Error comparing Mem reads

[jemcmahan13]

Right now a read port returns a _MemIndexed. Normal operations are not defined on this class, so attempting to compare two reads returns pyrtl.core.PyrtlError: error, attempt to covert wirevector to compile-time boolean, and attempting a logical operations returns an unsupported operand types error.

import sys
sys.path.append("..")
from pyrtl import *


regfile = MemBlock(8, 3)
# Both of the following lines will fail
regfile[0] == regfile[1]
regfile[0] & regfile[1]

[timsherwood]

I was working on this -- the basic problem is that a _MemIndexed will be converted to a WireVector when it is combined with another WireVector or assigned to a WireVector ( as_wires() does the conversion ). But when two _MemIndexed are combined it does not know what to do. 3 options that I see:

1. Implement the full set of WireVector operations converting from _MemIndexed. This results in a lot of boilerplate code and I worry about keeping it in sync with WireVectors

2. Use multiple-inheritance to inherit from WireVector. This is kind of a hack, but would make the code shorter and more consistent.

3. Define a _WireVectorInterface from which WireVector and _MemIndexed can both be derived. This is a reasonably serious change to WireVector but should not change any of the other code as far as I can think.

[timsherwood]

Added a test but commented out for with """ for now.

[Gamrix]

Option 2 is not possible.
http://stackoverflow.com/questions/16885299/subclass-of-type-and-tuple

[timsherwood]

Neat. Does the same restriction hold for namedtuples? Those are implemented in the collections library and are not technically builtin.

[Gamrix]

According to the docstring in the collections source code, they actually "subclass" tuple.

[timsherwood]

Maybe that is why when I tried it acted all weird. Any thoughts on 1 or 3?

[Gamrix]

actually
a = collections.namedtuple("a","b")
In[5]: isinstance(a,tuple)
Out[5]: False
In[6]: isinstance(a,object)
Out[6]: True
In[19]: isinstance(a,type)
Out[19]: True

In[10]: b = tuple([1,2])
In[11]: isinstance(b,tuple)
Out[11]: True
In[12]: isinstance(b,object)
Out[12]: True
In[21]: isinstance(b,type)
Out[21]: False

Now I don't know what to believe

[Gamrix]

I think that 3 is the better choice, and I think it won't be as hard as you anticipate. For ROMs, I just lifted the read related calls from memblock and put them into _MemReadBase. The hardest part about the whole thing was tracking down the few places where there was a call to isinstance(xxx, Memblock) where I wanted the code to apply to ROMs as well.

[timsherwood]

works for me. Once you have ROMs committed I will work on that -- I think there is a good chance of conflicts if I don't wait.

[jemcmahan13]

Issue has resurfaced where a _MemIndexed is lacking attributes a WireVector has (in this case, block).

from pyrtl import *
RF = MemBlock(8, 3)
val1 = RF[0]
test1 = concat(0, val1)

[jemcmahan13]

This is not a critical fix, by the way; for now I can just pre-declare read-values as WireVectors and assign the result of the port.

[timsherwood]

fixed (it could have showed up a couple of places, such as in a mux as well) 11577a8