randomize with constraint fail.

Question

Hi.

I'm trying to randomize with constrainted variable of Systemverilog as the below

class C;
  rand int q[];
  rand int wsize;
  randc int index;
 
  //int queue[$urandom_range(queue.size()-1)];
  constraint select {
  wsize inside {[0:10]};
    q.size() == wsize;
  index inside{[0:q.size()-1]};                                       
  }
 
endclass                                                                 
 
module DA;
  initial begin
      C c; 
      c = new();                                                        
 
    if(c.randomize())
      $info("randc success");                                           
    else
      $error("fail");
 
    foreach(c.q[i])
      $display("Randc:%0d", c.q[i]);
 
 
  end
endmodule

But I keep faced fail, How do I success the randomization?

#  vsim -voptargs=+acc=npr
# run -all
# testbench.sv(49): randomize() failed due to conflicts between the following constraints:
# 	testbench.sv(11): select { (wsize inside { [0:10] }); }
# 	testbench.sv(12): select { (q.size == wsize); }
# 	testbench.sv(13): select { (index inside { [0:(q.size - 1)] }); }
# Where:
# 	index = 663679149 /* random  */
# Given:
# 	bit signed [31:0] q.size
# 	bit signed [31:0] wsize
# ** Note: (vsim-7130) Enabling enhanced debug (-solvefaildebug=2) may generate a more descriptive constraint contradiction report and -solvefaildebug testcase.
# ** Note: (vsim-7106) Use vsim option '-solvefailtestcase[=filename]' to generate a simplified testcase that will reproduce the failure.
# ** Warning: (vsim-7084) No solutions exist which satisfy the specified constraints; randomize() failed.
# 
#    Time: 0 ns  Iteration: 0  Process: /DA/#INITIAL#22(#ublk#1153#22) File: testbench.sv Line: 49
# ** Error: fail

Answer 1

Solution

cgales

Forum Moderator

1986 posts

December 05, 2022 at 4:46 am

In reply to UVM_LOVE:

The state space of randc variables is based on the variable type. In this case, you have a 32bit variable which has a huge randomization space for randc to try and work within.

Also, from the LRM:
The semantics of random-cyclical variables requires that they be solved before other random variables. A set of constraints that includes both rand and randc variables shall be solved so that the randc variables are solved first, and this can sometimes cause randomize() to fail.

You should size your variables to match the desired range of variables.

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