[FFmpeg-devel] [PATCH 5/9] avcodec/dvdec: Avoid stack buffers

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

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>
---
 libavcodec/dvdata.h |  2 ++
 libavcodec/dvdec.c  | 26 +++++++++++++-------------
 2 files changed, 15 insertions(+), 13 deletions(-)

diff --git a/libavcodec/dvdata.h b/libavcodec/dvdata.h
index ae104096ad..31191a8475 100644
--- a/libavcodec/dvdata.h
+++ b/libavcodec/dvdata.h
@@ -27,6 +27,8 @@ extern const uint8_t ff_dv_quant_shifts[22][4];
 extern const uint8_t ff_dv_quant_offset[4];
 
 #define NB_DV_VLC 409
+/* The number of entries with value zero in ff_dv_vlc_level. */
+#define NB_DV_ZERO_LEVEL_ENTRIES 72
 
 extern const uint8_t ff_dv_vlc_len[NB_DV_VLC];
 extern const uint8_t ff_dv_vlc_run[NB_DV_VLC];
diff --git a/libavcodec/dvdec.c b/libavcodec/dvdec.c
index 3857ed1266..32085a3ba6 100644
--- a/libavcodec/dvdec.c
+++ b/libavcodec/dvdec.c
@@ -137,31 +137,30 @@ 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);
 
@@ -174,8 +173,9 @@ 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;
-- 
2.34.1

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