[Pixman] Performance of radial gradients

Siarhei Siamashka siarhei.siamashka at gmail.com
Mon Feb 22 08:39:29 PST 2010

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.

Best regards,
Siarhei Siamashka

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