Contact / Report an issue

SystemVerilog Bidirectional Constraints

Bidirectional Constraints

SystemVerilog constraints are solved bidirectionally, which means constraints on all random variables will be solved parallel.

Consider a constraint example,

constraint c_name { if(a == 0) b == 1;
                    else       b == 0; }

We see that 'b' is dependent on 'a'.
but constraint solver see's it as 'a' is dependent on 'b' and 'b' is dependent on 'a'.
i.e if 'b' is inline constrained as '1', in order to satisfy, 'a' should take the value '0'.

As constraints are considered from all the aspects, SystemVerilog constraints are called as bidirectional constraints.

Bidirectional constraint example 

In the example below,

Value of a should be sum of b and c, b should be greater than 6 and c should be less than 8. so in this case constraint solver will choose a value to 'a' in such a way that it should be sum of b and c, also 'b' and 'c' should satisfies their constraint.

class packet;
  rand bit [3:0] a;
  rand bit [3:0] b;
  rand bit [3:0] c;

  constraint a_value { a == b + c; } 
  constraint b_value { b > 6; }
  constraint c_value { c < 8; }

module bidirectional_constr;
  initial begin
    packet pkt;
    pkt = new();
    repeat(5) begin
      $display("Value of a = %0d \tb = %0d \tc =%0d",pkt.a,pkt.b,pkt.c);

 Simulator Output  

Value of a = 8 b = 8 c =0
Value of a = 0 b = 14 c =2
Value of a = 14 b = 14 c =0
Value of a = 6 b = 15 c =7
Value of a = 13 b = 11 c =2
Execute the above code on 


In below example,

Generation of value for b is depending on value of a.
i.e if(a == 0)  b = 1;

this condition can be re-written as,
    if(b == 1)  a = 0;

but there is a constraint for 'a' that value for 'a' should be always '1'. so 'b' should not take value of '1' (to satisfy constraint if(a == 0) b == 1;)

What if we make 'b' value as '1' with inline constraint?
See the below example for the answer.

class packet;
  rand bit a;
  rand bit b;

  constraint a_value { a == 1; } 
  constraint b_value { if(a == 0) b == 1;
                       else       b == 0; }

module bidirectional_const;
  initial begin
    packet pkt;
    pkt = new();
    pkt.randomize() with { b == 1; };
    $display("Value of a = %0d \tb = %0d",pkt.a,pkt.b);

 Simulator Output  

Error-[CNST-CIF] Constraints inconsistency failure, 18
  Constraints are inconsistent and cannot be solved.
  Please check the inconsistent constraints being printed above and rewrite
Execute the above code on