[Numpy-discussion] Problems with Numexpr and discontiguous arrays

[Travis Oliphant]

Tim Hochberg wrote:

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>>That would be easy to do. Right now the opcodes should work correctly 
>>on data that is spaced in multiples of the itemsize on the last axis. 
>>Other arrays are copied (no opcode required, it's embedded at the top 
>>of interp_body lines 64-80). The record array case apparently slips 
>>through the cracks when we're checking whether an array is suitable to 
>>be used correctly (interpreter.c 1086-1103). It would certainly not be 
>>any harder to only allow contiguous arrays than to correctly deal with 
>>record arrays. Only question I have is whether the extra copy will 
>>overwhelm the savings of that operating on contiguous data gives.  The 
>>thing to do is probably try it and see what happens.
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>OK, I've checked in a fix for this that makes a copy when the array is 
>not strided in an even multiple of the itemsize. I first tried copying 
>for all discontiguous array, but this resulted in a large speed hit for 
>vanilla strided arrays (a=arange(10)[::2], etc.), so I was more frugal 
>with my copying. I'm not entirely certain that I caught all of the 
>problematic cases, so let me know if you run into any more issues like this.
>
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There is an ElementStrides check and similar requirement flag you can 
use to make sure that you have an array whose strides are multiples of 
it's itemsize.

-Travis