ClearNonLiveTransitions has to hold on to non-map values.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 1544836fe3bac3c4e796f1abd22d9b1a74874eec..4133f97cae7f79018b2e2eb3e0d6b4c6a031e52d 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -7399,7 +7399,10 @@ void String::PrintOn(FILE* file) {
 // Return true in case a back pointer has been cleared and false otherwise.
 // Set *keep_entry to true when a live map transition has been found.
 static bool ClearBackPointer(Heap* heap, Object* target, bool* keep_entry) {
-  if (!target->IsMap()) return false;
+  if (!target->IsMap()) {
+    *keep_entry = true;

[Michael Starzinger, 2012/06/05 09:12:48]

I don't think this fix is completely correct, because in the ELEMENTS_TRANSITION
case we might have a slot in the backing store that is undefined. In this case
the backing store might be kept even though it is empty.

[Toon Verwaest, 2012/06/05 10:29:16]

Done.

+    return false;
+  }
   Map* map = Map::cast(target);
   if (Marking::MarkBitFrom(map).Get()) {
     *keep_entry = true;