RE: [sv-ec] semantics of wildcard bins - #1474

Arturo, Shalom,

The argument for ignoring X,Z values makes sense.

So the LRM should be updated to:
a) remove the parenthetical comment about ==?
b) remove the statement about the range.

I have added mantis #1474 for this issue with a 
proposal to remove those parts of the text.

- Ray

> -----Original Message-----
> From: Arturo Salz [mailto:Arturo.Salz@synopsys.com] 
> Sent: Thursday, May 11, 2006 10:45 AM
> To: Ryan, Ray; Bresticker, Shalom; sv-ec@eda.org
> Subject: RE: [sv-ec] semantics of wildcard bins
> 
> Ray,
> 
> The parenthetical comment in the LRM that mentions the ==? 
> operator is incorrect. In version 3.1a, that comment read 
> "(similar to the =?= operator)". That operator was not 
> asymmetrical, hence, the comment only intended to draw a 
> parallel between the two syntactical forms, that is, an X, Z, 
> or ? would act as a wildcard. It was not intended to specify 
> the semantics of the operator. Later, when P1800 changed the 
> symmetrical =?= operator into its asymmetrical form ==?, that 
> section was simply changed.
> 
> Not incrementing bin counts upon matching an X or Z value is 
> something that was explicitly requested by users. From a 
> coverage perspective, those are typically unimportant or 
> error conditions that users do not wish to include in 
> coverage. As Shalom pointed out, the LRM is very specific 
> about matching only 0 and 1 values, not X or Z. If we modify 
> the wildcard semantics, we will render this coverage feature 
> largely useless. If this is creating any confusion, I would 
> much rather remove the parenthetical comment. It was only 
> there in the first place because wildcard equality was new to Verilog.
> 
> 	Arturo
> 
> -----Original Message-----
> From: owner-sv-ec@eda.org [mailto:owner-sv-ec@eda.org] On 
> Behalf Of Ryan, Ray
> Sent: Wednesday, May 10, 2006 12:42 PM
> To: Bresticker, Shalom; sv-ec@eda.org
> Subject: RE: [sv-ec] semantics of wildcard bins
> 
> Hi Shalom,
> 
>  
> For #2 the sample exclusion is different from the semantics 
> of '==?'.  
> 
> The '==?' operator is asymmetric. The '?', 'x' or 'z' values 
> are wildcards in the right operand. An 'x' in the right 
> operand matches any value in the left operand (including 'x' or 'z'). 
> An 'x' in the left operand only matches a wildcard ('x' 'z' 
> or '?') in the right operand.
> 
> When applied to the context of bin matching, the bin value 
> correspond to the right operand (wildcards recognized) and 
> the sample value is the left operand. If the sample value 
> contains an 'x', and the corresponding bit of the bin value 
> is '?' (as in my example) application of 'sample ==? bin_value' 
> would yield TRUE - indicating a match and presumably the bin 
> should be incremented.
> 
> However, the LRM text seems to state that if the SAMPLED 
> value contains an 'x' or 'z' value, the sample value is 
> excluded - and thus the bin would NOT be incremented.
> 
> I would expect the behavior to be that of the '==?' operator 
> and the bin should be incremented. That is, a wildcard 
> matches any value not just 1 or 0.
> 
> Ray
> 
> > -----Original Message-----
> > From: Bresticker, Shalom [mailto:shalom.bresticker@intel.com]
> > Sent: Wednesday, May 10, 2006 4:15 AM
> > To: Ryan, Ray; sv-ec@server.eda.org
> > Subject: RE: [sv-ec] semantics of wildcard bins
> > 
> > Hi, Ray.
> > 
> > As a user reading this section for the first time, here are the 
> > interpretations I would come to as answers to your questions:
> > 
> > 
> > > -----Original Message-----
> > > From: owner-sv-ec@server.eda.org 
> [mailto:owner-sv-ec@server.eda.org]
> > On
> > > Behalf Of Ryan, Ray
> > > Sent: Wednesday, May 10, 2006 3:49 AM
> > > To: sv-ec@server.eda.org
> > > Subject: [sv-ec] semantics of wildcard bins
> > > 
> > > The last last paragraph of the section on wildcard bins (18.4.3)
> > states:
> > > 
> > > "A wildcard bin definition only considers 2-state values; sampled
> > values
> > > containing X or Z are excluded. Thus, the range of values
> > covered by a
> > > wildcard bin is established by replacing every wildcard
> > digit by 0 to
> > > compute the low bound and 1 to compute the high bound."
> > > 
> > > 
> > > 1) The statement about the range of values is not right or is
> > confusing.
> > > 
> > >    For example:
> > > 	wildcard bins bx = { 4'b?0?0 };
> > >    This does not cover the values in the range 0000 to 1010. It
> > doesn't
> > > cover 0001.
> > > 
> > 
> > [Shalom] Agree.
> > 
> > 
> > > 2) What does it mean that sampled values containing X or Z are
> > excluded.
> > > 
> > >    For example:
> > > 	wildcard bins bx = { 4'b?0?0 };
> > >    I would expect that if the sample value is:
> > >         0000	- increment the bin count
> > >         1000      - increment the bin count
> > >         0001      - don't increment
> > >         000X 	- don't increment
> > >         0010      - increment
> > >         00X0      - increment
> > >         00Z0      - increment
> > >     However, the text seems to indicate that the last two sample
> > values
> > > are excluded and
> > >     so don't increment the bin count.
> > > 
> > 
> > [Shalom] Yes, they are excluded. The text explicitly says 
> that the ? 
> > is a wildcard for 0 and 1 and that it works like ==?.
> > 
> > 
> > > 3) How many bins are created for
> > >     	wildcard bins bx[] = { 4'b?0?0, 4'b?000, 4'b00?0 }
> > >    I would expect that 3 bins are created.
> > > 
> > 
> > [Shalom] Agree.
> > 
> > 
> > > 4) How are wildcard ignore_bins handled?
> > >     For example:
> > > 	bins BA[2] = { 2'b00, 2'b1x };
> > >       bins BB[2] = { 2'b10, 2'b0x };
> > > 	wildcard bins BC = { 2'b0?, 2'b11 }
> > >       wildcard ignore_bins ivals = { 2'b00, 2'b?0, 2'b0? };
> > >       ignore_bins vals = { 2'b01, 2'b00, 2'b0x, 2'b0z };
> > > 
> > >     The ignore value 2'b00 will be removed from the first BA bin
> > causing
> > > the
> > >     bin to be ignored.
> > >     Will the ignore value 2'b?0 remove the 2'b10 value from BB ?
> > 
> > [Shalom] Yes.
> > 
> > 
> > >     Will the list of values in the last ignore_bins remove the
> > wildcard
> > >     value 2'b0 from BC ?
> > 
> > [Shalom] Yes.
> > 
> > Two more editorial quibbles:
> > 
> > The first example in this section is called g12_16 and increments 
> > "when the sampled value is between 12 and 16".
> > Actually the range is only 12-15.
> > 
> > Also, the first and third paragraphs in this section refer to 
> > "wildcard bins", with both in Courier font. As a reader, I 
> would be in 
> > doubt whether or not this implies that 'wildcard'
> > may only be used with 'bins'
> > and not with 'ignore_bins' and 'illegal_bins' (going back 
> to the rule 
> > that the BNF does not contain all the information and the text 
> > supplements it). The simplest correction would be to de-Courierize 
> > "bins' in those two places.
> > 
> > Shalom
> >  
> > 
> 
>