RE: [sv-ec] handling x's in expressions in constrants

Daniel,

In my opinion,
the constraint:
               ( (a/b) == 1) || (b==0)
Is equivalent to:
               If (b != 0) { (a/b) == 1; }

The solver should allow solutions with b == 0.

In the second case, you have effectively have the constraint:
               (a / 0) == 1;
There isn't explicitly an 'x' - rather for any value of 'a' the result of "a / 0" is 'x'. Since 'x' is not allowed, this implies that there is no legal value for 'a'. Thus the randomize result is 0.

Regards,
Ray Ryan

From: owner-sv-ec@eda.org [mailto:owner-sv-ec@eda.org] On Behalf Of Daniel Mlynek
Sent: Wednesday, November 16, 2011 4:35 AM
To: sv-ec@eda.org
Subject: [sv-ec] handling x's in expressions in constrants

I've doubts on how x's should be handled by solver. LRM say only that x's are illegal in constraints but give no more detail on complex expr behaviour

Please see examples - my questions are in comments
class C;
 rand reg[1:0] a,b;
 constraint c1{(a/b==1) || (b==0);};
 function void post_randomize;
   $display("%d %d",a,b);
 endfunction
endclass

module top;
  C c =new;
  initial begin
    repeat (15)//should solver allow b==0 to be randomized or should not (bcos of x result in 2nd part of expr - a/b==1)??
      assert(c.randomize) else $display("failure");
    c.b=0;
    repeat (15) //should fail immediately bcos of x's or only x's on top level expr should results in runtime failure???
      assert(c.randomize(a)) else $display("failure");
  end
endmodule

DANiel

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