verilator/test_regress/t/t_comb_input_2.v

// DESCRIPTION: Verilator: Verilog Test module
//
// Copyright 2022 by Geza Lore. 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

`ifdef VERILATOR
// The '$c1(1)' is there to prevent inlining of the signal by V3Gate
`define IMPURE_ONE ($c(1))
`else
// Use standard $random (chaces of getting 2 consecutive zeroes is zero).
`define IMPURE_ONE (|($random | $random))
`endif

module top(
  clk,
  inc
);

  input clk;
  input [31:0] inc;

  // Cycle count
  reg [31:0] cyc = 0;

  /* verilator lint_off UNOPTFLAT */
  // Circular combinational logic driven from primary input, but with the
  // cycle itself not involving the primary input
  wire [31:0] dup = `IMPURE_ONE ? inc : 32'd0;
  wire [31:0] feedback;
  wire [31:0] sum = cyc + dup + feedback;
  wire msb = sum[31];  // Always 0, but Verilator cannot know that
  assign feedback = {32{msb}};


  always @(posedge clk) begin
    $display("cyc: %d sum: %d", cyc, sum);
    if (sum != 2*cyc + 1) $stop;
    cyc <= cyc + 1;
    if (cyc == 100) begin
      $write("*-* All Finished *-*\n");
      $finish;
    end
  end

endmodule