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

src/x64/lithium-codegen-x64.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 3911 matching lines @@
 
   Register write_register = object;
   if (!access.IsInobject()) {
     write_register = ToRegister(instr->temp());
     __ movp(write_register, FieldOperand(object, JSObject::kPropertiesOffset));
   }
 
   if (representation.IsSmi() &&
       hinstr->value()->representation().IsInteger32()) {
     ASSERT(hinstr->store_mode() == STORE_TO_INITIALIZED_ENTRY);
+#ifdef DEBUG
+    __ movq(kScratchRegister, FieldOperand(write_register, offset));
+    __ AssertSmi(kScratchRegister);
+#endif
     // Store int value directly to upper half of the smi.
     STATIC_ASSERT(kSmiTag == 0);
     STATIC_ASSERT(kSmiTagSize + kSmiShiftSize == 32);
     offset += kPointerSize / 2;
     representation = Representation::Integer32();
   }
 
   Operand operand = FieldOperand(write_register, offset);
 
   if (instr->value()->IsRegister()) {
@@ skipping 1603 matching lines @@
                                FixedArray::kHeaderSize - kPointerSize));
   __ bind(&done);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64