Re: [myhdl-list] intbv wrap

[Christopher F. <chr...@gm...>]

On 5/11/2011 7:31 AM, Ben wrote:
> Hi Chris,
>
> I am following your struggle with this feature with great interest, as
> I do think this would be an interesting addition to MyHDL, however, as
> an hypothetical future extension to MyHDL, I think this would deserve
> a MEP ... For now, If I want to understand your design decisions, I
> have to dig into my mails and look for the relevant part. I for one
> have for instance a question about the restrictions you enforce. See
> below.
Here is the original MEP I wrote.  It is not complete and has not been 
updated based on the conversations had here.  Also, this MEP simply 
exists in my user space right now.

http://www.myhdl.org/doku.php/user:cfelton:projects:wrap

I will try and update the MEP based on the latest developments when time 
permits.  More comment below.
>
> On Tue, May 10, 2011 at 13:27, Christopher Felton
> <chr...@gm...>  wrote:
>> I made a couple of minor adjustments to this patch.
>>
>>   1. Made the wrap function private (_wrap) and moved _wrap to __wrap.  The
>> wrap function should not be used directly because it will not covert
>> correctly.  I think for a future (todo) enhancement the wrap function can be
>> made public and handled by the converter.  This is an involved modification.
>>   For now I think it is good to advertise the wrap function was private.
>
> I don't really understand the reason of such an enforcement on min and
> max for the wrapping. Let's be for a moment the devil's advocate and
> say I want my intbv to be between -3 and 12. Why would MyHDL prevent
> me to have it wrapped ?

The reason is, we want to model natural behavior of a n-bit word.  An 
n-bit word automatically wraps, by truncating the most-significant bits, 
when assigned.  We want to prevent the automatic wrap in the example 
because additional logic is required.  This is the same for the 
saturate.  This enhancement is simply trying to add the natural 
occurring wrap of a n-bit binary word.  To model this behavior correctly 
the min and max need to be the full range of the binary word.

This is a little confusing because the intbv lets you assign arbitrary 
min and max for design checking (which is a good thing).  But note, this 
checking (bound checking) is only enforced during the design, MyHDL 
simulations.

To do the wrapping in your example, the logic would explicitly need to 
be added to the design.  IMO is good and why I think the wrap() will be 
public in the future once direct use of the function is convertible.

This enhancement is intended to handle the generic wrap case.  That 
should not be part of the intbv object.  But rather model the inherent 
wrapping of a bit-vector.

>
>>
>>   2. Added myrange to a slot, _range.
>
> This is a coding style decision. This _range is used at only one
> place, and completely redundant with the existence of _nrbits. I don't
> think it deserves a slot.
>
>>
>>   3. Modified a couple comments.
>>
>
> BTW, did you got the testsuite to run ? To run the "core" one, I don't
> think you would need py.py. It uses unittest, which is included in
> your python distribution.

Yes, your additions are perfect and IMO correct.  I reran the tests in 
the core test suite and they are part of the patch.  Thanks again for 
doing this work!

>
> Best Regards
> Benoît
>
> ------------------------------------------------------------------------------
> Achieve unprecedented app performance and reliability
> What every C/C++ and Fortran developer should know.
> Learn how Intel has extended the reach of its next-generation tools
> to help boost performance applications - inlcuding clusters.
> http://p.sf.net/sfu/intel-dev2devmay