Fix eliminating assignments to DPI-read vaiables (#7158)

src/V3AstNodeOther.h

@@ -1933,6 +1933,7 @@ class AstVar final : public AstNode {
     bool m_isLatched : 1;  // Not assigned in all control paths of combo always
     bool m_isForceable : 1;  // May be forced/released externally from user C code
     bool m_isForcedByCode : 1;  // May be forced/released from AstAssignForce/AstRelease
+    bool m_isReadByDpi : 1;  // This variable can be read by a DPI Export
     bool m_isWrittenByDpi : 1;  // This variable can be written by a DPI Export
     bool m_isWrittenBySuspendable : 1;  // This variable can be written by a suspendable process
     bool m_ignorePostRead : 1;  // Ignore reads in 'Post' blocks during ordering
@@ -1987,6 +1988,7 @@ class AstVar final : public AstNode {
         m_isLatched = false;
         m_isForceable = false;
         m_isForcedByCode = false;
+        m_isReadByDpi = false;
         m_isWrittenByDpi = false;
         m_isWrittenBySuspendable = false;
         m_ignorePostRead = false;
@@ -2152,6 +2154,8 @@ public:
     void setForceable() { m_isForceable = true; }
     void setForcedByCode() { m_isForcedByCode = true; }
     bool isForced() const { return m_isForceable || m_isForcedByCode; }
+    bool isReadByDpi() const { return m_isReadByDpi; }
+    void setReadByDpi() { m_isReadByDpi = true; }
     bool isWrittenByDpi() const { return m_isWrittenByDpi; }
     void setWrittenByDpi() { m_isWrittenByDpi = true; }
     bool isWrittenBySuspendable() const { return m_isWrittenBySuspendable; }

src/V3AstNodes.cpp

@@ -2880,6 +2880,8 @@ void AstVar::dump(std::ostream& str) const {
     if (isSigPublic()) str << " [P]";
     if (isSigUserRdPublic()) str << " [PRD]";
     if (isSigUserRWPublic()) str << " [PWR]";
+    if (isReadByDpi()) str << " [DPIRD]";
+    if (isWrittenByDpi()) str << " [DPIWR]";
     if (isInternal()) str << " [INTERNAL]";
     if (isLatched()) str << " [LATCHED]";
     if (isUsedLoopIdx()) str << " [LOOPIDX]";
@@ -2927,6 +2929,8 @@ void AstVar::dumpJson(std::ostream& str) const {
     if (dtypep()) dumpJsonStr(str, "dtypeName", dtypep()->name());
     dumpJsonBoolFuncIf(str, isSigUserRdPublic);
     dumpJsonBoolFuncIf(str, isSigUserRWPublic);
+    dumpJsonBoolFuncIf(str, isReadByDpi);
+    dumpJsonBoolFuncIf(str, isWrittenByDpi);
     dumpJsonBoolFuncIf(str, isGParam);
     dumpJsonBoolFuncIf(str, isParam);
     dumpJsonBoolFuncIf(str, attrScBv);

src/V3Life.cpp

@@ -129,6 +129,7 @@ public:
         const AstVar* const varp = vscp->varp();
         // We don't optimize any public sigs
         if (varp->isSigPublic()) return;
+        if (varp->isReadByDpi()) return;
         if (varp->sensIfacep()) return;
         // Check the var entry, and remove if appropriate
         AstNodeStmt* const oldassp = entr.assignp();

src/V3Task.cpp

@@ -1428,15 +1428,22 @@ class TaskVisitor final : public VNVisitor {
             // Mark non-local variables written by the exported function
             bool writesNonLocals = false;
             cfuncp->foreach([&writesNonLocals](AstVarRef* refp) {
-                if (refp->access().isReadOnly()) return;  // Ignore read reference
                 AstVar* const varp = refp->varScopep()->varp();
                 // We are ignoring function locals as they should not be referenced anywhere
                 // outside the enclosing AstCFunc, hence they are irrelevant for code ordering.
                 if (varp->isFuncLocal()) return;
-                // Mark it as written by DPI export
-                varp->setWrittenByDpi();
-                // Remember we had some
-                writesNonLocals = true;
+                // Check if written
+                if (refp->access().isWriteOrRW()) {
+                    // Mark it as written by DPI export
+                    varp->setWrittenByDpi();
+                    // Remember we had some
+                    writesNonLocals = true;
+                }
+                // Check if read
+                if (refp->access().isReadOrRW()) {
+                    // Mark it as read by DPI export
+                    varp->setReadByDpi();
+                }
             });
 
             // If this DPI export writes some non-local variables, set the DPI Export Trigger flag

test_regress/t/t_opt_life_dpi_read.cpp

@@ -0,0 +1,22 @@
+// -*- 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" int getDpi();
+
+extern "C" int getViaDpi() {
+    const int value = getDpi();
+    std::cout << "getDpi " << value << std::endl;
+    return value;
+}

test_regress/t/t_opt_life_dpi_read.py

@@ -0,0 +1,18 @@
+#!/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.passes()

test_regress/t/t_opt_life_dpi_read.v

@@ -0,0 +1,35 @@
+// 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
+
+// verilog_format: off
+`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)
+// verilog_format: on
+
+module t;
+
+  int dpiGet = 0;
+  function automatic int getDpi();
+    return dpiGet;
+  endfunction
+  export "DPI-C" function getDpi;
+  import "DPI-C" context function int getViaDpi();  // calls getDpi()
+
+  int tmp1, tmp2, tmp3;
+
+  initial begin
+    dpiGet = 13;
+    tmp1 = getViaDpi();
+    dpiGet = 14;
+    tmp2 = getViaDpi();
+    dpiGet = 15;
+    tmp3 = getViaDpi();
+    `check(tmp1, 13);
+    `check(tmp2, 14);
+    `check(tmp3, 15);
+  end
+
+endmodule