RE: [sv-ec] Functions in constraints

Thanks, Ray.

One question. You mentioned random variables used as arguments to a function. What if a random variable is used inside a function, but is not an argument to the function?

Thanks,
Shalom

________________________________
From: owner-sv-ec@server.eda.org [mailto:owner-sv-ec@server.eda.org] On Behalf Of Ryan, Ray
Sent: Wednesday, June 17, 2009 3:50 AM
To: sv-ec@server.eda.org
Subject: RE: [sv-ec] Functions in constraints

Shalom,

In a general sense here is how the variable ordering works.

First values are determined for all randc variable. Each gets the
next value in the cycle for that variable. These values are then
treated as state (constant) values for the remainder of the randomization.

Next consider function calls. Randomize does NOT look into function calls. Therefore,
if a constraint contains a function call, the function must be evaluated before the
constraint can be evaluated. In order to evaluate (call) a function, all the arguments
must be known (constant). If any of the arguments to a function are random variables,
randomized values must first be choosen for those variable.

So consider the constraints:
        x < f1 ( y );
        y < f1 ( z );
        z < 10;
        z < x;

This requires that:
        a value is choosen for 'y' before evaluating the constraint "x < f1 ( y )"
        a value is choosen for 'z' before evaluating the constraint "y < f1 ( z )"
Therefore the solver will
        first choose a value for 'z'
        then choose a value for 'y'
        then coose a value for 'x'
[NOTE, I am not using the word 'solved' because the ordering discussed in 18.5.11
 is different from the ordering defined by "solve-before" constraints. ]

In choosing a value for 'z', the solve will consider any constraints on 'z' that do not
reference random variables to be choosen after 'z'. Here a value for 'z' is choosen
meeting the constraint "z < 10". Say the value 5 is selected (z is now 5)

Then the call to "f1 (z)" is evaluated [ie. f1(5)]. Say the result is 23;

Next a value for 'y' is choose. Here the constraint "y < 23" applies. Say the
value 20 is selected (y is now 20).

Then the call to " f1 (y)" is evaluated [ie. f2(20)]. Say the result is 7.

Now there are no remaing function calls, so the rest can be solved. That is,
a value is choosen for 'x' with the constraints:
        "x < f1(y)"  which is now  "x < 7"
        "z < x"       which is now "5 < x"


As an additional note, the ordering introduced by function calls can
cause the solver to fail as a result of an earlier choosen value. For
example, say the value 8 is choosen for 'z' and then based the value
choosen for 'y' the result of "f1(y)" is 4. Now the constraints for
choosing a value for 'x' are:
        "x < f1(y)"  which is  "x < 4"
        " z < x "     which is "8 < x"
In this case, the solver will fail and return a status of 0 AND none
of the random variables will have changed value (including the randc variables).

Ray Ryan


________________________________
From: owner-sv-ec@server.eda.org [mailto:owner-sv-ec@server.eda.org] On Behalf Of Bresticker, Shalom
Sent: Tuesday, June 16, 2009 5:40 AM
To: sv-ec@server.eda.org
Subject: [sv-ec] Functions in constraints

Hi,

Chris Spear pointed out that the text in 18.5.11 on functions in constraints is unclear.

The text says,

"- Functions shall be called before constraints are solved, and their return values shall be treated as state variables.

- Random variables used as function arguments shall establish an implicit variable ordering or priority. Constraints that include only variables with higher priority are solved before other, lower priority constraints. Random variables solved as part of a higher priority set of constraints become
state variables to the remaining set of constraints. For example:

class B;
rand int x, y;
constraint C { x <= F(y); }
constraint D { y inside { 2, 4, 8 } ; }
endclass

forces y to be solved before x. Thus, constraint D is solved separately before constraint C, which uses the values of y and F(y) as state variables. In SystemVerilog, the behavior for variable ordering implied by function arguments differs from the behavior for ordering specified using the "solve...before..." constraint; function argument variable ordering subdivides the solution space thereby changing it. Because constraints on higher priority variables are solved without considering lower priority constraints at all, this subdivision can cause the overall constraints to fail. Within each prioritized set of constraints, cyclical (randc) variables are solved first."

These two dashed items seem contradictory. The first says that functions are called before constraints are solved, but the example shows constraint D being solved before the call to F(y).

Apparently a simple, and at first glance reasonable, interpretation of the first dashed item, that ALL functions in constraints are solved before ANY constraints are solved, is not the correct interpretation.

What is the correct interpretation?

Is the second dashed item to be considered a continuation of the first, denoting an exception to the general rule, and saying that only those constraints needed to be solved to order to evaluate the function calls are solved before the functions?

Or is the first dashed item to be interpreted as saying that functions in a certain constraint are called before that particular constraint is solved, but there is no necessary order between a function in one constraint and another, unrelated constraint?

Is there a difference between these two interpretations?

If a function reads the value of a random variable which has no constraint on it, I assume it is simply randomly assigned a value?

Also, the last sentence mentions randc variables. Is the interpretation that first function calls are evaluated, then randc variables are solved for, then rand variables?

Thanks,
Shalom

Shalom Bresticker
Intel LAD DA
Jerusalem, Israel
+972  2 589 6582 (office)
+972 54 721 1033 (cell)



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