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

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index a527d282a3eeeb756e527cb5470ce0d3284e59af..152bbffbfb875ac6c69f53551521e55a301b28fd 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -11719,6 +11719,17 @@ static void EndPerformSplice(Handle<JSArray> object) {
 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()) {