SystemVerilog Inline Constraints

Consider that a class already has well-written constraints, and there's a need to randomize the class variables with a different set of constraints, decided by the user. Using the with construct, you can declare inline constraints at the point randomize() is called. These additional constraints are considered alongside the object's original constraints by the solver — not instead of them.

10 min read | Intermediate Level

What You'll Learn

• What an inline constraint is, and why it's applied on top of a class's existing constraints rather than replacing them
• What happens when an inline constraint and an existing class constraint are genuinely incompatible
• Why inline constraints can never override an existing constraint, only add to it
• When constraint_mode() is the right tool instead, if a constraint actually needs to be replaced rather than added to

Basic Inline Constraint

class Item;
  rand bit [7:0] id;
  
  constraint c_id { id < 25; }
  
endclass

module tb;
  
  initial begin
    Item itm = new ();
    itm.randomize() with { id == 10; }; 		// In-line constraint using with construct
    $display ("Item Id = %0d", itm.id);
  end
endmodule

The inline constraint has been applied here, and the randomized id is 10:

run -all;
# KERNEL: Item Id = 10
# KERNEL: Simulation has finished. There are no more test vectors to simulate.

This works because id == 10 and id < 25 are both true at once — the solver has no trouble satisfying both simultaneously.

When Inline and Class Constraints Conflict

If the original constraint fixes id to a single value, and the inline constraint asks for something incompatible, randomization fails:

class Item;
  rand bit [7:0] id;
  
  constraint c_id { id == 25; }
endclass

module tb;
  initial begin
    Item itm = new ();
    if (! itm.randomize() with { id < 10; })
    	$display ("Randomization failed");
    $display ("Item Id = %0d", itm.id);
  end
endmodule

Output

ncsim> run
    if (! itm.randomize() with { id < 10; })
                      |
ncsim: *W,SVRNDF (./testbench.sv,10|22): The randomize method call failed.
Observed simulation time : 0 FS + 0
ncsim: *W,RNDOCS: These constraints contribute to the set of conflicting constraints:

  constraint c_id { id == 25; }; (./testbench.sv,4)
    if (! itm.randomize() with { id < 10; }) (./testbench.sv,10)
ncsim: *W,RNDOCS: These variables contribute to the set of conflicting constraints:

rand variables:
       id [./testbench.sv, 2]

Randomization failed
Item Id = 0
ncsim: *W,RNQUIE: Simulation is complete.

id can't simultaneously equal exactly 25 and be less than 10 — the solver correctly reports failure, and id is left at its stale default value, 0, rather than any value that actually satisfied either constraint.

The same failure happens even when both sides try to pin id to specific, individually valid values:

...
if (! itm.randomize() with { id == 10; })
...

Output

ncsim> run
    if (! itm.randomize() with { id == 10; })
                      |
ncsim: *W,SVRNDF (./testbench.sv,10|22): The randomize method call failed.
Observed simulation time : 0 FS + 0
ncsim: *W,RNDOCS: These constraints contribute to the set of conflicting constraints:

  constraint c_id { id == 25; }; (./testbench.sv,4)
    if (! itm.randomize() with { id == 10; }) (./testbench.sv,10)
ncsim: *W,RNDOCS: These variables contribute to the set of conflicting constraints:

rand variables:
       id [./testbench.sv, 2]

Randomization failed
Item Id = 0
ncsim: *W,RNQUIE: Simulation is complete.

Even though id == 10 would be a perfectly legal value all on its own, c_id still requires exactly 25 — and a variable can't equal two different specific values at the same time.

Inline Constraints Add To, Never Replace

The takeaway from both failing examples: constraints provided inline don't override the class's original constraints — they're considered alongside them by the solver, as an addition, not a replacement. If a class constraint is genuinely incompatible with what a specific test needs, an inline constraint can never fix that on its own; the original constraint has to be disabled first, using constraint_mode():

// If a test genuinely needs to REPLACE c_id rather than add to it:
itm.c_id.constraint_mode(0);              // disable the original constraint first
itm.randomize() with { id < 10; };        // now this alone governs id
itm.c_id.constraint_mode(1);              // re-enable it afterward, if needed

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