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..15af8af4586debbf877d8f5e3f90df3059f66d05 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;
 }
 
@@ -624,18 +629,20 @@ 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());
+  std::vector<int> unused_layers;
+  for (LayerCountMap::iterator it = used_layer_counts_.begin();
+       it != used_layer_counts_.end();
+       ++it) {
+    if (it->second == 0)
+      unused_layers.push_back(it->first);
+  }
 
   // Now calculate the set of layers in |image_decode_tasks_| that are not used
   // by any tile.
-  std::vector<int> unused_layers;
   for (LayerPixelRefTaskMap::iterator it = image_decode_tasks_.begin();
        it != image_decode_tasks_.end();
        ++it) {

[reveman, 2013/09/04 18:18:52]

do we need this loop at all?

[tomhudson, 2013/09/04 18:38:34]

Is it there just for safety? (If we somehow have a task whose layer isn't in the
used_layer_counts_ list at all...)

It'd be less necessary insurance if we didn't do the used_layer_counts_.erase()
in 654, but if the set of layer IDs can get large that's no good.

[vmpstr, 2013/09/04 19:36:40]

Yeah I think this was just insurance. The layer ids in used_layer_counts
contains all of the layer ids in image_decode_tasks (and possibly then some). So
it's sufficient to just check the "refcount" on the layer counts to determine if
the layer is not used. If it isn't, we can delete it from both maps.

-    if (used_layers.find(it->first) == used_layers.end())
+    if (used_layer_counts_.find(it->first) == used_layer_counts_.end())
       unused_layers.push_back(it->first);
   }
 
@@ -644,6 +651,7 @@ void TileManager::CleanUpUnusedImageDecodeTasks() {
        it != unused_layers.end();
        ++it) {
     image_decode_tasks_.erase(*it);
+    used_layer_counts_.erase(*it);
   }
 }