[Pixman] Performance of radial gradients

[Rodrigo Kumpera]

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.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.freedesktop.org/archives/pixman/attachments/20100222/1c7d84d8/attachment.htm>