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

[Michael Niedermayer <michael@niedermayer.cc>]

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On Sat, Jul 23, 2022 at 07:44:40AM +0200, Andreas Rheinhardt wrote:
> 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@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?

sure, ok

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

If you think the mosad wants you dead since a long time then you are either
wrong or dead since a long time.

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