[FFmpeg-devel] [PATCH 14/18] avcodec/hevcdec: Don't allocate redundant HEVCContexts

[Andreas Rheinhardt]

Michael Niedermayer:
> On Fri, Jul 01, 2022 at 12:29:45AM +0200, Andreas Rheinhardt wrote:
>> The HEVC decoder has both HEVCContext and HEVCLocalContext
>> structures. The latter is supposed to be the structure
>> containing the per-slicethread state.
>>
>> Yet up until now that is not how it is handled in practice:
>> Each HEVCLocalContext has a unique HEVCContext allocated for it
>> and each of these coincides except in exactly one field: The
>> corresponding HEVCLocalContext. This makes it possible to pass
>> the HEVCContext everywhere where logically a HEVCLocalContext
>> should be used. And up until recently, this is how it has been done.
>>
>> Yet the preceding patches changed this, making it possible
>> to avoid allocating redundant HEVCContexts.
>>
>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>
>> ---
>>  libavcodec/hevcdec.c | 40 ++++++++++++++++------------------------
>>  libavcodec/hevcdec.h |  2 --
>>  2 files changed, 16 insertions(+), 26 deletions(-)
>>
>> diff --git a/libavcodec/hevcdec.c b/libavcodec/hevcdec.c
>> index 9d1241f293..048fcc76b4 100644
>> --- a/libavcodec/hevcdec.c
>> +++ b/libavcodec/hevcdec.c
>> @@ -2548,13 +2548,12 @@ static int hls_decode_entry_wpp(AVCodecContext *avctxt, void *hevc_lclist,
>>  {
>>      HEVCLocalContext *lc = ((HEVCLocalContext**)hevc_lclist)[self_id];
>>      const HEVCContext *const s = lc->parent;
>> -    HEVCContext *s1  = avctxt->priv_data;
>> -    int ctb_size    = 1<< s1->ps.sps->log2_ctb_size;
>> +    int ctb_size    = 1 << s->ps.sps->log2_ctb_size;
>>      int more_data   = 1;
>>      int ctb_row = job;
>> -    int ctb_addr_rs = s1->sh.slice_ctb_addr_rs + ctb_row * ((s1->ps.sps->width + ctb_size - 1) >> s1->ps.sps->log2_ctb_size);
>> -    int ctb_addr_ts = s1->ps.pps->ctb_addr_rs_to_ts[ctb_addr_rs];
>> -    int thread = ctb_row % s1->threads_number;
>> +    int ctb_addr_rs = s->sh.slice_ctb_addr_rs + ctb_row * ((s->ps.sps->width + ctb_size - 1) >> s->ps.sps->log2_ctb_size);
>> +    int ctb_addr_ts = s->ps.pps->ctb_addr_rs_to_ts[ctb_addr_rs];
>> +    int thread = ctb_row % s->threads_number;
>>      int ret;
>>  
>>      if(ctb_row) {
>> @@ -2572,7 +2571,7 @@ static int hls_decode_entry_wpp(AVCodecContext *avctxt, void *hevc_lclist,
>>  
>>          ff_thread_await_progress2(s->avctx, ctb_row, thread, SHIFT_CTB_WPP);
>>  
>> -        if (atomic_load(&s1->wpp_err)) {
>> +        if (atomic_load(&s->wpp_err)) {
>>              ff_thread_report_progress2(s->avctx, ctb_row , thread, SHIFT_CTB_WPP);
> 
> the consts in "const HEVCContext *const " make clang version 6.0.0-1ubuntu2 unhappy
> (this was building shared libs)
> 
> 
> CC	libavcodec/hevcdec.o
> src/libavcodec/hevcdec.c:2574:13: error: address argument to atomic operation must be a pointer to non-const _Atomic type ('const atomic_int *' (aka 'const _Atomic(int) *') invalid)
>         if (atomic_load(&s->wpp_err)) {
>             ^           ~~~~~~~~~~~
> /usr/lib/llvm-6.0/lib/clang/6.0.0/include/stdatomic.h:134:29: note: expanded from macro 'atomic_load'
> #define atomic_load(object) __c11_atomic_load(object, __ATOMIC_SEQ_CST)
>                             ^                 ~~~~~~
> 1 error generated.
> src/ffbuild/common.mak:81: recipe for target 'libavcodec/hevcdec.o' failed
> make: *** [libavcodec/hevcdec.o] Error 1
> 
> thx
> 

Thanks for testing this. atomic_load is indeed declared without const in
7.17.7.2:

C atomic_load(volatile A *object);

Upon reflection this makes sense, because if atomics are implemented via
mutexes, even a read may involve a preceding write. So I'll cast const
away here, too, and add a comment. (It works when casting const away,
doesn't it?)

- Andreas