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

[Andreas Rheinhardt]

Andreas Rheinhardt:
> Michael Niedermayer:
>> On Sat, Jul 02, 2022 at 08:32:06AM +0200, Andreas Rheinhardt wrote:
>>> 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?)
>>
>> This doesnt feel "right". These pointers should not be coming from a const
>> if they are written to
>>
> 
> The HEVCContext is not const because the underlying object is const; the
> HEVCContext is const when accessed from any part of the code that may be
> run from slice threads, because if a slice thread modifies it, you have
> a data race in case any of the other slice threads reads this field or
> modifies it itself. But this is by definition not true for atomic
> operations, so casting const away for them is fine.
> 
>> The compiler accepts it with an explicit cast though. With an implicit cast
>> it produces a warning
>>

Did the above explanation satisfy you? Or do you want something else?

- Andreas