[Libre-soc-dev] adding PartitionedSignal support to nmigen's If/Switch/Case

[Jacob Lifshay]

On Mon, Sep 20, 2021, 18:50 Luke Kenneth Casson Leighton <lkcl at lkcl.net>
wrote:

> ---
> crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68
>
> On Tue, Sep 21, 2021 at 1:36 AM Jacob Lifshay <programmerjake at gmail.com>
> wrote:
> >
> > from what I can see, nmigen threads the If and Switch/Case signals all
> the
> > way through the IR


> yes.  by which time they are just Signals.


actually, by adding a part and
> mux function to class Value, then converting nmigen.Part and nmigen.Mux
> defined as "def Mux(a, b): return a.mux(b)" EXACTLY as is done in
> the python operator module, not even the nmigen IR sees different
> types because it is the object.part() and object.mux()'s responsibility
> to convert down to standard nmigen Signals.
>

If/Switch/Case are fundamentally different than Mux/Part/+/*/etc. and can't
be handled in the same way -- they are control-flow-based, whereas
Mux/Part/etc. are all only data-flow based. yosys's process passes are the
code that does the control-flow to data-flow translation. This difference
was apparently not noticed, hence why you apparently thought we could do
the same kind of thing as was done for mux/part/etc.

To implement PartitionedSignal support for If/Switch/Case, we would need a
full-on vectorization pass -- this is similar to translating SIMT gpu code
into explicit-vectors & predication.


> > and ends up leaving them to yosys's process_* passes to
> > translate to logic -- adding support for new types will be a major job.
>
> nope, not at all.  if you believe this then you have fundamentally
> misunderstood
> the scope and design.


you've fundamentally missed my point...the code we would end up writing *in
PartitionedSignal* would be a functional duplicate (actually even more
complex due to vectorization) of what currently exists in yosys's process
passes. nmigen passes the switch/if/case statements *unmodified* to yosys.
nmigen *does not* do any lowering to muxes that we could hook into, it
leaves that all to yosys.

>
nmigen's If/Switch/Case fundamentally assume the boolean conditions (the
output of the if condition/case matching logic) it uses are 1 bit wide, it
doesn't support vectors of boolean conditions. it would require major
refactoring to be usable for our purposes, the  alternative is to implement
the same logic ourselves (essentially reimplementing yosys's process
passes).

>
> > From what I see, we have several options available:
> > 1. take the do-it-ourselves approach of having nmigen If, Switch, and
> Case
> > all just defer to PartitionedSignal methods, this requires reimplementing
> > inside PartitionedSignal what is currently a major portion of nmigen and
> > basically all of yosys's process logic.
>
> no.  look again.  it is *absolutely not* the case that yosys or nmigen
> itself
> need know *anything* about data types other than those that yosys and
> the IR generated and targetted by nmigen.
>

it *does* need to know since a vector of boolean conditions is
fundamentally different than a 1-bit-wide boolean condition. we *can* take
the option of bypassing nmigen and yosys's if/switch/case logic completely,
but we'd need to reimplement it all in PartitionedSignal.

Jacob