Preliminary compositor disabling patch.

cc/output/direct_renderer.cc

 // Copyright 2012 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/output/direct_renderer.h"
 
 #include <utility>
 #include <vector>
 
 #include "base/containers/hash_tables.h"
@@ skipping 191 matching lines @@
   TRACE_EVENT0("cc", "DirectRenderer::DrawFrame");
   UMA_HISTOGRAM_COUNTS("Renderer4.renderPassCount",
                        render_passes_in_draw_order->size());
 
   const RenderPass* root_render_pass = render_passes_in_draw_order->back();
   DCHECK(root_render_pass);
 
   DrawingFrame frame;
   frame.render_passes_in_draw_order = render_passes_in_draw_order;
   frame.root_render_pass = root_render_pass;
+
+  overlay_processor_->ProcessForOverlays(render_passes_in_draw_order,
+                                         &frame.overlay_list);
+
+  // Merge quad damage for the root pass.
+  render_passes_in_draw_order->back()->UnionOverlayDamage();
+
   frame.root_damage_rect = Capabilities().using_partial_swap
                                ? root_render_pass->damage_rect
                                : root_render_pass->output_rect;
   frame.root_damage_rect.Intersect(gfx::Rect(device_viewport_rect.size()));
   frame.device_viewport_rect = device_viewport_rect;
   frame.device_clip_rect = device_clip_rect;
   frame.disable_picture_quad_image_filtering =
       disable_picture_quad_image_filtering;
 
-  overlay_processor_->ProcessForOverlays(render_passes_in_draw_order,
-                                         &frame.overlay_list);
+  if (root_render_pass->damage_rect.IsEmpty()) {
+    // If we have a scene that is composed of only overlays then we can
+    // potentially avoid drawing anything. However, if there are any copy
+    // requests we must do the drawing.
+    auto copy_pass = std::find_if(
+        render_passes_in_draw_order->begin(),
+        render_passes_in_draw_order->end(),
+        [](const RenderPass* pass) { return !pass->copy_requests.empty(); });
+    if (copy_pass == render_passes_in_draw_order->end()) {
+      ScheduleOverlays(&frame);
+      render_passes_in_draw_order->clear();
+      return;
+    }
+  }
 
   EnsureBackbuffer();
 
   // Only reshape when we know we are going to draw. Otherwise, the reshape
   // can leave the window at the wrong size if we never draw and the proper
   // viewport size is never set.
   output_surface_->Reshape(device_viewport_rect.size(), device_scale_factor);
 
   BeginDrawingFrame(&frame);
   for (size_t i = 0; i < render_passes_in_draw_order->size(); ++i) {
     RenderPass* pass = render_passes_in_draw_order->at(i);
     DrawRenderPass(&frame, pass);
 
     for (ScopedPtrVector<CopyOutputRequest>::iterator it =
              pass->copy_requests.begin();
          it != pass->copy_requests.end();
          ++it) {
       if (it != pass->copy_requests.begin()) {
         // Doing a readback is destructive of our state on Mac, so make sure
         // we restore the state between readbacks. http://crbug.com/99393.
         UseRenderPass(&frame, pass);
       }
       CopyCurrentRenderPassToBitmap(&frame, pass->copy_requests.take(it));
     }
   }
   FinishDrawingFrame(&frame);
+  ScheduleOverlays(&frame);
 
   render_passes_in_draw_order->clear();
 }
 
 gfx::Rect DirectRenderer::ComputeScissorRectForRenderPass(
     const DrawingFrame* frame) {
   gfx::Rect render_pass_scissor = frame->current_render_pass->output_rect;
 
   if (frame->root_damage_rect == frame->root_render_pass->output_rect ||
       !frame->current_render_pass->copy_requests.empty())
@@ skipping 235 matching lines @@
   ScopedResource* texture = render_pass_textures_.get(id);
   return texture && texture->id();
 }
 
 // static
 gfx::Size DirectRenderer::RenderPassTextureSize(const RenderPass* render_pass) {
   return render_pass->output_rect.size();
 }
 
 }  // namespace cc