Use xorps to break the cvtsi2sd unnecessary dependence due to its partially written

src/ia32/macro-assembler-ia32.h

 // 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.
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@@ skipping 348 matching lines @@
   // Store the function for the given builtin in the target register.
   void GetBuiltinFunction(Register target, Builtins::JavaScript id);
 
   // Store the code object for the given builtin in the target register.
   void GetBuiltinEntry(Register target, Builtins::JavaScript id);
 
   // Expression support
   void Set(Register dst, const Immediate& x);
   void Set(const Operand& dst, const Immediate& x);
 
+  // Use xorps to clear out the destination register before cvtsi2sd

[Yang, 2013/09/12 11:56:49]

Could you add a comment on why we do this? I mean, it's not entirely obvious why
we need to clear out the destination register, so it would be great if somebody
working on this code in the future can figure out what the background of this
change is.

+  void Cvtsi2sd(XMMRegister dst, Register src) { Cvtsi2sd(dst, Operand(src)); }
+  void Cvtsi2sd(XMMRegister dst, const Operand& src);
+
   // Support for constant splitting.
   bool IsUnsafeImmediate(const Immediate& x);
   void SafeSet(Register dst, const Immediate& x);
   void SafePush(const Immediate& x);
 
   // Compare object type for heap object.
   // Incoming register is heap_object and outgoing register is map.
   void CmpObjectType(Register heap_object, InstanceType type, Register map);
 
   // Compare instance type for map.
@@ skipping 695 matching lines @@
   }                                                                       \
   masm->
 #else
 #define ACCESS_MASM(masm) masm->
 #endif
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_IA32_MACRO_ASSEMBLER_IA32_H_