Subject: RE: [sv-ec] Can an ordering constraint cause the solver to fail ?
From: Ryan, Ray (Ray_Ryan@mentorg.com)
Date: Wed Dec 03 2003 - 10:55:14 PST
Arturo,
In the new section 12.4.10 (Functions in Constraints) there is a
different definition or usage of 'variable ordering' and 'solve before'.
The 5th bullit at the end of the section states:
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 a state
variables.
Within each prioritized set of constrained, cyclical (randc) variables
are solved first.
This section rather clearly states the behavior I expected for ordering
of variables. Specifically:
(a) Constraints that include only variables with higher priority are
solved before other, lower priority constraints.
(b) Random variables solved as part of a higher priority set of
constraints become state variables to the remaining set of constraints.
In the above example, if we change "F(y)" to simply "y" and explicitly
add the constraint "solve y before x", then:
- First y is solved using only constraint D - as per (a).
- Second x is solved using constraint C with y as a state variable - as
per (b).
If constraint C were like the constraint in my original email, x might
not be solveable.
Although this section does not specifically state that an "implicit
variable ordering" is the same as an "explicit variable ordering" (ie a
solve-before constraint), it would seem intuitive that this is the case.
Furthermore, I believe it is very useful for the User to be able to
specify a solve ordering (possibly in addition to distribution ordering)
to reduce constraint complexity (ie potentially reduce solver time).
Ray
-----Original Message-----
From: owner-sv-ec@eda.org [mailto:owner-sv-ec@eda.org] On Behalf Of
Arturo Salz
Sent: Tuesday, December 02, 2003 6:15 PM
To: Ryan, Ray; sv-ec@eda.org
Subject: Re: [sv-ec] Can an ordering constraint cause the solver to fail
?
Ray,
The short answer is no. Ordering constraints affects only the
distribution, not the
solution space. Hence, a "solve before" constraint can never cause a
failure.
Note that the statement in question:
"The constraints provide a mechanism for ordering variables so that
s can be
chosen independently of d."
Uses the word "chosen", not "solved". This choice is intended to mean
that the
selection (i.e., the distribution) of s is independent of d. The
solution space is
still constrained by both s and d.
Using your example, the ordering constraint does not change the solution
space.
Therefore, x can only have even values. With the ordering constraint all
even
values of x have the same probablity (1/128). Without the ordering
constraint, the
cross-product of all possible values of x and y would have the same
probability.
Perhaps that sentence should state:
"The constraints provide a mechanism for ordering variables so that
s can be
chosen with a distribution that is independent of d."
Arturo
----- Original Message -----
From: "Ryan, Ray" < <mailto:Ray_Ryan@mentorg.com> Ray_Ryan@mentorg.com>
To: < <mailto:sv-ec@eda.org> sv-ec@eda.org>
Sent: Tuesday, December 02, 2003 4:02 PM
Subject: [sv-ec] Can an ordering constraint cause the solver to fail ?
In section 12.4.8 Variable ordering, referring to ordering constraints,
the
LRM states that:
"The constraints provide a mechanism for ordering variables so that s
can be
chosen independently of d."
This statement is referring to the constraint "solve s before d".
Since s is "chosen independently of d", can the choice for 's' result in
a
constraint for 'd' than that cannot be met. For example:
class B
rand bit[7:0] x, y;
constraint c1 { 2*y == x; }
constraint ord { solve x before y; }
end class
When 'x' is solved before 'y', if an even value is choosen for 'x', then
when 'y' is
solved there is a solution. However, if an odd value is choosen, then
the solver
cannot choose a value for 'y' that meets constraint 'c1'.
My expectation is case is unstable / over-constrainted and it is valid
for
the solver to sometimes fail.
Thanks,
Ray
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