cc: Speed up CleanUpImageDecodeTasks a bit.

cc/resources/tile_manager.cc

Index: cc/resources/tile_manager.cc
diff --git a/cc/resources/tile_manager.cc b/cc/resources/tile_manager.cc
index 50668c01f8dc52cabaae57d9363e0e96e0a62ebc..b2131b57dc2b4082c406f046675d6bac7dc6d8f3 100644
--- a/cc/resources/tile_manager.cc
+++ b/cc/resources/tile_manager.cc
@@ -190,6 +190,7 @@ void TileManager::RegisterTile(Tile* tile) {
   DCHECK(tiles_.find(tile->id()) == tiles_.end());
 
   tiles_[tile->id()] = tile;
+  used_layer_counts_[tile->layer_id()]++;
   prioritized_tiles_dirty_ = true;
 }
 
@@ -198,6 +199,10 @@ void TileManager::UnregisterTile(Tile* tile) {
 
   DCHECK(tiles_.find(tile->id()) != tiles_.end());
   tiles_.erase(tile->id());
+
+  DCHECK_GT(used_layer_counts_[tile->layer_id()], 0);
+  used_layer_counts_[tile->layer_id()]--;
+
   prioritized_tiles_dirty_ = true;
 }
 
@@ -624,26 +629,22 @@ void TileManager::CleanUpUnusedImageDecodeTasks() {
   if (image_decode_tasks_.empty())
     return;
 
-  // Calculate a set of layers that are used by at least one tile.
-  base::hash_set<int> used_layers;
-  for (TileMap::iterator it = tiles_.begin(); it != tiles_.end(); ++it)
-    used_layers.insert(it->second->layer_id());
-
-  // Now calculate the set of layers in |image_decode_tasks_| that are not used
-  // by any tile.
+  // Unused layers are layers whose use count is zero.
   std::vector<int> unused_layers;
-  for (LayerPixelRefTaskMap::iterator it = image_decode_tasks_.begin();
-       it != image_decode_tasks_.end();
+  for (LayerCountMap::iterator it = used_layer_counts_.begin();
+       it != used_layer_counts_.end();
        ++it) {
-    if (used_layers.find(it->first) == used_layers.end())
+    if (it->second == 0)
       unused_layers.push_back(it->first);
   }
 
-  // Erase unused layers from |image_decode_tasks_|.
+  // Erase unused layers from |image_decode_tasks_| and from
+  // |used_layer_counts_|.
   for (std::vector<int>::iterator it = unused_layers.begin();
        it != unused_layers.end();
        ++it) {
     image_decode_tasks_.erase(*it);
+    used_layer_counts_.erase(*it);

[reveman, 2013/09/04 21:25:17]

can we move these two lines to UnregisterTile and remove this function
completely?

[vmpstr, 2013/09/04 22:03:49]

Good idea. Done.

   }
 }