Fix coroutines without awaits to have a co_return (#4208) (#5175)

After the V3Timing refactoring the V3SchedTiming phase could apparently
move away all awaits from a coroutine without adding a co_return
statement.

Signed-off-by: Arkadiusz Kozdra <akozdra@antmicro.com>

src/V3SchedTiming.cpp

@@ -283,6 +283,7 @@ void transformForks(AstNetlist* const netlistp) {
         // STATE
         bool m_inClass = false;  // Are we in a class?
         bool m_beginHasAwaits = false;  // Does the current begin have awaits?
+        bool m_awaitMoved = false;  // Has the current function lost awaits?
         AstFork* m_forkp = nullptr;  // Current fork
         AstCFunc* m_funcp = nullptr;  // Current function
 
@@ -344,7 +345,13 @@ void transformForks(AstNetlist* const netlistp) {
         }
         void visit(AstCFunc* nodep) override {
             m_funcp = nodep;
+            m_awaitMoved = false;
             iterateChildren(nodep);
+            if (nodep->isCoroutine() && m_awaitMoved
+                && !nodep->stmtsp()->exists([](AstCAwait*) { return true; })) {
+                // co_return at the end (either that or a co_await is required in a coroutine
+                nodep->addStmtsp(new AstCStmt{nodep->fileline(), "co_return;\n"});
+            }
             m_funcp = nullptr;
         }
         void visit(AstVar* nodep) override {
@@ -399,6 +406,8 @@ void transformForks(AstNetlist* const netlistp) {
                 if (!m_beginHasAwaits) {
                     // co_return at the end (either that or a co_await is required in a coroutine
                     newfuncp->addStmtsp(new AstCStmt{nodep->fileline(), "co_return;\n"});
+                } else {
+                    m_awaitMoved = true;
                 }
                 remapLocals(newfuncp, callp);
             } else {

test_regress/t/t_fork_join_none_virtual.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 2023 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 => ["--exe --main --timing"],
+    );
+
+execute(
+    check_finished => 1,
+    );
+
+ok(1);
+1;

test_regress/t/t_fork_join_none_virtual.v

@@ -0,0 +1,62 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2024 by Antmicro Ltd.
+// SPDX-License-Identifier: CC0-1.0
+
+event evt1;
+
+typedef enum {ENUM_VALUE} enum_t;
+
+class Foo;
+  int m_member;
+  enum_t m_en;
+
+  virtual task do_something();
+    fork
+      #20 begin
+        m_member++;
+        $display("this's m_member: %0d  m_en: %s", m_member, m_en.name());
+        if (m_member != 3)
+          $stop;
+        ->evt1;
+      end
+      #10 begin
+        m_member++;
+        bar(this);
+      end
+    join_none
+  endtask
+
+  static task bar(Foo foo);
+    fork
+      begin
+        foo.m_member++;
+        $display("foo's m_member: %0d  m_en: %s", foo.m_member, foo.m_en.name());
+        if (foo.m_member != 2)
+          $stop;
+      end
+    join_none
+  endtask
+endclass
+
+class Subfoo extends Foo;
+  virtual task do_something();#5;endtask
+endclass
+
+module t();
+  initial begin
+    Subfoo subfoo;
+    Foo foo;
+    subfoo = new;
+    subfoo.do_something();
+    foo = new;
+    foo.m_member = 0;
+    foo.do_something();
+  end
+
+  always @(evt1) begin
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule