Wrap forked processes in begins early (#6343) (#6354)

Signed-off-by: Krzysztof Bieganski <kbieganski@antmicro.com>

src/V3AstNodes.cpp

@@ -3232,7 +3232,8 @@ AstAlways* AstAssignW::convertToAlways() {
     if (hasTimingControl) {
         // If there's a timing control, put the assignment in a fork..join_none. This process won't
         // get marked as suspendable and thus will be scheduled normally
-        AstFork* forkp = new AstFork{flp, "", bodysp};
+        AstBegin* const beginp = new AstBegin{flp, "", bodysp};
+        AstFork* const forkp = new AstFork{flp, "", beginp};
         forkp->joinType(VJoinType::JOIN_NONE);
         bodysp = forkp;
     }

src/V3Fork.cpp

@@ -459,10 +459,11 @@ class DynScopeVisitor final : public VNVisitor {
                 })) {
             nodep->user2(true);
             // Put it in a fork to prevent lifetime issues with the local
-            AstFork* const forkp = new AstFork{nodep->fileline(), "", nullptr};
+            AstBegin* const beginp = new AstBegin{nodep->fileline(), "", nullptr};
+            AstFork* const forkp = new AstFork{nodep->fileline(), "", beginp};
             forkp->joinType(VJoinType::JOIN_NONE);
             nodep->replaceWith(forkp);
-            forkp->addStmtsp(nodep);
+            beginp->addStmtsp(nodep);
             UINFO(9, "assign new fork " << forkp);
         } else {
             visit(static_cast<AstNodeStmt*>(nodep));

src/V3Timing.cpp

@@ -1030,7 +1030,10 @@ class TimingControlVisitor final : public VNVisitor {
                 if (!controlp) controlp = nbaEventControlp;
             }
             controlp->replaceWith(forkp);
-            forkp->addStmtsp(controlp);
+            AstBegin* beginp = VN_CAST(controlp, Begin);
+            if (!beginp) beginp = new AstBegin{nodep->fileline(), "", controlp};
+            forkp->addStmtsp(beginp);
+            controlp = forkp;
         }
         UASSERT_OBJ(nodep, controlp, "Assignment should have timing control");
         addCLocalScope(flp, insertBeforep);
@@ -1225,14 +1228,9 @@ class TimingControlVisitor final : public VNVisitor {
         AstNode* stmtp = nodep->stmtsp();
         // Put each statement in a begin
         while (stmtp) {
-            if (!VN_IS(stmtp, Begin)) {
-                auto* const beginp = new AstBegin{stmtp->fileline(), "", nullptr};
-                if (hasFlags(stmtp, T_HAS_PROC)) addFlags(beginp, T_HAS_PROC);
-                stmtp->replaceWith(beginp);
-                beginp->addStmtsp(stmtp);
-                stmtp = beginp;
-            }
-            auto* const beginp = VN_AS(stmtp, Begin);
+            UASSERT_OBJ(VN_IS(stmtp, Begin), nodep,
+                        "All statements under forks must be begins at this point");
+            AstBegin* const beginp = VN_AS(stmtp, Begin);
             stmtp = beginp->nextp();
             iterate(beginp);
             // Even if we do not find any awaits, we cannot simply inline the process here, as new

test_regress/t/t_timing_intra_assign_func.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2025 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()
+
+test.passes()

test_regress/t/t_timing_intra_assign_func.v

@@ -0,0 +1,26 @@
+// 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
+
+module t;
+  reg [3:0] ia = 4'd1;
+  wire signed [3:0] iufunc;
+
+  // verilator lint_off WIDTH
+  assign #1 iufunc = int_func(ia);
+  // verilator lint_on WIDTH
+
+  function [31:0] int_func;
+    input [31:0] in;
+    int_func = in * 2;
+  endfunction
+
+  initial begin
+      #1
+      if (iufunc != 4'd2) $stop;
+      $write("*-* All Finished *-*\n");
+      $finish;
+  end
+endmodule