Do not compact weak fixed array when re-allocating new backing store.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 1f25a5d11b7dcd788a6541e41781eb98e826f6b7..ed394ad0ca8c79c75cd6f964c151e111afaef537 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -7856,28 +7856,22 @@ Handle<WeakFixedArray> WeakFixedArray::Allocate(
   DCHECK(0 <= size);
   Handle<FixedArray> result =
       isolate->factory()->NewUninitializedFixedArray(size + kFirstIndex);
-  Handle<WeakFixedArray> casted_result = Handle<WeakFixedArray>::cast(result);
-  if (initialize_from.is_null()) {
-    for (int i = 0; i < result->length(); ++i) {
-      result->set(i, Smi::FromInt(0));
-    }
-  } else {
+  int index = 0;
+  if (!initialize_from.is_null()) {
     DCHECK(initialize_from->Length() <= size);
     Handle<FixedArray> raw_source = Handle<FixedArray>::cast(initialize_from);
-    int target_index = kFirstIndex;
-    for (int source_index = kFirstIndex; source_index < raw_source->length();
-         ++source_index) {
-      // The act of allocating might have caused entries in the source array
-      // to be cleared. Copy only what's needed.
-      if (initialize_from->IsEmptySlot(source_index - kFirstIndex)) continue;
-      result->set(target_index++, raw_source->get(source_index));
-    }
-    casted_result->set_last_used_index(target_index - 1 - kFirstIndex);
-    for (; target_index < result->length(); ++target_index) {
-      result->set(target_index, Smi::FromInt(0));
+    // Copy the entries without compacting, since the PrototypeInfo relies on
+    // the index of the entries not to change.
+    while (index < raw_source->length()) {
+      result->set(index, raw_source->get(index));
+      index++;
     }

[mvstanton, 2015/08/19 13:52:12]

How about calling set_last_used_index(index); here.

   }
-  return casted_result;
+  while (index < result->length()) {
+    result->set(index, Smi::FromInt(0));
+    index++;
+  }
+  return Handle<WeakFixedArray>::cast(result);
 }