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.rheinha...@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
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