Use EphemeronHashTable as backing store for JSWeakCollection

JSWeakCollection should use EphemeronHashTable as backing store instead of
ObjectHashTable such that the GC can handle these structures differently in
the future.

Bug: chromium:844008
Change-Id: Icc6df60c975a942877e2507ef45e0d235e5f72be
Reviewed-on: https://chromium-review.googlesource.com/1089063
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53572}

src/builtins/builtins-collections-gen.cc

@@ -1967,7 +1967,7 @@ class WeakCollectionsBuiltinsAssembler : public BaseCollectionsAssembler {
   TNode<Smi> CreateIdentityHash(TNode<Object> receiver);
   TNode<IntPtrT> EntryMask(TNode<IntPtrT> capacity);
 
-  // Builds code that finds the ObjectHashTable entry for a {key} using the
+  // Builds code that finds the EphemeronHashTable entry for a {key} using the
   // comparison code generated by {key_compare}. The key index is returned if
   // the {key} is found.
   typedef std::function<void(TNode<Object> entry_key, Label* if_same)>
@@ -1976,12 +1976,13 @@ class WeakCollectionsBuiltinsAssembler : public BaseCollectionsAssembler {
                               TNode<IntPtrT> entry_mask,
                               const KeyComparator& key_compare);
 
-  // Builds code that finds an ObjectHashTable entry available for a new entry.
+  // Builds code that finds an EphemeronHashTable entry available for a new
+  // entry.
   TNode<IntPtrT> FindKeyIndexForInsertion(TNode<HeapObject> table,
                                           TNode<IntPtrT> key_hash,
                                           TNode<IntPtrT> entry_mask);
 
-  // Builds code that finds the ObjectHashTable entry with key that matches
+  // Builds code that finds the EphemeronHashTable entry with key that matches
   // {key} and returns the entry's key index. If {key} cannot be found, jumps to
   // {if_not_found}.
   TNode<IntPtrT> FindKeyIndexForKey(TNode<HeapObject> table, TNode<Object> key,
@@ -2011,13 +2012,13 @@ class WeakCollectionsBuiltinsAssembler : public BaseCollectionsAssembler {
 void WeakCollectionsBuiltinsAssembler::AddEntry(
     TNode<HeapObject> table, TNode<IntPtrT> key_index, TNode<Object> key,
     TNode<Object> value, TNode<IntPtrT> number_of_elements) {
-  // See ObjectHashTable::AddEntry().
+  // See EphemeronHashTable::AddEntry().
   TNode<IntPtrT> value_index = ValueIndexFromKeyIndex(key_index);
   StoreFixedArrayElement(table, key_index, key);
   StoreFixedArrayElement(table, value_index, value);
 
   // See HashTableBase::ElementAdded().
-  StoreFixedArrayElement(table, ObjectHashTable::kNumberOfElementsIndex,
+  StoreFixedArrayElement(table, EphemeronHashTable::kNumberOfElementsIndex,
                          SmiFromIntPtr(number_of_elements), SKIP_WRITE_BARRIER);
 }
 
@@ -2034,14 +2035,15 @@ TNode<Object> WeakCollectionsBuiltinsAssembler::AllocateTable(
   TNode<FixedArray> table =
       AllocateFixedArray(HOLEY_ELEMENTS, length, kAllowLargeObjectAllocation);
 
-  Heap::RootListIndex map_root_index =
-      static_cast<Heap::RootListIndex>(ObjectHashTableShape::GetMapRootIndex());
+  Heap::RootListIndex map_root_index = static_cast<Heap::RootListIndex>(
+      EphemeronHashTableShape::GetMapRootIndex());
   StoreMapNoWriteBarrier(table, map_root_index);
-  StoreFixedArrayElement(table, ObjectHashTable::kNumberOfElementsIndex,
+  StoreFixedArrayElement(table, EphemeronHashTable::kNumberOfElementsIndex,
                          SmiConstant(0), SKIP_WRITE_BARRIER);
-  StoreFixedArrayElement(table, ObjectHashTable::kNumberOfDeletedElementsIndex,
+  StoreFixedArrayElement(table,
+                         EphemeronHashTable::kNumberOfDeletedElementsIndex,
                          SmiConstant(0), SKIP_WRITE_BARRIER);
-  StoreFixedArrayElement(table, ObjectHashTable::kCapacityIndex,
+  StoreFixedArrayElement(table, EphemeronHashTable::kCapacityIndex,
                          SmiFromIntPtr(capacity), SKIP_WRITE_BARRIER);
 
   TNode<IntPtrT> start = KeyIndexFromEntry(IntPtrConstant(0));
@@ -2126,22 +2128,22 @@ TNode<IntPtrT> WeakCollectionsBuiltinsAssembler::KeyIndexFromEntry(
   // See HashTable::KeyAt().
   // (entry * kEntrySize) + kElementsStartIndex + kEntryKeyIndex
   return IntPtrAdd(
-      IntPtrMul(entry, IntPtrConstant(ObjectHashTable::kEntrySize)),
-      IntPtrConstant(ObjectHashTable::kElementsStartIndex +
-                     ObjectHashTable::kEntryKeyIndex));
+      IntPtrMul(entry, IntPtrConstant(EphemeronHashTable::kEntrySize)),
+      IntPtrConstant(EphemeronHashTable::kElementsStartIndex +
+                     EphemeronHashTable::kEntryKeyIndex));
 }
 
