[FFmpeg-devel] [PATCH 3/3] lavc/cbrt_tablegen: unroll table generation loop

[Ronald S. Bultje]

Hi,

On Thu, Dec 31, 2015 at 11:39 AM, Ganesh Ajjanagadde <gajjanagadde at gmail.com
> wrote:

> This patch does not seem to have measurable impact, at least on x86-64,
> though
> there could be benefits for less than stellar branch predictors.
>
[..]

> -        for (i = 0; i < 1<<13; i++) {
> -            if (!(i & 7))
> -                cbrt_tab[i].f = 16 * cbrt_tab[i>>3].f;
> -            else
> -                cbrt_tab[i].f = i * cbrt(i);
> +        for (i = 0; i < 1<<13; i+=8) {
> +            cbrt_tab[i].f = 16 * cbrt_tab[i>>3].f;
> +            cbrt_tab[i+1].f = (i+1) * cbrt(i+1);
> +            cbrt_tab[i+2].f = (i+2) * cbrt(i+2);
> +            cbrt_tab[i+3].f = (i+3) * cbrt(i+3);
> +            cbrt_tab[i+4].f = (i+4) * cbrt(i+4);
> +            cbrt_tab[i+5].f = (i+5) * cbrt(i+5);
> +            cbrt_tab[i+6].f = (i+6) * cbrt(i+6);
> +            cbrt_tab[i+7].f = (i+7) * cbrt(i+7);


gcc (and most other compilers) will unroll the loop automatically, I
suspect. Check disassembly to confirm?

(That doesn't mean the patch shouldn't go in, I'm just trying to help you
explain the result. I have no comment on the patch itself.)

Ronald