Consider that a class already has well written constraints and there is a need to randomize the class variables with a set of different constraints decided by the user. By using the with construct, users can declare in-line constraints at the point where the randomize() method is called. These additional constraints will be considered along with the object's original constraints by the solver.
Example
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
Notice that inline constraint has been applied here and hence the randomized id is 10.
run -all; # KERNEL: Item Id = 10 # KERNEL: Simulation has finished. There are no more test vectors to simulate.
If the original constraint c_id is fixed to 25 as follows and we provide a conflicting in-line value, then the randomization will fail.
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
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.
For the next case, we'll see what happens when the original constraint is fixed and the in-line value also tries to fix the variable to another value.
...
if (! itm.randomize() with { id == 10; })
...
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.
The takeaway here is that constraints provided should not conflict with each other and in-line method of providing constraints does not override but instead is also considered along with the original by the solver.
