[dv] VCS UNR misses simple cases

[matutem]

The VCS UNR flow misses the following simple cases for clkmgr:

• The clkmgr output cg_en_o.aon_infra, cg_en_o.aon_powerup, and a few other members of cg_en_o were not found to be unreachable for toggle, even if they are driven by constants, like
  assign cg_en_o.aon_infra = MuBi4False;

• The u_idle_cnt instance in module clkmgr_trans ties input step_i to a constant, but UNR fails to detect it won't toggle.

These issues were flagged to Synopsys.

Effort estimate is difficult to make, as it largely depends on vendor support.

estimate 8

[weicaiyang]

Found 2 similar cases in sram_ctrl.

1. the inputs to u_seed_anchor are constant. anything uncovered in this instance should be unreachable, but VCS UNR can't catch it.
2. This also has all constant inputs, which UNR fails to catch as well.

More are found in this file with annotation "[UNR] all inputs are constant".

Reported this to AE.

[weicaiyang]

Found 2 similar cases in sram_ctrl.

1. the inputs to u_seed_anchor are constant. anything uncovered in this instance should be unreachable, but VCS UNR can't catch it.
2. This also has all constant inputs, which UNR fails to catch as well.

The VCS 2022.06-SP2 can't fix above issues, but it can catch the unreachable toggle coverage as inputs are constant, like this

[andreaskurth]

Triaged for sram_ctrl. Suggest M2.5 with Priority:P3 Priority: low because coverage numbers for that block are already well above 90%. If coverage would become a problem, though, this issue should be kept in mind.

[andreaskurth]

Triaged for clkmgr, ditto, though even Priority:P4 Priority: propose to move to backlog would suffice.

[msfschaffner]

Given that this seems to track a tooling issue and that we are otherwise in pretty good shape coverage-wise, I suggest to defer this to after M2.5.

[matutem]

Since this is a tooling issue I think the only impact it has for us is that we may need to add additional coverage exclusions manually. This means it is irrelevant, since we can reach adequate coverage with or without the vendor fixing this.

[vogelpi]

@matutem , do you think this still persists with VCS U-2023.03-SP2 (the version most partners are using today)?