Add more specific metrics for main thread scrolling reasons

third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp

 /*
  * Copyright (C) 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 10 matching lines @@
  * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
  * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #include "core/layout/compositing/CompositingRequirementsUpdater.h"
 
 #include "core/layout/LayoutView.h"
 #include "core/layout/compositing/PaintLayerCompositor.h"
+#include "core/page/Page.h"
+#include "core/page/scrolling/ScrollingCoordinator.h"
 #include "core/paint/PaintLayer.h"
 #include "core/paint/PaintLayerStackingNode.h"
 #include "core/paint/PaintLayerStackingNodeIterator.h"
 #include "platform/tracing/TraceEvent.h"
 
 namespace blink {
 
 class OverlapMapContainer {
  public:
   void add(const IntRect& bounds) {
@@ skipping 202 matching lines @@
     Vector<PaintLayer*>& unclippedDescendants,
     IntRect& absoluteDescendantBoundingBox) {
   PaintLayerCompositor* compositor = m_layoutView.compositor();
 
   layer->stackingNode()->updateLayerListsIfNeeded();
 
   CompositingReasons reasonsToComposite = CompositingReasonNone;
   CompositingReasons directReasons =
       m_compositingReasonFinder.directReasons(layer);
 
+  if (!compositor->preferCompositingToLCDTextEnabled())
+    updateMainThreadScrollingReasonsFromStyle(layer);

[bokan, 2016/12/16 23:06:01]

Sorry, one last thing. I think this is the wrong place for this.
CompositingRequirementsUpdater is collecting reasons we *should* composite. This
looks like it rather belongs in PaintLayerScrollableArea's
layerNeedsCompositedScrolling, here particularily, where we return false:
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/paint/Pai...

[yigu, 2016/12/18 15:05:11]

Done.

+
   // Video is special. It's the only PaintLayer type that can both have
   // PaintLayer children and whose children can't use its backing to render
   // into. These children (the controls) always need to be promoted into their
   // own layers to draw on top of the accelerated video.
   if (currentRecursionData.m_compositingAncestor &&
       currentRecursionData.m_compositingAncestor->layoutObject()->isVideo())
     directReasons |= CompositingReasonVideoOverlay;
 
   bool hasCompositedScrollingAncestor =
       layer->ancestorScrollingLayer() &&
@@ skipping 274 matching lines @@
 
     descendantHas3DTransform |=
         anyDescendantHas3DTransform || layer->has3DTransform();
   }
 
   // At this point we have finished collecting all reasons to composite this
   // layer.
   layer->setCompositingReasons(reasonsToComposite);
 }
 
+// Check if current layer has layout object that contains properties that cause
+// main thread scrolling.
+void CompositingRequirementsUpdater::updateMainThreadScrollingReasonsFromStyle(
+    const PaintLayer* layer) const {
+  // If current layer has opacity and has not been recorded, add the layer into
+  // frameView and vice versa.
+  if (!layer || !layer->getScrollableArea())
+    return;
+
+  if (ScrollingCoordinator* scrollingCoordinator =

[bokan, 2016/12/16 23:06:01]

nit: we generally prefer the above pattern of early-return on null to prevent
excessive indenting.

[yigu, 2016/12/18 15:05:12]

Done.

+          m_layoutView.document().page()->scrollingCoordinator()) {
+    if (layer->compositesWithOpacity() !=
+        layer->getScrollableArea()->hasMainThreadScrollingReason(
+            MainThreadScrollingReason::kHasOpacity)) {
+      layer->getScrollableArea()->flipMainThreadScrollingReason(
+          MainThreadScrollingReason::kHasOpacity);
+
+      scrollingCoordinator->adjustMainThreadReasonsFromStyle(
+          MainThreadScrollingReason::kHasOpacity,
+          layer->compositesWithOpacity());
+    }
+  }
+}
+
 }  // namespace blink