cc : Add UMA metric for finding closest matching layer to a point

cc/trees/layer_tree_impl.cc

 // Copyright 2011 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 "cc/trees/layer_tree_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 1863 matching lines @@
   // closer in this case, counterintuitively.
   float closest_distance;
 };
 
 template <typename Functor>
 static void FindClosestMatchingLayer(const gfx::PointF& screen_space_point,
                                      LayerImpl* root_layer,
                                      const Functor& func,
                                      FindClosestMatchingLayerState* state) {
   // We want to iterate from front to back when hit testing.
-  for (auto* layer : base::Reversed(*root_layer->layer_tree_impl())) {
-    if (!func(layer))
-      continue;
+  {
+    base::ElapsedTimer timer;
+    for (auto* layer : base::Reversed(*root_layer->layer_tree_impl())) {
+      if (!func(layer))
+        continue;
 
-    float distance_to_intersection = 0.f;
-    bool hit = false;
-    if (layer->Is3dSorted())
-      hit =
-          PointHitsLayer(layer, screen_space_point, &distance_to_intersection);
-    else
-      hit = PointHitsLayer(layer, screen_space_point, nullptr);
+      float distance_to_intersection = 0.f;
+      bool hit = false;
+      if (layer->Is3dSorted())
+        hit = PointHitsLayer(layer, screen_space_point,
+                             &distance_to_intersection);
+      else
+        hit = PointHitsLayer(layer, screen_space_point, nullptr);
 
-    if (!hit)
-      continue;
+      if (!hit)
+        continue;
 
-    bool in_front_of_previous_candidate =
-        state->closest_match &&
-        layer->GetSortingContextId() ==
-            state->closest_match->GetSortingContextId() &&
-        distance_to_intersection >
-            state->closest_distance + std::numeric_limits<float>::epsilon();
+      bool in_front_of_previous_candidate =
+          state->closest_match &&
+          layer->GetSortingContextId() ==
+              state->closest_match->GetSortingContextId() &&
+          distance_to_intersection >
+              state->closest_distance + std::numeric_limits<float>::epsilon();
 
-    if (!state->closest_match || in_front_of_previous_candidate) {
-      state->closest_distance = distance_to_intersection;
-      state->closest_match = layer;
+      if (!state->closest_match || in_front_of_previous_candidate) {
+        state->closest_distance = distance_to_intersection;
+        state->closest_match = layer;
+      }
     }
+    UMA_HISTOGRAM_COUNTS("Compositing.LayerTreeImpl.FindClosestMatchingLayerUs",
+                         timer.Elapsed().InMicroseconds());
   }
 }
 
 static bool IsScrollableOrDrawnScrollbarLayer(LayerImpl* layer) {
   return layer->scrollable() ||
          (layer->ToScrollbarLayer() &&
           layer->is_drawn_render_surface_layer_list_member());
 }
 
 struct FindScrollingLayerOrScrollbarLayerFunctor {
@@ skipping 175 matching lines @@
 
 void LayerTreeImpl::ResetAllChangeTracking() {
   layers_that_should_push_properties_.clear();
   // Iterate over all layers, including masks.
   for (auto& layer : *layers_)
     layer->ResetChangeTracking();
   property_trees_.ResetAllChangeTracking();
 }
 
 }  // namespace cc