Fix V3Gate crash on circular logic

The recent patch to defer substitutions on V3Gate crashes on circular
logic that has cycle length >= 3 with all inlineable signals (cycle
length 2 is detected correctly and is not inlined). Fix by stopping
recursion at the loop-back edge.

Fixes #3543

Changes

@@ -13,6 +13,8 @@ Verilator 4.227 devel
 
 **Minor:**
 
+Fix crash in gate optimization of circular logic (#3543). [Bill Flynn]
+
 
 Verilator 4.226 2022-08-31
 ==========================

src/V3Gate.cpp

@@ -985,11 +985,18 @@ static void eliminate(AstNode* logicp,
                       const std::unordered_map<AstVarScope*, AstNode*>& substitutions,
                       GateDedupeVarVisitor* varVisp) {
 
-    const std::function<void(AstNodeVarRef*)> visit
+    // Recursion filter holding already replaced variables
-        = [&substitutions, &visit, varVisp](AstNodeVarRef* nodep) -> void {
+    std::unordered_set<const AstVarScope*> replaced(substitutions.size() * 2);
+
+    const std::function<void(AstNodeVarRef*)> visit = [&, varVisp](AstNodeVarRef* nodep) -> void {
         // See if this variable has a substitution
-        const auto& it = substitutions.find(nodep->varScopep());
+        AstVarScope* const vscp = nodep->varScopep();
+        const auto& it = substitutions.find(vscp);
         if (it == substitutions.end()) return;
+
+        // Do not substitute circular logic
+        if (!replaced.insert(vscp).second) return;
+
         AstNode* const substp = it->second;
 
         // Substitute in the new tree
@@ -1016,6 +1023,9 @@ static void eliminate(AstNode* logicp,
         VL_DO_DANGLING(nodep->deleteTree(), nodep);
         // Recursively substitute the new tree
         newp->foreach<AstNodeVarRef>(visit);
+
+        // Remove from recursion filter
+        replaced.erase(vscp);
     };
 
     logicp->foreach<AstNodeVarRef>(visit);

test_regress/t/t_gate_loop.pl

@@ -0,0 +1,18 @@
+#!/usr/bin/env perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2022 by Geza Lore. 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 or the Perl Artistic License
+# Version 2.0.
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+scenarios(simulator => 1);
+
+compile(
+    verilator_flags2 => ["-Wno-UNOPTFLAT"]
+    );
+
+ok(1);
+1;

test_regress/t/t_gate_loop.v

@@ -0,0 +1,14 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2022 by Geza Lore.
+// SPDX-License-Identifier: CC0-1.0
+
+module t;
+  wire a;
+  wire b;
+  wire c;
+  assign a = b;
+  assign b = c;
+  assign c = a;
+endmodule