Don't track Smi->Double->Object element transitions for small undefined arrays.

src/parser.cc

Index: src/parser.cc
diff --git a/src/parser.cc b/src/parser.cc
index 7e648ede5a61dc030b58718e39e3d8c5a4db88ed..c83a88644d3bedccb978a93c56b42d94e7ab1fa6 100644
--- a/src/parser.cc
+++ b/src/parser.cc
@@ -3377,7 +3377,9 @@ Expression* Parser::ParseArrayLiteral(bool* ok) {
   Handle<FixedArray> object_literals =
       isolate()->factory()->NewFixedArray(values->length(), TENURED);
   Handle<FixedDoubleArray> double_literals;
+
   ElementsKind elements_kind = FAST_SMI_ONLY_ELEMENTS;
+  bool has_ony_undefined_values = true;

[Jakob Kummerow, 2011/12/06 10:01:12]

s/ony/only/

 
   // Fill in the literals.
   bool is_simple = true;
@@ -3401,6 +3403,7 @@ 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_ony_undefined_values = false;

[Jakob Kummerow, 2011/12/06 10:01:12]

s/ony/only/

       object_literals->set(i, *boilerplate_value);
       if (elements_kind == FAST_SMI_ONLY_ELEMENTS) {
         // Smi only elements. Notice if a transition to FAST_DOUBLE_ELEMENTS or
@@ -3439,6 +3442,13 @@ 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_ony_undefined_values && values->length() <= 2) {

[Jakob Kummerow, 2011/12/06 10:01:12]

s/ony/only/

+    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 &&