 TNode<IntPtrT> WeakCollectionsBuiltinsAssembler::LoadNumberOfElements(
     TNode<HeapObject> table, int offset) {
-  TNode<IntPtrT> number_of_elements = SmiUntag(CAST(
-      LoadFixedArrayElement(table, ObjectHashTable::kNumberOfElementsIndex)));
+  TNode<IntPtrT> number_of_elements = SmiUntag(CAST(LoadFixedArrayElement(
+      table, EphemeronHashTable::kNumberOfElementsIndex)));
   return IntPtrAdd(number_of_elements, IntPtrConstant(offset));
 }
 
 TNode<IntPtrT> WeakCollectionsBuiltinsAssembler::LoadNumberOfDeleted(
     TNode<HeapObject> table, int offset) {
   TNode<IntPtrT> number_of_deleted = SmiUntag(CAST(LoadFixedArrayElement(
-      table, ObjectHashTable::kNumberOfDeletedElementsIndex)));
+      table, EphemeronHashTable::kNumberOfDeletedElementsIndex)));
   return IntPtrAdd(number_of_deleted, IntPtrConstant(offset));
 }
 
@@ -2153,7 +2155,7 @@ TNode<HeapObject> WeakCollectionsBuiltinsAssembler::LoadTable(
 TNode<IntPtrT> WeakCollectionsBuiltinsAssembler::LoadTableCapacity(
     TNode<HeapObject> table) {
   return SmiUntag(
-      CAST(LoadFixedArrayElement(table, ObjectHashTable::kCapacityIndex)));
+      CAST(LoadFixedArrayElement(table, EphemeronHashTable::kCapacityIndex)));
 }
 
 TNode<Word32T> WeakCollectionsBuiltinsAssembler::InsufficientCapacityToAdd(
@@ -2177,16 +2179,17 @@ TNode<Word32T> WeakCollectionsBuiltinsAssembler::InsufficientCapacityToAdd(
 void WeakCollectionsBuiltinsAssembler::RemoveEntry(
     TNode<HeapObject> table, TNode<IntPtrT> key_index,
     TNode<IntPtrT> number_of_elements) {
-  // See ObjectHashTable::RemoveEntry().
+  // See EphemeronHashTable::RemoveEntry().
   TNode<IntPtrT> value_index = ValueIndexFromKeyIndex(key_index);
   StoreFixedArrayElement(table, key_index, TheHoleConstant());
   StoreFixedArrayElement(table, value_index, TheHoleConstant());
 
   // See HashTableBase::ElementRemoved().
   TNode<IntPtrT> number_of_deleted = LoadNumberOfDeleted(table, 1);
-  StoreFixedArrayElement(table, ObjectHashTable::kNumberOfElementsIndex,
+  StoreFixedArrayElement(table, EphemeronHashTable::kNumberOfElementsIndex,
                          SmiFromIntPtr(number_of_elements), SKIP_WRITE_BARRIER);
-  StoreFixedArrayElement(table, ObjectHashTable::kNumberOfDeletedElementsIndex,
+  StoreFixedArrayElement(table,
+                         EphemeronHashTable::kNumberOfDeletedElementsIndex,
                          SmiFromIntPtr(number_of_deleted), SKIP_WRITE_BARRIER);
 }
 
@@ -2216,8 +2219,8 @@ TNode<Word32T> WeakCollectionsBuiltinsAssembler::ShouldShrink(
 TNode<IntPtrT> WeakCollectionsBuiltinsAssembler::ValueIndexFromKeyIndex(
     TNode<IntPtrT> key_index) {
   return IntPtrAdd(key_index,
-                   IntPtrConstant(ObjectHashTableShape::kEntryValueIndex -
-                                  ObjectHashTable::kEntryKeyIndex));
+                   IntPtrConstant(EphemeronHashTableShape::kEntryValueIndex -
+                                  EphemeronHashTable::kEntryKeyIndex));
 }
 
 TF_BUILTIN(WeakMapConstructor, WeakCollectionsBuiltinsAssembler) {
@@ -2291,7 +2294,7 @@ TF_BUILTIN(WeakMapHas, WeakCollectionsBuiltinsAssembler) {
 }
 
 // Helper that removes the entry with a given key from the backing store
-// (ObjectHashTable) of a WeakMap or WeakSet.
+// (EphemeronHashTable) of a WeakMap or WeakSet.
 TF_BUILTIN(WeakCollectionDelete, WeakCollectionsBuiltinsAssembler) {
   TNode<Context> context = CAST(Parameter(Descriptor::kContext));
   TNode<HeapObject> collection = CAST(Parameter(Descriptor::kCollection));
@@ -2320,8 +2323,8 @@ TF_BUILTIN(WeakCollectionDelete, WeakCollectionsBuiltinsAssembler) {
                      SmiTag(hash)));
 }
 
