diff -r 000000000000 -r 6474c204b198 gfx/layers/composite/FPSCounter.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gfx/layers/composite/FPSCounter.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,93 @@
+/* -*- Mode: C++; tab-width: 20; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_opengl_FPSCounter_h_
+#define mozilla_layers_opengl_FPSCounter_h_
+
+#include <stddef.h>                     // for size_t
+#include <algorithm>                    // for min
+#include "GLDefs.h"                     // for GLuint
+#include "mozilla/TimeStamp.h"          // for TimeStamp, TimeDuration
+#include "nsTArray.h"                   // for nsAutoTArray, nsTArray_Impl, etc
+#include "VBOArena.h"                   // for gl::VBOArena
+
+namespace mozilla {
+namespace gl {
+class GLContext;
+}
+namespace layers {
+
+class DataTextureSource;
+class ShaderProgramOGL;
+
+const double kFpsWindowMs = 250.0;
+const size_t kNumFrameTimeStamps = 16;
+struct FPSCounter {
+  FPSCounter() : mCurrentFrameIndex(0) {
+      mFrames.SetLength(kNumFrameTimeStamps);
+  }
+
+  // We keep a circular buffer of the time points at which the last K
+  // frames were drawn.  To estimate FPS, we count the number of
+  // frames we've drawn within the last kFPSWindowMs milliseconds and
+  // divide by the amount time since the first of those frames.
+  nsAutoTArray<TimeStamp, kNumFrameTimeStamps> mFrames;
+  size_t mCurrentFrameIndex;
+
+  void AddFrame(TimeStamp aNewFrame) {
+    mFrames[mCurrentFrameIndex] = aNewFrame;
+    mCurrentFrameIndex = (mCurrentFrameIndex + 1) % kNumFrameTimeStamps;
+  }
+
+  double AddFrameAndGetFps(TimeStamp aCurrentFrame) {
+    AddFrame(aCurrentFrame);
+    return EstimateFps(aCurrentFrame);
+  }
+
+  double GetFpsAt(TimeStamp aNow) {
+    return EstimateFps(aNow);
+  }
+
+private:
+  double EstimateFps(TimeStamp aNow) {
+    TimeStamp beginningOfWindow =
+      (aNow - TimeDuration::FromMilliseconds(kFpsWindowMs));
+    TimeStamp earliestFrameInWindow = aNow;
+    size_t numFramesDrawnInWindow = 0;
+    for (size_t i = 0; i < kNumFrameTimeStamps; ++i) {
+      const TimeStamp& frame = mFrames[i];
+      if (!frame.IsNull() && frame > beginningOfWindow) {
+        ++numFramesDrawnInWindow;
+        earliestFrameInWindow = std::min(earliestFrameInWindow, frame);
+      }
+    }
+    double realWindowSecs = (aNow - earliestFrameInWindow).ToSeconds();
+    if (realWindowSecs == 0.0 || numFramesDrawnInWindow == 1) {
+      return 0.0;
+    }
+    return double(numFramesDrawnInWindow - 1) / realWindowSecs;
+  }
+};
+
+struct FPSState {
+  FPSCounter mCompositionFps;
+  FPSCounter mTransactionFps;
+
+  FPSState() {}
+
+  void DrawFPS(TimeStamp, unsigned, Compositor* aCompositor);
+
+  void NotifyShadowTreeTransaction() {
+    mTransactionFps.AddFrame(TimeStamp::Now());
+  }
+
+private:
+  RefPtr<DataTextureSource> mFPSTextureSource;
+};
+
+}
+}
+
+#endif // mozilla_layers_opengl_FPSCounter_h_