Refactor byte_io to avoid potentially undefined behavior

webrtc/modules/rtp_rtcp/source/byte_io.h

 /*
  *  Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 70 matching lines @@
   }
 
   // If number of bytes is less than native data type (eg 24 bit, in int32_t),
   // and the most significant bit of the actual data is set, we must sign
   // extend the remaining byte(s) with ones so that the correct negative
   // number is retained.
   // Ex: 0x810A0B -> 0xFF810A0B, but 0x710A0B -> 0x00710A0B
   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;
   }
 };
 
 // Class for writing integers to a sequence of bytes
 // T = type of integer, B = bytes to write
 template<typename T, unsigned int B = sizeof(T)>
 class ByteWriter {
  public:
@@ skipping 121 matching lines @@
     data[4] = val >> 32;
     data[5] = val >> 40;
     data[6] = val >> 48;
     data[7] = val >> 56;
   }
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_RTP_RTCP_SOURCE_BYTE_IO_H_