Fix tautological-compare warnings.

Changes

@@ -44,6 +44,7 @@ Verilator 5.021 devel
 * Fix NOT when checking EQ/NEQ under AND/OR tree (#4857) (#4863). [Yutetsu TAKATSUKASA]
 * Fix tracing chandles (#4860). [Nathan Graybeal]
 * Fix $fwrite of null (#4862). [Jose Tejada]
+* Fix GCC tautological-compare warnings.
 
 
 Verilator 5.020 2024-01-01

configure.ac

@@ -428,6 +428,7 @@ m4_foreach([cflag],[
         [-Wno-shadow],
         [-Wno-sign-compare],
         [-Wno-tautological-bitwise-compare],
+        [-Wno-tautological-compare],
         [-Wno-uninitialized],
         [-Wno-unused-but-set-parameter],
         [-Wno-unused-but-set-variable],

src/V3Const.cpp

@@ -1441,16 +1441,22 @@ class ConstVisitor final : public VNVisitor {
 
     static bool operandsSame(AstNode* node1p, AstNode* node2p) {
         // For now we just detect constants & simple vars, though it could be more generic
-        if (VN_IS(node1p, Const) && VN_IS(node2p, Const)) {
-            return node1p->sameGateTree(node2p);
-        } else if (VN_IS(node1p, VarRef) && VN_IS(node2p, VarRef)) {
+        if (VN_IS(node1p, Const) && VN_IS(node2p, Const)) return node1p->sameGateTree(node2p);
+        if (VN_IS(node1p, VarRef) && VN_IS(node2p, VarRef)) {
             // Avoid comparing widthMin's, which results in lost optimization attempts
             // If cleanup sameGateTree to be smarter, this can be restored.
             // return node1p->sameGateTree(node2p);
             return node1p->isSame(node2p);
-        } else {
-            return false;
         }
+        // Pattern created by coverage-line; avoid compiler tautological-compare warning
+        if (AstAnd* const and1p = VN_CAST(node1p, And)) {
+            if (AstAnd* const and2p = VN_CAST(node2p, And)) {
+                if (VN_IS(and1p->lhsp(), Const) && VN_IS(and1p->rhsp(), NodeVarRef)
+                    && VN_IS(and2p->lhsp(), Const) && VN_IS(and2p->rhsp(), NodeVarRef))
+                    return node1p->sameGateTree(node2p);
+            }
+        }
+        return false;
     }
     bool ifSameAssign(const AstNodeIf* nodep) {
         const AstNodeAssign* const thensp = VN_CAST(nodep->thensp(), NodeAssign);

test_regress/t/t_cover_const_compare.pl

@@ -0,0 +1,22 @@
+#!/usr/bin/env perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2003-2009 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(simulator => 1);
+
+compile(
+    verilator_flags2 => ['--coverage-line'],
+    );
+
+execute(
+    check_finished => 1,
+    );
+
+ok(1);
+1;

test_regress/t/t_cover_const_compare.v

@@ -0,0 +1,32 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2024 by Wilson Snyder.
+// SPDX-License-Identifier: CC0-1.0
+
+module t (/*AUTOARG*/
+   // Inputs
+   clk
+   );
+
+   input clk;
+
+   integer cyc; initial cyc=1;
+
+   wire a = cyc[0];
+   wire b = cyc[0];
+
+   always @ (posedge clk) begin
+      cyc <= cyc + 1;
+      // Before this was optimized, with --coverage-line
+      // error: self-comparison always evaluates to true [-Werror=tautological-compare]
+      //        if (((1U & vlSelf->t__DOT__cyc) == (1U & vlSelf->t__DOT__cyc)))
+      if (a != cyc[0]) $stop;  // Becomes cyc == cyc after substitution
+      if (b != cyc[0]) $stop;
+      if (cyc==10) begin
+         $write("*-* All Finished *-*\n");
+         $finish;
+      end
+   end
+
+endmodule