"solve before" construct

UVM
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1

Hello,

“solve A before B” means, using the existing constrains get a random value for A and then get a random value for B. - I hope that I am correct on this one.
However, what if there is one more variable C? Is it solved with A or is it solved with B?

My understanding is that the simulator creates a table of all possible combinations of A,B,C, e.t.c. and after that chooses one line from the table.

If “A is solved before B” then:

  1. column of variable B is removed from the table and the simulator picks a line
  2. A is determined → a new table is constructed with all possible values for B

I am not sure though if variable C is present in the table from item 1 or the table contains just variable A.

Thanks,
Alex

2

In reply to Alex K.:

Wanted to rephrase the question:
bit A;
bit[1:0] B, C, D;

constraint my_cnstr_1 {
(A == 1) → (B == 3); }

constraint my_cnstr_2 {
(A == 0) → (D == 0);}

constraint my_cnstr_3 {
(D == 1) → (C == 0); }

solve A before B;
solve A before C;

A is randomized first.
After that based on the result of A, the values for B and C are chosen randomly.

But how about D? Is it randomized with B and C? Or with A?
my_cnstr_2 and my_cnstr_3 will have a different meaning based on when D is randomized.
Is it up to the simulator?

Thanks,
Alex

3

In reply to Alex K.:

It always helps to provide an example. As soon as you add a constraint connecting two variables.

The LRM says “Variables that are not explicitly ordered shall be solved with the last set of ordered variables”. So a value gets picked for A, then B,C, and D are picked together.

Note that solve before has no affect on the solution, just the distribution of values picked. Please see constrain random variable result to enumerated type | Verification Academy

4

In reply to dave_59:

Thank you, Dave.

Does the above mean that the randomization is done in 2 stages?

  1. Split all the variables into groups, when in every group the variables are randomized together (defined by “solved before”).
  2. The groups are randomized sequentially. For every group the randomization is done simultaneously.

Thanks,
Alex