-// Helper that sets the key and value to the backing store (ObjectHashTable) of
-// a WeakMap or WeakSet.
+// Helper that sets the key and value to the backing store (EphemeronHashTable)
+// of a WeakMap or WeakSet.
 TF_BUILTIN(WeakCollectionSet, WeakCollectionsBuiltinsAssembler) {
   TNode<Context> context = CAST(Parameter(Descriptor::kContext));
   TNode<HeapObject> collection = CAST(Parameter(Descriptor::kCollection));

src/heap/mark-compact.cc

@@ -1859,16 +1859,17 @@ void MarkCompactCollector::ProcessWeakCollections() {
     JSWeakCollection* weak_collection =
         reinterpret_cast<JSWeakCollection*>(weak_collection_obj);
     DCHECK(non_atomic_marking_state()->IsBlackOrGrey(weak_collection));
-    if (weak_collection->table()->IsHashTable()) {
-      ObjectHashTable* table = ObjectHashTable::cast(weak_collection->table());
+    if (weak_collection->table()->IsEphemeronHashTable()) {
+      EphemeronHashTable* table =
+          EphemeronHashTable::cast(weak_collection->table());
       for (int i = 0; i < table->Capacity(); i++) {
         HeapObject* heap_object = HeapObject::cast(table->KeyAt(i));
         if (non_atomic_marking_state()->IsBlackOrGrey(heap_object)) {
           Object** key_slot =
-              table->RawFieldOfElementAt(ObjectHashTable::EntryToIndex(i));
+              table->RawFieldOfElementAt(EphemeronHashTable::EntryToIndex(i));
           RecordSlot(table, key_slot, *key_slot);
-          Object** value_slot =
-              table->RawFieldOfElementAt(ObjectHashTable::EntryToValueIndex(i));
+          Object** value_slot = table->RawFieldOfElementAt(
+              EphemeronHashTable::EntryToValueIndex(i));
           if (V8_UNLIKELY(FLAG_track_retaining_path) &&
               (*value_slot)->IsHeapObject()) {
             heap()->AddEphemeralRetainer(heap_object,
@@ -1889,8 +1890,9 @@ void MarkCompactCollector::ClearWeakCollections() {
     JSWeakCollection* weak_collection =
         reinterpret_cast<JSWeakCollection*>(weak_collection_obj);
     DCHECK(non_atomic_marking_state()->IsBlackOrGrey(weak_collection));
-    if (weak_collection->table()->IsHashTable()) {
-      ObjectHashTable* table = ObjectHashTable::cast(weak_collection->table());
+    if (weak_collection->table()->IsEphemeronHashTable()) {
+      EphemeronHashTable* table =
+          EphemeronHashTable::cast(weak_collection->table());
       for (int i = 0; i < table->Capacity(); i++) {
         HeapObject* key = HeapObject::cast(table->KeyAt(i));
         if (!non_atomic_marking_state()->IsBlackOrGrey(key)) {

src/objects-inl.h

@@ -505,6 +505,8 @@ bool HeapObject::IsMapCache() const { return IsHashTable(); }
 
 bool HeapObject::IsObjectHashTable() const { return IsHashTable(); }
 
+bool HeapObject::IsEphemeronHashTable() const { return IsHashTable(); }
+
 bool HeapObject::IsOrderedHashSet() const {
   return map() == GetHeap()->ordered_hash_set_map();
 }
@@ -597,6 +599,7 @@ CAST_ACCESSOR(BoilerplateDescription)
 CAST_ACCESSOR(Cell)
 CAST_ACCESSOR(ConstantElementsPair)
 CAST_ACCESSOR(DescriptorArray)
+CAST_ACCESSOR(EphemeronHashTable)
 CAST_ACCESSOR(EnumCache)
 CAST_ACCESSOR(FeedbackCell)
 CAST_ACCESSOR(Foreign)
@@ -3311,10 +3314,6 @@ Handle<Object> ObjectHashTableShape::AsHandle(Isolate* isolate,
   return key;
 }
 
-Handle<ObjectHashTable> ObjectHashTable::Shrink(Handle<ObjectHashTable> table) {
-  return DerivedHashTable::Shrink(table);
-}
-
 Relocatable::Relocatable(Isolate* isolate) {
   isolate_ = isolate;
   prev_ = isolate->relocatable_top();

src/objects.cc

@@ -18000,23 +18000,24 @@ Object* Dictionary<Derived, Shape>::SlowReverseLookup(Object* value) {
   return isolate->heap()->undefined_value();
 }
 
-
-Object* ObjectHashTable::Lookup(Isolate* isolate, Handle<Object> key,
-                                int32_t hash) {
+template <typename Derived, typename Shape>
+Object* ObjectHashTableBase<Derived, Shape>::Lookup(Isolate* isolate,
+                                                    Handle<Object> key,
+                                                    int32_t hash) {
   DisallowHeapAllocation no_gc;
-  DCHECK(IsKey(isolate, *key));
+  DCHECK(this->IsKey(isolate, *key));
 
-  int entry = FindEntry(isolate, key, hash);
+  int entry = this->FindEntry(isolate, key, hash);
   if (entry == kNotFound) return isolate->heap()->the_hole_value();
-  return get(EntryToIndex(entry) + 1);
+  return this->get(Derived::EntryToIndex(entry) + 1);
 }
 
