Fix a numerical issue in NetEq delay plotting

webrtc/modules/audio_coding/neteq/tools/neteq_delay_analyzer.cc

Index: webrtc/modules/audio_coding/neteq/tools/neteq_delay_analyzer.cc
diff --git a/webrtc/modules/audio_coding/neteq/tools/neteq_delay_analyzer.cc b/webrtc/modules/audio_coding/neteq/tools/neteq_delay_analyzer.cc
index a75da49b2248a3f271ab34e8f238328b0d17a33d..aedf067278f6efc3e884ccc065edc8bdeff289a2 100644
--- a/webrtc/modules/audio_coding/neteq/tools/neteq_delay_analyzer.cc
+++ b/webrtc/modules/audio_coding/neteq/tools/neteq_delay_analyzer.cc
@@ -14,6 +14,8 @@
 #include <limits>
 #include <utility>
 
+#include <webrtc/base/checks.h>
+
 namespace webrtc {
 namespace test {
 namespace {
@@ -115,8 +117,9 @@ void NetEqDelayAnalyzer::CreateGraphs(
   // This loop traverses data_ and populates rtp_timestamps_ms as well as
   // calculates the base offset.
   for (auto& d : data_) {
-    rtp_timestamps_ms.push_back(unwrapper.Unwrap(d.first) /
-                                (last_sample_rate_hz_ / 1000.f));

[ivoc, 2017/06/12 10:19:19]

Are you sure this change has any effect? I tried this division for 8000.f,
16000.f, 32000.f and 48000.f, but the results are always exact (floats can
exactly represent all integers up to 16 million).

[hlundin-webrtc, 2017/06/12 11:54:58]

The machine instructions change quite a bit: https://godbolt.org/g/kfXdqX

If I understand correctly, the inner division by 1000 is transformed from a
floating point divss to a multiplication and shifts. The outer division is
changed from a floating point divss to an idiv. So, it does make a change.

With the old code, I get an alternating +/- offset of the results, which causes
a saw-tooth graph, without motivation. This change resolves that.

+    rtp_timestamps_ms.push_back(
+        unwrapper.Unwrap(d.first) /
+        rtc::CheckedDivExact(last_sample_rate_hz_, 1000));
     offset =
         std::min(offset, d.second.arrival_time_ms - rtp_timestamps_ms.back());
   }