Keep maps when resetting elements.

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/v8.h"
 
 #include "src/arguments.h"
 #include "src/conversions.h"
 #include "src/elements.h"
 #include "src/objects.h"
@@ skipping 1418 matching lines @@
             if (details.IsDontDelete()) new_length = number + 1;
           }
         }
       }
       if (new_length != length) {
         length_object = isolate->factory()->NewNumberFromUint(new_length);
       }
     }
 
     if (new_length == 0) {
-      // If the length of a slow array is reset to zero, we clear
-      // the array and flush backing storage. This has the added
-      // benefit that the array returns to fast mode.
+      // Flush the backing store.
       JSObject::ResetElements(array);
     } else {
       DisallowHeapAllocation no_gc;
       // Remove elements that should be deleted.
       int removed_entries = 0;
       Handle<Object> the_hole_value = isolate->factory()->the_hole_value();
       for (int i = 0; i < capacity; i++) {
         Object* key = dict->KeyAt(i);
         if (key->IsNumber()) {
           uint32_t number = static_cast<uint32_t>(key->Number());
@@ skipping 540 matching lines @@
       UNREACHABLE();
       break;
   }
 
   array->set_elements(*elms);
   array->set_length(Smi::FromInt(number_of_elements));
   return array;
 }
 
 } }  // namespace v8::internal