Promote double arrays to FAST_ELEMENT that use generic KeyedLoadIC

src/ic.cc

 // 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:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1102 matching lines @@
       } else if (object->IsJSObject()) {
         Handle<JSObject> receiver = Handle<JSObject>::cast(object);
         if (receiver->elements()->map() ==
             isolate()->heap()->non_strict_arguments_elements_map()) {
           stub = non_strict_arguments_stub();
         } else if (receiver->HasIndexedInterceptor()) {
           stub = indexed_interceptor_stub();
         } 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");
     }
     if (!stub.is_null()) set_target(*stub);
   }
 
   TRACE_IC("KeyedLoadIC", key, state, target());
 
   // Get the property.
@@ skipping 1294 matching lines @@
 #undef ADDR
 };
 
 
 Address IC::AddressFromUtilityId(IC::UtilityId id) {
   return IC_utilities[id];
 }
 
 
 } }  // namespace v8::internal