[PATCH] Add a lock() function in PassRegistry to speed up multi-thread synchronization.

[Erik Eckstein]

Committed in r231276.

Thanks all for reviewing!


================
Comment at: lib/IR/PassRegistry.cpp:46
@@ +45,3 @@
+  Optional<sys::SmartScopedReader<true>> Guard;
+  if (!locked)
+    Guard.emplace(Lock);
----------------
joker.eph wrote:
> "Pedantic" comment: since your motivation is purely performance here, you may want to replace all uses of  ```locked``` with ```locked.load(memory_order_consume)```. Maybe in a separate private method like that:
> ```
> bool isLocked() { return locked.load(memory_order_consume); }
> ```
> Especially since I don't think there is a guarantee that atomic<bool> is lock_free.
Thanks for this suggestion. But I'm not sure if memory_order_consume is OK here. I think it must be memory_order_acquire.
I prefer to be conservative in this case and let it as it is now.

http://reviews.llvm.org/D7787

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