avcodec/hevcdec: Add stat_coeffs to HEVCABACState

The HEVC decoder has both HEVCContext and HEVCLocalContext
structures. The latter is supposed to be the structure
containing the per-slicethread state.

Yet that is not how it is handled in practice: Each HEVCLocalContext
has a unique HEVCContext allocated for it and each of these
coincides with the main HEVCContext except in exactly one field:
The corresponding HEVCLocalContext.
This makes it possible to pass the HEVCContext everywhere where
logically a HEVCLocalContext should be used.

This led to confusion in the first version of what eventually became
commit c8bc0f66a875bc3708d8dc11b757f2198606ffd7:
Before said commit, the initialization of the Rice parameter derivation
state was incorrect; the fix for single-threaded as well as
frame-threaded decoding was to add backup stats to HEVCContext
that are used when the cabac state is updated*, see
https://ffmpeg.org/pipermail/ffmpeg-devel/2020-August/268861.html
Yet due to what has been said above, this does not work for
slice-threading, because the each HEVCLocalContext has its own
HEVCContext, so the Rice parameter state would not be transferred
between threads.

This is fixed in c8bc0f66a875bc3708d8dc11b757f2198606ffd7
by a hack: It rederives what the previous thread was and accesses
the corresponding HEVCContext.

Fix this by treating the Rice parameter state the same way
the ordinary CABAC parameters are shared between threads:
Make them part of the same struct that is shared between
slice threads. This does not cause races, because
the parts of the code that access these Rice parameters
are a subset of the parts of code that access the CABAC parameters.

*: And if the persistent_rice_adaptation_enabled_flag is set.

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

1 files changed, 5 insertions, 5 deletions

diff --git a/libavcodec/hevcdec.c b/libavcodec/hevcdec.c
index b0212a10a5..de1688cd2e 100644
--- a/libavcodec/hevcdec.c
+++ b/libavcodec/hevcdec.c
@@ -2502,7 +2502,7 @@ static int hls_decode_entry(AVCodecContext *avctxt, void *isFilterThread)
         y_ctb = (ctb_addr_rs / ((s->ps.sps->width + ctb_size - 1) >> s->ps.sps->log2_ctb_size)) << s->ps.sps->log2_ctb_size;
         hls_decode_neighbour(s, x_ctb, y_ctb, ctb_addr_ts);
 
-        ret = ff_hevc_cabac_init(s, ctb_addr_ts, 0);
+        ret = ff_hevc_cabac_init(s, ctb_addr_ts);
         if (ret < 0) {
             s->tab_slice_address[ctb_addr_rs] = -1;
             return ret;
@@ -2580,7 +2580,7 @@ static int hls_decode_entry_wpp(AVCodecContext *avctxt, void *input_ctb_row, int
             return 0;
         }
 
-        ret = ff_hevc_cabac_init(s, ctb_addr_ts, thread);
+        ret = ff_hevc_cabac_init(s, ctb_addr_ts);
         if (ret < 0)
             goto error;
         hls_sao_param(s, x_ctb >> s->ps.sps->log2_ctb_size, y_ctb >> s->ps.sps->log2_ctb_size);
@@ -3598,7 +3598,7 @@ static av_cold int hevc_decode_free(AVCodecContext *avctx)
 
     av_freep(&s->md5_ctx);
 
-    av_freep(&s->cabac_state);
+    av_freep(&s->cabac);
 
     for (i = 0; i < 3; i++) {
         av_freep(&s->sao_pixel_buffer_h[i]);
@@ -3652,8 +3652,8 @@ static av_cold int hevc_init_context(AVCodecContext *avctx)
     s->HEVClcList[0] = s->HEVClc;
     s->sList[0] = s;
 
-    s->cabac_state = av_malloc(HEVC_CONTEXTS);
-    if (!s->cabac_state)
+    s->cabac = av_malloc(sizeof(*s->cabac));
+    if (!s->cabac)
         return AVERROR(ENOMEM);
 
     s->output_frame = av_frame_alloc();