Re: [FFmpeg-devel] [PATCH] avutil/aarch64/float_dsp_neon: Refactor ff_vector_fmul_add_neon

[Krzysztof Pyrkosz via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

On Sun, Jan 19, 2025 at 10:57:57PM +0200, Martin Storsjö wrote:
> On Sun, 19 Jan 2025, Krzysztof Pyrkosz via ffmpeg-devel wrote:
> 
> > Removed a branch, unrolled loop. Speed increase bumped from 3.95 to 5.60.
> 
> On what core is that? Please quote the actual output including the absolute
> numbers.
> 
> I'm getting much more inconclusive numbers for this one:
> 
> Before:            Cortex A53    A72     A73    A78
> vector_fmul_add_neon:   620.0  257.2   624.5  162.8
> After:
> vector_fmul_add_neon:   767.0  259.2   767.5  110.5
> 
> This seems to make things quite a lot slower on 2 of these 4 cores. On the
> A78, I'm getting numbers that look like yours though.
> 
> So while it makes things better on one kind of core, it also regresses
> things quite a bit on others, so I'm not quite as convinced about this one.

Indeed, I conducted the tests and gathered the benchmark results on the
A78, Rock5B to be more specific.

> Doing that change, i.e.
> 
>          fmla           v4.4s, v0.4s, v2.4s
>          fmla           v5.4s, v1.4s, v3.4s
> +        subs           w4, w4, #16
>          stp            q4, q5, [x0], #32
> -        sub            w4, w4, #16
> -        cbnz           w4, 1b
> +        b.gt           1b
>          ret
> 
> has this effect on numbers:
> 
> Before:            Cortex A53    A72     A73    A78
> vector_fmul_add_neon:   767.0  259.2   769.5  109.0
> After:
> vector_fmul_add_neon:   751.0  254.5   751.0  109.2
> 

Below are my results for different cores I have lying around. First is
the mainline version, second uses the patch from the original email,
third improves things by reordering instructions:

- vector_fmul_neon
                 A72      A78      Thinkpad x13s
Mainline:        218.42   114.7    62.85
Original patch:  223.92   86.16    64.77
Reordered instr: 221.75   85.66    61.16

- vector_fmul_add_neon
                 A72      A78      Thinkpad x13s
Mainline:        269.49   163.43   84.48
Original patch:  269.42   114.16   85.31
Reordered instr: 266.92   111.62   85.04

> That makes things a little bit better on A53,A72,A73, but it's still overall
> a notable regression on the A53 and A73.

Let's skip this patch and two others then, thank you for the review.

Krzysztof
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