Add support to load/store byte fields.

src/x64/lithium-x64.cc

 // 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.
 //     * 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 2050 matching lines @@
     context = UseRegister(instr->context());
     value = UseRegister(instr->value());
     temp = NULL;
   }
   LInstruction* result = new(zone()) LStoreContextSlot(context, value, temp);
   return instr->RequiresHoleCheck() ? AssignEnvironment(result) : result;
 }
 
 
 LInstruction* LChunkBuilder::DoLoadNamedField(HLoadNamedField* instr) {
-  if (instr->access().IsExternalMemory() && instr->access().offset() == 0) {
+  // Use the special mov rax, moffs64 encoding for external
+  // memory accesses with 64-bit word-sized values.
+  if (instr->access().IsExternalMemory() &&
+      instr->access().offset() == 0 &&
+      (instr->access().representation().IsSmi() ||
+       instr->access().representation().IsTagged() ||
+       instr->access().representation().IsHeapObject() ||
+       instr->access().representation().IsExternal())) {
     LOperand* obj = UseRegisterOrConstantAtStart(instr->object());
     return DefineFixed(new(zone()) LLoadNamedField(obj), rax);
   }
   LOperand* obj = UseRegisterAtStart(instr->object());
   return DefineAsRegister(new(zone()) LLoadNamedField(obj));
 }
 
 
 LInstruction* LChunkBuilder::DoLoadNamedGeneric(HLoadNamedGeneric* instr) {
   LOperand* object = UseFixed(instr->object(), rax);
@@ skipping 464 matching lines @@
 LInstruction* LChunkBuilder::DoLoadFieldByIndex(HLoadFieldByIndex* instr) {
   LOperand* object = UseRegister(instr->object());
   LOperand* index = UseTempRegister(instr->index());
   return DefineSameAsFirst(new(zone()) LLoadFieldByIndex(object, index));
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64