[FFmpeg-devel] SH4: optimization attempts

[Ronald S. Bultje]

Hi,

On Thu, Jan 27, 2011 at 4:27 AM, Guennadi Liakhovetski
<g.liakhovetski at gmx.de> wrote:
> Some of the proposed patches are generic, like the vp8, replacing
> multiplication by addition in several filter functions, but, as you see
> below, it didn't bring any results.
[..]
> -    a0 = (27*w + 63) >> 7;
> -    a1 = (18*w + 63) >> 7;
> -    a2 = ( 9*w + 63) >> 7;
> +    w9 = 9 * w;
> +    w9_63 = w9 + 63;
> +
> +    a2 = w9_63 >> 7;
> +    w9_63 += w9;
> +    a1 = w9_63 >> 7;
> +    w9_63 += w9;
> +    a0 = w9_63 >> 7;

I might be advocating the devil here, but please do check the
disassembly before and after your patches. You'd be (pleasantly!)
surprised at the kind of code that gcc sometimes generates for this
kind of stuff. (Then again, it might just be that your code does
improve it and you just can't measure the effect...)

Ronald