From 39c3e79a3cd01e0c0677439269296e58e2bce506 Mon Sep 17 00:00:00 2001
From: Geza Lore <gezalore@gmail.com>
Date: Fri, 28 Mar 2025 17:04:09 +0000
Subject: [PATCH] Fix V3Gate assertion on eliminated circular logic (#5889)
 (#5898)

Fixes #5889
---
 src/V3Gate.cpp                      | 16 ++++++++++++--
 test_regress/t/t_gate_elim_cycle.py | 16 ++++++++++++++
 test_regress/t/t_gate_elim_cycle.v  | 33 +++++++++++++++++++++++++++++
 3 files changed, 63 insertions(+), 2 deletions(-)
 create mode 100755 test_regress/t/t_gate_elim_cycle.py
 create mode 100644 test_regress/t/t_gate_elim_cycle.v

diff --git a/src/V3Gate.cpp b/src/V3Gate.cpp
index abb8fff7d..b5269027a 100644
--- a/src/V3Gate.cpp
+++ b/src/V3Gate.cpp
@@ -869,8 +869,20 @@ class GateInline final {
 
                 if (debug() >= 9) dstVtxp->nodep()->dumpTree("      inside: ");
 
-                UASSERT_OBJ(logicp != dstVtxp->nodep(), logicp,
-                            "Circular logic should have been rejected by okVisitor");
+                if (logicp == dstVtxp->nodep()) {
+                    // This is a bit involved. The graph tells us that the logic is circular
+                    // (driver is same as sink), however, okVisitor rejects a circular driver
+                    // and we would not reach here if the driver logic was actually circular.
+                    // The reason we end up here is because during graph building, the driver
+                    // was ciruclar, however, after committing some substituions to it, it
+                    // has become non-circualr due to V3Const being applied inside
+                    // 'commitSubstitutions'. We will trust GateOkVisitor telling the truth
+                    // that the logic is not actually circular, meaning this edge is not
+                    // actually needed, can just delete it and move on.
+                    VL_DO_DANGLING(edgep->unlinkDelete(), edgep);
+                    continue;
+                }
+
                 recordSubstitution(vscp, substp, dstVtxp->nodep());
 
                 // If the new replacement referred to a signal,
diff --git a/test_regress/t/t_gate_elim_cycle.py b/test_regress/t/t_gate_elim_cycle.py
new file mode 100755
index 000000000..147fe6faf
--- /dev/null
+++ b/test_regress/t/t_gate_elim_cycle.py
@@ -0,0 +1,16 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2025 by Wilson Snyder. 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
+
+import vltest_bootstrap
+
+test.scenarios('simulator')
+
+test.compile()
+
+test.passes()
diff --git a/test_regress/t/t_gate_elim_cycle.v b/test_regress/t/t_gate_elim_cycle.v
new file mode 100644
index 000000000..adc3ed1f0
--- /dev/null
+++ b/test_regress/t/t_gate_elim_cycle.v
@@ -0,0 +1,33 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2025 by Wilson Snyder.
+// SPDX-License-Identifier: CC0-1.0
+
+module GND(output G);
+  assign G = 0;
+endmodule
+
+module CARRY2(
+  output [1:0] CO,
+  input        CI,
+  input  [1:0] DI, S
+);
+  assign CO[0] = S[0] ? CI    : DI[0];
+  assign CO[1] = S[1] ? CO[0] : DI[1];
+endmodule
+
+module A;
+  wire const0;
+  wire ci;
+  GND GND (
+    .G(const0)
+  );
+  CARRY2 CARRY2 (
+    .CO(),
+
+    .CI(ci),
+    .DI({const0,const0}),
+    .S({const0,const0})
+  );
+endmodule