[FFmpeg-devel] [PATCH 13/21] avcodec/smacker: Remove redundant checks when reading VLC codes

[Andreas Rheinhardt]

The VLC codes in question originate from a Huffmann tree and so every
sequence of bits that is longer than the longest code contains an
initial sequence that is a valid code. Given that it has been checked
during reading said tree (and once again in ff_init_vlc_sparse()) that
the length of each code is <= 3 * the number of bits read at once when
reading codes, get_vlc2() will always find a matching entry.

These checks have been added in 71d3c25a7ef442ac2dd7b6fbf7c489ebc0b58e9b
at a time when the length of the codes had not been checked when parsing
the tree.

For GCC 9 and the sample from ticket #2425 this led to a slight
performance regression: The time for one call to smka_decode_frame()
increased from 2053671 to 2064529 decicycles; for Clang 9, performance
improved from 1521288 to 1508459 decicycles.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
---
 libavcodec/smacker.c | 32 --------------------------------
 1 file changed, 32 deletions(-)

diff --git a/libavcodec/smacker.c b/libavcodec/smacker.c
index 991622eb0e..5e1b8c59ff 100644
--- a/libavcodec/smacker.c
+++ b/libavcodec/smacker.c
@@ -140,8 +140,6 @@ static int smacker_decode_bigtree(GetBitContext *gb, HuffContext *hc,
         int val, i1, i2;
         i1 = ctx->v1->table ? get_vlc2(gb, ctx->v1->table, SMKTREE_BITS, 3) : 0;
         i2 = ctx->v2->table ? get_vlc2(gb, ctx->v2->table, SMKTREE_BITS, 3) : 0;
-        if (i1 < 0 || i2 < 0)
-            return AVERROR_INVALIDDATA;
         val = ctx->recode1[i1] | (ctx->recode2[i2] << 8);
         if(val == ctx->escapes[0]) {
             ctx->last[0] = hc->current;
@@ -678,21 +676,11 @@ static int smka_decode_frame(AVCodecContext *avctx, void *data,
                     res = get_vlc2(&gb, vlc[2].table, SMKTREE_BITS, 3);
                 else
                     res = 0;
-                if (res < 0) {
-                    av_log(avctx, AV_LOG_ERROR, "invalid vlc\n");
-                    ret = AVERROR_INVALIDDATA;
-                    goto error;
-                }
                 val  = h[2].values[res];
                 if(vlc[3].table)
                     res = get_vlc2(&gb, vlc[3].table, SMKTREE_BITS, 3);
                 else
                     res = 0;
-                if (res < 0) {
-                    av_log(avctx, AV_LOG_ERROR, "invalid vlc\n");
-                    ret = AVERROR_INVALIDDATA;
-                    goto error;
-                }
                 val |= h[3].values[res] << 8;
                 pred[1] += (unsigned)sign_extend(val, 16);
                 *samples++ = pred[1];
@@ -701,21 +689,11 @@ static int smka_decode_frame(AVCodecContext *avctx, void *data,
                     res = get_vlc2(&gb, vlc[0].table, SMKTREE_BITS, 3);
                 else
                     res = 0;
-                if (res < 0) {
-                    av_log(avctx, AV_LOG_ERROR, "invalid vlc\n");
-                    ret = AVERROR_INVALIDDATA;
-                    goto error;
-                }
                 val  = h[0].values[res];
                 if(vlc[1].table)
                     res = get_vlc2(&gb, vlc[1].table, SMKTREE_BITS, 3);
                 else
                     res = 0;
-                if (res < 0) {
-                    av_log(avctx, AV_LOG_ERROR, "invalid vlc\n");
-                    ret = AVERROR_INVALIDDATA;
-                    goto error;
-                }
                 val |= h[1].values[res] << 8;
                 pred[0] += (unsigned)sign_extend(val, 16);
                 *samples++ = pred[0];
@@ -736,11 +714,6 @@ static int smka_decode_frame(AVCodecContext *avctx, void *data,
                     res = get_vlc2(&gb, vlc[1].table, SMKTREE_BITS, 3);
                 else
                     res = 0;
-                if (res < 0) {
-                    av_log(avctx, AV_LOG_ERROR, "invalid vlc\n");
-                    ret = AVERROR_INVALIDDATA;
-                    goto error;
-                }
                 pred[1] += sign_extend(h[1].values[res], 8);
                 *samples8++ = pred[1];
             } else {
@@ -748,11 +721,6 @@ static int smka_decode_frame(AVCodecContext *avctx, void *data,
                     res = get_vlc2(&gb, vlc[0].table, SMKTREE_BITS, 3);
                 else
                     res = 0;
-                if (res < 0) {
-                    av_log(avctx, AV_LOG_ERROR, "invalid vlc\n");
-                    ret = AVERROR_INVALIDDATA;
-                    goto error;
-                }
                 pred[0] += sign_extend(h[0].values[res], 8);
                 *samples8++ = pred[0];
             }
-- 
2.20.1