[Libre-soc-dev] fantastically-weird regfile

[Jacob Lifshay]

On Sun, Dec 13, 2020, 16:12 Luke Kenneth Casson Leighton <lkcl at lkcl.net>
wrote:

> i had an idea about the CR regfile when it's vectorised.  this is one
> of the oddest designs but there is a reason for it. see
> https://libre-soc.org/openpower/sv/svp_rewrite/svp64/discussion/ for
> full details
>
> the issue we've got is that scalar CRs were not intended to be
> vectorised.  so in scalar OpenPOWER there's only 8 of them.  they're
> "dedicated": CR0 is for INT/Logical, CR1 for FP, CR6 for SIMD VSX.
>
> so here's the problem when we apply SV, which normally applies
> "sequential" increments: any SV-Vectorised Rc=1 INT operation is
> *automatically* going to wipe out CR1.
>
> what if, then, the numbering went:
>
>     0 8 16 24 32 40 48 56
>     1 9 17 ....
>
> that way it would only be vector INT operations longer than 8 that
> would destroy CR1.
>
> that in turn would imply that the CRs would be treated as an 8x8
> matrix, dual-ported, reading horizontally *and vertically*!  which is
> just so spectacularly weird i feel it has merit just for fits and
> giggles.
>

That's very similar to what I already did for int and fp registers, see:

https://libre-soc.org/openpower/sv/svp_rewrite/svp64/