[Libre-soc-bugs] [Bug 713] PartitionedSignal enhancement to add partition-context-aware lengths

bugzilla-daemon at libre-soc.org bugzilla-daemon at libre-soc.org
Fri Oct 8 22:43:52 BST 2021


--- Comment #40 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Jacob Lifshay from comment #36)

> only if you never run `opt`...expression deduplication is a pretty trivial
> compiler optimization. In my experience, yosys deduplicates built-in
> expressions just fine.

not on TestIssuer or other complex desigbs it doesn't

> > 
> > it was the submodules.
> we can deduplicate expressions ourselves pretty easily:

ooo niiiice, that's really valuable and exciting.

i bet a similar trick would be possible to deploy for autocreating
intermediary temporary Signals.

can you drop this into nmutil?

> > the next problem is: how do you now reduce the number of Case() statements
> > needed to four, or, more to the point, on doing so which bits of data do
> > you copy from which partition, without the prerequisite information?
> easy,

it was a rhetorical-ish question as part of the reasoning chain, but hey :)
let's go through it.

> each `part_wid` wide bit slice in any PartitionedSignal can be
> independently muxed -- as long as the condition is first converted to the
> layout:
> {
>     ElWid.I8: 1,
>     ElWid.I16: 1,
>     ElWid.I32: 1,
>     ElWid.I64: 1,
> }
> with padding being a sign-extended version of each lane... Aka. the result
> of PartitionedSignal.bool(). (or what *should* be the result if design were
> left to me).

ah.  ok, so with so much code already having been written, and it taking 5
months, and you weren't contributing at the time, we have to be pragmatic
and fit with the existing design (and, it's simply good engineering
practice to finish something first rather than discard it 90% the
way to completion)

redesigning the entire suite of submodules (12 and counting) to fit elwidth
is both unrealistic and undesirable, but there *is* a way to retrofit by
having an adapter - layout() - create the values that the PartPoints
mask *should* be set to for any given elwidth.

in this way we do *not* have to discard 5 months of work.

the example you gave, i apologise i couldn't follow it, it appears
to be a different design paradigm which is not the focus of the

tomorrow i will drop the layout() function into the mix and i think
i have enough of a handle on things to convert to elwidth.

however because it would involve a massive duplication of the
test suite, or a disruptive alteration of the unit tests, i'll do it
as a separate class that inherits from PartitionedSignal, and
think about how to go from there

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