Re: [sv-bc] FW: interpretation of priority if-else or case statement

Hi, Antara -

You are correct that case statements and if-else-if statements are already 
priority. I noted in section 4.1 of a recent conference paper (read the 
section for my full argument) that I really dislike the keyword "priority" 
because it is confusing

http://www.sunburst-design.com/papers/CummingsSNUG2005Israel_SystemVerilog_UniquePriority.pdf

What priority case offers is run-time checking to see if the case 
expression matches one of the defined case items. In essence, by adding the 
priority keyword, you have said that the case statement defines all 
possible cases; hence, a better keyword would have been "full," "fullcase," 
or "all_possible," but alas, we are now stuck with the confusing "priority."

If during simulation or if a formal tool can prove that it is possible to 
find a case expression that does not match one of the case items, then you 
have violated the assertion that all possible cases have been defined in 
the case statement (or if-else-if statement) and accord to the P1800 
standard a warning is issued (according to the SV 3.1a standard it was an 
error - I have noted this in my ballot comments - when an assertion fails, 
and "priority" really is an assertion, it should be an error, and the SNUG 
San Jose conference attendees also raised this point).

Also note that either adding a "default" to a case statement or a final 
"else" to an if-else-if statement will nullify the "priority" keyword 
because you have now said, "and for ALL other cases, do the following."

I don't like the "priority" keyword, but we are now stuck with it. Just 
think of it as an engineer stating the assertion that all possible cases of 
else-if's have been defined. The paper above gives many examples and details.

Hope this helps.

Regards - Cliff

At 10:22 PM 3/28/2005, Maidment, Matthew R wrote:
> >From: Antara Ghosh <antarag@interrasystems.com>
> >To: sv-bc@server.eda.org
> >Subject: interpreation of priority if-else or case statement
> >
> >Hello,
> >
> >What is the difference between the behavior of priority if-else/case
> >statement and normal (without unique or priority attribute) if-else
> >statement?
> >
> >LRM states "A priority if indicates that a series of if ...
> >else ... if
> >conditions shall be evaluated in the order listed." But that is the
> >inherent behavior of an if-else if statements. The only difference
> >between normal if and priority if seems to be the fact that the later
> >errors out if there is a condition which is not present in
> >if-else ladder.
> >Please let me know if I am missing something.
> >
> >Thanks
> >Antara Ghosh

----------------------------------------------------
Cliff Cummings - Sunburst Design, Inc.
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