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, 7 insertions, 3 deletions

diff --git a/libavcodec/hevcdec.h b/libavcodec/hevcdec.h
index e2dba54f26..b97b7d1354 100644
--- a/libavcodec/hevcdec.h
+++ b/libavcodec/hevcdec.h
@@ -226,6 +226,11 @@ enum ScanType {
     SCAN_VERT,
 };
 
+typedef struct HEVCCABACState {
+    uint8_t state[HEVC_CONTEXTS];
+    uint8_t stat_coeff[HEVC_STAT_COEFFS];
+} HEVCCABACState;
+
 typedef struct LongTermRPS {
     int     poc[32];
     uint8_t poc_msb_present[32];
@@ -482,8 +487,7 @@ typedef struct HEVCContext {
     int                 width;
     int                 height;
 
-    uint8_t *cabac_state;
-    uint8_t stat_coeff[HEVC_STAT_COEFFS];
+    HEVCCABACState *cabac;
 
     /** 1 if the independent slice segment header was successfully parsed */
     uint8_t slice_initialized;
@@ -601,7 +605,7 @@ int ff_hevc_frame_rps(HEVCContext *s);
 int ff_hevc_slice_rpl(HEVCContext *s);
 
 void ff_hevc_save_states(HEVCContext *s, int ctb_addr_ts);
-int ff_hevc_cabac_init(HEVCContext *s, int ctb_addr_ts, int thread);
+int ff_hevc_cabac_init(HEVCContext *s, int ctb_addr_ts);
 int ff_hevc_sao_merge_flag_decode(HEVCContext *s);
 int ff_hevc_sao_type_idx_decode(HEVCContext *s);
 int ff_hevc_sao_band_position_decode(HEVCContext *s);