-
-Object* ObjectHashTable::Lookup(Handle<Object> key) {
+template <typename Derived, typename Shape>
+Object* ObjectHashTableBase<Derived, Shape>::Lookup(Handle<Object> key) {
   DisallowHeapAllocation no_gc;
 
-  Isolate* isolate = GetIsolate();
-  DCHECK(IsKey(isolate, *key));
+  Isolate* isolate = this->GetIsolate();
+  DCHECK(this->IsKey(isolate, *key));
 
   // If the object does not have an identity hash, it was never used as a key.
   Object* hash = key->GetHash();
@@ -18026,18 +18027,21 @@ Object* ObjectHashTable::Lookup(Handle<Object> key) {
   return Lookup(isolate, key, Smi::ToInt(hash));
 }
 
-Object* ObjectHashTable::ValueAt(int entry) {
-  return get(EntryToValueIndex(entry));
+template <typename Derived, typename Shape>
+Object* ObjectHashTableBase<Derived, Shape>::Lookup(Handle<Object> key,
+                                                    int32_t hash) {
+  return Lookup(this->GetIsolate(), key, hash);
 }
 
-Object* ObjectHashTable::Lookup(Handle<Object> key, int32_t hash) {
-  return Lookup(GetIsolate(), key, hash);
+template <typename Derived, typename Shape>
+Object* ObjectHashTableBase<Derived, Shape>::ValueAt(int entry) {
+  return this->get(EntryToValueIndex(entry));
 }
 
-
-Handle<ObjectHashTable> ObjectHashTable::Put(Handle<ObjectHashTable> table,
-                                             Handle<Object> key,
-                                             Handle<Object> value) {
+template <typename Derived, typename Shape>
+Handle<Derived> ObjectHashTableBase<Derived, Shape>::Put(Handle<Derived> table,
+                                                         Handle<Object> key,
+                                                         Handle<Object> value) {
   Isolate* isolate = table->GetIsolate();
   DCHECK(table->IsKey(isolate, *key));
   DCHECK(!value->IsTheHole(isolate));
@@ -18045,14 +18049,14 @@ Handle<ObjectHashTable> ObjectHashTable::Put(Handle<ObjectHashTable> table,
   // Make sure the key object has an identity hash code.
   int32_t hash = key->GetOrCreateHash(isolate)->value();
 
-  return Put(table, key, value, hash);
+  return ObjectHashTableBase<Derived, Shape>::Put(table, key, value, hash);
 }
 
-
-Handle<ObjectHashTable> ObjectHashTable::Put(Handle<ObjectHashTable> table,
-                                             Handle<Object> key,
-                                             Handle<Object> value,
-                                             int32_t hash) {
+template <typename Derived, typename Shape>
+Handle<Derived> ObjectHashTableBase<Derived, Shape>::Put(Handle<Derived> table,
+                                                         Handle<Object> key,
+                                                         Handle<Object> value,
+                                                         int32_t hash) {
   Isolate* isolate = table->GetIsolate();
   DCHECK(table->IsKey(isolate, *key));
   DCHECK(!value->IsTheHole(isolate));
@@ -18061,7 +18065,7 @@ Handle<ObjectHashTable> ObjectHashTable::Put(Handle<ObjectHashTable> table,
 
   // Key is already in table, just overwrite value.
   if (entry != kNotFound) {
-    table->set(EntryToIndex(entry) + 1, *value);
+    table->set(Derived::EntryToIndex(entry) + 1, *value);
     return table;
   }
 
@@ -18086,15 +18090,14 @@ Handle<ObjectHashTable> ObjectHashTable::Put(Handle<ObjectHashTable> table,
   }
 
   // Check whether the hash table should be extended.
-  table = EnsureCapacity(table, 1);
+  table = Derived::EnsureCapacity(table, 1);
   table->AddEntry(table->FindInsertionEntry(hash), *key, *value);
   return table;
 }
 
-
-Handle<ObjectHashTable> ObjectHashTable::Remove(Handle<ObjectHashTable> table,
-                                                Handle<Object> key,
-                                                bool* was_present) {
+template <typename Derived, typename Shape>
+Handle<Derived> ObjectHashTableBase<Derived, Shape>::Remove(
+    Handle<Derived> table, Handle<Object> key, bool* was_present) {
   DCHECK(table->IsKey(table->GetIsolate(), *key));
 
   Object* hash = key->GetHash();
@@ -18106,11 +18109,10 @@ Handle<ObjectHashTable> ObjectHashTable::Remove(Handle<ObjectHashTable> table,
   return Remove(table, key, was_present, Smi::ToInt(hash));
 }
 
-
-Handle<ObjectHashTable> ObjectHashTable::Remove(Handle<ObjectHashTable> table,
-                                                Handle<Object> key,
-                                                bool* was_present,
-                                                int32_t hash) {
+template <typename Derived, typename Shape>
+Handle<Derived> ObjectHashTableBase<Derived, Shape>::Remove(
+    Handle<Derived> table, Handle<Object> key, bool* was_present,
+    int32_t hash) {
   Isolate* isolate = table->GetIsolate();
   DCHECK(table->IsKey(isolate, *key));
 
@@ -18122,21 +18124,22 @@ Handle<ObjectHashTable> ObjectHashTable::Remove(Handle<ObjectHashTable> table,
 
