[FFmpeg-devel] [PATCH] swscale/output: Altivec-optimize yuv2plane1_8

[Carl Eugen Hoyos]

2018-11-16 13:59 GMT+01:00, Lauri Kasanen <cand at gmx.com>:
> ./ffmpeg_g -f rawvideo -pix_fmt rgb24 -s hd1080 -i /dev/zero -pix_fmt
> yuv420p \
> -f null -vframes 100 -v error -nostats -
>
> 1158 UNITS in planar1,   65528 runs,      8 skips
>
> -cpuflags 0
>
> 19082 UNITS in planar1,   65533 runs,      3 skips
>
> 16.48 speedup ratio. On x86, SSE2 is ~7. Curiously, the Power C version
> takes as many cycles as the x86 SSE2 version, yikes it's fast.

> Note that this function uses VSX instructions, but is not marked so.
> This is because several existing functions also make that mistake.
> I'll submit a patch moving them all once this is reviewed.

(This is less important atm, but I believe all functions currently
in libswscale/ppc compile and run fine on - old - 32bit be hardware
as your new function does.
My completely inexperienced suspicion is that the instruction that
you call "VSX" also exists on Altivec.)

> No BE support since I can only test LE. LE is however the common case
> for POWER8 and POWER9.
>
> Signed-off-by: Lauri Kasanen <cand at gmx.com>
> ---
>  libswscale/ppc/swscale_altivec.c | 55
> ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 55 insertions(+)
>
> diff --git a/libswscale/ppc/swscale_altivec.c
> b/libswscale/ppc/swscale_altivec.c
> index 2fb2337..a064016 100644
> --- a/libswscale/ppc/swscale_altivec.c
> +++ b/libswscale/ppc/swscale_altivec.c
> @@ -324,6 +324,53 @@ static void hScale_altivec_real(SwsContext *c, int16_t
> *dst, int dstW,
>              }
>          }
>  }
> +
> +static void yuv2plane1_8_u(const int16_t *src, uint8_t *dest, int dstW,
> +                           const uint8_t *dither, int offset, int start)
> +{
> +    int i;
> +    for (i = start; i < dstW; i++) {
> +        int val = (src[i] + dither[(i + offset) & 7]) >> 7;
> +        dest[i] = av_clip_uint8(val);
> +    }
> +}
> +
> +static void yuv2plane1_8_altivec(const int16_t *src, uint8_t *dest, int
> dstW,
> +                           const uint8_t *dither, int offset)
> +{
> +    const int dst_u = -(uintptr_t)dest & 15;
> +    int i, j;
> +    LOCAL_ALIGNED(16, int16_t, val, [16]);
> +    const vector uint16_t shifts = (vector uint16_t) {7, 7, 7, 7, 7, 7, 7,
> 7};
> +    vector int16_t vi, vileft, ditherleft, ditherright;
> +    vector uint8_t vd;
> +
> +    for (j = 0; j < 16; j++) {
> +        val[j] = dither[(dst_u + offset + j) & 7];
> +    }
> +
> +    ditherleft = vec_ld(0, val);
> +    ditherright = vec_ld(0, &val[8]);
> +
> +    yuv2plane1_8_u(src, dest, dst_u, dither, offset, 0);
> +
> +    for (i = dst_u; i < dstW - 15; i += 16) {
> +
> +        vi = vec_vsx_ld(0, &src[i]);
> +        vi = vec_adds(ditherleft, vi);
> +        vileft = vec_sra(vi, shifts);
> +
> +        vi = vec_vsx_ld(0, &src[i + 8]);
> +        vi = vec_adds(ditherright, vi);
> +        vi = vec_sra(vi, shifts);
> +
> +        vd = vec_packsu(vileft, vi);
> +        vec_st(vd, 0, &dest[i]);
> +    }
> +
> +    yuv2plane1_8_u(src, dest, dstW, dither, offset, i);
> +}
> +
>  #endif /* HAVE_ALTIVEC */
>
>  av_cold void ff_sws_init_swscale_ppc(SwsContext *c)
> @@ -367,6 +414,14 @@ av_cold void ff_sws_init_swscale_ppc(SwsContext *c)
>              c->yuv2packedX = ff_yuv2rgb24_X_altivec;
>              break;
>          }

> +
> +        switch (c->dstBpc) {
> +        case 8:
> +#if !HAVE_BIGENDIAN
> +            c->yuv2plane1 = yuv2plane1_8_altivec;
> +            break;
> +#endif /* !HAVE_BIGENDIAN */
> +        }

I wanted to write that this hunk breaks compilation on big-endian
(you should be able to test with "#if 0" instead of "#if !HAVE_BIGENDIAN")
but the good news is that your patch works fine on big-endian,
just remove the if-endif block. (Tested visually with lena on 32 and 64bit be.)

Are you aware of the bounty that is offered for this task?
https://trac.ffmpeg.org/ticket/5568
(and #5569, #5570)

There is a bug report about one altivec routine that works on
big-endian but breaks the output visually on little-endian while
many other functions work on both, could you have a look?
https://trac.ffmpeg.org/ticket/7124

Carl Eugen