Optimize loads from root-array in X64.

src/x64/code-stubs-x64.cc

 // Copyright 2011 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 3543 matching lines @@
   // TODO(X64): On Win64, if we ever use XMM6-XMM15, the low low 64 bits are
   // callee save as well.
 
   // Save copies of the top frame descriptor on the stack.
   ExternalReference c_entry_fp(Top::k_c_entry_fp_address);
   __ load_rax(c_entry_fp);
   __ push(rax);
 
   // Set up the roots and smi constant registers.
   // Needs to be done before any further smi loads.
-  ExternalReference roots_address = ExternalReference::roots_address();
-  __ movq(kRootRegister, roots_address);
+  __ InitializeRootRegister();
   __ InitializeSmiConstantRegister();
 
 #ifdef ENABLE_LOGGING_AND_PROFILING
   // If this is the outermost JS call, set js_entry_sp value.
   ExternalReference js_entry_sp(Top::k_js_entry_sp_address);
   __ load_rax(js_entry_sp);
   __ testq(rax, rax);
   __ j(not_zero, &not_outermost_js);
   __ movq(rax, rbp);
   __ store_rax(js_entry_sp);
@@ skipping 1565 matching lines @@
           FieldOperand(elements, PixelArray::kExternalPointerOffset));
   __ movb(Operand(external_pointer, untagged_key, times_1, 0), untagged_value);
   __ ret(0);  // Return value in eax.
 }
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64