Stores to external references don't need write barriers.

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 2219 matching lines @@
 
 LInstruction* LChunkBuilder::DoStoreNamedField(HStoreNamedField* instr) {
   bool is_in_object = instr->access().IsInobject();
   bool is_external_location = instr->access().IsExternalMemory() &&
       instr->access().offset() == 0;
   bool needs_write_barrier = instr->NeedsWriteBarrier();
   bool needs_write_barrier_for_map = !instr->transition().is_null() &&
       instr->NeedsWriteBarrierForMap();
 
   LOperand* obj;
-  if (needs_write_barrier) {
-    obj = is_in_object
-        ? UseRegister(instr->object())
-        : UseTempRegister(instr->object());
-  } else if (is_external_location) {
+  if (is_external_location) {
     ASSERT(!is_in_object);
     ASSERT(!needs_write_barrier);
     ASSERT(!needs_write_barrier_for_map);
     obj = UseRegisterOrConstant(instr->object());
+  } else if (needs_write_barrier) {
+    obj = is_in_object
+        ? UseRegister(instr->object())
+        : UseTempRegister(instr->object());
   } else {
     obj = needs_write_barrier_for_map
         ? UseRegister(instr->object())
         : UseRegisterAtStart(instr->object());
   }
 
   bool can_be_constant = instr->value()->IsConstant() &&
       HConstant::cast(instr->value())->NotInNewSpace() &&
       !(FLAG_track_double_fields && instr->field_representation().IsDouble());
 
@@ skipping 285 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