The Elements pointer in a JSObject can have a filler map instead of a

src/objects-inl.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 5498 matching lines @@
   ASSERT(index >= kDataIndex);  // Only implementation data can be set this way.
   FixedArray::cast(data())->set(index, value);
 }
 
 
 ElementsKind JSObject::GetElementsKind() {
   ElementsKind kind = map()->elements_kind();
 #if DEBUG
   FixedArrayBase* fixed_array =
       reinterpret_cast<FixedArrayBase*>(READ_FIELD(this, kElementsOffset));
-  Map* map = fixed_array->map();
-  ASSERT((IsFastSmiOrObjectElementsKind(kind) &&
-          (map == GetHeap()->fixed_array_map() ||
-           map == GetHeap()->fixed_cow_array_map())) ||
-         (IsFastDoubleElementsKind(kind) &&
-          (fixed_array->IsFixedDoubleArray() ||
-           fixed_array == GetHeap()->empty_fixed_array())) ||
-         (kind == DICTIONARY_ELEMENTS &&
+
+  // If a GC was caused while constructing this object, the elements
+  // pointer may point to a one pointer filler map.
+  if (ElementsAreSafeToExamine()) {
+    Map* map = fixed_array->map();
+    ASSERT((IsFastSmiOrObjectElementsKind(kind) &&
+            (map == GetHeap()->fixed_array_map() ||
+             map == GetHeap()->fixed_cow_array_map())) ||
+           (IsFastDoubleElementsKind(kind) &&
+            (fixed_array->IsFixedDoubleArray() ||
+             fixed_array == GetHeap()->empty_fixed_array())) ||
+           (kind == DICTIONARY_ELEMENTS &&
             fixed_array->IsFixedArray() &&
-          fixed_array->IsDictionary()) ||
-         (kind > DICTIONARY_ELEMENTS));
-  ASSERT((kind != NON_STRICT_ARGUMENTS_ELEMENTS) ||
-         (elements()->IsFixedArray() && elements()->length() >= 2));
+            fixed_array->IsDictionary()) ||
+           (kind > DICTIONARY_ELEMENTS));
+    ASSERT((kind != NON_STRICT_ARGUMENTS_ELEMENTS) ||
+           (elements()->IsFixedArray() && elements()->length() >= 2));
+  }
 #endif
   return kind;
 }
 
 
 ElementsAccessor* JSObject::GetElementsAccessor() {
   return ElementsAccessor::ForKind(GetElementsKind());
 }
 
 
@@ skipping 851 matching lines @@
 #undef WRITE_UINT32_FIELD
 #undef READ_SHORT_FIELD
 #undef WRITE_SHORT_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_INL_H_