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SystemVerilog Constraints Examples

Here’s a cheatsheet of SystemVerilog constraints patterns. Some are from the LRM and other are code examples of what I’ve used.

Set Membership

/* using other variables as a part of a constraint */
rand integer x, y, z;
constraint c1 {x inside {3, 5, [9:15], [24:32], [y:2*y], z};}

rand integer a, b, c;
constraint c2 {a inside {b, c};}

/* constraint a variable to values from an array */
integer fives[4] = '{ 5, 10, 15, 20 }; rand integer v;
constraint c3 { v inside {fives}; }

Distribution

The := operator assigns the specified weight to the item or, if the item is a range, to every value in the range. The :/ operator assigns the specified weight to the item or, if the item is a range, to the range as a whole. If there are n values in the range, the weight of each value is range_weight / n.

/* 
 * x is equal to 100, 101, 102, 200, or 300 with a 
 * weighted ratio of 1-1-1-2-5
 * i.e., probability of (x=100) is 1/10 and that of (x=300) is 5/10 
 */
x dist { [100:102] := 1, 200 := 2, 300 := 5};

/* x is equal to one of 100, 101, 102, 200, or 300 with 
 * a weighted ratio of 1/3-1/3-1/3-2-5
 */
x dist { [100:102] :/ 1, 200 := 2, 300 := 5}

Using foreach within a constraint

In this example

  • An IP packet is constructed
  • hdr_chain is an array and the constraint iterates over the array using a foreach loop
  • Each array entry is randomized depending upon what the previous entry was randomized to
constraint ip_pkt_cnstr {
  foreach (hdr_chain[i]) {
    if (i == 0) {
      hdr_chain[i] == ETHERNET;
    } else {
      if (hdr_chain[i-1] == ETHERNET) {
        hdr_chain[i] inside {VLAN, IPV4, IPV6};
      } else if(hdr_chain[i-1] == VLAN) {
        hdr_chain[i] inside {IPV4, IPV6};
      } else if(hdr_chain[i-1] inside {IPV4, IPV6}) {
        hdr_chain[i] inside {TCP, UDP};
      } else if(hdr_chain[i-1] inside {TCP, UDP}) {
        hdr_chain[i] inside {PAYLOAD};
      } else if(hdr_chain[i-1] == PAYLOAD) {
        hdr_chain[i] inside {END};
      } else {
        hdr_chain[i] inside {END};
      }
    }
  }
}