RE: [sv-ac] Re: [sv-bc] Re: if-else

I thought we would use 'b0?
ed 

> -----Original Message-----
> From: owner-sv-ac@eda.org [mailto:owner-sv-ac@eda.org] On 
> Behalf Of John Havlicek
> Sent: Thursday, November 01, 2007 12:13 PM
> To: sharp@cadence.com
> Cc: sharp@cadence.com; john.havlicek@freescale.com; 
> sv-ac@eda.org; sv-bc@eda.org
> Subject: [sv-ac] Re: [sv-bc] Re: if-else
> 
> Hi Steven:
> 
> Thanks for your clarification.
> 
> The only other issues that are nagging me involve stuff that
> I guess is near the boundary of the realm of what is useful.  
> For example:
> 
> - In "expresson != 0", the constant 0 is signed 32-bit.  Are there
>   any cases in which we should use 1'b0 instead of 0?
> 
> - Are there any cases where "expression" has no bit at all?  If so,
>   is sign-extension defined?  I am worried about void members of 
>   tagged unions and similar things.  Perhaps a tagged union with 
>   only one member and such that the one member is void type.
> 
> 
> J.H.
> 
> > Date: Thu, 1 Nov 2007 10:51:06 -0400 (EDT)
> > From: Steven Sharp <sharp@cadence.com>
> > Reply-To: Steven Sharp <sharp@cadence.com>
> > Cc: sv-ac@eda.org, sv-bc@eda.org
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> > X-Received: By mx-sanjose.cadence.com as lA1EpoHs026736 at 
> Thu Nov  1 07:51:50 2007
> > X-OriginalArrivalTime: 01 Nov 2007 14:55:02.0620 (UTC) 
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> > 
> > John,
> > 
> > As you have noted,
> > 
> > 	if (expression)
> > 	
> > is equivalent to
> > 
> > 	if (expression != 0)
> > 
> > The problem is then that the results of the equality 
> operator are not
> > clearly specified when unknowns are involved.  The correct answer is
> > that (value1 != value2) is equivalent to (|(value1 ^ 
> value2)).  So for
> > (expression != 0), that is equivalent to (|expression), as you have
> > suggested.
> > 
> > This definition makes sense.  If any of the bits are 1, 
> then the value
> > is definitely not equal to zero, no matter what any unknown 
> bits are.
> > If all of the bits are 0, then the value is definitely 
> equal to zero.
> > If the bits are a mixture of 0 and X/Z, then it is unknown 
> whether the
> > value is equal to zero.  It depends on the value of the 
> unknown bits.
> > 
> > After that, there is the issue of how if-else deals with an unknown
> > condition.  It rather arbitrarily treats an unknown 
> condition as false.
> > Jonathan's suggestion of a cast to "bit" has this same effect.  This
> > treatment can cause simulation to produce optimistic 
> results, which is
> > undesirable, but that is how it works.
> > 	
> > Steven Sharp
> > sharp@cadence.com
> > 
> 
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