RE: [sv-bc] Pre-Proposal to handle X-problems in RTL coding

Hi Cliff,

I like this proposal.  

I have a few questions which I am sure you have thought about, I just
want to hear them addressed by you.  

1. unique0 could potentially create scenarios where the designer forgets
to place his default assignment before the case statement.  This results
in a latching element and no indication of such until synthesis.  I like
how it is used in conjunction with always*x blocks, I just worry that we
won't catch these issues sooner now. 

2. I think we all agree that removing optimism is always good even at
the expense of pessimism, but how much pessimism are we introducing now
and do you see this as an issue we'll have to address later?  The more I
think about this, and I have done plenty of x debug, I don't think it's
going to be that bad.  Our debug tools today are much better at tracing
x's back in simulation, whether through automation or manually.  This
may be the motivation to fine tune those tools.

3. If designers move to always_*x it will require rewritting existing
legacy code, beyond just tacking on the x suffix. The case statement
examples require that the default be removed and placed as a
pre-assignment.  I like the results of doing this, don't get me wrong,
but what I'd _really_ like is for this to work semantically as you
describe without moving the default. Is there a way to make this
enhancement to the proposal?

Thanks, -Tom

-----Original Message-----
From: owner-sv-bc@server.eda.org [mailto:owner-sv-bc@server.eda.org] On
Behalf Of Clifford E. Cummings
Sent: Monday, November 05, 2007 6:07 PM
To: sv-bc@server.eda.org
Subject: [sv-bc] Pre-Proposal to handle X-problems in RTL coding

Hi, All -

Could you peek at this pre-proposal to handle X-problems in RTL 
coding? I have run this past engineers at ARM and they agree that 
this would solve their RTL coding related X-problems.

In short:
(1) parallel_case equivalent using a new keyword: unique0
(2) X-trapping using new keywords: initialx, alwaysx, always_combx, 
always_latchx, always_ffx

Addresses Mantis items: 99, 2115, 2129, 2131, 2132

Regards - Cliff

----------------------------------------------------
Cliff Cummings - Sunburst Design, Inc.
14314 SW Allen Blvd., PMB 501, Beaverton, OR 97005
Phone: 503-641-8446 / FAX: 503-641-8486
cliffc@sunburst-design.com / www.sunburst-design.com
Expert Verilog, SystemVerilog, Synthesis and Verification Training

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