Fix Dfg reusing volatile variables

Forceable/externally written variables cannot be used as the canonical
result variable for a Dfg value as the variables value can be
inconsistent with its Dfg drivers (e.g. when forced).

src/V3Dfg.cpp

@@ -552,6 +552,10 @@ DfgVertexVar* DfgVertex::getResultVar() {
     this->foreachSink([&resp](DfgVertex& sink) {
         DfgVertexVar* const varp = sink.cast<DfgVertexVar>();
         if (!varp) return false;
+
+        // Do not use it if value might differ from its drivers
+        if (varp->isVolatile()) return false;
+
         // First variable found
         if (!resp) {
             resp = varp;
@@ -559,12 +563,8 @@ DfgVertexVar* DfgVertex::getResultVar() {
         }
 
         // Prefer those variables that must be kept anyway
-        if (resp->hasExtRefs() != varp->hasExtRefs()) {
-            if (!resp->hasExtRefs()) resp = varp;
-            return false;
-        }
-        if (resp->hasModWrRefs() != varp->hasModWrRefs()) {
-            if (!resp->hasModWrRefs()) resp = varp;
+        if (resp->hasExtRdRefs() != varp->hasExtRdRefs()) {
+            if (!resp->hasExtRdRefs()) resp = varp;
             return false;
         }
         if (resp->hasDfgRefs() != varp->hasDfgRefs()) {

test_regress/t/t_dfg_result_var_ext_write.py

@@ -0,0 +1,18 @@
+#!/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('vlt')
+
+test.compile(verilator_flags2=["--binary"])
+
+test.execute()
+
+test.passes()

test_regress/t/t_dfg_result_var_ext_write.v

@@ -0,0 +1,65 @@
+// 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
+
+`define stop $stop
+`define check(got ,exp) do if ((got) !== (exp)) begin $write("%%Error: %s:%0d: $time=%0t got='h%x exp='h%x\n", `__FILE__,`__LINE__, $time, (got), (exp)); `stop; end while(0)
+
+module top;
+
+    logic clk = 0;
+    always #5 clk = ~clk;
+
+    int cnt = 0;
+    always @(posedge clk) cnt <= cnt + 1;
+
+    wire forcedVar = &cnt[2:1];
+    wire sharedTerm = &cnt[2:1];
+    wire otherVar = ~sharedTerm;
+
+    initial begin
+      repeat (50) begin
+        @(posedge clk);
+        `check(otherVar == ~forcedVar, 1);
+      end
+
+      force forcedVar = 1'b1;
+
+      repeat (50) begin
+        @(posedge clk);
+        `check(otherVar == ~forcedVar, cnt[2] & cnt[1]);
+      end
+
+      release forcedVar;
+
+      repeat (50) begin
+        @(posedge clk);
+        `check(otherVar == ~forcedVar, 1);
+      end
+
+      force forcedVar = 1'b0;
+
+      repeat (50) begin
+        @(posedge clk);
+        `check(otherVar == ~forcedVar, ~cnt[2] | ~cnt[1]);
+      end
+
+      release forcedVar;
+
+      repeat (50) begin
+        @(posedge clk);
+        `check(otherVar == ~forcedVar, 1);
+      end
+
+
+      $write("*-* All Finished *-*\n");
+      $finish;
+    end
+
+    always @(sharedTerm or otherVar) begin
+      `check(otherVar, ~sharedTerm);
+    end
+
+endmodule

test_regress/t/t_inst_tree_inl0_pub1.py

@@ -42,7 +42,7 @@ if test.vlt_all:
     # We expect to combine sequent functions across multiple instances of
     # l2, l3, l4, l5. If this number drops, please confirm this has not broken.
     test.file_grep(test.stats, r'Optimizations, Combined CFuncs\s+(\d+)',
-                   (99 if test.vltmt else 82))
+                   (85 if test.vltmt else 67))
 
     # Everything should use relative references
     check_relative_refs("t", True)