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

[Tomas Härdin]

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.

We could disable strict aliasing, but that will likely hurt performance
of other parts of the code.

/Tomas