Adding tracing to measure synthetic smooth scrolling correctness

cc/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/layer_tree_host_impl.h"
 
 #include <algorithm>
 
 #include "base/basictypes.h"
 #include "base/debug/trace_event.h"
@@ skipping 782 matching lines @@
 {
     TRACE_EVENT0("cc", "LayerTreeHostImpl::drawLayers");
     DCHECK(canDraw());
     DCHECK(!frame.renderPasses.empty());
 
     // FIXME: use the frame begin time from the overall compositor scheduler.
     // This value is currently inaccessible because it is up in Chromium's
     // RenderWidget.
     m_fpsCounter->markBeginningOfFrame(base::TimeTicks::Now());
 
+    // Get the TranslateY value for a sanity-check on scrolling benchmark
+    // metrics. Specifically, we want to make sure
+    // BasicMouseWheelSmoothScrollGesture has proper scroll curves.
+    TRACE_COUNTER_ID1("gpu", "frame_counter_translate_y", this,

[nduca, 2012/11/27 23:50:27]

maybe root_layer_y_translate?

[shawnsingh, 2012/11/27 23:59:58]

I also like nduca's proposed rename.

What assumptions are we allowed to make about the transform that the root
scrollLayer will ever have?  Are we certain it will only ever have translation? 
If this layer has more complicated transforms, are you sure this is the value
you want to trace?

assuming you know that's the value you want, it LGTM.

+                      m_rootScrollLayerImpl->drawTransform().matrix().getDouble(

[jamesr, 2012/11/28 00:02:16]

cc::LayerImpl has a scrollOffset, can't you use that instead of looking at the
draw transform?

+                          1, 3));
+
     if (m_settings.showDebugRects())
         m_debugRectHistory->saveDebugRectsForCurrentFrame(m_rootLayerImpl.get(), *frame.renderSurfaceLayerList, frame.occludingScreenSpaceRects, frame.nonOccludingScreenSpaceRects, settings());
 
     // Because the contents of the HUD depend on everything else in the frame, the contents
     // of its texture are updated as the last thing before the frame is drawn.
     if (m_hudLayerImpl)
         m_hudLayerImpl->updateHudTexture(m_resourceProvider.get());
 
     m_renderer->drawFrame(frame.renderPasses, frame.renderPassesById);
 
@@ skipping 785 matching lines @@
     ScrollbarAnimationController* scrollbarController = layer->scrollbarAnimationController();
     double monotonicTime = (time - base::TimeTicks()).InSecondsF();
     if (scrollbarController && scrollbarController->animate(monotonicTime))
         m_client->setNeedsRedrawOnImplThread();
 
     for (size_t i = 0; i < layer->children().size(); ++i)
         animateScrollbarsRecursive(layer->children()[i], time);
 }
 
 }  // namespace cc