON(heap)) {
	VLOG(gc) << "Marked " << count << " references in mod union table";
	}
	}

	void ModUnionTableCardCache::ClearCards() {
	CardTable* card_table = GetHeap()->GetCardTable();
	ModUnionClearCardSetVisitor visitor(&cleared_cards_);
	// Clear dirty cards in the this space and update the corresponding mod-union bits.
	card_table->ModifyCardsAtomic(space_->Begin(), space_->End(), AgeCardVisitor(), visitor);
	}

	// Mark all references to the alloc space(s).
	void ModUnionTableCardCache::UpdateAndMarkReferences(MarkHeapReferenceCallback* callback,
	void* arg) {
	CardTable* card_table = heap_->GetCardTable();
	ModUnionScanImageRootVisitor scan_visitor(callback, arg);
	ContinuousSpaceBitmap* bitmap = space_->GetLiveBitmap();
	for (const byte* card_addr : cleared_cards_) {
	uintptr_t start = reinterpret_cast<uintptr_t>(card_table->AddrFromCard(card_addr));
	DCHECK(space_->HasAddress(reinterpret_cast<Object*>(start)));
	bitmap->VisitMarkedRange(start, start + CardTable::kCardSize, scan_visitor);
	}
	}

	void ModUnionTableCardCache::Dump(std::ostream& os) {
	CardTable* card_table = heap_->GetCardTable();
	os << "ModUnionTable dirty cards: [";
	for (const byte* card_addr : cleared_cards_) {
	auto start = reinterpret_cast<uintptr_t>(card_table->AddrFromCard(card_addr));
	auto end = start + CardTable::kCardSize;
	os << reinterpret_cast<void>(start) << "-" << reinterpret_cast<void>(end) << "\n";
	}
	os << "]";
	}

	} // namespace accounting
	} // namespace gc
	} // namespace art