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 11:42:23PM +0200, Andreas Rheinhardt wrote:
> Michael Niedermayer:
> > 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
> > 
> > [...]
> > 
> 
> Good to hear. This patchset (namely patch 11/18: "avcodec/hevcpred: Pass
> HEVCLocalContext when slice-threading") includes modifications to mips
> code that I created blindly. Can you please test it? Here is a branch of
> this rebased on top of current git master:
> https://github.com/mkver/FFmpeg/commits/hevc_wpp
> (Said branch actually contains a bit of further work which also modifies
> mips code (in particular,
> https://github.com/mkver/FFmpeg/commit/cf441e559b8d4bf2c05c29483ccf49e82fc6b863
> does so); you may also test this.)

what exact tests do we need ?
simple fate ? any specific thread type count ?
also note i can only test qemu mips not real hw.
I stopped maintaining the MIPS hw and sofar noone volunteered to take its 
maintaince over

thx

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

The smallest minority on earth is the individual. Those who deny 
individual rights cannot claim to be defenders of minorities. - Ayn Rand

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