Fix undefined reference to log2 on android

webrtc/modules/video_coding/utility/quality_scaler.cc

Index: webrtc/modules/video_coding/utility/quality_scaler.cc
diff --git a/webrtc/modules/video_coding/utility/quality_scaler.cc b/webrtc/modules/video_coding/utility/quality_scaler.cc
index 99bc6dad223bdaa58fcfcb56da6d451cf0ab0ad4..ba43be54ddda1199f0e8ab730c68fb3253ad4575 100644
--- a/webrtc/modules/video_coding/utility/quality_scaler.cc
+++ b/webrtc/modules/video_coding/utility/quality_scaler.cc
@@ -10,8 +10,14 @@
 
 #include "webrtc/modules/video_coding/utility/quality_scaler.h"
 
+#include <math.h>

[nisse-webrtc, 2016/09/15 08:22:12]

Why switching from cmath to math.h? Just to drop the std:: prefix?

[kthelgason, 2016/09/15 08:25:15]

Yep.

+
 #include <algorithm>
-#include <cmath>
+
+// Android does not support log2

[nisse-webrtc, 2016/09/15 08:22:12]

[citation needed]

Is there any documentation or bug report about this? 

Both my local man-page and http://en.cppreference.com/w/c/numeric/math/log2
claims that log2 is part of c99, which ought to be supported by Android.

Wild guess, but have you checked if only changing to math.h solves the problem?
I'm thinking, maybe c99 log2 works, but c++ std::log2 doesn't? They might
correspond to different symbols when linking.

[kthelgason, 2016/09/15 08:25:15]

I found some references to this on the internet, including:
https://code.google.com/p/android/issues/detail?id=212634
I had the same hunch, unfortunately that didn't work when I tried it.

+#if defined(WEBRTC_ANDROID)
+#define log2(x) (log(x) / log(2))
+#endif
 
 namespace webrtc {
 
@@ -182,7 +188,7 @@ void QualityScaler::UpdateTargetResolution(int width, int height) {
     maximum_shift_ = 0;
   } else {
     maximum_shift_ = static_cast<int>(
-        std::log2(std::min(width, height) / kMinDownscaleDimension));
+        log2(std::min(width, height) / kMinDownscaleDimension));
   }
   target_res_ = Resolution{width, height};
 }