[Numpy-discussion] New iterator APIs (nditer / MapIter): Overlap detection in NumPy

[Pauli Virtanen]

Mon, 12 Sep 2016 11:31:07 +0200, Sebastian Berg kirjoitti:
>> * NPY_ITER_COPY_IF_OVERLAP, NPY_ITER_OVERLAP_NOT_SAME
>>   flags for NpyIter_New.
>> 
>> * New API function PyArray_MapIterArrayCopyIfOverlap,
>>   as ufunc.at needs to check overlaps for index arrays before
>>   constructing iterators, and the parsing is done in multiarray.
> 
> I think here Nathaniels point might be right. It could be we can assume
> that copying is always fine, there is probably only one or two
> downstream projects using this function, plus it seems harder to create
> abusing structures that actually do something useful.
> It was only exposed for usage in `ufunc.at` if I remember right. I know
> theano uses it though, but not sure about anyone else, maybe numba. On
> the other hand.... It is not the worst API clutter in history.

Do you suggest that I break the PyArray_MapIterArray API?

One issue here is that the function doesn't make distinction between read-
only access and read-write access, so copying may give unnecessary 
slowdown. The second thing is that it will result to a bit uglier code, as 
I need to manage the overlap with the second operation in ufunc_at.

For NpyIter, I'd still be wary about copying by default, because it's not 
needed everywhere (the may_share_memory checks are better done earlier), 
and since the semantic change can break things inside Numpy.

	Pauli