The -nocells flag skips cell equivalence checking and only checks wire
equivalence. This is useful when optimizations preserve functionality but
rename or restructure cells, avoiding false negatives in equivalence checking.
I am getting weird crashes on `main` in `tests/sdc/alu_sub.ys` which I traced to a null `Wire*`
in `SdcObjects::constrained_ports`. The null `Wire*` is being set in the `SdcObjects`
constructor. I don't understand what's going on here, so I added this check to detect the
missing wire early ... and that made the crash go away. Compiler bug maybe? I have
`Debian clang version 19.1.7 (3+build5)`, default build configuration.
Anyway this code seems fine to have.
I'm not sure why but this is actually faster than existing `opt_merge` even with
YOSYS_MAX_THREADS=1, for the jpeg synthesis test. 16.0s before, 15.5s after for
end-to-end synthesis.
This avoids changing `assign_map` and `initvals`, which are inputs to the hash function for `known_cells`,
while `known_cells` exists. Changing the hash function for a hashtable while it exists leads to
confusing behavior. That also means the exact behavior of `opt_merge` cannot be reproduced by a
parallel implementation.
When IdString refcounting was expensive, it made sense to pass it by const reference
instead of by value, to avoid refcount churn. Now that IdString is not refcounted,
it's slightly more efficient to pass it by value.
Fixes these build errors I'm getting locally with `tcl-devel-9.0.0-7.fc42.x86_64`.
I guess Tcl 9 broke this.
```
passes/cmds/sdc/sdc.cc:438:6: error: no matching function for call to 'Tcl_ListObjLength'
438 | if (Tcl_ListObjLength(interp, listObj, &listLength) == TCL_OK) {
| ^~~~~~~~~~~~~~~~~
/usr/include/tclDecls.h:1788:13: note: candidate function not viable: no known conversion from 'int *' to 'Tcl_Size *' (aka 'long *') for 3rd argument
1788 | EXTERN int Tcl_ListObjLength(Tcl_Interp *interp,
| ^
1789 | Tcl_Obj *listPtr, Tcl_Size *lengthPtr);
| ~~~~~~~~~~~~~~~~~~~
passes/cmds/sdc/sdc.cc:446:8: error: no matching function for call to 'Tcl_ListObjLength'
446 | if (Tcl_ListObjLength(interp, subListObj, &subListLength) == TCL_OK) {
| ^~~~~~~~~~~~~~~~~
/usr/include/tclDecls.h:1788:13: note: candidate function not viable: no known conversion from 'int *' to 'Tcl_Size *' (aka 'long *') for 3rd argument
1788 | EXTERN int Tcl_ListObjLength(Tcl_Interp *interp,
| ^
1789 | Tcl_Obj *listPtr, Tcl_Size *lengthPtr);
| ~~~~~~~~~~~~~~~~~~~
```
