Only emit a fallback SurfaceDrawQuad if differs from primary

cc/layers/surface_layer_impl.cc

 // 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 "cc/layers/surface_layer_impl.h"
 
 #include <stdint.h>
 
 #include "base/trace_event/trace_event_argument.h"
 #include "cc/debug/debug_colors.h"
@@ skipping 48 matching lines @@
   layer_impl->SetPrimarySurfaceInfo(primary_surface_info_);
   layer_impl->SetFallbackSurfaceInfo(fallback_surface_info_);
   layer_impl->SetStretchContentToFillBounds(stretch_content_to_fill_bounds_);
 }
 
 void SurfaceLayerImpl::AppendQuads(RenderPass* render_pass,
                                    AppendQuadsData* append_quads_data) {
   AppendRainbowDebugBorder(render_pass);
   auto* primary = CreateSurfaceDrawQuad(
       render_pass, SurfaceDrawQuadType::PRIMARY, primary_surface_info_);
-  if (primary) {
+  // Emitting a fallback SurfaceDrawQuad is unnecessary if the primary and
+  // fallback surface Ids match.
+  if (primary && fallback_surface_info_.id() != primary_surface_info_.id()) {

[vmpstr, 2017/02/16 00:52:12]

I'm guessing this comparison takes into account that fallback_surface_info_.id()
may not be valid

[Fady Samuel, 2017/02/16 01:16:10]

If the fallback SurfaceId is invalid  then CreateSurfaceDrawQuad will exit
early.

     primary->fallback_quad = CreateSurfaceDrawQuad(
         render_pass, SurfaceDrawQuadType::FALLBACK, fallback_surface_info_);
   }
 }
 
 SurfaceDrawQuad* SurfaceLayerImpl::CreateSurfaceDrawQuad(
     RenderPass* render_pass,
     SurfaceDrawQuadType surface_draw_quad_type,
     const SurfaceInfo& surface_info) {
   if (!surface_info.id().is_valid())
@@ skipping 136 matching lines @@
   dict->SetString("surface_id", primary_surface_info_.id().ToString());
   dict->SetString("fallback_surface_id",
                   fallback_surface_info_.id().ToString());
 }
 
 const char* SurfaceLayerImpl::LayerTypeAsString() const {
   return "cc::SurfaceLayerImpl";
 }
 
 }  // namespace cc