[simulator] Make reference redirection thread-safe.

src/arm/simulator-arm.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 
 // Declares a Simulator for ARM instructions if we are not generating a native
 // ARM binary. This Simulator allows us to run and debug ARM code generation on
 // regular desktop machines.
 // V8 calls into generated code by "calling" the CALL_GENERATED_CODE macro,
 // which will start execution in the Simulator or forwards to the real entry
@@ skipping 343 matching lines @@
   void InstructionDecode(Instruction* instr);
 
   // ICache.
   static void CheckICache(base::CustomMatcherHashMap* i_cache,
                           Instruction* instr);
   static void FlushOnePage(base::CustomMatcherHashMap* i_cache, intptr_t start,
                            int size);
   static CachePage* GetCachePage(base::CustomMatcherHashMap* i_cache,
                                  void* page);
 
-  // Runtime call support.
+  // Runtime call support. Uses the isolate in a thread-safe way.
   static void* RedirectExternalReference(
       Isolate* isolate, void* external_function,
       v8::internal::ExternalReference::Type type);
 
   // Handle arguments and return value for runtime FP functions.
   void GetFpArgs(double* x, double* y, int32_t* z);
   void SetFpResult(const double& result);
   void TrashCallerSaveRegisters();
 
   template<class ReturnType, int register_size>
@@ skipping 193 matching lines @@
   static inline void UnregisterCTryCatch(v8::internal::Isolate* isolate) {
     Simulator::current(isolate)->PopAddress();
   }
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // !defined(USE_SIMULATOR)
 #endif  // V8_ARM_SIMULATOR_ARM_H_