Fix for left shift of negative value in NetEq.

webrtc/common_audio/signal_processing/levinson_durbin.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 28 matching lines @@
 
     // Normalize the autocorrelation R[0]...R[order+1]
 
     norm = WebRtcSpl_NormW32(R[0]);
 
     for (i = 0; i <= order; ++i)
     {
         temp1W32 = R[i] * (1 << norm);
         // Put R in hi and low format
         R_hi[i] = (int16_t)(temp1W32 >> 16);
         R_low[i] = (int16_t)((temp1W32 - ((int32_t)R_hi[i] * 65536)) >> 1);

[hlundin-webrtc, 2016/12/14 08:24:05]

I don't understand this code...

[ivoc, 2016/12/14 09:10:44]

It's pretty hard to read, but I think it's just splitting temp1W32 into the
upper 16 and lower 16 bits.

     }
 
     // K = A[1] = -R[1] / R[0]
 
-    temp2W32 = WEBRTC_SPL_LSHIFT_W32((int32_t)R_hi[1],16)
-            + WEBRTC_SPL_LSHIFT_W32((int32_t)R_low[1],1); // R[1] in Q31

[ivoc, 2016/12/14 09:10:44]

This code is putting the upper and lower bits together again into one number,
but only for R[1]. It seems a bit pointless to me. Maybe the author thought
shifting and adding is cheaper than multiplying?

+    temp2W32 = (R[1] * (1 << norm)); // R[1] in Q31

[hlundin-webrtc, 2016/12/14 08:24:05]

You can drop the outer parentheses.

[hlundin-webrtc, 2016/12/14 08:24:05]

Since the code at line 49 is quite obscure, did you verify that you get the same
value for temp2W32 before and after your change?

[ivoc, 2016/12/14 09:10:44]

I verified that it gives exactly the same values. It also still passes the
bitexactness tests.

     temp3W32 = WEBRTC_SPL_ABS_W32(temp2W32); // abs R[1]
     temp1W32 = WebRtcSpl_DivW32HiLow(temp3W32, R_hi[0], R_low[0]); // abs(R[1])/R[0] in Q31
     // Put back the sign on R[1]
     if (temp2W32 > 0)
     {
         temp1W32 = -temp1W32;
     }
 
     // Put K in hi and low format
     K_hi = (int16_t)(temp1W32 >> 16);
@@ skipping 171 matching lines @@
     for (i = 1; i <= order; i++)
     {
         // temp1W32 in Q27
         temp1W32 = (int32_t)A_hi[i] * 65536
                 + WEBRTC_SPL_LSHIFT_W32((int32_t)A_low[i], 1);
         // Round and store upper word
         A[i] = (int16_t)(((temp1W32 * 2) + 32768) >> 16);
     }
     return 1; // Stable filters
 }