randomizing 32 bit value in systemverilog with xilinx vivado 2018.2

Question

I have written a test bench for my parameterized design in which I need to randomize the input. I got very surprised when I found out that if I run the following code, I get a nice random number for my 16 bit val:

val = $urandom_range(16'hffff, 0);

I use the following code to test my design with a 32 bit number (val[31:0]):

val = $urandom_range(32'hffff_ffff, 0);

however, in this case, I always get either 0 or 32'hffff_ffff. I don't know if this is a known bug in xilinx tools. I reduced my test code to the following and I still get same result:

initial begin
    static print_verbosity verbosity = VERB_LOW;
    static int unsigned num_tests = 1000;
    int unsigned val;
    for (int test_id=0; test_id<num_tests; test_id++) begin
        val = $urandom_range(32'hffff_ffff, 0);
        `test_print("INFO", $sformatf("val=%0d", val), verbosity)
    end
    $finish();
end  

and the output I get is the following pattern:

[ INFO]  val=0 
[ INFO]  val=4294967295 
[ INFO]  val=4294967295 
[ INFO]  val=0 
[ INFO]  val=4294967295 
[ INFO]  val=0 
[ INFO]  val=4294967295 
[ INFO]  val=0 

I changed the upper boundary for urandom_range as follow:

    val = $urandom_range(32'hefff_ffff, 0);

This time I got random numbers. I am wondering if someone can help me to find out where I am making mistake. I am using the latest Vivado 2018.2 simulator.

Answer 1

Confirmed using Vivado 2017.2:

module tst_urandom;
reg [31:0] numb1,numb2,numb3;
integer i;

initial
begin
  for (i=0; i<20; i=i+1)
  begin
    numb1 = $urandom_range(32'hFFFFFFFF,0);
    numb2 = $urandom_range(32'hFFFFFFFE,1);
    numb3 = $urandom();
    #10;
    $display("0x%08X  0x%08X  0x%08X",numb1,numb2,numb3);
  end
end

endmodule

Produces:

Vivado Simulator 2017.2
Time resolution is 1 ps
0x00000000  0xffffffff  0x8484d609
0xffffffff  0x00000000  0x46df998d
0xffffffff  0xfffffffe  0x00f3e301
0x00000000  0x00000000  0x1e8dcd3d
0x00000000  0x00000001  0x7cfde9f9
0xffffffff  0xffffffff  0xd513d2aa
0x00000000  0xfffffffe  0x8932d612
0x00000000  0x00000001  0xe77696ce
0xffffffff  0xffffffff  0x2e58495c
0xffffffff  0xfffffffe  0xb2a72665
0xffffffff  0x00000000  0xc03b2280
0x00000000  0x00000001  0xcecccc9d
0xffffffff  0xfffffffe  0x86bc380d
0xffffffff  0x00000000  0xeaa62ad5
0xffffffff  0xffffffff  0x0effe91d
0xffffffff  0x00000000  0x0509650a

I think you should file a bug report.