avcodec/smacker: Use unsigned for prediction values

Up until now, the Smacker decoder has pretended that the prediction
values are signed in code like 'pred[0] += (unsigned)sign_extend(val, 16)'
(the cast has been added to this code later to fix undefined behaviour).
This has been even done in case the PCM format is u8.

Yet in case of 8/16 bit samples, only the lower 8/16 bit of the predicition
values are ever used, so one can just as well just use unsigned and
remove the sign extensions. This is what this commit does.

For GCC 9 the time for one call to smka_decode_frame() for the sample from
ticket #2425 decreased from 1709043 to 1693619 decicycles; for Clang 9
it went up from 1355273 to 1369089 decicycles.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/smacker.c

@@ -595,11 +595,10 @@ static int smka_decode_frame(AVCodecContext *avctx, void *data,
     int16_t *samples;
     uint8_t *samples8;
     uint8_t values[4];
-    int val;
     int i, res, ret;
     int unp_size;
     int bits, stereo;
-    int pred[2] = {0, 0};
+    unsigned pred[2], val;
 
     if (buf_size <= 4) {
         av_log(avctx, AV_LOG_ERROR, "packet is too small\n");
@@ -668,7 +667,7 @@ static int smka_decode_frame(AVCodecContext *avctx, void *data,
     /* this codec relies on wraparound instead of clipping audio */
     if(bits) { //decode 16-bit data
         for(i = stereo; i >= 0; i--)
-            pred[i] = sign_extend(av_bswap16(get_bits(&gb, 16)), 16);
+            pred[i] = av_bswap16(get_bits(&gb, 16));
         for(i = 0; i <= stereo; i++)
             *samples++ = pred[i];
         for(; i < unp_size / 2; i++) {
@@ -687,7 +686,7 @@ static int smka_decode_frame(AVCodecContext *avctx, void *data,
                 else
                     res = values[3];
                 val |= res << 8;
-                pred[1] += (unsigned)sign_extend(val, 16);
+                pred[1] += val;
                 *samples++ = pred[1];
             } else {
                 if(vlc[0].table)
@@ -700,7 +699,7 @@ static int smka_decode_frame(AVCodecContext *avctx, void *data,
                 else
                     res = values[1];
                 val |= res << 8;
-                pred[0] += (unsigned)sign_extend(val, 16);
+                pred[0] += val;
                 *samples++ = pred[0];
             }
         }
@@ -719,14 +718,14 @@ static int smka_decode_frame(AVCodecContext *avctx, void *data,
                     res = get_vlc2(&gb, vlc[1].table, SMKTREE_BITS, 3);
                 else
                     res = values[1];
-                pred[1] += sign_extend(res, 8);
+                pred[1] += res;
                 *samples8++ = pred[1];
             } else {
                 if(vlc[0].table)
                     res = get_vlc2(&gb, vlc[0].table, SMKTREE_BITS, 3);
                 else
                     res = values[0];
-                pred[0] += sign_extend(res, 8);
+                pred[0] += res;
                 *samples8++ = pred[0];
             }
         }