[Pixman] Performance of radial gradients

[Jeff Muizelaar]

On 22-Feb-10, at 10:35 AM, Rodrigo Kumpera wrote:

>
>
> On Mon, Feb 22, 2010 at 1:39 PM, Siarhei Siamashka <siarhei.siamashka at gmail.com 
> > wrote:
> On Friday 19 February 2010, Luca Barbato wrote:
> > On 02/19/2010 12:57 PM, Siarhei Siamashka wrote:
> > > Adding small increments to the values at the end of loop  
> iteration could
> > > be the biggest source of precision loss. Replacing this with  
> explicit
> > > calculation like 'pdx = pdx0 + cx * n' should improve precision  
> and maybe
> > > allow to use floats freely. And floats work better with SIMD on  
> any
> > > platforms.
> >
> > And all the SIMD we are covering have a multiply-accumulate  
> instruction
> > that would be in use, I'm a bit more concerned about the sqrt usage
> > though...
>
> The usage of sqrt is probably not a fatal performance problem.
>
> ARM11 VFP has a separate DS pipeline which can calculate divides or  
> square
> roots simultaneously with the other operations. So it's only a  
> matter of
> hiding very high square root calculation latency.
>
> ARM Cortex-A8 has special SIMD instructions intended to help  
> calculating
> reciprocals and reciprocal square roots using Newton-Raphson method.
>
> SSE has SIMD instructions for calculating square roots.
>
>
> Note that SSE functions for square root and reciprocal are unfit for  
> this task as they have
> very little precision (12 bits). At least one Newton-Raphson  
> iteration must be done to have
> usable values.

rsqrt is an approximation, however the regular square root  
instructions are not and are fine replacements for the sqrt()  
function. In fact, gcc on OS X already does this substitution.

-Jeff


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.freedesktop.org/archives/pixman/attachments/20100222/a260821a/attachment-0001.htm>