From e4c5eb5e69ddaf16f024d2b0939f706420466e89 Mon Sep 17 00:00:00 2001
From: Wilson Snyder <wsnyder@wsnyder.org>
Date: Sat, 1 Jan 2022 18:37:34 -0500
Subject: [PATCH] Fix spurious UNUSED by ignoring inout pin connections
 (#3242).

---
 Changes                             |  1 +
 src/V3Undriven.cpp                  | 14 +++++++++++++-
 test_regress/t/t_lint_unused_tri.pl | 18 ++++++++++++++++++
 test_regress/t/t_lint_unused_tri.v  | 26 ++++++++++++++++++++++++++
 4 files changed, 58 insertions(+), 1 deletion(-)
 create mode 100755 test_regress/t/t_lint_unused_tri.pl
 create mode 100644 test_regress/t/t_lint_unused_tri.v

diff --git a/Changes b/Changes
index e699ac150..1c8de8ffb 100644
--- a/Changes
+++ b/Changes
@@ -33,6 +33,7 @@ Verilator 4.217 devel
 * Fix associative array foreach loop (#3229).
 * Fix $fclose not accepting expressions (#3237). [Julie Schwartz]
 * Fix $random not updating seed (#3238). [Julie Schwartz]
+* Fix spurious UNUSED by ignoring inout pin connections (#3242). [Julie Schwartz]
 * Fix splitting of _eval and other top level functions. [Geza Lore, Shunyao CAD]
 
 
diff --git a/src/V3Undriven.cpp b/src/V3Undriven.cpp
index be909a9e8..4618af23c 100644
--- a/src/V3Undriven.cpp
+++ b/src/V3Undriven.cpp
@@ -250,6 +250,7 @@ private:
     bool m_inBBox = false;  // In black box; mark as driven+used
     bool m_inContAssign = false;  // In continuous assignment
     bool m_inProcAssign = false;  // In procedural assignment
+    bool m_inInoutPin = false;  // Connected to pin that is inout
     const AstNodeFTask* m_taskp = nullptr;  // Current task
     const AstAlways* m_alwaysCombp = nullptr;  // Current always if combo, otherwise nullptr
 
@@ -374,7 +375,13 @@ private:
                 }
                 entryp->drivenWhole();
             }
-            if (m_inBBox || nodep->access().isReadOrRW() || fdrv) entryp->usedWhole();
+            if (m_inBBox || nodep->access().isReadOrRW()
+                || fdrv
+                // Inouts have only isWrite set, as we don't have more
+                // information and operating on module boundry, treat as
+                // both read and writing
+                || m_inInoutPin)
+                entryp->usedWhole();
         }
     }
 
@@ -430,6 +437,11 @@ private:
             iterateChildren(nodep);
         }
     }
+    virtual void visit(AstPin* nodep) override {
+        VL_RESTORER(m_inInoutPin);
+        m_inInoutPin = nodep->modVarp()->isInoutish();
+        iterateChildren(nodep);
+    }
 
     // Until we support tables, primitives will have undriven and unused I/Os
     virtual void visit(AstPrimitive*) override {}
diff --git a/test_regress/t/t_lint_unused_tri.pl b/test_regress/t/t_lint_unused_tri.pl
new file mode 100755
index 000000000..3679f5264
--- /dev/null
+++ b/test_regress/t/t_lint_unused_tri.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 2008 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
+
+scenarios(vlt => 1);
+
+lint(
+    verilator_flags2 => ["--lint-only -Wall -Wno-DECLFILENAME"],
+    );
+
+ok(1);
+1;
diff --git a/test_regress/t/t_lint_unused_tri.v b/test_regress/t/t_lint_unused_tri.v
new file mode 100644
index 000000000..4d2646a00
--- /dev/null
+++ b/test_regress/t/t_lint_unused_tri.v
@@ -0,0 +1,26 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2022 by Wilson Snyder.
+// SPDX-License-Identifier: CC0-1.0
+
+module Receiver(in);
+   inout [31:0] in;
+   always @(in) $display(in);
+endmodule
+
+module Sender(out);
+   inout [31:0] out;
+   assign out = 12;
+endmodule
+
+module t;
+   // ports of submodule recv
+   tri [31 : 0] recvIn;
+
+   // submodule recv
+   Receiver recv(.in(recvIn));
+
+   // submodule send
+   Sender send(.out(recvIn));
+endmodule