Move map retaining to finalization of incremental marking.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index bd521794ede771ce1d48363d80803c6bc5154165..c6ebc0dccea50d4bb4eae4fe8e346b2c1a9282d5 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -5332,6 +5332,9 @@ DependentCode* Heap::LookupWeakObjectToCodeDependency(Handle<HeapObject> obj) {
 void Heap::AddRetainedMap(Handle<Map> map) {
   Handle<WeakCell> cell = Map::WeakCellForMap(map);
   Handle<ArrayList> array(retained_maps(), isolate());
+  if (array->IsFull()) {
+    CompactRetainedMaps(*array);
+  }
   array = ArrayList::Add(
       array, cell, handle(Smi::FromInt(FLAG_retain_maps_for_n_gc), isolate()),
       ArrayList::kReloadLengthAfterAllocation);
@@ -5341,6 +5344,35 @@ void Heap::AddRetainedMap(Handle<Map> map) {
 }
 
 
+void Heap::CompactRetainedMaps(ArrayList* retained_maps) {
+  DCHECK_EQ(retained_maps, this->retained_maps());
+  int length = retained_maps->Length();
+  int new_length = 0;
+  int new_number_of_disposed_maps = 0;
+  // This loop compacts the array by removing cleared weak cells.
+  for (int i = 0; i < length; i += 2) {
+    DCHECK(retained_maps->Get(i)->IsWeakCell());
+    WeakCell* cell = WeakCell::cast(retained_maps->Get(i));
+    Object* age = retained_maps->Get(i + 1);
+    if (cell->cleared()) continue;
+    if (i != new_length) {
+      retained_maps->Set(new_length, cell);
+      retained_maps->Set(new_length + 1, age);
+    }
+    if (i < number_of_disposed_maps_) {
+      new_number_of_disposed_maps += 2;
+    }
+    new_length += 2;
+  }
+  number_of_disposed_maps_ = new_number_of_disposed_maps;
+  Object* undefined = undefined_value();
+  for (int i = new_length; i < length; i++) {
+    retained_maps->Clear(i, undefined);
+  }
+  if (new_length != length) retained_maps->SetLength(new_length);
+}
+
+
 void Heap::FatalProcessOutOfMemory(const char* location, bool take_snapshot) {
   v8::internal::V8::FatalProcessOutOfMemory(location, take_snapshot);
 }