[runtime] do not left-trim short arrays

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/isolate-inl.h"
@@ skipping 2132 matching lines @@
     *nof_items = count;
     return Just(true);
   }
 
   static void MoveElements(Isolate* isolate, Handle<JSArray> receiver,
                            Handle<FixedArrayBase> backing_store, int dst_index,
                            int src_index, int len, int hole_start,
                            int hole_end) {
     Heap* heap = isolate->heap();
     Handle<BackingStore> dst_elms = Handle<BackingStore>::cast(backing_store);
-    if (heap->CanMoveObjectStart(*dst_elms) && dst_index == 0) {
+    if (heap->CanMoveObjectStart(*dst_elms) && dst_index == 0 &&
+        len > JSArray::kMaxCopyElements) {
       // Update all the copies of this backing_store handle.
       *dst_elms.location() =
           BackingStore::cast(heap->LeftTrimFixedArray(*dst_elms, src_index));
       receiver->set_elements(*dst_elms);
       // Adjust the hole offset as the array has been shrunk.
       hole_end -= src_index;
       DCHECK_LE(hole_start, backing_store->length());
       DCHECK_LE(hole_end, backing_store->length());
     } else if (len != 0) {
       if (IsFastDoubleElementsKind(KindTraits::Kind)) {
@@ skipping 2137 matching lines @@
     insertion_index += len;
   }
 
   DCHECK_EQ(insertion_index, result_len);
   return result_array;
 }
 
 ElementsAccessor** ElementsAccessor::elements_accessors_ = NULL;
 }  // namespace internal
 }  // namespace v8