[Numpy-discussion] "aligned" matrix / ctypes

Zachary Pincus zachary.pincus@yale....
Fri Apr 25 11:09:55 CDT 2008


Hello all,

Attached is code (plus tests) for allocating aligned arrays -- I think  
this addresses all the requests in this thread, with regard to  
allowing for different kinds of alignment. Thanks Robert and Anne for  
your help and suggestions. Hopefully this will be useful.

The core is a function for allocating arrays with totally arbitrary  
alignment along each dimension (e.g. you could allocate an 10x20 array  
of uint16's where each uint16 is aligned to 4-byte boundaries and each  
row of 20 uint16's is aligned to 32-byte boundaries, and the entire  
buffer is aligned to a 128-byte boundary.) I've also included helper  
functions for two common cases: when you want everything aligned to a  
particular multiple (every element, row, etc. as well as the whole  
buffer), and when you want an array where the rows (second-fastest  
moving index) are so aligned (this was my original use case, for fast  
image-blitting).

Zach


def aligned_empty(shape, dtype, dim_alignments, array_alignment):
   '''Allocate an empty array with the given shape and dtype, where  
the array
   buffer starts at a memory address evenly-divisible by  
array_alignment, and
   where items along each dimension are offset from the first item on  
that
   dimension by a byte offset that is an integer multiple of the  
corresponding
   value in the dim_alignments tuple.

   Example: To allocate a 20x30 array of floats32s, where individual  
data
   elements are aligned to 16-bute boundaries, each row is aligned to  
a 64-byte
   boundary, and the array's buffer starts on a 128-byte boundary, call:
     aligned_empty((20,30), numpy.float32, (64, 16), 128)
   '''

def aligned_rows_empty(shape, dtype, alignment, order='C'):
   '''Return an array where the rows (second-fastest-moving index) are  
aligned
   to byte boundaries evenly-divisible by 'alignment'. If 'order' is  
'C', then
   the indexing is such that the fastest-moving index is the last one;  
if the
   order is 'F', then the fastest-moving index is the first.'''

def aligned_elements_empty(shape, dtype, alignment, order='C'):
   '''Return an array where each element is aligned to byte boundaries  
evenly-
   divisible by 'alignment'.'''


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