Use a root array register for constant loads and stack guards on x64....

src/x64/macro-assembler-x64.h

 // Copyright 2006-2008 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 48 matching lines @@
   TRY_FINALLY_HANDLER,
   JS_ENTRY_HANDLER
 };
 
 
 // MacroAssembler implements a collection of frequently used macros.
 class MacroAssembler: public Assembler {
  public:
   MacroAssembler(void* buffer, int size);
 
+  void LoadRoot(Register destination, Heap::RootListIndex index);
+  void CompareRoot(Register with, Heap::RootListIndex index);
+  void PushRoot(Heap::RootListIndex index);
+
   // ---------------------------------------------------------------------------
   // GC Support
 
   // Set the remembered set bit for [object+offset].
   // object is the object being stored into, value is the object being stored.
   // If offset is zero, then the scratch register contains the array index into
   // the elements array represented as a Smi.
   // All registers are clobbered by the operation.
   void RecordWrite(Register object,
                    int offset,
@@ skipping 370 matching lines @@
   }                                                                       \
   masm->
 #else
 #define ACCESS_MASM(masm) masm->
 #endif
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_MACRO_ASSEMBLER_X64_H_