iSAC entropy coder: Avoid signed integer overflow

webrtc/modules/audio_coding/codecs/isac/fix/source/entropy_coding.c

 /*
  *  Copyright (c) 2011 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 374 matching lines @@
   int16_t dither1_Q7, dither2_Q7, dither_gain_Q14, shft;
 
   if (AvgPitchGain_Q12 < 614)  /* this threshold should be equal to that in decode_spec() */
   {
     for (k = 0; k < length-2; k += 3)
     {
       /* new random unsigned int32_t */
       seed = WEBRTC_SPL_UMUL(seed, 196314165) + 907633515;
 
       /* fixed-point dither sample between -64 and 64 (Q7) */
-      dither1_Q7 = (int16_t)(((int32_t)seed + 16777216) >> 25);
+      dither1_Q7 = (int16_t)(((int32_t)(seed + 16777216)) >> 25);
 
       /* new random unsigned int32_t */
       seed = WEBRTC_SPL_UMUL(seed, 196314165) + 907633515;
 
       /* fixed-point dither sample between -64 and 64 */
-      dither2_Q7 = (int16_t)((seed + 16777216) >> 25);
+      dither2_Q7 = (int16_t)(((int32_t)(seed + 16777216)) >> 25);

[kwiberg-webrtc, 2016/02/25 11:57:00]

Here, I'm actually changing the behavior. The old code did an unsigned right
shift, while the new does a signed right shift (like in all the other places).
The new behavior is obviously the correct one; the old code gave results in the
range [0,127] instead of [-64,63].

[tlegrand-webrtc, 2016/02/26 10:21:40]

I think this changes should be fine.

 
       shft = (int16_t)(WEBRTC_SPL_RSHIFT_U32(seed, 25) & 15);
       if (shft < 5)
       {
         bufQ7[k]   = dither1_Q7;
         bufQ7[k+1] = dither2_Q7;
         bufQ7[k+2] = 0;
       }
       else if (shft < 10)
       {
@@ skipping 13 matching lines @@
   {
     dither_gain_Q14 = (int16_t)(22528 - WEBRTC_SPL_MUL(10, AvgPitchGain_Q12));
 
     /* dither on half of the coefficients */
     for (k = 0; k < length-1; k += 2)
     {
       /* new random unsigned int32_t */
       seed = WEBRTC_SPL_UMUL(seed, 196314165) + 907633515;
 
       /* fixed-point dither sample between -64 and 64 */
-      dither1_Q7 = (int16_t)(((int32_t)seed + 16777216) >> 25);
+      dither1_Q7 = (int16_t)(((int32_t)(seed + 16777216)) >> 25);
 
       /* dither sample is placed in either even or odd index */
       shft = (int16_t)(WEBRTC_SPL_RSHIFT_U32(seed, 25) & 1);     /* either 0 or 1 */
 
       bufQ7[k + shft] = (int16_t)((dither_gain_Q14 * dither1_Q7 + 8192) >> 14);
       bufQ7[k + 1 - shft] = 0;
     }
   }
 }
 
@@ skipping 1579 matching lines @@
 
     index_gQQ[k] = pos2QQ + WebRtcIsacfix_kQuantMinGain[k]; //ATTN: ok?
     if (index_gQQ[k] < 0) {
       index_gQQ[k] = 0;
     }
     else if (index_gQQ[k] > WebRtcIsacfix_kMaxIndGain[k]) {
       index_gQQ[k] = WebRtcIsacfix_kMaxIndGain[k];
     }
   }
 }