Do not overwrite already optimized code.

src/optimizing-compiler-thread.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 240 matching lines @@
         info->closure()->PrintName();
         PrintF(" is ready for install and entry at AST id %d]\n",
                info->osr_ast_id().ToInt());
       }
       job->WaitForInstall();
       // Remove stack check that guards OSR entry on original code.
       Handle<Code> code = info->unoptimized_code();
       uint32_t offset = code->TranslateAstIdToPcOffset(info->osr_ast_id());
       BackEdgeTable::RemoveStackCheck(code, offset);
     } else {
-      Handle<Code> code = Compiler::GetConcurrentlyOptimizedCode(job);
-      function->ReplaceCode(
-          code.is_null() ? function->shared()->code() : *code);
+      if (function->IsOptimized()) {
+        DisposeOptimizedCompileJob(job, false);
+      } else {
+        Handle<Code> code = Compiler::GetConcurrentlyOptimizedCode(job);
+        function->ReplaceCode(
+            code.is_null() ? function->shared()->code() : *code);
+      }
     }
   }
 }
 
 
 void OptimizingCompilerThread::QueueForOptimization(OptimizedCompileJob* job) {
   ASSERT(IsQueueAvailable());
   ASSERT(!IsOptimizerThread());
   CompilationInfo* info = job->info();
   if (info->is_osr()) {
@@ skipping 107 matching lines @@
 
 
 bool OptimizingCompilerThread::IsOptimizerThread() {
   LockGuard<Mutex> lock_guard(&thread_id_mutex_);
   return ThreadId::Current().ToInteger() == thread_id_;
 }
 #endif
 
 
 } }  // namespace v8::internal