Re: [FFmpeg-devel] Misc mpegvideo patches

[Kacper Michajlow <kasper93@gmail.com>]

On Sun, 9 Mar 2025 at 14:55, Andreas Rheinhardt
<andreas.rheinhardt@outlook.com> wrote:
>
> Kacper Michajlow:
> > On Thu, 6 Mar 2025 at 14:31, Andreas Rheinhardt
> > <andreas.rheinhardt@outlook.com> wrote:
> >>
> >> Andreas Rheinhardt:
> >>> Ramiro Polla:
> >>>> On Tue, Mar 4, 2025 at 6:05 PM Andreas Rheinhardt
> >>>> <andreas.rheinhardt@outlook.com> wrote:
> >>>>> Ramiro Polla:
> >>>>>>
> >>>>>> On 3/4/25 14:42, Andreas Rheinhardt wrote:
> >>>>>>> (Mostly trivial) patches attached. A branch is at
> >>>>>>> https://github.com/mkver/FFmpeg/tree/mpegvideo_misc
> >>>>>>
> >>>>>>
> >>>>>> [PATCH 10/40] avcodec/mpegvideo_enc: Move default_mv_penalty to h261enc.c
> >>>>>>
> >>>>>>> diff --git a/libavcodec/h261enc.c b/libavcodec/h261enc.c
> >>>>>>> index dabab9d80a..e33bf35a8a 100644
> >>>>>>> --- a/libavcodec/h261enc.c
> >>>>>>> +++ b/libavcodec/h261enc.c
> >>>>>>> @@ -46,6 +46,7 @@ static struct VLCLUT {
> >>>>>>>      uint16_t code;
> >>>>>>>  } vlc_lut[H261_MAX_RUN + 1][32 /* 0..2 * H261_MAX_LEN are used */];
> >>>>>>>
> >>>>>>> +static uint8_t mv_penalty[MAX_FCODE + 1][MAX_DMV * 2 + 1];
> >>>>>>>  static uint8_t uni_h261_rl_len     [64 * 128];
> >>>>>>>  static uint8_t uni_h261_rl_len_last[64 * 128];
> >>>>>>>  static uint8_t h261_mv_codes[64][2];
> >>>>>>> @@ -370,6 +371,8 @@ av_cold int ff_h261_encode_init(MpegEncContext *s)
> >>>>>>>      s->max_qcoeff       = 127;
> >>>>>>>      s->ac_esc_length    = H261_ESC_LEN;
> >>>>>>>
> >>>>>>> +    s->me.mv_penalty = mv_penalty;
> >>>>>>> +
> >>>>>>>      s->intra_ac_vlc_length      = s->inter_ac_vlc_length      =
> >>>>>>> uni_h261_rl_len;
> >>>>>>>      s->intra_ac_vlc_last_length = s->inter_ac_vlc_last_length =
> >>>>>>> uni_h261_rl_len_last;
> >>>>>>>      ff_thread_once(&init_static_once, h261_encode_init_static);
> >>>>>>
> >>>>>> This global mv_penalty doesn't seem to be ever initialized; it could be
> >>>>>> declared const.
> >>>>>
> >>>>> But then it would no longer be placed in .bss, but instead in .rodata
> >>>>> and increase binary size.
> >>>>
> >>>> Wow, that's a huge array.
> >>>>
> >>>>>> But it also makes me think that whatever code is using this mv_penalty,
> >>>>>> which is always set to zero, might be calculating things wrong.
> >>>>>>
> >>>>>
> >>>>> It is obviously done to avoid branches for the codecs that matter. H.261
> >>>>> does not matter much. Apart from that, it is a very cheap workaround
> >>>>> given that this table is .bss.
> >>>>
> >>>> Could you add some comments (either next to the declaration or the
> >>>> commit message) to reflect this? (save space from .rodata, and this
> >>>> being a noop for h.261, which doesn't matter that much)
> >>>>
> >>>>>> [PATCH 15/40] avcodec/ituh263enc: Make SVQ1+Snowenc stop calling
> >>>>>> ff_h263_encode_init()
> >>>>>>
> >>>>>>> diff --git a/libavcodec/ituh263enc.c b/libavcodec/ituh263enc.c
> >>>>>>> index 02da090ba4..8313b2c2c1 100644
> >>>>>>> --- a/libavcodec/ituh263enc.c
> >>>>>>> +++ b/libavcodec/ituh263enc.c
> >>>>>>> @@ -65,6 +65,127 @@ static uint8_t  uni_h263_inter_rl_len [64*64*2*2];
> >>>>>> [...]
> >>>>>>> +static av_cold void h263_encode_init_static(void)
> >>>>>>> +{
> >>>>>>> +    static uint8_t rl_intra_table[2][2 * MAX_RUN + MAX_LEVEL + 3];
> >>>>>>> +
> >>>>>>> +    ff_rl_init(&ff_rl_intra_aic, rl_intra_table);
> >>>>>>> +    ff_h263_init_rl_inter();
> >>>>>>> +
> >>>>>>> +    init_uni_h263_rl_tab(&ff_rl_intra_aic,  uni_h263_intra_aic_rl_len);
> >>>>>>> +    init_uni_h263_rl_tab(&ff_h263_rl_inter, uni_h263_inter_rl_len);
> >>>>>>> +
> >>>>>>> +    init_mv_penalty_and_fcode();
