Replacing SkTDArray with SkAutoSTArray to reduce memory allocation overhead in SkTileGrid::search

src/core/SkTileGrid.h

 
 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkTileGrid_DEFINED
 #define SkTileGrid_DEFINED
 
 #include "SkBBoxHierarchy.h"
 #include "SkPictureStateTree.h"
 #include "SkTileGridPicture.h" // for TileGridInfo
 
 /**
  * Subclass of SkBBoxHierarchy that stores elements in buckets that correspond
  * to tile regions, disposed in a regular grid.  This is useful when the tile
  * structure that will be use in search() calls is known prior to insertion.
  * Calls to search will return in constant time.
  *
  * Note: Current implementation of search() only supports looking-up regions
  * that are an exact match to a single tile.  Implementation could be augmented
  * to support arbitrary rectangles, but performance would be sub-optimal.
  */
 class SkTileGrid : public SkBBoxHierarchy {
 public:
-    typedef void* (*SkTileGridNextDatumFunctionPtr)(SkTDArray<void*>** tileData, SkTDArray<int>& tileIndices);
+    enum {
+        // Number of tiles for which data is allocated on the stack in
+        // SkTileGrid::search. If malloc becomes a bottleneck, we may consider
+        // increasing this number. Typical large web page, say 2k x 16k, would
+        // require 512 tiles of size 256 x 256 pixels.
+        kStackAllocationTileCount = 1024
+    };
+
+    typedef void* (*SkTileGridNextDatumFunctionPtr)(SkTDArray<void*>** tileData, SkAutoSTArray<kStackAllocationTileCount, int>& tileIndices);
 
     SkTileGrid(int xTileCount, int yTileCount, const SkTileGridPicture::TileGridInfo& info,
         SkTileGridNextDatumFunctionPtr nextDatumFunction);
 
     virtual ~SkTileGrid();
 
     /**
      * Insert a data pointer and corresponding bounding box
      * @param data The data pointer, may be NULL
      * @param bounds The bounding box, should not be empty
@@ skipping 45 matching lines @@
  * calls to this method must reflect the order in which the were originally
  * recorded into the tile grid.
  *
  * \param tileData array of pointers to arrays of tile data
  * \param tileIndices per-tile data indices, indices are incremented for tiles that contain
  *     the next datum.
  * \tparam T a type to which it is safe to cast a datum and that has an operator <
  *     such that 'a < b' is true if 'a' was inserted into the tile grid before 'b'.
  */
 template <typename T>
-void* SkTileGridNextDatum(SkTDArray<void*>** tileData, SkTDArray<int>& tileIndices) {
+void* SkTileGridNextDatum(SkTDArray<void*>** tileData, SkAutoSTArray<SkTileGrid::kStackAllocationTileCount, int>& tileIndices) {
     T* minVal = NULL;
     int tileCount = tileIndices.count();
     int minIndex = tileCount;
     int maxIndex = 0;
     // Find the next Datum; track where it's found so we reduce the size of the second loop.
     for (int tile = 0; tile < tileCount; ++tile) {
         int pos = tileIndices[tile];
         if (pos != SkTileGrid::kTileFinished) {
             T* candidate = (T*)(*tileData[tile])[pos];
             if (NULL == minVal || (*candidate) < (*minVal)) {
@@ skipping 16 matching lines @@
                     tileIndices[tile] = SkTileGrid::kTileFinished;
                 }
             }
         }
         return minVal;
     }
     return NULL;
 }
 
 #endif