cc: Optimize decode scheduling for checker-images.

cc/tiles/checker_image_tracker.h

Index: cc/tiles/checker_image_tracker.h
diff --git a/cc/tiles/checker_image_tracker.h b/cc/tiles/checker_image_tracker.h
index b2a16ecf9b2d60c9e4b9cc1371c47329c9fd406e..ac7161293ccecf35b65e3522c344793e1a1e562e 100644
--- a/cc/tiles/checker_image_tracker.h
+++ b/cc/tiles/checker_image_tracker.h
@@ -51,6 +51,12 @@ class CC_EXPORT CheckerImageTracker {
   // Returns the set of images to invalidate on the sync tree.
   const ImageIdFlatSet& TakeImagesToInvalidateOnSyncTree();
 
+  // Resets any pending image decode requests that have not been scheduled for
+  // decode already. Note that calling this means that tiles which have already
+  // been checker-imaged need to be filtered on the tracker again to
+  // re-rasterize them.
+  void ResetImageDecodeQueue();
+
   void DidActivateSyncTree();
 
  private:
@@ -65,6 +71,10 @@ class CC_EXPORT CheckerImageTracker {
 
   void ScheduleImageDecodeIfNecessary(const sk_sp<const SkImage>& image);
 
+  // Called when the next request in the |image_decode_queue_| should be
+  // scheduled with the image decode service.
+  void ScheduleNextImageDecode();
+
   ImageController* image_controller_;
   CheckerImageTrackerClient* client_;
   const bool enable_checker_imaging_;
@@ -76,12 +86,16 @@ class CC_EXPORT CheckerImageTracker {
   // A set of images which were invalidated on the current sync tree.
   ImageIdFlatSet invalidated_images_on_current_sync_tree_;
 
-  // A set of images which are currently pending decode from the image decode
-  // service.
-  // TODO(khushalsagar): This should be a queue that gets re-built each time we
-  // do a PrepareTiles? See crbug.com/689184.
+  // The set of images in the image decode queue.
   ImageIdFlatSet pending_image_decodes_;
 
+  // The queue of images pending decode. We maintain a queue to ensure that the
+  // order in which images are decoded is aligned with the priority of the tiles
+  // dependent on these images.
+  // The first image in this queue, if any, is the currently outstanding decode
+  // request given to the queue.
+  std::list<sk_sp<const SkImage>> image_decode_request_queue_;

[vmpstr, 2017/03/07 19:25:08]

Can you write a cc_perftest for repeated schedules/cancels to see if std::list
performs better than std::vector (this can be a one-off)

[Khushal, 2017/03/13 20:35:08]

I guess not necessary anymore. We iterate through the queue to pull the next
image to decode and just clear it at the end.

+
   // A set of images which have been decoded at least once from the
   // ImageDecodeService and should not be checkered again.
   // TODO(khushalsagar): Limit the size of this set.