> >>>>>>> +}
> >>>>>>> +
> >>>>>>> +av_cold const uint8_t (*ff_h263_get_mv_penalty(void))[MAX_DMV*2+1]
> >>>>>>> +{
> >>>>>>> +    static AVOnce init_static_once = AV_ONCE_INIT;
> >>>>>>> +
> >>>>>>> +    ff_thread_once(&init_static_once, h263_encode_init_static);
> >>>>>>> +
> >>>>>>> +    return mv_penalty;
> >>>>>>> +}
> >>>>>>> +
> >>>>>>
> >>>>>> This approach kind of hides the rest of h263_encode_init_static() inside
> >>>>>> ff_h263_get_mv_penalty(), so the name is a bit misleading. I'd expect
> >>>>>> h263 to still call some init function and ff_h263_get_mv_penalty(), and
> >>>>>> SVQ1 and Snow to only call ff_h263_get_mv_penalty(), which would only
> >>>>>> take care of the mv_penalty table.
> >>>>>>
> >>>>>
> >>>>> This would entail using another AVOnce etc. and this level of
> >>>>> granularity is just not worth it.
> >>>>
> >>>> Ok.
> >>>>
> >>>>> The name is chosen for what it does for an outsider (i.e. from the
> >>>>> perspective of svq1enc or snowenc, not the actual H.263 based encoders).
> >>>>
> >>>> I'm still not quite happy with the name and how it's used, but it's
> >>>> good enough so I won't insist.
> >>>>
> >>>>>> [PATCH 20/40] avcodec/mpeg4video: Split ff_mpeg4_pred_dc()
> >>>>>>
> >>>>>>> diff --git a/libavcodec/mpeg4videoenc.c b/libavcodec/mpeg4videoenc.c
> >>>>>>> index 64fb96a0cf..26f9b40ff7 100644
> >>>>>>> --- a/libavcodec/mpeg4videoenc.c
> >>>>>>> +++ b/libavcodec/mpeg4videoenc.c
> >>>>>>> @@ -806,8 +806,14 @@ void ff_mpeg4_encode_mb(MpegEncContext *s,
> >>>>>>> int16_t block[6][64],
> >>>>>>>          const uint8_t *scan_table[6];
> >>>>>>>          int i;
> >>>>>>>
> >>>>>>> -        for (i = 0; i < 6; i++)
> >>>>>>> -            dc_diff[i] = ff_mpeg4_pred_dc(s, i, block[i][0], &dir[i],
> >>>>>>> 1);
> >>>>>>> +        for (int i = 0; i < 6; i++) {
> >>>>>>
> >>>>>> Redeclaring i inside for.
> >>>>>
> >>>>> There are other loops which use this i as loop variable. The shadowing
> >>>>> is IMO less bad than keeping loops in their current form (with iterators
> >>>>> that don't have loop-scope).
> >>>>
> >>>> Agreed. I also prefer scoped iterators.
> >>>
> >>> I added a comment to #10 and modified #18 as described. I also changed
> >>> #21 to protect the macro in parentheses and simplified the FF_RC_OFFSET
> >>> macro in #31. Furthermore, there are now five more patches. All
> >>> attached. https://github.com/mkver/FFmpeg/tree/mpegvideo_misc has been
> >>> force-pushed.
> >>>
> >> Will apply this patchset (with the issues pointed out by Ramiro fixed)
> >> tomorrow unless there are objections.
> >
> > After the "Don't count errors from first thread twice" patch, there
> > are some new UBSAN warnings.
> >
> > libavcodec/mpeg12dec.c:2264:37: runtime error: signed integer
> > overflow: 2147483647 + 99 cannot be represented in type 'int'
> >
>
> That actually exposes a bug: If two threads have this INT_MAX set, then
> a third one could make the overall error count to zero (indicating no
> error) despite the presence of errors.
>
> > If we look around there are places where atomic_store(&s->error_count,
> > INT_MAX); is done.
> >
> > I don't think this change caused the issue, because overflows would
> > also happen before, but it looks like UBSAN doesn't instrument atomic
> > variables, so it was hidden.
>
> Overflow for atomic variables are not undefined: "For signed integer
> types, arithmetic is defined to use two’s complement representation with
> silent wrap-around on overflow; there are no undefined results."
>

Thanks for clearing this out, I guess I never thought about this. Good to know.

- Kacper
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