RE: [sv-bc] part selects on arbitrary expressions

Mac,

The impetus of this discussion was to do this in a context where
assigning to temps would be awkward, like assertions or continuous
assignments.

I have a suggestion that might clear up a number of ambiguities, and
that is to do the select as part of a cast. Rich now, we only allow
simple types as part of a cast, but we could interpret an index as a
select of that type. For example

typedef logic [1:8] t[5:2];

t[3][8]'(expr)

That means cast the expression to t, then take word 3, bit 8 of that
type.

This does mean that you do need to have an intermediate type, but a good
programmer should always be working with a named type.

There is also an existing request to have a generic packed type for
bit-stream casting, so

packed'(expr)

creates a packed array of bits that has the size of the total number of
bits in the expression with indexes normalized to n-1:0.

Then you could do 

packed[0]'(expr)

would always give you the lsb of the expression.

Dave


> -----Original Message-----
> From: owner-sv-bc@server.eda.org [mailto:owner-sv-bc@server.eda.org]
On
> Behalf Of Michael (Mac) McNamara
> Sent: Wednesday, March 07, 2007 6:54 AM
> To: Feldman, Yulik; Steven Sharp; Arturo.Salz@synopsys.com;
Bresticker,
> Shalom; sv-bc@server.eda.org
> Subject: RE: [sv-bc] part selects on arbitrary expressions
> 
> I will look for this explaination when I get into my office, as this
does
> makes sense to me.  What expression can be in () that cannot be in {},
for
> which a part or bit select makes sense?
> 
> Even if there is some small interesting class of such expressions, I
would
> submit that it would still be more reasonable to use {} for the
majority,
> and require assigning to a temp and selecting from that for these
complex
> datatype expressions you describe - for many reasons including
enabling
> clear understanding of these complex expressions.
> 
> -mac
> 
> mcnamara@cadence.com
> 
>  -----Original Message-----
> From: 	Feldman, Yulik [mailto:yulik.feldman@intel.com]
> Sent:	Wednesday, March 07, 2007 05:31 AM Pacific Standard Time
> To:	Michael (Mac) McNamara; Steven Sharp; Arturo.Salz@synopsys.com;
> Bresticker, Shalom; sv-bc@eda.org
> Subject:	RE: [sv-bc] part selects on arbitrary expressions
> 
> As Shalom has already pointed out, concatenation will not work for
> expressions that have a complex data type.
> 
> W.r.t. new modeling capabilities/succinctness, the main benefit this
> feature will provide is the ability to perform arbitrary slicing in a
> single expression tree, without introducing artificial variables and
> assignments. As of now, Verilog expressions provide means to express
> very complex logic, but for some reason they do not provide a way to
do
> a simple slicing operation of another expression.
> 
> --Yulik.
> 
> -----Original Message-----
> From: Michael (Mac) McNamara [mailto:mcnamara@cadence.com]
> Sent: Wednesday, March 07, 2007 2:25 AM
> To: Steven Sharp; Arturo.Salz@synopsys.com; Bresticker, Shalom;
Feldman,
> Yulik; sv-bc@eda.org
> Subject: RE: [sv-bc] part selects on arbitrary expressions
> 
> I agree that this enhancement adds no new modeling capability - it
just
> delivers some succinctness. (most every language feature after
turing's
> Mark and Advance commands are of this form)
> 
> To restate my point succinctly, were folks to still wish to deliver
this
> enhancement, I would guide to using the concatenate rather than
defining
> a new meaning for a parentetical grouping.
> 
> -mac
> 
> mcnamara@cadence.com
> 
>  -----Original Message-----
> From: 	Steven Sharp [mailto:sharp]
> Sent:	Tuesday, March 06, 2007 04:02 PM Pacific Standard Time
> To:	Steven Sharp; Arturo.Salz@synopsys.com;
> shalom.bresticker@intel.com; yulik.feldman@intel.com; sv-bc@eda.org;
> Michael (Mac) McNamara
> Subject:	RE: [sv-bc] part selects on arbitrary expressions
> 
> 
> >Are you suggesting we dispense with slicing altogether?
> >Shifting, masking, and casting are tedious and error prone.
> 
> Based on the discussion, there are complex issues that would
> need to be resolved before allowing part selects on arbitrary
> expressions.  Given that there is already a way of getting the
> proposed functionality, I don't think the effort to work this
> out is a high priority at this time.
> 
> In procedural code, an intermediate variable can be used to
> hold the expression value and allow a part select.  In any
> situation where an intermediate variable is not a possibility,
> the functionality can still be obtained with a shift and cast.
> 
> Steven Sharp
> sharp@cadence.com
> 
> --
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