Fix spurious FSM COVERIGN on datapath comparisons (#7900)

src/V3FsmDetect.cpp

@@ -988,21 +988,15 @@ class FsmDetectVisitor final : public VNVisitor {
         return assp;
     }
 
-    static AstVarRef* tryExtractVarRef(AstNodeExpr* const exprp) {
-        AstVarRef* const varp = VN_CAST(AstArraySel::baseFromp(exprp, true), VarRef);
-        if (!varp) {
-            exprp->v3warn(COVERIGN,
-                          "Ignoring unsupported: FSM coverage with " << exprp->prettyTypeName());
-            return nullptr;
-        }
-        return varp;
+    static AstVarRef* extractVarRefQuiet(AstNodeExpr* const exprp) {
+        return VN_CAST(AstArraySel::baseFromp(exprp, true), VarRef);
     }
 
     static AstNodeAssign* nodeStateVarAssign(AstNode* nodep, AstVarScope*& stateVscp,
                                              AstVarScope*& fromVscp) {
         AstNodeAssign* const assp = VN_CAST(nodep, NodeAssign);
         if (!assp) return nullptr;
-        AstVarRef* const lhsp = tryExtractVarRef(assp->lhsp());
+        AstVarRef* const lhsp = extractVarRefQuiet(assp->lhsp());
         AstVarRef* const rhsp = VN_CAST(assp->rhsp(), VarRef);
         if (!rhsp || !lhsp) return nullptr;
         stateVscp = lhsp->varScopep();
@@ -1016,7 +1010,7 @@ class FsmDetectVisitor final : public VNVisitor {
                                                    FsmStateValue& resetValue) {
         AstNodeAssign* const assp = VN_CAST(nodep, NodeAssign);
         if (!assp) return nullptr;
-        AstVarRef* const lhsp = tryExtractVarRef(assp->lhsp());
+        AstVarRef* const lhsp = extractVarRefQuiet(assp->lhsp());
         AstCond* const rhsp = VN_CAST(assp->rhsp(), Cond);
         if (!rhsp || !lhsp) return nullptr;
         if (AstVarRef* const elsep = VN_CAST(rhsp->elsep(), VarRef)) {
@@ -1228,7 +1222,7 @@ class FsmDetectVisitor final : public VNVisitor {
         AstVarRef* vrefp = VN_CAST(eqp->lhsp(), VarRef);
         AstNodeExpr* valuep = eqp->rhsp();
         if (!vrefp) {
-            vrefp = tryExtractVarRef(eqp->rhsp());
+            vrefp = extractVarRefQuiet(eqp->rhsp());
             if (!vrefp) { return false; }
             valuep = eqp->lhsp();
         }

test_regress/t/t_cover_fsm_concat_unsup.out

@@ -1,6 +0,0 @@
-%Warning-COVERIGN: t/t_cover_fsm_concat_unsup.v:12:17: Ignoring unsupported: FSM coverage with CONCAT
-   12 |   assign c = ({a, b} == 8'h00);
-      |                 ^
-                   ... For warning description see https://verilator.org/warn/COVERIGN?v=latest
-                   ... Use "/* verilator lint_off COVERIGN */" and lint_on around source to disable this message.
-%Error: Exiting due to

test_regress/t/t_cover_fsm_concat_unsup.v

@@ -1,18 +0,0 @@
-// DESCRIPTION: Verilator: Verilog Test module
-//
-// This file ONLY is placed under the Creative Commons Public Domain.
-// SPDX-FileCopyrightText: 2026 Antmicro
-// SPDX-License-Identifier: CC0-1.0
-
-module t (
-    input logic [6:0] a,
-    input logic b,
-    output logic c
-);
-  assign c = ({a, b} == 8'h00);
-
-  initial begin
-    $write("*-* All Finished *-*\n");
-    $finish;
-  end
-endmodule

test_regress/t/t_cover_fsm_datapath_cmp.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python3
-# DESCRIPTION: Verilator: FSM coverage concat as unsupported operation test
+# DESCRIPTION: Verilator: FSM coverage ignores unrelated datapath comparisons
 #
 # This program is free software; you can redistribute it and/or modify it
 # under the terms of either the GNU Lesser General Public License Version 3
@@ -11,6 +11,7 @@ import vltest_bootstrap
 
 test.scenarios('vlt')
 
-test.lint(fails=True, expect_filename=test.golden_filename, verilator_flags2=['--coverage'])
+# Fail if speculative FSM extraction emits COVERIGN for unrelated datapath comparisons.
+test.lint(verilator_flags2=['--coverage-fsm', '-Werror-COVERIGN'])
 
 test.passes()

test_regress/t/t_cover_fsm_datapath_cmp.v

@@ -0,0 +1,62 @@
+// DESCRIPTION: Verilator: FSM coverage ignores unrelated datapath comparisons
+//
+// This file ONLY is placed under the Creative Commons Public Domain.
+// SPDX-FileCopyrightText: 2026 Wilson Snyder
+// SPDX-License-Identifier: CC0-1.0
+
+module datapath_only #(
+  parameter logic [15:0] MASK = 16'hff00,
+  parameter logic [15:0] MATCH = 16'h1200
+) (
+  input logic [6:0] a,
+  input logic b,
+  input logic [15:0] data,
+  output logic concat_hit,
+  output logic masked_hit
+);
+  assign concat_hit = ({a, b} == 8'h00);
+  assign masked_hit = (data & MASK) == MATCH;
+endmodule
+
+module t #(
+  parameter logic [15:0] MASK = 16'hff00,
+  parameter logic [15:0] MATCH = 16'h1200
+) (
+  input logic clk,
+  input logic rst,
+  input logic go,
+  input logic [6:0] a,
+  input logic b,
+  input logic [15:0] data,
+  output logic busy,
+  output logic hit,
+  output logic concat_hit,
+  output logic masked_hit
+);
+  typedef enum logic [1:0] { IDLE, RUN, DONE } state_t;
+  state_t state;
+
+  always_ff @(posedge clk) begin
+    if (rst) begin
+      state <= IDLE;
+    end else begin
+      case (state)
+        IDLE: if (go) state <= RUN;
+        RUN: state <= DONE;
+        DONE: state <= IDLE;
+        default: state <= IDLE;
+      endcase
+    end
+  end
+
+  assign busy = (state != IDLE);
+  assign hit = (data & MASK) == MATCH;
+
+  datapath_only datapath_only_u (
+    .a(a),
+    .b(b),
+    .data(data),
+    .concat_hit(concat_hit),
+    .masked_hit(masked_hit)
+  );
+endmodule