[libre-riscv-dev] SV Prefix questions
Luke Kenneth Casson Leighton
lkcl at lkcl.net
Wed Jun 26 10:33:14 BST 2019
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>
> 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
it's not used at all, because the algorithm instead uses rs1==x0 for
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?
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