Fix error on empty generate with -O0 (#5250).

Changes

@@ -22,6 +22,7 @@ Verilator 5.027 devel
 * Fix unknown conversion on queues (#5220). [Alex Solomatnikov]
 * Fix top-level unpacked structure resets (#5221).
 * Fix concurrency for mailbox and semaphores (#5222). [Liam Braun]
+* Fix error on empty generate with -O0 (#5250).
 
 
 Verilator 5.026 2024-06-15

src/V3Unroll.cpp

@@ -356,7 +356,11 @@ class UnrollVisitor final : public VNVisitor {
             }
         }
         if (!newbodysp) {  // initp might have effects after the loop
-            newbodysp = initp;  // Maybe nullptr
+            if (m_generate && initp) {  // GENFOR(ASSIGN(...)) need to move under a new Initial
+                newbodysp = new AstInitial{initp->fileline(), initp};
+            } else {
+                newbodysp = initp;  // Maybe nullptr
+            }
             initp = nullptr;
         }
         // Replace the FOR()

test_regress/t/t_genfor_init_o0.pl

@@ -0,0 +1,20 @@
+#!/usr/bin/env perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# 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
+
+scenarios(vlt => 1);
+
+compile(
+    verilator_flags2 => ['-O0'],
+    make_main => 0,
+    verilator_make_gmake => 0,
+    );
+
+ok(1);
+1;

test_regress/t/t_genfor_init_o0.v

@@ -0,0 +1,10 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2024 by Wilson Snyder.
+// SPDX-License-Identifier: CC0-1.0
+
+module t(/*AUTOARG*/);
+   genvar i;
+   for (i = 0; i < 0; i = i + 1) begin end
+endmodule