   *was_present = true;
   table->RemoveEntry(entry);
-  return Shrink(table);
+  return Derived::Shrink(table);
 }
 
-
-void ObjectHashTable::AddEntry(int entry, Object* key, Object* value) {
-  set(EntryToIndex(entry), key);
-  set(EntryToIndex(entry) + 1, value);
-  ElementAdded();
+template <typename Derived, typename Shape>
+void ObjectHashTableBase<Derived, Shape>::AddEntry(int entry, Object* key,
+                                                   Object* value) {
+  this->set(Derived::EntryToIndex(entry), key);
+  this->set(Derived::EntryToIndex(entry) + 1, value);
+  this->ElementAdded();
 }
 
-
-void ObjectHashTable::RemoveEntry(int entry) {
-  set_the_hole(EntryToIndex(entry));
-  set_the_hole(EntryToIndex(entry) + 1);
-  ElementRemoved();
+template <typename Derived, typename Shape>
+void ObjectHashTableBase<Derived, Shape>::RemoveEntry(int entry) {
+  this->set_the_hole(Derived::EntryToIndex(entry));
+  this->set_the_hole(Derived::EntryToIndex(entry) + 1);
+  this->ElementRemoved();
 }
 
 
@@ -18168,7 +18171,7 @@ void JSMap::Clear(Handle<JSMap> map) {
 
 void JSWeakCollection::Initialize(Handle<JSWeakCollection> weak_collection,
                                   Isolate* isolate) {
-  Handle<ObjectHashTable> table = ObjectHashTable::New(isolate, 0);
+  Handle<EphemeronHashTable> table = EphemeronHashTable::New(isolate, 0);
   weak_collection->set_table(*table);
 }
 
@@ -18177,11 +18180,11 @@ void JSWeakCollection::Set(Handle<JSWeakCollection> weak_collection,
                            Handle<Object> key, Handle<Object> value,
                            int32_t hash) {
   DCHECK(key->IsJSReceiver() || key->IsSymbol());
-  Handle<ObjectHashTable> table(
-      ObjectHashTable::cast(weak_collection->table()));
+  Handle<EphemeronHashTable> table(
+      EphemeronHashTable::cast(weak_collection->table()));
   DCHECK(table->IsKey(table->GetIsolate(), *key));
-  Handle<ObjectHashTable> new_table =
-      ObjectHashTable::Put(table, key, value, hash);
+  Handle<EphemeronHashTable> new_table =
+      EphemeronHashTable::Put(table, key, value, hash);
   weak_collection->set_table(*new_table);
   if (*table != *new_table) {
     // Zap the old table since we didn't record slots for its elements.
@@ -18193,12 +18196,12 @@ void JSWeakCollection::Set(Handle<JSWeakCollection> weak_collection,
 bool JSWeakCollection::Delete(Handle<JSWeakCollection> weak_collection,
                               Handle<Object> key, int32_t hash) {
   DCHECK(key->IsJSReceiver() || key->IsSymbol());
-  Handle<ObjectHashTable> table(
-      ObjectHashTable::cast(weak_collection->table()));
+  Handle<EphemeronHashTable> table(
+      EphemeronHashTable::cast(weak_collection->table()));
   DCHECK(table->IsKey(table->GetIsolate(), *key));
   bool was_present = false;
-  Handle<ObjectHashTable> new_table =
-      ObjectHashTable::Remove(table, key, &was_present, hash);
+  Handle<EphemeronHashTable> new_table =
+      EphemeronHashTable::Remove(table, key, &was_present, hash);
   weak_collection->set_table(*new_table);
   if (*table != *new_table) {
     // Zap the old table since we didn't record slots for its elements.
@@ -18210,7 +18213,7 @@ bool JSWeakCollection::Delete(Handle<JSWeakCollection> weak_collection,
 Handle<JSArray> JSWeakCollection::GetEntries(Handle<JSWeakCollection> holder,
                                              int max_entries) {
   Isolate* isolate = holder->GetIsolate();
-  Handle<ObjectHashTable> table(ObjectHashTable::cast(holder->table()));
+  Handle<EphemeronHashTable> table(EphemeronHashTable::cast(holder->table()));
   if (max_entries == 0 || max_entries > table->NumberOfElements()) {
     max_entries = table->NumberOfElements();
   }
@@ -18870,6 +18873,12 @@ template class HashTable<CompilationCacheTable, CompilationCacheShape>;
 
 template class HashTable<ObjectHashTable, ObjectHashTableShape>;
 
+template class HashTable<EphemeronHashTable, EphemeronHashTableShape>;
+
+template class ObjectHashTableBase<ObjectHashTable, ObjectHashTableShape>;
+
+template class ObjectHashTableBase<EphemeronHashTable, EphemeronHashTableShape>;
+
 template class Dictionary<NameDictionary, NameDictionaryShape>;
 
 template class Dictionary<GlobalDictionary, GlobalDictionaryShape>;

src/objects.h

@@ -1054,6 +1054,7 @@ template <class C> inline bool Is(Object* obj);
   V(DeoptimizationData)                        \
   V(DependentCode)                             \
   V(DescriptorArray)                           \
+  V(EphemeronHashTable)                        \
   V(EnumCache)                                 \
   V(External)                                  \
   V(ExternalOneByteString)                     \

src/objects/hash-table.h

@@ -269,19 +269,9 @@ class ObjectHashTableShape : public BaseShape<Handle<Object>> {
   static const bool kNeedsHoleCheck = false;
 };
 
