Make sure when we shrink an object that we store a filler first into the free memory before updatin…

src/elements.cc

 // 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 985 matching lines @@
     // Check whether the backing store should be shrunk.
     if (length <= old_capacity) {
       if (array->HasFastSmiOrObjectElements()) {
         backing_store = JSObject::EnsureWritableFastElements(array);
       }
       if (2 * length <= old_capacity) {
         // If more than half the elements won't be used, trim the array.
         if (length == 0) {
           array->initialize_elements();
         } else {
-          backing_store->set_length(length);
+          int filler_size = (old_capacity - length) * ElementSize;
           Address filler_start = backing_store->address() +
               BackingStore::OffsetOfElementAt(length);
-          int filler_size = (old_capacity - length) * ElementSize;
           array->GetHeap()->CreateFillerObjectAt(filler_start, filler_size);
+
+          // We are storing the new length using release store after creating a
+          // filler for the left-over space to avoid races with the sweeper
+          // thread.
+          backing_store->synchronized_set_length(length);
         }
       } else {
         // Otherwise, fill the unused tail with holes.
         int old_length = FastD2IChecked(array->length()->Number());
         for (int i = length; i < old_length; i++) {
           Handle<BackingStore>::cast(backing_store)->set_the_hole(i);
         }
       }
       return length_object;
     }
@@ skipping 1057 matching lines @@
       UNREACHABLE();
       break;
   }
 
   array->set_elements(*elms);
   array->set_length(Smi::FromInt(number_of_elements));
   return array;
 }
 
 } }  // namespace v8::internal