SystemVerilog Randomization II

// Day 29 advanced constraints. IEEE 1800 SystemVerilog; full constraint solver required.
class packet;
  rand bit [7:0] addr;
  rand bit [7:0] payload;
  rand bit write_enable;
  rand bit [1:0] kind;
  rand bit [3:0] values[];
  rand bit [3:0] prio;
  bit error_mode = 0;

  constraint size_c { values.size() inside {[3:5]}; }
  constraint address_c {
    if (write_enable) addr inside {[8'h20:8'h3f]};
    else addr inside {[8'h00:8'h1f]};
  }
  constraint implication_c { (kind == 2) -> (payload == 8'hff); }
  constraint weighted_c { kind dist {0 := 6, 1 := 3, 2 := 1}; }
  constraint ordering_c {
    foreach (values[i]) {
      values[i] inside {[0:15]};
      if (i > 0) values[i] > values[i-1];
    }
  }
  constraint preferred_c { soft prio == 4'd3; }
  constraint legal_c {
    if (!error_mode) payload != 8'h00;
    else payload == 8'h00;
  }
  // Demonstrate solve-before: this influences distribution, not correctness.
  constraint solve_order_c { solve write_enable before addr; }

  function void post_randomize();
    if (values.size() < 3 || values.size() > 5)
      $fatal(1, "size outside policy");
  endfunction
endclass

class restricted_packet extends packet;
  constraint subclass_c { prio inside {[1:7]}; addr != 8'h00; }
endclass

module tb_advanced_constraints;
  packet p;
  restricted_packet q;
  int failures;
  initial begin
    p = new();
    for (int i = 0; i < 100; i++) begin
      if (!p.randomize()) $fatal(1, "legal randomize failed at %0d", i);
      if (p.write_enable && !(p.addr inside {[8'h20:8'h3f]}))
        $fatal(1, "write address illegal");
      if (!p.write_enable && !(p.addr inside {[8'h00:8'h1f]}))
        $fatal(1, "read address illegal");
      if (p.kind == 2 && p.payload != 8'hff) $fatal(1, "implication broken");
      if (p.payload == 0) $fatal(1, "legal profile produced error payload");
      foreach (p.values[j])
        if (j > 0 && p.values[j] <= p.values[j-1])
          $fatal(1, "array not strictly ordered");
    end
    p.error_mode = 1;
    // kind=2 forces payload FF, conflicting with error_mode payload 00.
    if (p.randomize() with { kind == 2; })
      $fatal(1, "expected UNSAT for conflicting illegal profile");
    // Error traffic can be generated when kind is not 2.
    if (!p.randomize() with { kind == 0; }) $fatal(1, "error profile UNSAT");
    if (p.payload != 0) $fatal(1, "error profile incorrect");
    p.error_mode = 0;
    p.preferred_c.constraint_mode(0);
    if (!p.randomize() with { prio == 9; })
      $fatal(1, "disabled soft preference should allow override");
    p.preferred_c.constraint_mode(1);
    p.prio.rand_mode(0);
    p.prio = 3;
    if (!p.randomize()) $fatal(1, "rand_mode test failed");
    if (p.prio != 3) $fatal(1, "rand_mode changed fixed field");
    p.prio.rand_mode(1);
    q = new();
    if (!q.randomize()) $fatal(1, "inherited constraint UNSAT");
    if (q.prio < 1 || q.prio > 7) $fatal(1, "inheritance violation");
    $display("DAY29_PASS advanced constraints");
    $finish;
  end
endmodule

// Five expected randomization failures: a failure to randomize is a PASS for each case.
class failure_case;
  rand int unsigned x, y;
  constraint base_c { x inside {[0:10]}; y inside {[0:10]}; }
endclass
module tb_unsat_cases;
  failure_case p;
  int detected = 0;
  initial begin
    p = new();
    if (!p.randomize() with { x > 10; }) detected++; else $fatal(1,"case1 not detected");
    if (!p.randomize() with { x < 0; }) detected++; else $fatal(1,"case2 not detected");
    if (!p.randomize() with { x == 2; x == 3; }) detected++; else $fatal(1,"case3 not detected");
    if (!p.randomize() with { x > y; y > x; }) detected++; else $fatal(1,"case4 not detected");
    if (!p.randomize() with { x == 11; y == 11; }) detected++; else $fatal(1,"case5 not detected");
    if (detected != 5) $fatal(1,"not all failures detected");
    $display("DAY29_UNSAT_PASS %0d cases", detected);
    $finish;
  end
endmodule