[Libre-soc-dev] new svp64 page

[Jacob Lifshay]

On Wed, Dec 9, 2020 at 6:45 PM Luke Kenneth Casson Leighton
<lkcl at lkcl.net> wrote:
>
> https://libre-soc.org/openpower/sv/svp_rewrite/svp64/discussion/
>
> i don't entirely know why we haven't yet raised this one as a
> bugreport :) perhaps i just like the different perspective.
>
> in looking at the AV opcodes i realised that DSP Audio saturation (if
> sum greater than max int range set to 0xffff) would either need a ton
> of ops (yuk) or an SPR bit (yuk) or, much better, a spare bit or two
> in the 24 bits from the SV Prefix

Couldn't we just use the already existing VSX instructions for some of
that and just SV-ify them, where the SV prefix just takes the base
operation:
using a SV-prefixed form of vaddshs would act as if the vaddshs was
just a scalar signed 16-bit saturating add, adding SV-style
vectorization if the src/dest registers are set as vectors.
using a SV-prefixed form of vavguw would act as if the vavguw was just
a scalar unsigned 16-bit average operation.

Using those (and other similar instructions) would make it waay easier
to get the operations we need with less friction, since the encodings
already exist.

Jacob