With one of our recent blog entries on SystemVerilog constraint solver (http://www.cvcblr.com/blog/?p=725) becoming so popular, several readers have contacted us via email to know little more about the puzzle. Specifically they wanted to understand how the solver ordering of variables is determined. Consider the same example as in that previous blog entry:

As noted in the previous blog, this creates an “implicit ordering” of variables – i.e. ‘v1” is solved BEFORE “v2”. A smart engineer (Muthurasu Sivaramakrishnan) asked this:

*Nice one. However, why cant we use Solve.. Before constraint in this scenario?*

The answer is a little involved with yet-another subtlety in the language, and hence this new entry:

This reader’s question boils down to whether the above constraint “** cst_ordered”** is same as the following;

constraint cst_guidance {solve v1 before v2;}

First intuition says YES, but the answer unfortunately is NO. In SV there are 2 kinds of solver ordering - an ordering constraint is more of a "guidance on probability" and does NOT change the solution space. Hence it can't lead to a failure from a success or vice-versa. This is what happens with a ** solve..before – **i.e. it is simply a “guidance” or suggestion to the solver.

However the ordering that gets enforced via function call is more **STRICT/DICTATIVE** in nature. It enforces the order by further "subdividing" the solution space and in a sense invokes the solver twice. In Questa you can actually see this in action via -solveverbose - you will see 2 "Working Set" prints for function based constraint:

1. First the solver gets “dictated” to solve “v1” INDEPENDENTLY. In a random choice, say it picked a value “1”

2. Now the solver takes up the next variable to be solved in THAT order, i.e. “v2” – you see in Questa the “Working Set’ print with details (note: ** randomize** is called only once per iteration in user code)

So this leads to a constraint solver failure. Whereas a mere “guidance” shown by a ** solve..before** would have solved both the variables TOGETHER, leading to a successful solving operation.

Bottomline: The function call “strictly enforces” the solve order, while the “** solve..before”** is more of a “guidance/suggestion”.

To learn more about this and other advanced SystemVerilog topics, join our training via www.cvcblr.com/trainings

Good Luck