Re: [sv-ac] RE: Call to vote: Due September 26 (resend with the correct Mantis number)

Hi, All -

Per Stu's comment, I am not completely up on the discussion, but I do
have concerns.

In general, adding extra unrelated (in an RTL sense) signals to a
sensitivity list would not affect the output value. Re-triggering
combinational logic additional times should still resolve to the correct
combinational outputs. Similarly, re-triggering latch-based logic would
either pass the combinational value through to the output during
transparent mode, and not impact the output during latching mode. We
have always encouraged engineers to avoid extra sensitivity list signals
mostly for simulation performance reasons because extra triggers
translates into re-evaluation of the expressions within the logic
blocks. Unsuspecting engineers could cause significant performance
degradation by adding multiple assertion-tested signals to the
sensitivity list.

I don't know what is legal or not, but my greater concern is if an
assertion action block is allowed to make assignments to the always_comb
and always_latch outputs. That would certainly be dangerous and could
cause outputs to be wrong and mismatch the synthesized logic. Under
those circumstances, non-input signals changing could cause the
assertion to assign an invalid output value that would not be fixed
until the next RTL input changes.

As an interesting side-note, Harry Foster and I have been strongly
recommending the placement of concurrent assertions into bind files as
opposed to putting them in the RTL code itself (which is a deviation
from Harry's recommendations in his Assertion Based Design book). The
reason is that RTL synthesis is often controlled by Makefiles, and every
time a new assertion is added to the RTL code, the file time-stamp
changes and could cause engineers to re-synthesize designs that in
reality have not changed. Keeping the separate bind file allows
engineers to add all the assertions they want without fear of triggering
a synthesis Makefile run.

Regards - Exiled SV Committee Member - Cliff

On 9/23/2011 2:26 PM, Stuart Sutherland wrote:
>
> In my opinion, even adding the condition(s) of an immediate assertion
> to the always_comb (or always_latch) sensistivity list is a BAD IDEA!
> It will force engineers to go back to the old days of having to deal
> with simulation and synthesis seeing very different things in the same
> block of code, and will lead to mismatches in RTL simulation and
> synthesized hardware. It is nasty, dangerous, and can risk expensive
> re-spins of a chip. Don't mess with the rules of inferred sensivity!!!!
>
> I would much rather see a rule in the standard that any type of
> assertion in always_comb and always_latch does not affect the inferred
> sensitivity list in any way. A cautionary note would be appropriate
> that if the immediate assertion reads an expression that are not read
> elsewhere in the procedure, the immediate assertion will not be
> evaluated when that expression changes value.
>
> Again, I am copying Cliff and Don, in case fellow real designers think
> I am off base.
>
> Stu
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Stuart Sutherland
>
> Sutherland HDL, Inc.
>
> stuart@sutherland-hdl.com
>
> 503-692-0898
> www.sutherland-hdl.com <http://www.sutherland-hdl.com>
>
> *From:* owner-sv-ac@eda.org [mailto:owner-sv-ac@eda.org] *On Behalf Of
> *Samik Sengupta
> *Sent:* Friday, September 23, 2011 11:53 AM
> *To:* Seligman, Erik; 'sv-ac@eda-stds.org'
> *Subject:* [sv-ac] RE: Call to vote: Due September 26 (resend with the
> correct Mantis number)
>
> Hello Erik,
>
> Thanks. I thought of that, although I did not check the mantis. I
> think it is okay to say just "immediate", although IMO in this case
> verbosity may be helpful. But I am fine either way.
>
> Regards,
> Samik
>
> *From:* owner-sv-ac@eda.org [mailto:owner-sv-ac@eda.org] *On Behalf Of
> *Seligman, Erik
> *Sent:* Friday, September 23, 2011 2:41 PM
> *To:* 'sv-ac@eda-stds.org'
> *Subject:* [sv-ac] RE: Call to vote: Due September 26 (resend with the
> correct Mantis number)
>
> Samik- wrt #2- remember that where we define deferred & final
> assertions, we explicitly state that they are kinds of immediate
> assertions, so I think we're covered as long as we just talk about
> immediate. (I remember writing the defs that way explicitly to avoid
> later verbosity in situations like this.)
>
> *From:* owner-sv-ac@eda.org [mailto:owner-sv-ac@eda.org] *On Behalf Of
> *Samik Sengupta
> *Sent:* Friday, September 23, 2011 11:39 AM
> *To:* Korchemny, Dmitry; 'sv-ac@eda-stds.org'
> *Subject:* [sv-ac] RE: Call to vote: Due September 26 (resend with the
> correct Mantis number)
>
> Mantis 3564 __X__ Yes ____ No
>
> http://www.eda-stds.org/mantis/view.php?id=3564
> <http://www.eda-stds.org/mantis/view.php?id=2093%20>
>
> http://www.eda-stds.org/mantis/file_download.php?file_id=5462&type=bug
> <http://www.eda-stds.org/mantis/file_download.php?file_id=5462&type=bug>
>
> Friendly amendments:
>
> 1. Only the condition checked in the immediate assertion should be
> added to the sensitivity list, i.e., nothing from action block should
> wake up the always_comb.
>
> 2. I think we should be more specific and say "Expressions used in
> immediate, final, and deferred assertions" -- not only immediate.
>
> I agree to Ed, Anupam, and Manisha that no expression from concurrent
> assertion, including the clock, should go into the sensitivity list.
>
> Thanks,
> Samik
>
> *From:* owner-sv-ac@eda.org [mailto:owner-sv-ac@eda.org] *On Behalf Of
> *Korchemny, Dmitry
> *Sent:* Wednesday, September 21, 2011 12:12 PM
> *To:* 'sv-ac@eda-stds.org'
> *Subject:* [sv-ac] Call to vote: Due September 26 (resend with the
> correct Mantis number)
>
> -You have until 11:59 pm PDT, Monday, September 26, 2011 to respond
>
> -An issue passes if there are zero NO votes and half of the eligible
> voters respond with a YES vote.
>
> -If you vote NO on any issue, your vote must be accompanied by a reason.
>
> The issue will then be up for discussion during a future conference call.
>
> As of the September 20, 2011 meeting, the eligible voters are
>
> Ashok Bhatt
>
> Eduard Cerny
>
> Ben Cohen
>
> Dana Fisman
>
> Tapan Kapoor
>
> Jacob Katz
>
> Scott Little
>
> Manisha Kulshrestha
>
> Anupam Prabhakar
>
> Erik Seligman
>
> Samik Sengupta
>
> Tom Thatcher
>
> Mantis 3564 ____ Yes ____ No
>
> http://www.eda-stds.org/mantis/view.php?id=3564
> <http://www.eda-stds.org/mantis/view.php?id=2093%20>
>
> http://www.eda-stds.org/mantis/file_download.php?file_id=5462&type=bug
> <http://www.eda-stds.org/mantis/file_download.php?file_id=5462&type=bug>
>
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-- 
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