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

When calling lock() after all passes are registered, the PassRegistry doesn't need a mutex anymore to look up passes.
This speeds up multithreaded llvm execution by ~5% (tested with 4 threads).
In an asserts build of llvm this has an even bigger impact.

Note that it's not required to use the lock function.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231276 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/PassRegistry.h

@@ -41,6 +41,9 @@ struct PassRegistrationListener;
 class PassRegistry {
   mutable sys::SmartRWMutex<true> Lock;
 
+  /// Only if false, synchronization must use the Lock mutex.
+  std::atomic<bool> locked;
+
   /// PassInfoMap - Keep track of the PassInfo object for each registered pass.
   typedef DenseMap<const void *, const PassInfo *> MapType;
   MapType PassInfoMap;
@@ -52,7 +55,7 @@ class PassRegistry {
   std::vector<PassRegistrationListener *> Listeners;
 
 public:
-  PassRegistry() {}
+  PassRegistry() : locked(false) {}
   ~PassRegistry();
 
   /// getPassRegistry - Access the global registry object, which is
@@ -60,6 +63,10 @@ public:
   /// llvm_shutdown.
   static PassRegistry *getPassRegistry();
 
+  /// Enables fast thread synchronization in getPassInfo().
+  /// After calling lock() no more passes may be registered.
+  void lock() { locked = true; }
+
   /// getPassInfo - Look up a pass' corresponding PassInfo, indexed by the pass'
   /// type identifier (&MyPass::ID).
   const PassInfo *getPassInfo(const void *TI) const;