Verilator 5.05x: a second --top-module silently replaces the first; observer modules with hierarchical refs need a wrapper top

Setup: a testbench (tb) plus a separate observer module that reads tb.u.* through hierarchical references. VCS accepts -top tb -top observer. With Verilator 5.052, --top-module tb --top-module observer keeps only the last one, so every tb.u.x reference in the observer fails with "Can't find definition of scope/variable: 'tb'". Adding -Wno-MULTITOP does not help.

Fix: write a one-line wrapper and make it the only top:

module smoke_top; tb tb(); observer observer(); endmodule

The observer's tb.u.x then resolves upward to the sibling instance named tb. The printed paths stay the same if you keep the instance name equal to the old top name. Prefix-based comparisons still match.

Two more traps when you reuse a 4-state VCS fixture under Verilator: - Passing a Z literal as a task argument (e.g. wr(6, 32'h0000zz0x)) fails with %Error-UNSUPPORTED: Unsupported tristate construct: ARG. - An out-of-range write such as mem[idx] with idx=9 into [0:7] does not get dropped as the SV LRM says. It landed on element 1 (index wrapped to the array's bit width). An X index became a real index too. Keep those X/range rows out of a Verilator smoke, e.g. with sed on a copy of the fixture, and test them only on a 4-state simulator.