[runtime] Speed up C++ version of ArrayPush

src/elements.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/elements.h"
 
 #include "src/arguments.h"
 #include "src/conversions.h"
 #include "src/factory.h"
 #include "src/messages.h"
@@ skipping 687 matching lines @@
       }
     }
 
     // Check whether the backing store should be shrunk.
     uint32_t capacity = backing_store->length();
     old_length = Min(old_length, capacity);
     if (length == 0) {
       array->initialize_elements();
     } else if (length <= capacity) {
       if (array->HasFastSmiOrObjectElements()) {
-        backing_store = JSObject::EnsureWritableFastElements(array);
+        JSObject::EnsureWritableFastElements(array);
+        if (array->elements() != *backing_store) {
+          backing_store = handle(array->elements(), isolate);
+        }
       }
       if (2 * length <= capacity) {
         // If more than half the elements won't be used, trim the array.
         isolate->heap()->RightTrimFixedArray<Heap::CONCURRENT_TO_SWEEPER>(
             *backing_store, capacity - length);
       } else {
         // Otherwise, fill the unused tail with holes.
         for (uint32_t i = length; i < old_length; i++) {
           BackingStore::cast(*backing_store)->set_the_hole(i);
         }
@@ skipping 1958 matching lines @@
     }
   }
 
   DCHECK(j == result_len);
   return result_array;
 }
 
 ElementsAccessor** ElementsAccessor::elements_accessors_ = NULL;
 }  // namespace internal
 }  // namespace v8