From d55790c91b0f91e73f5cee63a0af4af847939b91 Mon Sep 17 00:00:00 2001
From: Wilson Snyder <wsnyder@wsnyder.org>
Date: Sun, 29 Mar 2026 20:52:29 -0400
Subject: [PATCH] Tests: Add t_interface_update (#2765)

---
 test_regress/t/t_gen_for1.v          |  4 ++
 test_regress/t/t_interface_update.py | 18 +++++++++
 test_regress/t/t_interface_update.v  | 56 ++++++++++++++++++++++++++++
 3 files changed, 78 insertions(+)
 create mode 100755 test_regress/t/t_interface_update.py
 create mode 100644 test_regress/t/t_interface_update.v

diff --git a/test_regress/t/t_gen_for1.v b/test_regress/t/t_gen_for1.v
index 99e71f26e..276571dc7 100644
--- a/test_regress/t/t_gen_for1.v
+++ b/test_regress/t/t_gen_for1.v
@@ -61,6 +61,7 @@ module Testit (
           .clk(clk),
           .w(d[i])
       );
+      initial fnxtclk1.init();
     end
   endgenerate
 
@@ -90,4 +91,7 @@ module fnxtclk (
     end
   end
 
+  task init;
+  endtask
+
 endmodule
diff --git a/test_regress/t/t_interface_update.py b/test_regress/t/t_interface_update.py
new file mode 100755
index 000000000..9c3430fc9
--- /dev/null
+++ b/test_regress/t/t_interface_update.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_st')
+
+test.compile()
+
+#Nothing to test: test.execute()
+
+test.passes()
diff --git a/test_regress/t/t_interface_update.v b/test_regress/t/t_interface_update.v
new file mode 100644
index 000000000..a8d7157b4
--- /dev/null
+++ b/test_regress/t/t_interface_update.v
@@ -0,0 +1,56 @@
+// 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
+
+interface ShiftIf #(
+    parameter integer WIDTH,
+    parameter integer STAGE
+);
+  typedef struct packed {
+    logic [WIDTH-1:0] data;
+    logic valid;
+  } reg_t;
+
+  reg_t data;
+
+  function automatic reg_t update(input reg_t noChange, input logic enable);
+    return enable == 1 ? data : noChange;
+  endfunction
+endinterface
+
+module automatic t #(
+    parameter integer WIDTH = 8,
+    parameter integer SHIFT_WIDTH = 2
+) (
+    input logic logic_clk_in,
+    input logic sync_rst_in,
+    input logic [WIDTH-1:0] dataIn,
+    input logic validIn,
+    input logic enableIn,
+    output logic [WIDTH-1:0] dataOut,
+    output logic validOut
+);
+
+  ShiftIf #(WIDTH, 0) shift[SHIFT_WIDTH:0] ();
+  ShiftIf #(WIDTH, 1) shiftTmp1 ();
+  ShiftIf #(WIDTH, 2) shiftTmp2 ();
+
+  always_comb begin
+    shift[SHIFT_WIDTH].data.data = dataIn;
+    shift[SHIFT_WIDTH].data.valid = validIn;
+  end
+
+  for (genvar i = 0; i < SHIFT_WIDTH; ++i) begin
+    ShiftIf #(WIDTH, i) newValue ();
+    always_comb newValue.data = shift[i+1].data;
+
+    always_ff @(posedge logic_clk_in) shift[i].data = newValue.update(shift[i].data, enableIn);
+
+  end
+
+  assign dataOut = shift[0].data.data;
+  assign validOut = shift[0].data.valid;
+
+endmodule