Promote double arrays to FAST_ELEMENT that use generic KeyedLoadIC

src/ic.cc

Index: src/ic.cc
diff --git a/src/ic.cc b/src/ic.cc
index 9024605da54a6f111f54d180732906a083166807..b68fd38dced16b27eb7702e07df31ce5956c8915 100644
--- a/src/ic.cc
+++ b/src/ic.cc
@@ -1,4 +1,4 @@
-// Copyright 2011 the V8 project authors. All rights reserved.
+// Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
@@ -1120,6 +1120,17 @@ MaybeObject* KeyedLoadIC::Load(State state,
         } else if (key->IsSmi() && (target() != *non_strict_arguments_stub())) {
           stub = ComputeStub(receiver, LOAD, kNonStrictMode, stub);
         }
+        // If the IC is being replaced by the generic stub, loads from
+        // FAST_DOUBLE_ELEMENTS arrays will cause unboxing in Crankshafted
+        // code. To prevent these expensive allocations, proactively promote
+        // arrays to FAST_ELEMENTS ElementKinds.
+        if (*stub == *generic_stub()) {
+          if (receiver->HasFastDoubleElements()) {
+            MaybeObject* maybe_object =
+                receiver->TransitionElementsKind(FAST_ELEMENTS);
+            if (maybe_object->IsFailure()) return maybe_object;
+          }
+        }
       }
     } else {
       TRACE_GENERIC_IC("KeyedLoadIC", "force generic");