[FFmpeg-devel] [PATCH 21/21] avcodec/smacker: Avoid code duplication

[Andreas Rheinhardt]

Besides the obvious advantage of less code this also has a performance
impact: For GCC 9 the time spent on one call to smka_decode_frame() for
the sample from ticket #2425 decreased from 1693619 to 1498127
decicycles. For Clang 9, it decreased from 1369089 to 1366465
decicycles.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
---
The numbers for GCC surprised me (as did the fact that GCC was so much
worse than Clang).

 libavcodec/smacker.c | 62 ++++++++++++++------------------------------
 1 file changed, 20 insertions(+), 42 deletions(-)

diff --git a/libavcodec/smacker.c b/libavcodec/smacker.c
index d2b1c68162..ffd24c11e7 100644
--- a/libavcodec/smacker.c
+++ b/libavcodec/smacker.c
@@ -671,37 +671,23 @@ static int smka_decode_frame(AVCodecContext *avctx, void *data,
         for(i = 0; i <= stereo; i++)
             *samples++ = pred[i];
         for(; i < unp_size / 2; i++) {
+            unsigned idx = 2 * (i & stereo);
             if (get_bits_left(&gb) < 0) {
                 ret = AVERROR_INVALIDDATA;
                 goto error;
             }
-            if(i & stereo) {
-                if(vlc[2].table)
-                    res = get_vlc2(&gb, vlc[2].table, SMKTREE_BITS, 3);
-                else
-                    res = values[2];
-                val  = res;
-                if(vlc[3].table)
-                    res = get_vlc2(&gb, vlc[3].table, SMKTREE_BITS, 3);
-                else
-                    res = values[3];
-                val |= res << 8;
-                pred[1] += val;
-                *samples++ = pred[1];
-            } else {
-                if(vlc[0].table)
-                    res = get_vlc2(&gb, vlc[0].table, SMKTREE_BITS, 3);
-                else
-                    res = values[0];
-                val  = res;
-                if(vlc[1].table)
-                    res = get_vlc2(&gb, vlc[1].table, SMKTREE_BITS, 3);
-                else
-                    res = values[1];
-                val |= res << 8;
-                pred[0] += val;
-                *samples++ = pred[0];
-            }
+            if (vlc[idx].table)
+                res = get_vlc2(&gb, vlc[idx].table, SMKTREE_BITS, 3);
+            else
+                res = values[idx];
+            val  = res;
+            if (vlc[++idx].table)
+                res = get_vlc2(&gb, vlc[idx].table, SMKTREE_BITS, 3);
+            else
+                res = values[idx];
+            val |= res << 8;
+            pred[idx / 2] += val;
+            *samples++ = pred[idx / 2];
         }
     } else { //8-bit data
         for(i = stereo; i >= 0; i--)
@@ -709,25 +695,17 @@ static int smka_decode_frame(AVCodecContext *avctx, void *data,
         for(i = 0; i <= stereo; i++)
             *samples8++ = pred[i];
         for(; i < unp_size; i++) {
+            unsigned idx = i & stereo;
             if (get_bits_left(&gb) < 0) {
                 ret = AVERROR_INVALIDDATA;
                 goto error;
             }
-            if(i & stereo){
-                if(vlc[1].table)
-                    res = get_vlc2(&gb, vlc[1].table, SMKTREE_BITS, 3);
-                else
-                    res = values[1];
-                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] += res;
-                *samples8++ = pred[0];
-            }
+            if (vlc[idx].table)
+                val = get_vlc2(&gb, vlc[idx].table, SMKTREE_BITS, 3);
+            else
+                val = values[idx];
+            pred[idx] += val;
+            *samples8++ = pred[idx];
         }
     }
 
-- 
2.20.1