Fix latency calculation when multiple rendered buffers are in flight.

content/common/gpu/image_transport_surface.cc

Index: content/common/gpu/image_transport_surface.cc
diff --git a/content/common/gpu/image_transport_surface.cc b/content/common/gpu/image_transport_surface.cc
index b6367dd4cd8b7e74f153726f5b4a3e7d4b179170..5e7c2b0be71c3e08001b2a770a89c5f31231dec8 100644
--- a/content/common/gpu/image_transport_surface.cc
+++ b/content/common/gpu/image_transport_surface.cc
@@ -202,9 +202,11 @@ bool PassThroughImageTransportSurface::SwapBuffers() {
   // of this class. Callback will not be called once the instance of this class
   // is destroyed. However, this also means that the callback can be run on
   // the calling thread only.
+  std::vector<ui::LatencyInfo> latency_info;
+  latency_info.swap(latency_info_);
   return gfx::GLSurfaceAdapter::SwapBuffersAsync(
       base::Bind(&PassThroughImageTransportSurface::SwapBuffersCallBack,
-          weak_ptr_factory_.GetWeakPtr()));
+                 weak_ptr_factory_.GetWeakPtr(), latency_info));
 }
 
 bool PassThroughImageTransportSurface::PostSubBuffer(
@@ -221,21 +223,24 @@ bool PassThroughImageTransportSurface::PostSubBuffer(
   // of this class. Callback will not be called once the instance of this class
   // is destroyed. However, this also means that the callback can be run on
   // the calling thread only.
-  return gfx::GLSurfaceAdapter::PostSubBufferAsync(x, y, width, height,
+  std::vector<ui::LatencyInfo> latency_info;
+  latency_info.swap(latency_info_);

[brianderson, 2015/05/14 19:06:12]

Although this swap avoids one copy, I think there are still two copies after
this: once on Bind and again on SwapBuffersCallback. It would be nice to avoid
those copies since we're trying to keep the overhead of LatencyInfo low.

Since std::move isn't allowed, could we instead do:
std::vector<ui::LatencyInfo>* latency_info_ptr = new
std::vector<ui::LatencyInfo>;
latency_info_ptr->swap(latency_info_);

...Bind(..., Owned(latency_info_ptr));


Or scoped_ptr<vector<...>> and Passed(latency_info)?

[alexst (slow to review), 2015/05/14 19:32:41]

Sounds good, done.

+  return gfx::GLSurfaceAdapter::PostSubBufferAsync(
+      x, y, width, height,
       base::Bind(&PassThroughImageTransportSurface::SwapBuffersCallBack,
-          weak_ptr_factory_.GetWeakPtr()));
+                 weak_ptr_factory_.GetWeakPtr(), latency_info));
 }
 
-void PassThroughImageTransportSurface::SwapBuffersCallBack() {
+void PassThroughImageTransportSurface::SwapBuffersCallBack(
+    std::vector<ui::LatencyInfo> latency_info) {
   base::TimeTicks swap_ack_time = base::TimeTicks::Now();
-  for (auto& latency : latency_info_) {
+  for (auto& latency : latency_info) {
     latency.AddLatencyNumberWithTimestamp(
         ui::INPUT_EVENT_LATENCY_TERMINATED_FRAME_SWAP_COMPONENT, 0, 0,
         swap_ack_time, 1);
   }
 
-  helper_->stub()->SendSwapBuffersCompleted(latency_info_);
-  latency_info_.clear();
+  helper_->stub()->SendSwapBuffersCompleted(latency_info);
 }
 
 bool PassThroughImageTransportSurface::OnMakeCurrent(gfx::GLContext* context) {