[Numpy-discussion] Problems with Numexpr and discontiguous arrays
Tim Hochberg
tim.hochberg at ieee.org
Thu Oct 5 11:52:14 CDT 2006
Ivan Vilata i Balaguer wrote:
> En/na Tim Hochberg ha escrit::
>
>
>> Ivan Vilata i Balaguer wrote:
>>
>>> It seemed that discontiguous arrays worked OK in Numexpr since r1977 or
>>> so, but I have come across some alignment or striding problems which can
>>> be seen with the following code::
>>>
>> I looked at this just a little bit and clearly this bit from interp_body
>> cannot work in the presence of recor arrays:
>>
>> //....
>> intp sf1 = sb1 / sizeof(double); \
>> //...
>> #define f1 ((double *)x1)[j*sf1]
>>
>> There are clearly some assumptions that sb1 is evenly divisible by
>> sizeof(double). [...]
>>
>
> I noticed something strange in those statements when implementing
> support for strings, and I must confess that I didn't grasp their
> meaning, so I implemented it a little differently for strings::
>
> #define s1 ((char *)x1 + j*params.memsteps[arg1])
>
I believe that these approaches are the same as long as memstep is a
multiple of itemsize. I chose the indexing rather than the pointer foo
version[1] because there's a rumor that compilers will sometimes
generate faster code for it. One additional potential slowdown in the
above is the compiler may not be able to tell that memsteps[arg1]) is
constant and may do that lookup repeatedly. Or maybe not, I try not to
second guess compilers too much.
-tim
[1] I'm pretty sure I used the pointer foo version at least for a while.
and may have gone back and forth several times.
> That seemed to work, but it might not be right (though I tested a bit),
> and certainly it may not be efficient enough. Here you have my previous
> patches if you want to have a look at how I (try to) do it:
>
> 1.http://www.mail-archive.com/numpy-discussion%40lists.sourceforge.net/msg01551.html
> 2.http://www.mail-archive.com/numpy-discussion%40lists.sourceforge.net/msg02261.html
> 3.http://www.mail-archive.com/numpy-discussion%40lists.sourceforge.net/msg02644.html
>
> ::
>
> Ivan Vilata i Balaguer >qo< http://www.carabos.com/
> Cárabos Coop. V. V V Enjoy Data
> ""
>
>
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