Add thread-safety to creation of MemCopy and modulo functions.

src/x64/codegen-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 8806 matching lines @@
   Label clear_exceptions;
   __ testb(rax, Immediate(0x3f /* Any Exception*/));
   __ j(not_zero, &clear_exceptions);
   __ ret(0);
   __ bind(&clear_exceptions);
   __ fnclex();
   __ ret(0);
 
   CodeDesc desc;
   masm.GetCode(&desc);
-  // Call the function from C++.
+  // Make sure that the compiled code is visible to all threads before
+  // returning the pointer to it.
+  MemoryBarrier();
+  // Call the function from C++ through this pointer.
   return FUNCTION_CAST<ModuloFunction>(buffer);
 }
 
 #endif
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64