X64: Optimize access to external references.

test/cctest/test-macro-assembler-x64.cc

 // Copyright 2009 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 78 matching lines @@
 
 typedef int (*F0)();
 
 #define __ masm->
 
 
 static void EntryCode(MacroAssembler* masm) {
   // Smi constant register is callee save.
   __ push(v8::internal::kSmiConstantRegister);
   __ InitializeSmiConstantRegister();
+  __ InitializeRootRegister();
 }
 
 
 static void ExitCode(MacroAssembler* masm) {
   // Return -1 if kSmiConstantRegister was clobbered during the test.
   __ Move(rdx, Smi::FromInt(1));
   __ cmpq(rdx, v8::internal::kSmiConstantRegister);
   __ movq(rdx, Immediate(-1));
   __ cmovq(not_equal, rax, rdx);
   __ pop(v8::internal::kSmiConstantRegister);
@@ skipping 2378 matching lines @@
   CodeDesc desc;
   masm->GetCode(&desc);
   // Call the function from C++.
   int result = FUNCTION_CAST<F0>(buffer)();
   CHECK_EQ(0, result);
 }
 
 
 
 #undef __