Rehash and clear deleted entries in weak collections during GC

Otherwise, they'll just keep growing until we run out of memory or hit the FixedArray's maximum capacity.

BUG=v8:4909
R=hpayer@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1877233005

Cr-Commit-Position: refs/heads/master@{#35514}

src/heap/mark-compact.cc

@@ -2572,6 +2572,13 @@ void MarkCompactCollector::ClearWeakCollections() {
           table->RemoveEntry(i);
         }
       }
+      // Rehash if more than 25% of the entries are deleted entries.
+      // TODO(jochen): Consider to shrink the fixed array in place.
+      if ((table->NumberOfDeletedElements() << kJSWeakCollectionLoadFactorExp) >
+          table->NumberOfElements()) {
+        HandleScope scope(heap()->isolate());
+        table->Rehash(heap()->isolate()->factory()->undefined_value());
+      }
     }
     weak_collection_obj = weak_collection->next();
     weak_collection->set_next(heap()->undefined_value());

src/heap/mark-compact.h

@@ -512,6 +512,10 @@ class MarkCompactCollector {
   static const uint32_t kSingleFreeEncoding = 0;
   static const uint32_t kMultiFreeEncoding = 1;
 
+  // If the number of deleted slots in a JSWeakCollection exceeds the number
+  // of entries / 2^(factor), we rehash the table.
+  static const int kJSWeakCollectionLoadFactorExp = 1;
+
   static inline bool IsMarked(Object* obj);
   static bool IsUnmarkedHeapObjectWithHeap(Heap* heap, Object** p);
 

src/objects.cc

@@ -16860,6 +16860,16 @@ void HashTable<Derived, Shape, Key>::Rehash(Key key) {
       }
     }
   }
+  // Wipe deleted entries.
+  Heap* heap = GetHeap();
+  Object* the_hole = heap->the_hole_value();
+  Object* undefined = heap->undefined_value();
+  for (uint32_t current = 0; current < capacity; current++) {
+    if (get(EntryToIndex(current)) == the_hole) {
+      set(EntryToIndex(current), undefined);
+    }
+  }
+  SetNumberOfDeletedElements(0);
 }
 
 

test/cctest/test-weakmaps.cc

@@ -124,7 +124,7 @@ TEST(Weakness) {
   heap->CollectAllGarbage(false);
   CHECK_EQ(1, NumberOfWeakCalls);
   CHECK_EQ(0, ObjectHashTable::cast(weakmap->table())->NumberOfElements());
-  CHECK_EQ(2,
+  CHECK_EQ(0,
            ObjectHashTable::cast(weakmap->table())->NumberOfDeletedElements());
 }
 

test/cctest/test-weaksets.cc

@@ -123,8 +123,8 @@ TEST(WeakSet_Weakness) {
   heap->CollectAllGarbage(false);
   CHECK_EQ(1, NumberOfWeakCalls);
   CHECK_EQ(0, ObjectHashTable::cast(weakset->table())->NumberOfElements());
-  CHECK_EQ(
-      1, ObjectHashTable::cast(weakset->table())->NumberOfDeletedElements());
+  CHECK_EQ(0,
+           ObjectHashTable::cast(weakset->table())->NumberOfDeletedElements());
 }