[Libre-soc-dev] pia as cycle accurate simulator?

[Jacob Lifshay]

On Wed, Oct 14, 2020, 18:23 Luke Kenneth Casson Leighton <lkcl at lkcl.net>
wrote:

> On 10/15/20, Jacob Lifshay <programmerjake at gmail.com> wrote:
> > added the rest of the instructions in the Fixed-Point Arithmetic
> > Instructions section of the Power v3.1 spec to
> > power-instruction-analyzer, with the following exceptions:
> > paddi -- not supported on POWER9 since it's one of the new prefixed
> > instructions
>
> yeh we don't want those, they're V3.1B
>
> > addpcis -- results dependent on instruction's address, which
> > power-instruction-analyzer can't currently control.
> > darn -- results random -- not easily testable
> >
> > All implemented instructions pass the tests.
>
> fantastic.
>
> > Next, I guess I'll add the rest of the fixed-point instructions that
> > aren't loads, stores, branches, traps, or random or depend on the
> > instruction's address.
>
> that'd be great.
>
> my thoughts here are, it's pretty close to becoming the core of an
> actual simulator.
>

I think we should have the simulator be a separate crate built on top of
power-instruction-analyzer. we'd probably need a few kloc more to implement
a simulator, since pia doesn't have an instruction decoder and doesn't
handle reading/writing registers -- it just implements the ALU-portion of
the instructions.

>
> what do you reckon, and could we then drop SV on top in an
>

Sure, though it will require some thought to decide which level of the
simulator's architecture to implement SV at.

object-orientated fashion?
>

well, since it'll be written in Rust, it won't technically be object
oriented since rust doesn't really use that paradigm. Rust instead uses
something a bit different -- more like interface-oriented programming (I
don't know the actual name, other than Rust) since that's what Traits are.
Rust in particular doesn't really use inheritance or is-a relationships,
instead, it is more like types implement particular interfaces (called
traits in Rust).

Note that I think you should spend some time working on a decent-sized Rust
project, since I think most of the benefits you attribute to
object-oriented programming can be achieved in a cleaner and easier to
extend fashion using interface-oriented programming. Obviously this will
most likely have to be delayed till after the december tape-out.

>
> (in spike i did element width overrides through c++ OO overrides, it
> was a while ago now, i think you helped with that?)
>

I think I might have, a little, but you did the majority of the work on
spike.

Jacob