This used to be restricted to variable vertices, but now can handle
arbitrary circular vertices that represent packed values. It also
converges faster than the earlier version. Prep for resolving loops
through arrays.
SystemC variables are fairly special (they can only be assigned to/from,
but not otherwise participate in expressions), which complicates some
DFG code. These variables only ever appear as port on the top level
wrapper, so excluding them from DFG does not make us loose any
optimizations, but simplifies internals.
Previously DFG was limited to having a Sel, or an ArraySel potentially
under a Concat on the LHS of combinational assignments. Other forms or
combinations were not representable in the graph.
This adds support for arbitrary combinations of the above by
combining DfgSplicePacked and DfgSpliceArray vertices introduced in
#6176. In particular, Sel(ArraySel(VarRef,_),_) enables a lot more code
to be represented in DFG.
This is mostly a refactoring, but also enables handling some more
UNOPTFLAT, when the variable is only partially assigned in the cycle.
Previously the way partial assignments to variables were handled were
through the DfgVerexVar types themselves, which kept track of all
drivers. This has been replaced by DfgVertexSplice (which always drives
a DfgVeretexVar), and all DfgVertexVar now only have a single source,
either a DfgVertexSplice, if partially assigned, or an arbitrary
DfgVertex when updated as a whole.
Added a second algorithm to break cycles in DFG by identifying which
bits of a circular variable are actually independent of the variable,
then reuse the existing (but extended) driver tracing algorithm to
eliminate them.
This can fix up things like: `assign gray = binary ^ (gray >> 1)`
Added an algorithm that can break some combinational cycles in DFG, by
attempting to trace driver logic until we escape the cycle. This can
eliminate a decent portion of UNOPTFLAT warnings. E.g. due to this code:
```systemverilog
assign a[0] = .....;
assign a[1] = ~a[0];
```
Geza Lore
github action
Danny Oler
Igor Zaworski
Bartłomiej Chmiel
Max Wipfli
Wilson Snyder
Ibrahim Burak Yorulmaz
Yilou Wang
Ryszard Rozak
Artur Bieniek
Petr Nohavica
Artur Bieniek
Igor Zaworski