Only mark the descriptor that is valid for the map in question. If this map

src/objects-visiting-inl.h

Index: src/objects-visiting-inl.h
diff --git a/src/objects-visiting-inl.h b/src/objects-visiting-inl.h
index 7332a0c8d6ad591df61a2e7c3195e103de1cfdf5..45cb4504bf0057626a914bc54d0d2b454159a1f3 100644
--- a/src/objects-visiting-inl.h
+++ b/src/objects-visiting-inl.h
@@ -396,6 +396,33 @@ void StaticMarkingVisitor<StaticVisitor>::MarkMapContents(
     ASSERT(transitions->IsMap() || transitions->IsUndefined());
   }
 
+  // Since descriptor arrays are potentially shared, ensure that only the
+  // descriptors that appeared for this map are marked. The first time a
+  // non-empty descriptor array is marked, its header is also marked. The slot

[Michael Starzinger, 2013/01/31 10:44:09]

s/its header is also marked/its header is also visited/

[Toon Verwaest, 2013/01/31 10:50:02]

Done.

+  // holding the descriptor array will be implicitly recorded when the pointer
+  // fields of this map are visited.
+  DescriptorArray* descriptors = map->instance_descriptors();
+  if (StaticVisitor::MarkObjectWithoutPush(heap, descriptors) &&
+      descriptors->length() > 0) {
+    StaticVisitor::VisitPointers(heap,
+      descriptors->GetFirstElementAddress(),

[Michael Starzinger, 2013/01/31 10:44:09]

Indent by 4 instead of 2 characters.

[Toon Verwaest, 2013/01/31 10:50:02]

Done.

+      descriptors->GetDescriptorEndSlot(0));
+  }
+  int start = 0;
+  int end = map->NumberOfOwnDescriptors();
+  Object* back_pointer = map->GetBackPointer();
+  if (!back_pointer->IsUndefined()) {
+    Map* parent_map = Map::cast(back_pointer);
+    if (descriptors == parent_map->instance_descriptors()) {
+      start = parent_map->NumberOfOwnDescriptors();
+    }
+  }
+  if (start < end) {
+    StaticVisitor::VisitPointers(heap,
+        descriptors->GetDescriptorStartSlot(start),
+        descriptors->GetDescriptorEndSlot(end));
+  }
+
   // Mark prototype dependent codes array but do not push it onto marking
   // stack, this will make references from it weak. We will clean dead
   // codes when we iterate over maps in ClearNonLiveTransitions.