Re: [FFmpeg-devel] [RFC] flac_wasted32 vector implementation for VSX on ppc64le

[Sean McGovern <gseanmcg@gmail.com>]

On Thu, Jun 6, 2024, 05:53 Rémi Denis-Courmont <remi@remlab.net> wrote:

>
>
> Le 6 juin 2024 10:43:05 GMT+03:00, Sean McGovern <gseanmcg@gmail.com> a
> écrit :
> >Hi,
> >
> >Attached inline is a _non-working_ implementation of flac_wasted32 for
> >VSX developed on a POWER9 in little-endian mode but probably just as
> >usable on POWER{8,10}.
> >
> >I'm not sure why probably one of the simplest DSP functions in lavc
> >does not work for me, I imagine this is probably something endian
> >related even though IBM's documentation for vec_sl()[1] does not
> >suggest any.
>
> Mixing up bytes and elements in the iterator. But you should be able to
> track this down with gdb or good ol' printf().
>
> >Here's my code:
> >
> >#define VSX_STRIDE 16
> >
> >void ff_flac_wasted32_vsx(int32_t *decoded, int wasted, int len)
> >{
> >   register vec_s32 vec1;
> >   register vec_u32 vec2 = { wasted, wasted, wasted, wasted };
>
> There should be an instruction to splat a scalar to a vector. Better yet
> use vector-scalar shift, if VSX has it.
>

In the POWER ISA, vec_splat() only accepts an immediate, so I think this is
the only way to do it in flac_wasted32.


> >   register vec_s32 shifted;
> >
> >   for (int i = 0; i < len; i += VSX_STRIDE) {
> >       vec1 = vec_vsx_ld(i, decoded);
> >       shifted = vec_sl(vec1, vec2);
> >       vec_vsx_st(shifted, i, decoded);
> >   }
> >}
> >
> >Anyone with experience with AltiVec or VSX see something obvious I am
> missing?
> >
> >-- Sean McGovern
> >
> >[1]
> https://www.ibm.com/docs/en/xl-c-and-cpp-linux/16.1.1?topic=functions-vec-sl
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