Bug 1288618 - Part 6: Avoid unnecessarily allocating a TailDispatcher for XPCOMThreadWrapper. r=bholley

xpcom/threads/AbstractThread.cpp

@@ -94,6 +94,11 @@ public:
     return mTailDispatcher.ref();
   }
 
+  virtual bool MightHaveTailTasks() override
+  {
+    return mTailDispatcher.isSome();
+  }
+
   virtual nsIThread* AsXPCOMThread() override { return mTarget; }
 
 private:
@@ -101,6 +106,23 @@ private:
   Maybe<AutoTaskDispatcher> mTailDispatcher;
 };
 
+void
+AbstractThread::TailDispatchTasksFor(AbstractThread* aThread)
+{
+  if (MightHaveTailTasks()) {
+    TailDispatcher().DispatchTasksFor(aThread);
+  }
+}
+
+bool
+AbstractThread::HasTailTasksFor(AbstractThread* aThread)
+{
+  if (!MightHaveTailTasks()) {
+    return false;
+  }
+  return TailDispatcher().HasTasksFor(aThread);
+}
+
 bool
 AbstractThread::RequiresTailDispatch(AbstractThread* aThread) const
 {

xpcom/threads/AbstractThread.h

@@ -66,6 +66,16 @@ public:
   // threads which support it.
   virtual TaskDispatcher& TailDispatcher() = 0;
 
+  // Returns true if we have tail tasks scheduled, or if this isn't known.
+  // Returns false if we definitely don't have any tail tasks.
+  virtual bool MightHaveTailTasks() { return true; }
+
+  // Helper functions for methods on the tail TasklDispatcher. These check
+  // HasTailTasks to avoid allocating a TailDispatcher if it isn't
+  // needed.
+  void TailDispatchTasksFor(AbstractThread* aThread);
+  bool HasTailTasksFor(AbstractThread* aThread);
+
   // Returns true if this supports the tail dispatcher.
   bool SupportsTailDispatch() const { return mSupportsTailDispatch; }
 

xpcom/threads/TaskQueue.cpp

@@ -138,7 +138,7 @@ TaskQueue::AwaitIdleLocked()
   // Make sure there are no tasks for this queue waiting in the caller's tail
   // dispatcher.
   MOZ_ASSERT_IF(AbstractThread::GetCurrent(),
-                !AbstractThread::GetCurrent()->TailDispatcher().HasTasksFor(this));
+                !AbstractThread::GetCurrent()->HasTailTasksFor(this));
 
   mQueueMonitor.AssertCurrentThreadOwns();
   MOZ_ASSERT(mIsRunning || mTasks.empty());
@@ -154,7 +154,7 @@ TaskQueue::AwaitShutdownAndIdle()
   // Make sure there are no tasks for this queue waiting in the caller's tail
   // dispatcher.
   MOZ_ASSERT_IF(AbstractThread::GetCurrent(),
-                !AbstractThread::GetCurrent()->TailDispatcher().HasTasksFor(this));
+                !AbstractThread::GetCurrent()->HasTailTasksFor(this));
 
   MonitorAutoLock mon(mQueueMonitor);
   while (!mIsShutdown) {
@@ -169,7 +169,7 @@ TaskQueue::BeginShutdown()
   // Dispatch any tasks for this queue waiting in the caller's tail dispatcher,
   // since this is the last opportunity to do so.
   if (AbstractThread* currentThread = AbstractThread::GetCurrent()) {
-    currentThread->TailDispatcher().DispatchTasksFor(this);
+    currentThread->TailDispatchTasksFor(this);
   }
 
   MonitorAutoLock mon(mQueueMonitor);