Fix automatic task variables in unrolled loops with forks (#6194) (#6201).

docs/CONTRIBUTORS

@@ -37,6 +37,7 @@ Chuxuan Wang
 Chykon
 Conor McCullough
 Dan Petrisko
+Danny Oler
 Daniel Bates
 Dave Sargeant
 David Horton

src/V3Unroll.cpp

@@ -46,6 +46,7 @@ class UnrollVisitor final : public VNVisitor {
     bool m_varModeCheck;  // Just checking RHS assignments
     bool m_varModeReplace;  // Replacing varrefs
     bool m_varAssignHit;  // Assign var hit
+    bool m_forkHit; // Fork hit
     bool m_generate;  // Expand single generate For loop
     string m_beginName;  // What name to give begin iterations
     VDouble0 m_statLoops;  // Statistic tracking
@@ -133,6 +134,7 @@ class UnrollVisitor final : public VNVisitor {
         // Now, make sure there's no assignment to this variable in the loop
         m_varModeCheck = true;
         m_varAssignHit = false;
+        m_forkHit = false;
         m_ignoreIncp = incp;
         iterateAndNextNull(precondsp);
         iterateAndNextNull(bodysp);
@@ -141,6 +143,8 @@ class UnrollVisitor final : public VNVisitor {
         m_ignoreIncp = nullptr;
         if (m_varAssignHit) return cantUnroll(nodep, "genvar assigned *inside* loop");
 
+        if (m_forkHit) return cantUnroll(nodep, "fork inside loop");
+
         //
         if (m_forVscp) {
             UINFO(8, "   Loop Variable: " << m_forVscp);
@@ -463,6 +467,17 @@ class UnrollVisitor final : public VNVisitor {
         }
     }
 
+    void visit(AstFork* nodep) override {
+        if (m_varModeCheck) {
+            if (nodep->joinType().joinNone() || nodep->joinType().joinAny()) {
+                // Forks are not allowed to unroll for loops, so we just set a flag
+                m_forkHit = true;
+            }
+        } else {
+            iterateChildren(nodep);
+        }
+    }
+
     //--------------------
     // Default: Just iterate
     void visit(AstNode* nodep) override {
@@ -489,6 +504,7 @@ public:
         m_varModeCheck = false;
         m_varModeReplace = false;
         m_varAssignHit = false;
+        m_forkHit = false;
         m_generate = generate;
         m_beginName = beginName;
     }

test_regress/t/t_unroll_automatic_task_fork.out

@@ -0,0 +1,41 @@
+task_example start:  module 0, channel 0
+task_example end:  module 0, channel 0
+task_example start:  module 0, channel 1
+task_example end:  module 0, channel 1
+task_example start:  module 1, channel 0
+task_example end:  module 1, channel 0
+task_example start:  module 1, channel 1
+task_example end:  module 1, channel 1
+*-* Test 1 Finished *-*
+task_example start:  module 0, channel 0
+task_example end:  module 0, channel 0
+task_example start:  module 0, channel 1
+task_example end:  module 0, channel 1
+task_example start:  module 1, channel 0
+task_example end:  module 1, channel 0
+task_example start:  module 1, channel 1
+task_example end:  module 1, channel 1
+*-* Test 2 Finished *-*
+task_example start:  module 0, channel 0
+extra statement
+task_example start:  module 0, channel 1
+extra statement
+task_example start:  module 1, channel 0
+extra statement
+task_example start:  module 1, channel 1
+extra statement
+task_example end:  module 0, channel 0
+task_example end:  module 0, channel 1
+task_example end:  module 1, channel 0
+task_example end:  module 1, channel 1
+*-* Test 3 Finished *-*
+task_example start:  module 0, channel 0
+task_example start:  module 0, channel 1
+task_example start:  module 1, channel 0
+task_example start:  module 1, channel 1
+task_example end:  module 0, channel 0
+task_example end:  module 0, channel 1
+task_example end:  module 1, channel 0
+task_example end:  module 1, channel 1
+*-* Test 4 Finished *-*
+*-* All Finished *-*

test_regress/t/t_unroll_automatic_task_fork.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2024 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('simulator')
+
+test.compile(verilator_flags2=['--binary'])
+
+test.execute(expect_filename=test.golden_filename)
+
+test.passes()

test_regress/t/t_unroll_automatic_task_fork.v

@@ -0,0 +1,82 @@
+// 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
+
+// Targets issue 6194
+
+module t;
+
+  logic clk;
+
+  initial begin
+    clk = 0;
+    forever #(1) clk = ~clk;
+  end
+
+  task automatic task_example(int module_id, int channel);
+    $display("task_example start:  module %0d, channel %0d", module_id, channel);
+    @(posedge clk);
+    $display("task_example end:  module %0d, channel %0d", module_id, channel);
+  endtask
+
+  initial begin : initial_block
+    for (int m = 0; m < 2; m++) begin
+      for (int i = 0; i < 2; i++) begin : forked_loop
+        automatic int mod = m;
+        automatic int ch = i;
+        fork : forked_block
+          task_example(mod,  ch);
+        join
+      end
+    end
+
+    #10
+    $write("*-* Test 1 Finished *-*\n");
+
+    for (int m = 0; m < 2; m++) begin
+      for (int i = 0; i < 2; i++) begin : forked_loop
+        automatic int mod = m;
+        automatic int ch = i;
+        fork : forked_block
+          task_example(mod,  ch);
+        join_any
+      end
+    end
+
+    #10
+    $write("*-* Test 2 Finished *-*\n");
+
+    for (int m = 0; m < 2; m++) begin
+      for (int i = 0; i < 2; i++) begin : forked_loop
+        automatic int mod = m;
+        automatic int ch = i;
+        fork : forked_block
+          task_example(mod,  ch);
+          $display("extra statement");
+        join_any
+      end
+    end
+
+    #10
+    $write("*-* Test 3 Finished *-*\n");
+
+    for (int m = 0; m < 2; m++) begin
+      for (int i = 0; i < 2; i++) begin : forked_loop
+        automatic int mod = m;
+        automatic int ch = i;
+        fork : forked_block
+          task_example(mod,  ch);
+        join_none
+      end
+    end
+
+    #10
+    $write("*-* Test 4 Finished *-*\n");
+
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+
+endmodule