Make BitPat accept ChiselEnum

[carlosedp]

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
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Type of Improvement

• new feature/API

API Impact

No impact to current API, the PR makes BitPat accept ChiselEnum as input, converting to it's value.

Backend Code Generation Impact

No change.

Desired Merge Strategy

• Squash: The PR will be squashed and merged (choose this if you have no preference.

Release Notes

BitPat now accept ChiselEnum as an input.

Reviewer Checklist (only modified by reviewer)

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• Did you mark the proper milestone (Bug fix: 3.3.x, [small] API extension: 3.4.x, API modification or big change: 3.5.0)?
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[ekiwi]

Since apply is already overloaded for UInt, maybe it would make sense to also just overload it for EnumType. This way your example would be even easier to write.

[carlosedp]

That's neat @ekiwi! Even better... thanks!

[sequencer]

If directly cast to UInt, if might have negative influence to circuit PPA, since no mask is used, while actually there is mask for MuxOH, and we cannot ignore it.

Think about another option:
what if an API like this?

def apply(x: EnumType*): BitPat = ???

There is a common use case that we need match multiple Enum which a single circuit.
For example, we need to use BitPat match s1 and s0 at the same time.
if s1 and s0 is OneHot type, we just set correspond bits to 1, while set other bits to 0.

[sequencer]

This is an algorithm problem, if you have multiple encodings, for example, s0 -> 010, s1 -> 110, s2 -> 001.
How do you cover one(some) of them with minimal set.
It's simple in OneHot(just extract correspond bit).
However, for arbitrary user defined encoding, this problem is extended to the set cover problem, which is NP-hard.
Luckily, that's the problem, which is already solved by our decoder, you can construct a TruthTable in below:

s0 -> 1
s1 -> 1
s2 -> 0
default -> ?

to match a selection to s0 and s1
The generated circuit should be optimized to minimal circuit to use.
In this case, state is always small and we don't wanna this simple function depends on espresso, so you can directly select qmc to handle this job.

[carlosedp]

As an aside, I think we might need more documentation on espresso requirements, install instructions and Chisel dependency on it (like what can and can't be made without it).

[maartenboersma]

I am interested in this functionality. Do we know why this was never merged?