New hydrogen instruction to reduce cost of growing an array on keyed stores.

src/code-stubs-hydrogen.cc

Index: src/code-stubs-hydrogen.cc
diff --git a/src/code-stubs-hydrogen.cc b/src/code-stubs-hydrogen.cc
index d346e95af20411b48aadef4fedca2a977f531719..43224c1b77d818d926d638f1842c51f3ca96240f 100644
--- a/src/code-stubs-hydrogen.cc
+++ b/src/code-stubs-hydrogen.cc
@@ -578,13 +578,18 @@ Handle<Code> StoreScriptContextFieldStub::GenerateCode() {
 
 template <>
 HValue* CodeStubGraphBuilder<GrowArrayElementsStub>::BuildCodeStub() {
+  ElementsKind kind = casted_stub()->elements_kind();
+  if (IsFastDoubleElementsKind(kind)) {
+    info()->MarkAsSavesCallerDoubles();
+  }
+
   HValue* object = GetParameter(GrowArrayElementsDescriptor::kObjectIndex);
   HValue* key = GetParameter(GrowArrayElementsDescriptor::kKeyIndex);
-  HValue* current_capacity =
-      GetParameter(GrowArrayElementsDescriptor::kCapacityIndex);
-  ElementsKind kind = casted_stub()->elements_kind();
 
   HValue* elements = AddLoadElements(object);
+  HValue* current_capacity = Add<HLoadNamedField>(
+      elements, nullptr, HObjectAccess::ForFixedArrayLength());
+
   HValue* length =
       casted_stub()->is_js_array()
           ? Add<HLoadNamedField>(object, static_cast<HValue*>(NULL),