Separate mutex for megamorphic cache tables lookup in the background compiler.

runtime/vm/megamorphic_cache_table.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/megamorphic_cache_table.h"
 
 #include <stdlib.h>
 #include "vm/object.h"
 #include "vm/object_store.h"
 #include "vm/stub_code.h"
 #include "vm/symbols.h"
 
 namespace dart {
 
 RawMegamorphicCache* MegamorphicCacheTable::Lookup(Isolate* isolate,
                                                    const String& name,
                                                    const Array& descriptor) {
   // Multiple compilation threads could access this lookup.
-  SafepointMutexLocker ml(isolate->mutex());
+  SafepointMutexLocker ml(isolate->lookup_mutex());
   ASSERT(name.IsSymbol());
   // TODO(rmacnak): ASSERT(descriptor.IsCanonical());
 
   // TODO(rmacnak): Make a proper hashtable a la symbol table.
   GrowableObjectArray& table = GrowableObjectArray::Handle(
       isolate->object_store()->megamorphic_cache_table());
   MegamorphicCache& cache = MegamorphicCache::Handle();
   if (table.IsNull()) {
     table = GrowableObjectArray::New(Heap::kOld);
     isolate->object_store()->set_megamorphic_cache_table(table);
@@ skipping 115 matching lines @@
     cumulative_entries += probe_counts[i];
     OS::Print("Megamorphic probe %" Pd ": %" Pd " (%lf)\n",
               i, probe_counts[i],
               static_cast<double>(cumulative_entries) /
               static_cast<double>(entry_count));
   }
   delete[] probe_counts;
 }
 
 }  // namespace dart