[gmx-developers] 4xN kernel using Advanced NEON (VL=128 bits) and double-FP

[Szilárd Páll]

On Thu, Nov 26, 2020, 9:25 AM Guido Giuntoli <guido.giuntoli at huawei.com>
wrote:

> Hi,
>
>
>
> I am building a minikernel of the 4 x N kernel and testing it in different
> architectures. Currently I am trying with in an ARMv8 with Advanced NEON
> using double-FP.
>
>
>
> I see that this kernel operates with 4 particles at the time. When I use
> double-FP I can fill each SIMD unit with 2 FP numbers (128 bits long for
> Advanced NEON). I checked that that some interactions in this kernel
> between the 4 particles are not performed: particle 3 and 4 have net force
> = 0 (in this minikernel). Am I losing something here?
>
Not all interactions computed will evaluate to non-stop values.
E.g. there could be "dummy" particles used for padding or ( depending on
your inputs) part of excluded particles.

--
Szilárd


Is the kernel prepared to operate in double precision for vector lengths of
> 128 bits? Is the masking mechanism intended to solve this issue?
>
>
>
> Note: when I use single-FP the 4 forces are different from 0 as I expected.
>
>
>
> Thanks a lot for the help ;)
>
>
>
> *Best regards | Mit freundlichen Grüßen*
>
>
>
> *Guido Giuntoli*
>
>
>
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