cc: Optimize the tile map key hash for 4-byte size_t.

cc/tiles/picture_layer_tiling.h

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_TILES_PICTURE_LAYER_TILING_H_
 #define CC_TILES_PICTURE_LAYER_TILING_H_
 
 #include <map>
 #include <utility>
 #include <vector>
@@ skipping 34 matching lines @@
   virtual const Region* GetPendingInvalidation() = 0;
   virtual const PictureLayerTiling* GetPendingOrActiveTwinTiling(
       const PictureLayerTiling* tiling) const = 0;
   virtual TilePriority::PriorityBin GetMaxTilePriorityBin() const = 0;
   virtual bool RequiresHighResToDraw() const = 0;
 
  protected:
   virtual ~PictureLayerTilingClient() {}
 };
 
+struct TileMapKey {
+  TileMapKey(int x, int y) : index_x(x), index_y(y) {}
+  explicit TileMapKey(const std::pair<int, int>& index)
+      : index_x(index.first), index_y(index.second) {}
+
+  bool operator==(const TileMapKey& other) const {
+    return index_x == other.index_x && index_y == other.index_y;
+  }
+
+  int index_x;
+  int index_y;
+};
+
+}  // namespace cc
+
+namespace BASE_HASH_NAMESPACE {
+template <>
+struct hash<cc::TileMapKey> {
+  size_t operator()(const cc::TileMapKey& key) const {
+    uint16 value1 = static_cast<uint16>(key.index_x);
+    uint16 value2 = static_cast<uint16>(key.index_y);
+    uint32 value1_32 = value1;
+    return (value1_32 << 16) | value2;
+  }
+};
+}  // namespace BASE_HASH_NAMESPACE
+
+namespace cc {
+
 class CC_EXPORT PictureLayerTiling {
  public:
   static const int kBorderTexels = 1;
 
   PictureLayerTilingClient* client() const { return client_; }
   ~PictureLayerTiling();
 
   static float CalculateSoonBorderDistance(
       const gfx::Rect& visible_rect_in_content_space,
       float content_to_screen_scale);
@@ skipping 153 matching lines @@
   // here. Note that when processing a pending tree, this rect is the same as
   // the visible rect so no tiles are processed in this case.
   enum PriorityRectType {
     VISIBLE_RECT,
     PENDING_VISIBLE_RECT,
     SKEWPORT_RECT,
     SOON_BORDER_RECT,
     EVENTUALLY_RECT
   };
 
-  using TileMapKey = std::pair<int, int>;
   using TileMap = base::ScopedPtrHashMap<TileMapKey, ScopedTilePtr>;
 
   struct FrameVisibleRect {
     gfx::Rect visible_rect_in_content_space;
     double frame_time_in_seconds = 0.0;
   };
 
   PictureLayerTiling(WhichTree tree,
                      float contents_scale,
                      scoped_refptr<RasterSource> raster_source,
@@ skipping 123 matching lines @@
 
  private:
   DISALLOW_ASSIGN(PictureLayerTiling);
 
   RectExpansionCache expansion_cache_;
 };
 
 }  // namespace cc
 
 #endif  // CC_TILES_PICTURE_LAYER_TILING_H_