Fix constant propagating DPI-written variables (#7074)

src/V3Life.cpp

@@ -171,7 +171,8 @@ public:
             entr.init(false);
         } else {
             if (AstConst* const constp = entr.constNodep()) {
-                if (!varrefp->varp()->isSigPublic() && !varrefp->varp()->isVirtIface()) {
+                if (!varrefp->varp()->isSigPublic() && !varrefp->varp()->isWrittenByDpi()
+                    && !varrefp->varp()->isVirtIface()) {
                     // Aha, variable is constant; substitute in.
                     // We'll later constant propagate
                     UINFO(4, "     replaceconst: " << varrefp);

test_regress/t/t_opt_life_dpi_written.cpp

@@ -0,0 +1,21 @@
+// -*- mode: C++; c-file-style: "cc-mode" -*-
+//*************************************************************************
+//
+// 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-FileCopyrightText: 2026-2026 Wilson Snyder
+// SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+//
+//*************************************************************************
+
+#include "svdpi.h"
+
+#include <iostream>
+
+extern "C" void setDpi(int value);
+
+extern "C" void setViaDpi(int value) {
+    std::cout << "setViaDpi " << value << std::endl;
+    setDpi(value);
+}

test_regress/t/t_opt_life_dpi_written.py

@@ -0,0 +1,21 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# 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-FileCopyrightText: 2026 Wilson Snyder
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('vlt_all')
+
+test.compile(verilator_flags2=["--binary", "--stats", test.top_filename.replace(".v", ".cpp")])
+
+test.execute()
+
+test.file_grep_not(test.stats, r'Optimizations, Lifetime constant prop')
+test.file_grep_not(test.stats, r'Optimizations, Lifetime assign deletions')
+
+test.passes()

test_regress/t/t_opt_life_dpi_written.v

@@ -0,0 +1,31 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain.
+// SPDX-FileCopyrightText: 2026 Wilson Snyder
+// SPDX-License-Identifier: CC0-1.0
+
+`define stop $stop
+`define check(got ,exp) do if ((got) !== (exp)) begin $write("%%Error: %s:%0d: got='h%x exp='h%x\n", `__FILE__,`__LINE__, (got), (exp)); `stop; end while(0)
+
+module t;
+
+  int dpiSet = 0;
+  function automatic void setDpi(int value);
+    dpiSet = value;
+  endfunction
+  export "DPI-C" function setDpi;
+  import "DPI-C" context function void setViaDpi(int value); // calls setDpi(value)
+
+  initial begin
+    dpiSet = 13;
+    setViaDpi(14);
+    `check(dpiSet, 14);
+    dpiSet = 15;
+    setViaDpi(16);
+    `check(dpiSet, 16);
+    dpiSet = 17;
+    setViaDpi(18);
+    `check(dpiSet, 18);
+  end
+
+endmodule