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

["Rémi Denis-Courmont" <remi@remlab.net>]

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.

>   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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