Fix for a smi stores optimization on x64 with a regression test.

src/hydrogen.cc

 // Copyright 2013 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 9906 matching lines @@
         // 1) it's a child object of another object with a valid allocation site
         // 2) we can just use the mode of the parent object for pretenuring
         HInstruction* double_box =
             Add<HAllocate>(heap_number_constant, HType::HeapNumber(),
                 pretenure_flag, HEAP_NUMBER_TYPE);
         AddStoreMapConstant(double_box,
             isolate()->factory()->heap_number_map());
         Add<HStoreNamedField>(double_box, HObjectAccess::ForHeapNumberValue(),
                               Add<HConstant>(value));
         value_instruction = double_box;
-      } else if (representation.IsSmi() && value->IsUninitialized()) {
-        value_instruction = graph()->GetConstant0();
-        // Ensure that Constant0 is stored as smi.
+      } else if (representation.IsSmi()) {
+        value_instruction = value->IsUninitialized()
+            ? graph()->GetConstant0()
+            : Add<HConstant>(value);
+        // Ensure that value is stored as smi.
         access = access.WithRepresentation(representation);
       } else {
         value_instruction = Add<HConstant>(value);
       }
 
       Add<HStoreNamedField>(object, access, value_instruction);
     }
   }
 
   int inobject_properties = boilerplate_object->map()->inobject_properties();
@@ skipping 1352 matching lines @@
   if (ShouldProduceTraceOutput()) {
     isolate()->GetHTracer()->TraceHydrogen(name(), graph_);
   }
 
 #ifdef DEBUG
   graph_->Verify(false);  // No full verify.
 #endif
 }
 
 } }  // namespace v8::internal