Don't allow the composited-alpha folding optimization to cross subsequences

third_party/WebKit/Source/platform/graphics/paint/PaintController.cpp

 // Copyright 2014 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.
 
 #include "platform/graphics/paint/PaintController.h"
 
 #include "platform/graphics/GraphicsLayer.h"
 #include "platform/graphics/paint/DrawingDisplayItem.h"
 #include "platform/instrumentation/tracing/TraceEvent.h"
 #include "platform/wtf/AutoReset.h"
@@ skipping 145 matching lines @@
           "under-invalidation: new subsequence wrong length", client, start,
           end);
       CHECK(false);
     }
   }
 
   DCHECK(new_cached_subsequences_.Find(&client) ==
          new_cached_subsequences_.end());
 
   new_cached_subsequences_.insert(&client, SubsequenceMarkers(start, end));
+  last_cached_subsequence_end_ = end;
 }
 
 bool PaintController::LastDisplayItemIsNoopBegin() const {
   if (new_display_item_list_.IsEmpty())
     return false;
 
   const auto& last_display_item = new_display_item_list_.Last();
   return last_display_item.IsBegin() && !last_display_item.DrawsContent();
 }
 
+bool PaintController::LastDisplayItemIsSubsequenceEnd() const {
+  return last_cached_subsequence_end_ == new_display_item_list_.size() - 1;

[wkorman, 2017/04/11 20:52:25]

We initialize last_cached_subsequence_end_ to 0, and so I wonder whether if we
call this with a display item list of size 1, but where we never actually called
AddCachedSubsequence, we might return true here when in fact we shouldn't.
Possible? Unit tests to explore could be helpful.

[chrishtr, 2017/04/11 21:09:10]

Fixed.

+}
+
 void PaintController::RemoveLastDisplayItem() {
   if (new_display_item_list_.IsEmpty())
     return;
 
 #if DCHECK_IS_ON()
   // Also remove the index pointing to the removed display item.
   IndicesByClientMap::iterator it = new_display_item_indices_by_client_.Find(
       &new_display_item_list_.Last().Client());
   if (it != new_display_item_indices_by_client_.end()) {
     Vector<size_t>& indices = it->value;
@@ skipping 343 matching lines @@
       !new_display_item_list_.IsEmpty())
     GenerateChunkRasterInvalidationRects(new_paint_chunks_.LastChunk());
 
   SkPictureGpuAnalyzer gpu_analyzer;
 
   current_cache_generation_ =
       DisplayItemClient::CacheGenerationOrInvalidationReason::Next();
 
   new_cached_subsequences_.Swap(current_cached_subsequences_);
   new_cached_subsequences_.Clear();
+  last_cached_subsequence_end_ = 0;
   for (auto& item : current_cached_subsequences_) {
     item.key->SetDisplayItemsCached(current_cache_generation_);
 #if CHECK_DISPLAY_ITEM_CLIENT_ALIVENESS
     DisplayItemClient::EndShouldKeepAliveAllClients(item.key);
 #endif
   }
 
   Vector<const DisplayItemClient*> skipped_cache_clients;
   for (const auto& item : new_display_item_list_) {
     // No reason to continue the analysis once we have a veto.
@@ skipping 375 matching lines @@
               show_paint_records
                   ? (DisplayItemList::JsonOptions::kShowPaintRecords |
                      DisplayItemList::JsonOptions::kShowClientDebugName)
                   : DisplayItemList::JsonOptions::kShowClientDebugName)
           ->ToPrettyJSONString()
           .Utf8()
           .Data());
 }
 
 }  // namespace blink