[libre-riscv-dev] SV Prefix questions

[Luke Kenneth Casson Leighton]

On Wed, Jun 26, 2019 at 10:22 AM Jacob Lifshay <programmerjake at gmail.com> wrote:

> On Wed, Jun 26, 2019, 01:36 Luke Kenneth Casson Leighton <lkcl at lkcl.net>
> wrote:

> vsetvli's immediate starts at bit 15, not bit 20. for rvv that allows
> specifying more immediate bits. note that rvv may reserved some of the
> upper immediate bits and they may be later used by new brownfield-encoded
> instructions, so we should avoid setting the upper bits unless necessary.

 i believe they already use it for encoding vew and other "stuff".

> >  * rvv's vsetvli appears to be absolutely no different from vsetvl
> >
> bit 31 is the differentiating factor

yes.  however, in the variant list at
https://libre-riscv.org/simple_v_extension/specification/sv.setvl/
it's not used at all, because the algorithm instead uses rs1==x0 for
detection purposes.

a potential there is to use that bit 31 for what it's intended
(mirroring vsetvl's usage) and then reserving rs1==x0 for another
potential brownfield encoding.

what do you think?

l.