Fix disable fork and wait fork affecting pending NBA updates (#8483)

src/V3Timing.cpp

@@ -88,6 +88,7 @@ enum NodeFlag : uint8_t {
     T_FORCES_PROC = 1 << 3,  // Forces VlProcess allocation
     T_NEEDS_PROC = 1 << 4,  // Needs access to VlProcess if it's allocated
     T_HAS_PROC = 1 << 5,  // Has VlProcess argument in the signature
+    T_NBA_UPDATE = 1 << 6,  // Fork branch of a pending NBA update, which is not a subprocess
 };
 
 enum ForkType : uint8_t {
@@ -1222,6 +1223,7 @@ class TimingControlVisitor final : public VNVisitor {
             AstBegin* beginp = VN_CAST(controlp, Begin);
             if (!beginp) beginp = new AstBegin{nodep->fileline(), "", controlp, false};
             forkp->addForksp(beginp);
+            addFlags(beginp, T_NBA_UPDATE);
             controlp = forkp;
         }
         UASSERT_OBJ(nodep, controlp, "Assignment should have timing control");
@@ -1438,6 +1440,9 @@ class TimingControlVisitor final : public VNVisitor {
         VL_RESTORER(m_procp);
         VL_RESTORER(m_hasProcess);
         m_hasProcess |= hasFlags(nodep, T_HAS_PROC);
+        // A pending NBA update is not a subprocess of the process that scheduled it, so it is
+        // unaffected by 'disable fork' and 'wait fork' (IEEE 1800-2023 9.6.1, 9.6.3)
+        if (hasFlags(nodep, T_NBA_UPDATE)) m_hasProcess = false;
         m_procp = nodep;
         if (m_hasProcess) nodep->setNeedProcess();
         iterateChildren(nodep);

test_regress/t/t_disable_fork_nba.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Pending NBA updates are not subprocesses for disable/wait fork
+#
+# 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('simulator')
+
+test.compile(verilator_flags2=["--binary"])
+
+test.execute()
+
+test.passes()

test_regress/t/t_disable_fork_nba.v

@@ -0,0 +1,61 @@
+// DESCRIPTION: Verilator: Pending NBA updates are not subprocesses for disable/wait fork
+//
+// 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 checkd(gotv,expv) do if ((gotv) !== (expv)) begin $write("%%Error: %s:%0d:  got=%0d exp=%0d (%s !== %s)\n", `__FILE__,`__LINE__, (gotv), (expv), `"gotv`", `"expv`"); `stop; end while(0);
+// verilog_format: on
+
+interface bus_if;
+  int data;
+endinterface
+
+class Cls;
+  virtual bus_if vif;
+  task set(int v);
+    vif.data <= v;
+  endtask
+endclass
+
+module t;
+  bit clk;
+  int dly_value;
+  int wait_value;
+  bit [1:0] drive_value;
+  bus_if bus ();
+  Cls obj = new;
+
+  always #5 clk = ~clk;
+
+  default clocking cb @(posedge clk);
+    output drive_value;
+  endclocking
+
+  // 'disable fork' does not cancel pending updates (IEEE 1800-2023 9.6.3)
+  initial begin
+    obj.vif = bus;
+    dly_value <= #2 1;
+    obj.set(2);
+    cb.drive_value <= ##1 3;
+    disable fork;
+    #20;
+    `checkd(dly_value, 1)
+    `checkd(bus.data, 2)
+    `checkd(drive_value, 3)
+  end
+
+  // 'wait fork' does not wait for pending updates (IEEE 1800-2023 9.6.1)
+  initial begin
+    #30;
+    wait_value <= #5 4;
+    wait fork;
+    `checkd($time, 30)
+    #10;
+    `checkd(wait_value, 4)
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule