Trim fast elements tail on significant length decreases.

src/objects.cc

 // Copyright 2011 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 7489 matching lines @@
     if (value < 0) return ArrayLengthRangeError(GetHeap());
     switch (GetElementsKind()) {
       case FAST_ELEMENTS: {
         int old_capacity = FixedArray::cast(elements())->length();
         if (value <= old_capacity) {
           if (IsJSArray()) {
             Object* obj;
             { MaybeObject* maybe_obj = EnsureWritableFastElements();
               if (!maybe_obj->ToObject(&obj)) return maybe_obj;
             }
-            int old_length = FastD2I(JSArray::cast(this)->length()->Number());
-            // NOTE: We may be able to optimize this by removing the
-            // last part of the elements backing storage array and
-            // setting the capacity to the new size.
-            for (int i = value; i < old_length; i++) {
-              FixedArray::cast(elements())->set_the_hole(i);
+            FixedArray* fast_elements = FixedArray::cast(elements());
+            if (2 * value <= old_capacity) {
+              // If more than half the elements won't be used, trim the array.
+              if (value == 0) {
+                initialize_elements();
+              } else {
+                fast_elements->set_length(value);
+                Address filler_start = fast_elements->address() +
+                                       FixedArray::OffsetOfElementAt(value);
+                int filler_size = (old_capacity - value) * kPointerSize;
+                GetHeap()->CreateFillerObjectAt(filler_start, filler_size);
+              }
+            } else {
+              // Otherwise, fill the unused tail with holes.
+              int old_length = FastD2I(JSArray::cast(this)->length()->Number());
+              for (int i = value; i < old_length; i++) {
+                fast_elements->set_the_hole(i);
+              }
             }
             JSArray::cast(this)->set_length(Smi::cast(smi_length));
           }
           return this;
         }
         int min = NewElementsCapacity(old_capacity);
         int new_capacity = value > min ? value : min;
         if (new_capacity <= kMaxFastElementsLength ||
             !ShouldConvertToSlowElements(new_capacity)) {
           MaybeObject* result =
@@ skipping 4035 matching lines @@
   if (break_point_objects()->IsUndefined()) return 0;
   // Single beak point.
   if (!break_point_objects()->IsFixedArray()) return 1;
   // Multiple break points.
   return FixedArray::cast(break_point_objects())->length();
 }
 #endif
 
 
 } }  // namespace v8::internal