This code is quite confusing because there are two "is the cell known" filters
applied, one while building the cell vector and one after building the cell
vector, and they're subtly different. I'm preserving the actual behaviour here
but it looks like there is, or was, a bug here.
This ensures that entering and leaving bufnorm followed by `opt_clean`
is equivalent to just running `opt_clean`.
Also make sure that 'z-$buf cells get techmapped in a compatible way.
We call 'assign_map.set()' below which wipes the `SigMap` and reconstructs it.
This operation is expensive because it scans the whole module. I think it's
better to make heavyweight operations more visible so I'm removing
the less obvious operation.
The previous commit introduced code that optimizes the activation
patterns to be able to generate smaller activation logic. The resulting
supercell was then enqueued as shareable using those optimized
activation patterns. The condition represented by the optimized patterns
is an over-approximation of the actual activiation condition. This means
using it as activiation for the supercell loses precision and pessimises
sharing of the supercell with further cells, breaking the sat/share
test.
This commit fixes that by using the optimized activiation patterns only
for the generation of activation logic and using the original patterns
for enqueuing the supercell.
In case the two sets of activation patterns are mutually exclusive
without considering the logic feeding into the activation signals, an
activation condition can only be relevant if present in both sets with
opposite polarity.
This detects pattern-only mutual exclusion by running an additional SAT
query before importing the input cone logic. If that is already UNSAT,
we remove all non-relevant condition and re-simplify the remaining
patterns.
In cases of pattern-only mutual exclusion, this will often produce much
smaller selection logic and avoid the more costly SAT query that
includes the input cones.
If all the (non-select) inputs of a `$_MUX{4,8,16}_` are undefined, replace it, just like we do for `$mux` and `$_MUX_`.
Add `tests/opt/opt_expr_mux_undef.ys` to verify this.
This doesn't do any const folding on the wide muxes, or shrinking to less wide muxes. It only handles the case where all inputs are 'x and the mux can be completely removed.
