RE: [sv-ec] Functions in constraints

I believe the correct interpretation is "functions in a constraint are
called before that particular constraint is solved, but there is no
necessary order between a function in one constraint and another,
unrelated set of constraint" This is what subdivides the solution space
into an ordered set of solution spaces, each with their own set of
constraints.

 

Solve x before y constraints do not break up the solution space, they
only affect the distribution of results in the order that solutions are
picked for each variable. So "choose x before y" would have been a more
appropriate choice of keywords.

 

________________________________

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)

 

 

---------------------------------------------------------------------
Intel Israel (74) Limited
 
This e-mail and any attachments may contain confidential material for
the sole use of the intended recipient(s). Any review or distribution
by others is strictly prohibited. If you are not the intended
recipient, please contact the sender and delete all copies.

-- 
This message has been scanned for viruses and 
dangerous content by MailScanner <http://www.mailscanner.info/> , and is

believed to be clean. 

-- 
This message has been scanned for viruses and
dangerous content by MailScanner, and is
believed to be clean.