Trivial patch to set a max lookback on reordering

src/gpu/GrReorderCommandBuilder.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrReorderCommandBuilder.h"
 
 static bool intersect(const SkRect& a, const SkRect& b) {
     SkASSERT(a.fLeft <= a.fRight && a.fTop <= a.fBottom &&
              b.fLeft <= b.fRight && b.fTop <= b.fBottom);
     return a.fLeft < b.fRight && b.fLeft < a.fRight &&
            a.fTop < b.fBottom && b.fTop < a.fBottom;
 }
 
 GrTargetCommands::Cmd* GrReorderCommandBuilder::recordDrawBatch(State* state, GrBatch* batch) {
     // Check if there is a Batch Draw we can batch with by linearly searching back until we either
     // 1) check every draw
     // 2) intersect with something
     // 3) find a 'blocker'
+    // Experimentally we have found that most batching occurs within the first 10 comparisons.
+    static const int kMaxLookback = 10;
+    int i = 0;
     if (!this->cmdBuffer()->empty()) {
         GrTargetCommands::CmdBuffer::ReverseIter reverseIter(*this->cmdBuffer());
 
         do {
             if (Cmd::kDrawBatch_CmdType == reverseIter->type()) {
                 DrawBatch* previous = static_cast<DrawBatch*>(reverseIter.get());
 
                 if (previous->fState->getPipeline()->isEqual(*state->getPipeline()) &&
                     previous->fBatch->combineIfPossible(batch)) {
                     return NULL;
                 }
 
                 if (intersect(previous->fBatch->bounds(), batch->bounds())) {
                     break;
                 }
             } else {
                 // TODO temporary until we can navigate the other types of commands
                 break;
             }
-        } while (reverseIter.previous());
+        } while (reverseIter.previous() && ++i < kMaxLookback);
     }
 
     return GrNEW_APPEND_TO_RECORDER(*this->cmdBuffer(), DrawBatch, (state, batch,
                                                                     this->batchTarget()));
 }