Keep new arrays allocated with 'new Array(N)' in fast mode

src/objects.cc

 // Copyright 2013 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/accessors.h"
 #include "src/allocation-site-scopes.h"
 #include "src/api.h"
 #include "src/arguments.h"
@@ skipping 11701 matching lines @@
                   Handle<JSFunction>(isolate->observers_end_perform_splice()),
                   isolate->factory()->undefined_value(),
                   ARRAY_SIZE(args),
                   args).Assert();
 }
 
 
 MaybeHandle<Object> JSArray::SetElementsLength(
     Handle<JSArray> array,
     Handle<Object> new_length_handle) {
+  if (array->HasFastElements()) {
+    // If the new array won't fit in a some non-trivial fraction of the max old
+    // space size, then force it to go dictionary mode.
+    int max_fast_array_size = static_cast<int>(
+        (array->GetHeap()->MaxOldGenerationSize() / kDoubleSize) / 4);
+    if (new_length_handle->IsNumber() &&
+        NumberToInt32(*new_length_handle) >= max_fast_array_size) {
+      NormalizeElements(array);
+    }
+  }
+
   // We should never end in here with a pixel or external array.
   ASSERT(array->AllowsSetElementsLength());
   if (!array->map()->is_observed()) {
     return array->GetElementsAccessor()->SetLength(array, new_length_handle);
   }
 
   Isolate* isolate = array->GetIsolate();
   List<uint32_t> indices;
   List<Handle<Object> > old_values;
   Handle<Object> old_length_handle(array->length(), isolate);
@@ skipping 5209 matching lines @@
 #define ERROR_MESSAGES_TEXTS(C, T) T,
   static const char* error_messages_[] = {
       ERROR_MESSAGES_LIST(ERROR_MESSAGES_TEXTS)
   };
 #undef ERROR_MESSAGES_TEXTS
   return error_messages_[reason];
 }
 
 
 } }  // namespace v8::internal