Investigate improvements for assign_region

[pinkiebell]

At this moment, the SimpleFloorPlanner used in zkevm-circuits will call the closure for assign_region twice.
It might makes sense to deduplicate this call in halo2 because there a few compute heavy loops inside zkevm-circuits that not only does assignments but also data mapping / transformation and right now, doing the work twice. We can gain significant speedups by either
i) modifying zkevm-circuits to do the heavy work outside the closure and reference it instead
ii) find a way to avoid calling more than 1 times in halo2 - this generally improves performance for any circuit/crate

[pinkiebell]

ref: privacy-scaling-explorations/zkevm-circuits#1253 (comment)
cc/ @CPerezz

[jonathanpwang]

I have a PR for this here: halo2-ce#13
You can cherry-pick the last 2 commits and test it out.

[pinkiebell]

I have a PR for this here: halo2-ce#13
You can cherry-pick the last 2 commits and test it out.

I've tested that and while it makes the synthesize step faster (2x) it greatly increases the degree: privacy-scaling-explorations/zkevm-chain@f16e43d .

Maybe we can store all information from assignments and then do the shaping instead calling it multiple times?

[CPerezz]

I've tested that and while it makes the synthesize step faster (2x) it greatly increases the degree: privacy-scaling-explorations/zkevm-chain@f16e43d .

I don;t think that's an issue. While we keep the expr degree low (that's of course the case as it isn't touched) sacrifice 1 degree in the MockProver is not an issue at all IMO (Specially considering that this Layouter should never be used for final circuits/benchmarks).

Which is your proposal @pinkiebell ? I thought it's not a big deal to get a bigger degree (specially since we don't need a setup with the MockProver.

[pinkiebell]

It would be a great performance improvement for real & mock prover alike to improve halo2 to not calling these closures from assign_region etc. twice. Because changing all the circuits to take this into account is, I think, a lot of unnecessary work if that can be fixed in the halo2 crate. We have to keep in mind that for production circuits with a large fixed paddings the performance penalty goes into dozens of minutes like > 30mins.
I can take a look at the halo2 code because to assess this task I don't have enough information about it yet.