Fix timed nested fork block with disable (#6720) (#7743)

Fixes #6720.

src/V3LinkJump.cpp

@@ -262,6 +262,9 @@ class LinkJumpVisitor final : public VNVisitor {
         if (it != m_beginDisableBegins.end()) return it->second;
 
         AstBegin* const beginBodyp = new AstBegin{fl, "", nullptr, false};
+        // Disable-by-name rewrites kill this detached block-body process, so mark it as process
+        // backed to ensure fork/join kill-accounting hooks are always emitted.
+        beginBodyp->setNeedProcess();
         if (beginp->stmtsp()) beginBodyp->addStmtsp(beginp->stmtsp()->unlinkFrBackWithNext());
 
         AstFork* const forkp = new AstFork{fl, VJoinType::JOIN};

test_regress/t/t_disable_fork_nested.out

@@ -0,0 +1,3 @@
+Fast clock (200ns half-period): o_counter=7
+Slow clock (5400ns half-period): o_counter=3
+*-* All Finished *-*

test_regress/t/t_disable_fork_nested.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# 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", "-Wno-ZERODLY"])
+
+test.execute(expect_filename=test.golden_filename)
+
+test.passes()

test_regress/t/t_disable_fork_nested.v

@@ -0,0 +1,86 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain.
+// SPDX-FileCopyrightText: 2026 Wilson Snyder
+// SPDX-License-Identifier: CC0-1.0
+
+// Clock-period detector exercising a named-block `disable` of a block whose
+// body contains a nested fork..join. Disabling such a block from a sibling
+// process must release the enclosing fork..join so the surrounding `always`
+// block keeps iterating.
+
+module disable_fork (
+    input  logic i_clk,
+    output logic [2:0] o_counter
+);
+   time delay1 = 500ns;  // min period
+   time delay2 = 3333ns;  // max period
+
+   logic clk_re = 1'b0;  // rising edge of the clock
+   logic [2:0] counter = 3'b000;
+
+   always begin
+      fork
+         begin : check1
+            #delay1;
+            #1 disable check2;
+            fork
+               begin : check3
+                  #(delay2 - delay1);
+                  clk_re <= 1'b0;
+                  #1 disable check4;
+                  if (counter < 3'b111) counter <= counter + 3'b001;
+               end
+               begin : check4
+                  @(posedge i_clk);
+                  clk_re <= 1'b1;
+                  counter <= 3'b000;
+                  #1 disable check3;
+               end
+            join
+         end
+         begin : check2
+            @(posedge i_clk);
+            clk_re <= 1'b0;
+            #1 disable check1;
+            if (counter < 3'b111) counter <= counter + 3'b001;
+         end
+      join
+   end
+
+   assign o_counter = counter;
+endmodule
+
+// verilog_format: off
+`define stop $stop
+`define checkh(gotv,expv) do if ((gotv) !== (expv)) begin $write("%%Error: %s:%0d:  got=%0x exp=%0x (%s !== %s)\n", `__FILE__,`__LINE__, (gotv), (expv), `"gotv`", `"expv`"); `stop; end while(0);
+// verilog_format: on
+
+module t;
+   logic clk;
+   logic [2:0] counter;
+
+   task clk_cycle(input time half_period);
+      clk = 1'b1;
+      #half_period;
+      clk = 1'b0;
+      #half_period;
+   endtask : clk_cycle
+
+   initial begin
+      // Fast clock (period below delay1): every edge arrives before the
+      // min-period timeout, so the counter saturates at its max.
+      repeat (100) clk_cycle(200ns);
+      $display("Fast clock (200ns half-period): o_counter=%0d", counter);
+      `checkh(counter, 3'h7);
+      // Slow clock (period above delay2): the nested fork path runs, which
+      // only works if disabling check1 releases the inner fork..join.
+      repeat (100) clk_cycle(5400ns);
+      $display("Slow clock (5400ns half-period): o_counter=%0d", counter);
+      `checkh(counter, 3'h3);
+      $write("*-* All Finished *-*\n");
+      $finish;
+   end
+
+   disable_fork a_inst(.i_clk(clk), .o_counter(counter));
+endmodule