Compositor hit test performance improvement

Source/core/page/scrolling/ScrollingCoordinator.cpp

Index: Source/core/page/scrolling/ScrollingCoordinator.cpp
diff --git a/Source/core/page/scrolling/ScrollingCoordinator.cpp b/Source/core/page/scrolling/ScrollingCoordinator.cpp
index 0f5faec87e07500fae5a511398ccf52c3717d382..6d24bf271475ef03841d400c3298098dd5fc7fcd 100644
--- a/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -316,6 +316,8 @@ bool ScrollingCoordinator::scrollableAreaScrollLayerDidChange(ScrollableArea* sc
 
 static void convertLayerRectsToEnclosingCompositedLayer(const LayerHitTestRects& layerRects, LayerHitTestRects& compositorRects)
 {
+    TRACE_EVENT0("input", "ScrollingCoordinator::convertLayerRectsToEnclosingCompositedLayer");
+
     // We have a set of rects per RenderLayer, we need to map them to their bounding boxes in their
     // enclosing composited layer.
     for (LayerHitTestRects::const_iterator layerIter = layerRects.begin(); layerIter != layerRects.end(); ++layerIter) {
@@ -342,10 +344,9 @@ static void convertLayerRectsToEnclosingCompositedLayer(const LayerHitTestRects&
             compIter = compositorRects.add(compositedLayer, Vector<LayoutRect>()).iterator;
 
         // Transform each rect to the co-ordinate space of it's enclosing composited layer.
-        // Ideally we'd compute a transformation matrix once and re-use it for each rect, but
-        // there doesn't appear to be any easy way to do it (mapLocalToContainer will flatten
-        // the TransformState, so we can't use setQuad/mappedQuad over and over again). Perhaps
-        // RenderGeometryMap?
+        // Ideally we'd compute a transformation matrix once and re-use it for each rect.
+        // RenderGeometryMap can be used for this (but needs to be updated to support crossing
+        // iframe boundaries), but in practice doesn't appear to provide much performance benefit.
         for (size_t i = 0; i < layerIter->value.size(); ++i) {
             FloatQuad localQuad(layerIter->value[i]);
             TransformState transformState(TransformState::ApplyTransformDirection, localQuad);