Reland mysterious cast that improves performance.

webrtc/common_audio/signal_processing/cross_correlation.c

 /*
  *  Copyright (c) 2012 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.
  */
 
 #include "webrtc/common_audio/signal_processing/include/signal_processing_library.h"
 
 /* C version of WebRtcSpl_CrossCorrelation() for generic platforms. */
 void WebRtcSpl_CrossCorrelationC(int32_t* cross_correlation,
                                  const int16_t* seq1,
                                  const int16_t* seq2,
                                  int16_t dim_seq,
                                  int16_t dim_cross_correlation,
                                  int right_shifts,
                                  int step_seq2) {
   int i = 0, j = 0;
 
   for (i = 0; i < dim_cross_correlation; i++) {
     int32_t corr = 0;
-    /* Unrolling doesn't seem to improve performance. */
+    // Linux 64-bit performance is improved by the int16_t cast below.
+    // Presumably this is some sort of compiler bug, as there's no obvious
+    // reason why that should result in better code.
     for (j = 0; j < dim_seq; j++)
-      corr += (seq1[j] * seq2[j]) >> right_shifts;
+      corr += (seq1[j] * seq2[j]) >> (int16_t)right_shifts;
     seq2 += step_seq2;
     *cross_correlation++ = corr;
   }
 }