[libre-riscv-dev] Instruction sorta-prefixes for easier high-register access

[Jacob Lifshay]

On Fri, Jan 18, 2019, 01:33 Luke Kenneth Casson Leighton <lkcl at lkcl.net
wrote:

> hi jacob, a brief thought / reply, more later:
>
> * prefixes on instructions gives an opportunity to do key-hole
> analysis, assessing if the prefixes can be removed and use the CSR
> stack instead.
>
Haven't heard of key-hole analysis before -- wikipedia didn't seem to have
anything relevant

> * thank you for explaining about predicate registers.
> * one thought, have branch overwrite the src predicate.
>
that could work, but if we can specify a different dest I prefer to.

> * i like the VL multiplier idea.
> * we don't want to go completely overboard, just the main uses.  CSR
> stack can cover the "full" features
>
I actually think that having the prefixes implement everything and have the
compiler use prefixes instead of the rename table will greatly simplify the
compiler since we don't have to come up with entirely new algorithms to
handle the rename table.

There are already existing architectures that have well-known compilation
algorithms that are much more similar to the prefixed instruction approach
than to the rename table approach.

> * am going to be relying on you to give feedback and insight on what
> the main uses are!
>
will try my best

>
> l.
>
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