[myhdl-list] MEP 107 assessment

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

I have read and thought about MEP 107. Here is my initial
assessment.

This MEP is strictly about conversion. This is explained in
the first paragraph, but I think it should be mentioned in
the title also, because this has proven to be an eternal source
of confusion. It should be made clear that all modeling
aspects being discussed are supported by MyHDL as it is today.

One aspect of conversion is that it is unavoidable to consider
the capabilities and restrictions of the conversion targets
VHDL and Verilog, with special attention to synthesis. I think
that aspect is taken into account insufficiently.

Basically, the MEP proposes conversion support for various
types of Signal object lookup. The example given is:

     myObject.x.next = ListOfSignal[10] + DictOfSignal['fred']

and the proposed conversion is:

     myObject_x = ListOfSignal_10 + DictOfSignal_fred

Conceptually, these 3 cases are very similar: lookup based on
a manifest attribute name, index, string. The immediate question
is why we would need support for 3 almost identical cases.
The answer is that this example is not representative for
the main purpose of a List or a Dict data structure.

The essence of a List or a Dict is lookup based on a
*variable* index (integer or string). But this can only be
supported by a mapping to an equivalent data structure in
the target. Note that MyHDL conversion supports this today
for "homogenuous" lists. However, I don't know how this
would work for a "heterogenuous" list or for a dictionary.

There may be solutions based on pointers, access types,
or SystemVerilog data structures, but I doubt all this
is worth the trouble, especially because there is little
chance that such beasts would be supported by synthesis.
Synthesis is not the only, but an important application
of the converted code.

Of course conversion could be restricted to the case of
manifest indices. But that is a bad solution for the following
reasons:

1) when support for heterogenuous lists or dictionaries is
advertized, it is unavoidable that people will expect this
to work with variable indices. So it raises expectations that
cannot be met and will only result in frustrations.
2) there is a superior alternative available, which is
attribute lookup. Better direct people to that solution
immediately.

-- 
Jan Decaluwe - Resources bvba - http://www.jandecaluwe.com
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