Force small array literals to have FAST_ELEMENTs

src/parser.cc

Index: src/parser.cc
diff --git a/src/parser.cc b/src/parser.cc
index a7bb4e7985f7a3ac75de531d41415a4d73f47b5a..75fd7b73667eb611dd1393f26c1dc559addeff67 100644
--- a/src/parser.cc
+++ b/src/parser.cc
@@ -3718,6 +3718,7 @@ Expression* Parser::ParseArrayLiteral(bool* ok) {
       isolate()->factory()->NewFixedArray(values->length(), TENURED);
   Handle<FixedDoubleArray> double_literals;
   ElementsKind elements_kind = FAST_SMI_ELEMENTS;
+  bool has_only_undefined_values = true;
   bool has_hole_values = false;
 
   // Fill in the literals.
@@ -3749,6 +3750,7 @@ Expression* Parser::ParseArrayLiteral(bool* ok) {
       // FAST_DOUBLE_ELEMENTS and FAST_ELEMENTS as necessary.  Always remember
       // the tagged value, no matter what the ElementsKind is in case we
       // ultimately end up in FAST_ELEMENTS.
+      has_only_undefined_values = false;
       object_literals->set(i, *boilerplate_value);
       if (elements_kind == FAST_SMI_ELEMENTS) {
         // Smi only elements. Notice if a transition to FAST_DOUBLE_ELEMENTS or
@@ -3787,6 +3789,13 @@ Expression* Parser::ParseArrayLiteral(bool* ok) {
     }
   }
 
+  // Very small array literals that don't have a concrete hint about their type
+  // from a constant value should default to the slow case to avoid lots of
+  // elements transitions on really small objects.
+  if (has_only_undefined_values && values->length() <= 2) {
+    elements_kind = FAST_ELEMENTS;
+  }
+
   // Simple and shallow arrays can be lazily copied, we transform the
   // elements array to a copy-on-write array.
   if (is_simple && depth == 1 && values->length() > 0 &&