Consistent 30% improvement in audio mixer running time.

webrtc/modules/audio_mixer/audio_frame_manipulator.cc

Index: webrtc/modules/audio_mixer/audio_frame_manipulator.cc
diff --git a/webrtc/modules/audio_mixer/audio_frame_manipulator.cc b/webrtc/modules/audio_mixer/audio_frame_manipulator.cc
index ff4c31e6675d0642081878231864978b2e914323..8aa0b5ce02dc35c6bd3a9d4a90de48ca82f015d8 100644
--- a/webrtc/modules/audio_mixer/audio_frame_manipulator.cc
+++ b/webrtc/modules/audio_mixer/audio_frame_manipulator.cc
@@ -29,6 +29,9 @@ void Ramp(float start_gain, float target_gain, AudioFrame* audio_frame) {
   RTC_DCHECK(audio_frame);
   RTC_DCHECK_GE(start_gain, 0.0f);
   RTC_DCHECK_GE(target_gain, 0.0f);
+  if (start_gain == target_gain) {

[the sun, 2017/02/07 08:53:22]

Comparing floats is fuzzy and operator == is not to be trusted to do what you
intend. See e.g.:

http://stackoverflow.com/questions/41989570/in-c-is-exactly-one-of-and-guaran...
http://stackoverflow.com/questions/17333/what-is-the-most-effective-way-for-f...

hlundin@, do you know if we have any function in the codebase (outside of gtest)
for this?

[hlundin-webrtc, 2017/02/07 21:34:51]

No, I know of no such function in our codebase.

+    return;
+  }
 
   size_t samples = audio_frame->samples_per_channel_;
   RTC_DCHECK_LT(0, samples);