[FFmpeg-devel] [PATCH] avfilter: add wpsnr video filter

[Paul B Mahol]

On Fri, Oct 29, 2021 at 6:59 PM Tomas Härdin <tjoppen at acc.umu.se> wrote:

> fre 2021-10-29 klockan 17:00 +0200 skrev Paul B Mahol:
> > On Fri, Oct 29, 2021 at 4:46 PM Tomas Härdin <tjoppen at acc.umu.se>
> > wrote:
> >
> > > fre 2021-10-29 klockan 15:33 +0200 skrev Paul B Mahol:
> > > > On Fri, Oct 29, 2021 at 3:12 PM Tomas Härdin <tjoppen at acc.umu.se>
> > > > wrote:
> > > > >
> > > > > > +static double get_hx(const uint8_t *src, int linesize, int
> > > > > > w,
> > > > > > int h)
> > > > > > +{
> > > > > > +    int64_t sum = 0;
> > > > > > +
> > > > > > +    for (int y = 0; y < h; y++) {
> > > > > > +        for (int x = 0; x < w; x++) {
> > > > > > +            sum += 12 * src[x] -
> > > > > > +                    2 * (src[x-1] + src[x+1] +
> > > > > > +                         src[x + linesize] +
> > > > > > +                         src[x - linesize]) -
> > > > > > +                    1 * (src[x - 1 - linesize] +
> > > > > > +                         src[x + 1 - linesize] +
> > > > > > +                         src[x - 1 + linesize] +
> > > > > > +                         src[x + 1 + linesize]);
> > > > > > +        }
> > > > > > +
> > > > > > +        src += linesize;
> > > > > > +    }
> > > > > > +
> > > > > > +    return fabs(sum * 0.25);
> > > > > > +}
> > > > > > +
> > > > > > +static double get_hx16(const uint8_t *ssrc, int linesize,
> > > > > > int w,
> > > > > > int
> > > > > > h)
> > > > > > +{
> > > > > > +    const uint16_t *src = (const uint16_t *)ssrc;
> > > > >
> > > > > This is not -fstrict-aliasing safe
> > > > >
> > > >
> > > > How so? I get no warnings at all, and similar is used everywhere
> > > > else.
> > >
> > > Then those places should be fixed. We have macros like AV_RB16()
> > > for a
> > > reason. That gcc doesn't warn about this doesn't mean it isn't free
> > > to
> > > assume ssrc and src points to different non-overlapping regions of
> > > memory.
> > >
> > >
> > That is sub optimal and unacceptable solution.
>
> Did you do measurements to come to this conclusion?
>
> > Review remark ignored.
>
> Undefined behavior is *not* acceptable. If you want this file
> specifically to be compiled with strict aliasing disabled then you must
> at the very least change the build system accordingly
>

Cite source that can prove this.
Compilers are silent about this.
There are hundredths if not thousands of such examples across codebase.


>
> Type punning via union seems to be defined as of C99, so that should
> also be acceptable
>
> /Tomas
>
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