-// ObjectHashTable maps keys that are arbitrary objects to object values by
-// using the identity hash of the key for hashing purposes.
-class ObjectHashTable
-    : public HashTable<ObjectHashTable, ObjectHashTableShape> {
-  typedef HashTable<ObjectHashTable, ObjectHashTableShape> DerivedHashTable;
-
+template <typename Derived, typename Shape>
+class ObjectHashTableBase : public HashTable<Derived, Shape> {
  public:
-  DECL_CAST(ObjectHashTable)
-
-  // Attempt to shrink hash table after removal of key.
-  V8_WARN_UNUSED_RESULT static inline Handle<ObjectHashTable> Shrink(
-      Handle<ObjectHashTable> table);
-
   // Looks up the value associated with the given key. The hole value is
   // returned in case the key is not present.
   Object* Lookup(Handle<Object> key);
@@ -292,31 +282,51 @@ class ObjectHashTable
   Object* ValueAt(int entry);
 
   // Adds (or overwrites) the value associated with the given key.
-  static Handle<ObjectHashTable> Put(Handle<ObjectHashTable> table,
-                                     Handle<Object> key, Handle<Object> value);
-  static Handle<ObjectHashTable> Put(Handle<ObjectHashTable> table,
-                                     Handle<Object> key, Handle<Object> value,
-                                     int32_t hash);
+  static Handle<Derived> Put(Handle<Derived> table, Handle<Object> key,
+                             Handle<Object> value);
+  static Handle<Derived> Put(Handle<Derived> table, Handle<Object> key,
+                             Handle<Object> value, int32_t hash);
 
   // Returns an ObjectHashTable (possibly |table|) where |key| has been removed.
-  static Handle<ObjectHashTable> Remove(Handle<ObjectHashTable> table,
-                                        Handle<Object> key, bool* was_present);
-  static Handle<ObjectHashTable> Remove(Handle<ObjectHashTable> table,
-                                        Handle<Object> key, bool* was_present,
-                                        int32_t hash);
+  static Handle<Derived> Remove(Handle<Derived> table, Handle<Object> key,
+                                bool* was_present);
+  static Handle<Derived> Remove(Handle<Derived> table, Handle<Object> key,
+                                bool* was_present, int32_t hash);
 
   // Returns the index to the value of an entry.
   static inline int EntryToValueIndex(int entry) {
-    return EntryToIndex(entry) + ObjectHashTableShape::kEntryValueIndex;
+    return HashTable<Derived, Shape>::EntryToIndex(entry) +
+           Shape::kEntryValueIndex;
   }
 
  protected:
-  friend class MarkCompactCollector;
-
   void AddEntry(int entry, Object* key, Object* value);
   void RemoveEntry(int entry);
 };
 
+// ObjectHashTable maps keys that are arbitrary objects to object values by
+// using the identity hash of the key for hashing purposes.
+class ObjectHashTable
+    : public ObjectHashTableBase<ObjectHashTable, ObjectHashTableShape> {
+ public:
+  DECL_CAST(ObjectHashTable)
+};
+
+typedef ObjectHashTableShape EphemeronHashTableShape;
+
+// EphemeronHashTable is similar to ObjectHashTable but gets special treatment
+// by the GC. The GC treats its entries as ephemerons: both key and value are
+// weak references, however if the key is strongly reachable its corresponding
+// value is also kept alive.
+class EphemeronHashTable
+    : public ObjectHashTableBase<EphemeronHashTable, EphemeronHashTableShape> {
+ public:
+  DECL_CAST(EphemeronHashTable)
+
+ protected:
+  friend class MarkCompactCollector;
+};
+
 class ObjectHashSetShape : public ObjectHashTableShape {
  public:
   static const int kPrefixSize = 0;

src/profiler/heap-snapshot-generator.cc

@@ -1038,7 +1038,7 @@ void V8HeapExplorer::ExtractJSCollectionReferences(int entry,
 void V8HeapExplorer::ExtractJSWeakCollectionReferences(int entry,
                                                        JSWeakCollection* obj) {
   if (obj->table()->IsHashTable()) {
-    ObjectHashTable* table = ObjectHashTable::cast(obj->table());
+    EphemeronHashTable* table = EphemeronHashTable::cast(obj->table());
     TagFixedArraySubType(table, JS_WEAK_COLLECTION_SUB_TYPE);
   }
   SetInternalReference(obj, entry, "table", obj->table(),
@@ -1361,11 +1361,11 @@ void V8HeapExplorer::ExtractFixedArrayReferences(int entry, FixedArray* array) {
   }
   switch (it->second) {
     case JS_WEAK_COLLECTION_SUB_TYPE: {
-      ObjectHashTable* table = ObjectHashTable::cast(array);
+      EphemeronHashTable* table = EphemeronHashTable::cast(array);
       for (int i = 0, capacity = table->Capacity(); i < capacity; ++i) {
-        int key_index =
-            ObjectHashTable::EntryToIndex(i) + ObjectHashTable::kEntryKeyIndex;
-        int value_index = ObjectHashTable::EntryToValueIndex(i);
+        int key_index = EphemeronHashTable::EntryToIndex(i) +
+                        EphemeronHashTable::kEntryKeyIndex;
+        int value_index = EphemeronHashTable::EntryToValueIndex(i);
         Object* key = table->get(key_index);
         Object* value = table->get(value_index);
         SetWeakReference(table, entry, key_index, key,

