avcodec/dvdec: Avoid stack buffers

Instead reuse the destination RL VLC as scratch space.
This is possible, because the (implicit) codes here are already
ordered from left-to-right in the tree and because the codelengths
are increasing, which implies that mapping from VLC entries to the
corresponding entries used to initialize the VLC is monotonically
increasing. This means that one can reuse the right end of the
destination RL VLC to store the tables used to initialize the VLC
with.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

1 files changed, 13 insertions, 13 deletions

diff --git a/libavcodec/dvdec.c b/libavcodec/dvdec.c
index 7692300bdd..afc4bb0bcd 100644
--- a/libavcodec/dvdec.c
+++ b/libavcodec/dvdec.c
@@ -152,31 +152,30 @@ static av_cold void dv_init_static(void)
 {
     VLCElem vlc_buf[FF_ARRAY_ELEMS(dv_rl_vlc)] = { 0 };
     VLC dv_vlc = { .table = vlc_buf, .table_allocated = FF_ARRAY_ELEMS(vlc_buf) };
-    uint8_t    new_dv_vlc_len[NB_DV_VLC * 2];
-    uint8_t    new_dv_vlc_run[NB_DV_VLC * 2];
-    int16_t  new_dv_vlc_level[NB_DV_VLC * 2];
+    const unsigned offset = FF_ARRAY_ELEMS(dv_rl_vlc) - (2 * NB_DV_VLC - NB_DV_ZERO_LEVEL_ENTRIES);
+    RL_VLC_ELEM *tmp = dv_rl_vlc + offset;
     int i, j;
 
     /* it's faster to include sign bit in a generic VLC parsing scheme */
     for (i = 0, j = 0; i < NB_DV_VLC; i++, j++) {
-        new_dv_vlc_len[j]   = ff_dv_vlc_len[i];
-        new_dv_vlc_run[j]   = ff_dv_vlc_run[i];
-        new_dv_vlc_level[j] = ff_dv_vlc_level[i];
+        tmp[j].len   = ff_dv_vlc_len[i];
+        tmp[j].run   = ff_dv_vlc_run[i];
+        tmp[j].level = ff_dv_vlc_level[i];
 
         if (ff_dv_vlc_level[i]) {
-            new_dv_vlc_len[j]++;
+            tmp[j].len++;
 
             j++;
-            new_dv_vlc_len[j]   =  ff_dv_vlc_len[i] + 1;
-            new_dv_vlc_run[j]   =  ff_dv_vlc_run[i];
-            new_dv_vlc_level[j] = -ff_dv_vlc_level[i];
+            tmp[j].len   =  ff_dv_vlc_len[i] + 1;
+            tmp[j].run   =  ff_dv_vlc_run[i];
+            tmp[j].level = -ff_dv_vlc_level[i];
         }
     }
 
     /* NOTE: as a trick, we use the fact the no codes are unused
      * to accelerate the parsing of partial codes */
     ff_init_vlc_from_lengths(&dv_vlc, TEX_VLC_BITS, j,
-                             new_dv_vlc_len, 1,
+                             &tmp[0].len, sizeof(tmp[0]),
                              NULL, 0, 0, 0, INIT_VLC_USE_NEW_STATIC, NULL);
     av_assert1(dv_vlc.table_size == 1664);
 
@@ -189,8 +188,9 @@ static av_cold void dv_init_static(void)
             run   = 0;
             level = code;
         } else {
-            run   = new_dv_vlc_run[code] + 1;
-            level = new_dv_vlc_level[code];
+            av_assert1(i <= code + offset);
+            run   = tmp[code].run + 1;
+            level = tmp[code].level;
         }
         dv_rl_vlc[i].len   = len;
         dv_rl_vlc[i].level = level;