This is the beginning of the process to enable full floor planning
capabilities in `halo2`. For now, all that a floor planner can do is
synthesize a circuit, which makes it no more powerful than a layouter,
but easier to use (as moving to a multi-pass layouter no longer requires
changes to `Circuit::synthesize`).
This layouter improves on the single-pass strategy in two ways:
- Regions are layed out with a "first fit" strategy (using the algorithm
described in https://github.com/zcash/halo2/issues/93) instead of a
"last fit" strategy. This enables gaps in the circuit to be filled.
- Regions are sorted by their "advice area" (columns * rows), and are
then layed out biggest-first. This takes advantage of the regularity
of circuits, and that the advice columns have the most contention. It
also leads to larger gaps between early layed-out regions that are more
easily filled by subsequent smaller regions.
The V1 layouter is a dual-pass layouter, that has visibility into the
entire `Circuit::synthesize` step.
This first commit implements the same strategy as `SingleChipLayouter`,
behaving as if it were a single-pass layouter.
This enables chips that e.g. want to load multiple lookup tables into
the same columns to store state about where each table was layed out.
Co-authored-by: Jack Grigg <jack@electriccoin.co>
Jack Grigg
therealyingtong