High resolution timer fix for Windows

base/message_loop/incoming_task_queue.h

Index: base/message_loop/incoming_task_queue.h
diff --git a/base/message_loop/incoming_task_queue.h b/base/message_loop/incoming_task_queue.h
index 56c5638295b8440cdcad41871f28e71eb7ee16e8..d533cafd1026069f8345516bcf8369d0e6b5ebbe 100644
--- a/base/message_loop/incoming_task_queue.h
+++ b/base/message_loop/incoming_task_queue.h
@@ -38,16 +38,17 @@ class BASE_EXPORT IncomingTaskQueue
                           TimeDelta delay,
                           bool nestable);
 
-  // Returns true if the message loop has high resolution timers enabled.
-  // Provided for testing.
-  bool IsHighResolutionTimerEnabledForTesting();
+  // Returns true if the queue contains tasks that require higher than default
+  // timer resolution. Currently only needed for Windows.
+  bool HasHighResolutionTasks();
 
   // Returns true if the message loop is "idle". Provided for testing.
   bool IsIdleForTesting();
 
   // Loads tasks from the |incoming_queue_| into |*work_queue|. Must be called
-  // from the thread that is running the loop.
-  void ReloadWorkQueue(TaskQueue* work_queue);
+  // from the thread that is running the loop. Returns the number of tasks that
+  // require high resolution timers.
+  int ReloadWorkQueue(TaskQueue* work_queue);
 
   // Disconnects |this| from the parent message loop.
   void WillDestroyCurrentMessageLoop();
@@ -65,9 +66,9 @@ class BASE_EXPORT IncomingTaskQueue
   // does not retain |pending_task->task| beyond this function call.
   bool PostPendingTask(PendingTask* pending_task);
 
-#if defined(OS_WIN)
-  TimeTicks high_resolution_timer_expiration_;
-#endif
+  // Number of tasks that require high resolution timing. This value is kept
+  // so that ReloadWorkQueue() completes in constant time.
+  int high_res_task_count_;

[darin (slow to review), 2014/07/18 04:07:58]

can this number be negative? should we use unsigned math?

[cpu_(ooo_6.6-7.5), 2014/07/18 22:53:10]

I rather use signed math to make sure we count down to zero correctly, there is
a CHECK() for that on decrement.

 
   // The lock that protects access to |incoming_queue_|, |message_loop_| and

[darin (slow to review), 2014/07/18 04:07:58]

This lock also protects access to high_res_task_count_ now. The comment that
enumerates all other member variables is sort of silly. The comment should just
say that it protects all members of this class or something like that instead.

[cpu_(ooo_6.6-7.5), 2014/07/18 22:53:10]

Done.

   // |next_sequence_num_|.