Surface Synchronization: Distinguish between dependencies and references

cc/trees/layer_tree_host_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_host_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 1606 matching lines @@
   }
 
   if (GetDrawMode() == DRAW_MODE_RESOURCELESS_SOFTWARE) {
     metadata.is_resourceless_software_draw_with_scroll_or_animation =
         IsActivelyScrolling() || mutator_host_->NeedsTickAnimations();
   }
 
   for (LayerImpl* surface_layer : active_tree_->SurfaceLayers()) {
     SurfaceLayerImpl* surface_layer_impl =
         static_cast<SurfaceLayerImpl*>(surface_layer);
-    metadata.referenced_surfaces.push_back(
-        surface_layer_impl->primary_surface_info().id());
-    // We need to retain a reference to the fallback surface too so that it's
-    // guaranteed to be available when needed.
-    if (surface_layer_impl->fallback_surface_info().is_valid()) {
+    if (!settings_.enable_surface_synchronization) {

[vmpstr, 2017/04/11 18:35:17]

nit: can you flip this to the positive and exchange if/else blocks

[Fady Samuel, 2017/04/11 19:39:24]

Done.

       metadata.referenced_surfaces.push_back(
-          surface_layer_impl->fallback_surface_info().id());
+          surface_layer_impl->primary_surface_info().id());
+    } else {
+      if (surface_layer_impl->fallback_surface_info().is_valid()) {
+        metadata.referenced_surfaces.push_back(
+            surface_layer_impl->fallback_surface_info().id());

[vmpstr, 2017/04/11 18:35:17]

hmm why do we not reference primary surface info if we have surface
synchronization enabled?

[Fady Samuel, 2017/04/11 19:39:24]

As the description says, fallbacks are references and primaries are
dependencies. Dependencies eventually become references when the CompositorFrame
activates AND the parent refers to the child in a fallback.

+      }
     }
   }
   if (!InnerViewportScrollLayer())
     return metadata;
 
   metadata.root_overflow_x_hidden |=
       !InnerViewportScrollLayer()->user_scrollable_horizontal();
   metadata.root_overflow_y_hidden |=
       !InnerViewportScrollLayer()->user_scrollable_vertical();
 
@@ skipping 2653 matching lines @@
 }
 
 void LayerTreeHostImpl::UpdateScrollSourceInfo(bool is_wheel_scroll) {
   if (is_wheel_scroll)
     has_scrolled_by_wheel_ = true;
   else
     has_scrolled_by_touch_ = true;
 }
 
 }  // namespace cc