Re: [myhdl-list] [PATCH] Remove redundant copy operators

[Jan D. <ja...@ja...>]

I'm not entirely sure I understand the issue, but here are some
upfront remarks.

Something like a VHDL alias in its full generality would probably
require Python language support. What we can do is try to
accomodate for some limited but interesting cases.

One issue that I have addressed myself is the fact the MyHDL signal
slices aren't signals. I consider this a major weakness, and I have
come up with a proposal to address what I consider the most important
issues:

    http://www.myhdl.org/doku.php/meps:mep-105

An implementation is present in the development trunk. I would be
interested to hear whether this can be a solution for you. E.g:

class ControlWord(object):
	pass

ctrlword = ControlWord()

a = intbv(0)[32:]

ctrlword.subpart = a(4, 2) # shadow signal, only for reading, note round brackets

Then: with Python you can of course do what you want, very clever
tricks. This is abvolutely fine for modeling. 
However, when you also want Verilog/VHDL convertibility, your
options are very limited. E.g. something like the above wouldn't work
because signals as attributes are not yet supported by the convertor.
However, somethink like:

subpart = a(4, 2)

would be convertible, because the convertor explicitly supports
dedicated conversion code for shadow signals.

Finally: the copy methods. I agree that they are not as they should be
as they are not compatible with subclassing now. However, there was a
a reason to have dedicated copy methods instead of the generic ones:
performance. The Signal class is using copy all the time, and it
makes a difference. However, I have to think about it again, because
now that I look at it it may be done much better for the intbv case.

Jan


Ben wrote:
> Hi there,
> 
> First of all, hello to everyone, and thanks for this great initiative
> which is MyHDL, till now, I enjoyed using it.
> 
> I ran into trouble though when I was looking for VHDL 'alias' ... I
> was trying to make my code more readable when dealing with
> ControlWords.
> 
> The first solution I found is to make a always_comb process, but
> still, we have to use a .next, so we don't have strictly the 'alias'
> behavior. This solution is sort of driven by my "think hardware" mind,
> or should I say, education.
> 
> The second solution I found, and there, MyHDL shows its supremacy, is
> to use Python as a tool, and not define my ControlWord as intbv, but
> to define my own class, say ctrlword that inherit from intbv, redefine
> "__getattr__", such that "ctrlword.subpart3" return the right slice
> from my intbv. THis also has the advantage that I keep complete
> compatibility (think conversion) with the intbv type. Clever isnt't it
> ? The only problem I got is that the intbv __copy__ function was
> forcing the return value as being an intbv, thus, removing all the
> information from my ctrlword.
> 
> As both __copy__ and __deepcopy__ are behaving the same as the Python
> default one, I was able to remove them, without influencing the
> testsuite result. (By the way on my MacOSX, I have some test failing
> with regard to set and Set)
> 
> Here is a patch that does what I described here.
> 
> Best Regards
> Benoit
> 
> # HG changeset patch
> # User Benoit Allard <ben...@gm...>
> # Date 1257160071 -3600
> # Node ID bd6b60949b4e68023e41f49ce853c0058e02019b
> # Parent  2ff17ece613127b174cb7a6a9aa8ba765ffcbd01
> Remove useless copy operators
> 
> The default one are making the same, and even beter.
> Those one were preventing us to subclass the intbv class, as the copy were
> forcing the return type to intbv, removing information about our subclass.
> 
> diff -r 2ff17ece6131 -r bd6b60949b4e myhdl/_intbv.py
> --- a/myhdl/_intbv.py	Tue Oct 06 07:02:37 2009 -0500
> +++ b/myhdl/_intbv.py	Mon Nov 02 12:07:51 2009 +0100
> @@ -90,12 +90,6 @@
>      def __hash__(self):
>          raise TypeError("intbv objects are unhashable")
> 
> -    # copy methods
> -    def __copy__(self):
> -        return intbv(self)
> -    def __deepcopy__(self, visit):
> -        return intbv(self)
> -
>      # iterator method
>      def __iter__(self):
>          if not self._nrbits:
> 
> 
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-- 
Jan Decaluwe - Resources bvba - http://www.jandecaluwe.com
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