Refactor byte_io to avoid potentially undefined behavior

webrtc/modules/rtp_rtcp/source/byte_io.h

Index: webrtc/modules/rtp_rtcp/source/byte_io.h
diff --git a/webrtc/modules/rtp_rtcp/source/byte_io.h b/webrtc/modules/rtp_rtcp/source/byte_io.h
index 2617806dd941205c813a0b0c535f949429a6c97e..4ac47898baf1ee34f7e2226f9676210e5e6b4db5 100644
--- a/webrtc/modules/rtp_rtcp/source/byte_io.h
+++ b/webrtc/modules/rtp_rtcp/source/byte_io.h
@@ -88,13 +88,8 @@ class ByteReader {
   static T SignExtend(const T val) {
     uint8_t msb = static_cast<uint8_t>(val >> ((B - 1) * 8));
     if (msb & 0x80) {
-      // Sign extension is -1 (all ones) shifted left B bytes.
-      // The "B % sizeof(T)"-part is there to avoid compiler warning for
-      // shifting the whole size of the data type.
-      T sign_extend = (sizeof(T) == B ? 0 :
-          (static_cast<T>(-1L) << ((B % sizeof(T)) * 8)));
-
-      return val | sign_extend;
+      T used_bits_mask = (1 << (B * 8)) - 1;
+      return ~used_bits_mask | val;

[stefan-webrtc, 2015/07/08 12:01:50]

I think this deserves a comment. :)

[sprang_webrtc, 2015/07/08 12:57:03]

Done.

     }
     return val;
   }