src/runtime/runtime-collections.cc

@@ -98,9 +98,9 @@ RUNTIME_FUNCTION(Runtime_WeakCollectionDelete) {
 
 #ifdef DEBUG
   DCHECK(key->IsJSReceiver());
-  DCHECK(ObjectHashTableShape::IsLive(isolate, *key));
-  Handle<ObjectHashTable> table(
-      ObjectHashTable::cast(weak_collection->table()));
+  DCHECK(EphemeronHashTableShape::IsLive(isolate, *key));
+  Handle<EphemeronHashTable> table(
+      EphemeronHashTable::cast(weak_collection->table()));
   // Should only be called when shrinking the table is necessary. See
   // HashTable::Shrink().
   DCHECK(table->NumberOfElements() - 1 <= (table->Capacity() >> 2) &&
@@ -130,11 +130,11 @@ RUNTIME_FUNCTION(Runtime_WeakCollectionSet) {
 
 #ifdef DEBUG
   DCHECK(key->IsJSReceiver());
-  DCHECK(ObjectHashTableShape::IsLive(isolate, *key));
-  Handle<ObjectHashTable> table(
-      ObjectHashTable::cast(weak_collection->table()));
+  DCHECK(EphemeronHashTableShape::IsLive(isolate, *key));
+  Handle<EphemeronHashTable> table(
+      EphemeronHashTable::cast(weak_collection->table()));
   // Should only be called when rehashing or resizing the table is necessary.
-  // See ObjectHashTable::Put() and HashTable::HasSufficientCapacityToAdd().
+  // See EphemeronHashTable::Put() and HashTable::HasSufficientCapacityToAdd().
   DCHECK((table->NumberOfDeletedElements() << 1) > table->NumberOfElements() ||
          !table->HasSufficientCapacityToAdd(1));
 #endif

test/cctest/test-weakmaps.cc

@@ -85,14 +85,14 @@ TEST(Weakness) {
     int32_t object_hash = object->GetOrCreateHash(isolate)->value();
     JSWeakCollection::Set(weakmap, object, smi, object_hash);
   }
-  CHECK_EQ(2, ObjectHashTable::cast(weakmap->table())->NumberOfElements());
+  CHECK_EQ(2, EphemeronHashTable::cast(weakmap->table())->NumberOfElements());
 
   // Force a full GC.
   CcTest::CollectAllGarbage(Heap::kAbortIncrementalMarkingMask);
   CHECK_EQ(0, NumberOfWeakCalls);
-  CHECK_EQ(2, ObjectHashTable::cast(weakmap->table())->NumberOfElements());
+  CHECK_EQ(2, EphemeronHashTable::cast(weakmap->table())->NumberOfElements());
   CHECK_EQ(
-      0, ObjectHashTable::cast(weakmap->table())->NumberOfDeletedElements());
+      0, EphemeronHashTable::cast(weakmap->table())->NumberOfDeletedElements());
 
   // Make the global reference to the key weak.
   std::pair<Handle<Object>*, int> handle_and_id(&key, 1234);
@@ -103,9 +103,9 @@ TEST(Weakness) {
 
   CcTest::CollectAllGarbage(Heap::kAbortIncrementalMarkingMask);
   CHECK_EQ(1, NumberOfWeakCalls);
-  CHECK_EQ(0, ObjectHashTable::cast(weakmap->table())->NumberOfElements());
-  CHECK_EQ(2,
-           ObjectHashTable::cast(weakmap->table())->NumberOfDeletedElements());
+  CHECK_EQ(0, EphemeronHashTable::cast(weakmap->table())->NumberOfElements());
+  CHECK_EQ(
+      2, EphemeronHashTable::cast(weakmap->table())->NumberOfDeletedElements());
 }
 
 
@@ -117,7 +117,7 @@ TEST(Shrinking) {
   Handle<JSWeakMap> weakmap = isolate->factory()->NewJSWeakMap();
 
   // Check initial capacity.
-  CHECK_EQ(32, ObjectHashTable::cast(weakmap->table())->Capacity());
+  CHECK_EQ(32, EphemeronHashTable::cast(weakmap->table())->Capacity());
 
   // Fill up weak map to trigger capacity change.
   {
@@ -132,19 +132,20 @@ TEST(Shrinking) {
   }
 
   // Check increased capacity.
-  CHECK_EQ(128, ObjectHashTable::cast(weakmap->table())->Capacity());
+  CHECK_EQ(128, EphemeronHashTable::cast(weakmap->table())->Capacity());
 
