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

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 701579cd8793aa95f34f485038fc9ac3529f791a..1d3b69cf4ca91e3944c3d55c20caae78ab91e570 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -282,6 +282,7 @@ class Isolate : public BaseIsolate {
   Mutex* constant_canonicalization_mutex() const {
     return constant_canonicalization_mutex_;
   }
+  Mutex* lookup_mutex() const { return lookup_mutex_; }
 
   Debugger* debugger() const {
     if (!FLAG_support_debugger) {
@@ -717,6 +718,7 @@ class Isolate : public BaseIsolate {
   Mutex* symbols_mutex_;  // Protects concurrent access to the symbol table.
   Mutex* type_canonicalization_mutex_;  // Protects type canonicalization.
   Mutex* constant_canonicalization_mutex_;  // Protects const canonicalization.
+  Mutex* lookup_mutex_;  // Protects megamorphic table lookup.

[siva, 2016/07/01 17:39:29]

can we call it megamorphic_lookup_mutex_;
so that it is more clear and people don't use it for other lookups.

[Florian Schneider, 2016/07/01 19:31:58]

Done.

   MessageHandler* message_handler_;
   IsolateSpawnState* spawn_state_;
   bool is_runnable_;