   // Force a full GC.
-  CHECK_EQ(32, ObjectHashTable::cast(weakmap->table())->NumberOfElements());
+  CHECK_EQ(32, EphemeronHashTable::cast(weakmap->table())->NumberOfElements());
   CHECK_EQ(
-      0, ObjectHashTable::cast(weakmap->table())->NumberOfDeletedElements());
+      0, EphemeronHashTable::cast(weakmap->table())->NumberOfDeletedElements());
   CcTest::CollectAllGarbage(Heap::kAbortIncrementalMarkingMask);
-  CHECK_EQ(0, ObjectHashTable::cast(weakmap->table())->NumberOfElements());
+  CHECK_EQ(0, EphemeronHashTable::cast(weakmap->table())->NumberOfElements());
   CHECK_EQ(
-      32, ObjectHashTable::cast(weakmap->table())->NumberOfDeletedElements());
+      32,
+      EphemeronHashTable::cast(weakmap->table())->NumberOfDeletedElements());
 
   // Check shrunk capacity.
-  CHECK_EQ(32, ObjectHashTable::cast(weakmap->table())->Capacity());
+  CHECK_EQ(32, EphemeronHashTable::cast(weakmap->table())->Capacity());
 }
 
 

test/cctest/test-weaksets.cc

@@ -53,7 +53,7 @@ static Handle<JSWeakSet> AllocateJSWeakSet(Isolate* isolate) {
   // Do not leak handles for the hash table, it would make entries strong.
   {
     HandleScope scope(isolate);
-    Handle<ObjectHashTable> table = ObjectHashTable::New(isolate, 1);
+    Handle<EphemeronHashTable> table = EphemeronHashTable::New(isolate, 1);
     weakset->set_table(*table);
   }
   return weakset;
@@ -96,14 +96,14 @@ TEST(WeakSet_Weakness) {
     int32_t hash = key->GetOrCreateHash(isolate)->value();
     JSWeakCollection::Set(weakset, key, smi, hash);
   }
-  CHECK_EQ(1, ObjectHashTable::cast(weakset->table())->NumberOfElements());
+  CHECK_EQ(1, EphemeronHashTable::cast(weakset->table())->NumberOfElements());
 
   // Force a full GC.
   CcTest::CollectAllGarbage(Heap::kAbortIncrementalMarkingMask);
   CHECK_EQ(0, NumberOfWeakCalls);
-  CHECK_EQ(1, ObjectHashTable::cast(weakset->table())->NumberOfElements());
+  CHECK_EQ(1, EphemeronHashTable::cast(weakset->table())->NumberOfElements());
   CHECK_EQ(
-      0, ObjectHashTable::cast(weakset->table())->NumberOfDeletedElements());
+      0, EphemeronHashTable::cast(weakset->table())->NumberOfDeletedElements());
 
   // Make the global reference to the key weak.
   std::pair<Handle<Object>*, int> handle_and_id(&key, 1234);
@@ -114,9 +114,9 @@ TEST(WeakSet_Weakness) {
 
   CcTest::CollectAllGarbage(Heap::kAbortIncrementalMarkingMask);
   CHECK_EQ(1, NumberOfWeakCalls);
-  CHECK_EQ(0, ObjectHashTable::cast(weakset->table())->NumberOfElements());
+  CHECK_EQ(0, EphemeronHashTable::cast(weakset->table())->NumberOfElements());
   CHECK_EQ(
-      1, ObjectHashTable::cast(weakset->table())->NumberOfDeletedElements());
+      1, EphemeronHashTable::cast(weakset->table())->NumberOfDeletedElements());
 }
 
 
@@ -128,7 +128,7 @@ TEST(WeakSet_Shrinking) {
   Handle<JSWeakSet> weakset = AllocateJSWeakSet(isolate);
 
   // Check initial capacity.
-  CHECK_EQ(32, ObjectHashTable::cast(weakset->table())->Capacity());
+  CHECK_EQ(32, EphemeronHashTable::cast(weakset->table())->Capacity());
 
   // Fill up weak set to trigger capacity change.
   {
@@ -143,19 +143,20 @@ TEST(WeakSet_Shrinking) {
   }
 
   // Check increased capacity.
-  CHECK_EQ(128, ObjectHashTable::cast(weakset->table())->Capacity());
+  CHECK_EQ(128, EphemeronHashTable::cast(weakset->table())->Capacity());
 
   // Force a full GC.
-  CHECK_EQ(32, ObjectHashTable::cast(weakset->table())->NumberOfElements());
+  CHECK_EQ(32, EphemeronHashTable::cast(weakset->table())->NumberOfElements());
   CHECK_EQ(
-      0, ObjectHashTable::cast(weakset->table())->NumberOfDeletedElements());
+      0, EphemeronHashTable::cast(weakset->table())->NumberOfDeletedElements());
   CcTest::CollectAllGarbage(Heap::kAbortIncrementalMarkingMask);
-  CHECK_EQ(0, ObjectHashTable::cast(weakset->table())->NumberOfElements());
+  CHECK_EQ(0, EphemeronHashTable::cast(weakset->table())->NumberOfElements());
   CHECK_EQ(
-      32, ObjectHashTable::cast(weakset->table())->NumberOfDeletedElements());
+      32,
+      EphemeronHashTable::cast(weakset->table())->NumberOfDeletedElements());
 
   // Check shrunk capacity.
-  CHECK_EQ(32, ObjectHashTable::cast(weakset->table())->Capacity());
+  CHECK_EQ(32, EphemeronHashTable::cast(weakset->table())->Capacity());
 }