[Numpy-discussion] Byte aligned arrays

Charles R Harris charlesr.harris at gmail.com
Wed Dec 19 09:57:51 EST 2012

On Wed, Dec 19, 2012 at 7:43 AM, Nathaniel Smith <njs at pobox.com> wrote:

> On Wed, Dec 19, 2012 at 8:40 AM, Henry Gomersall <heng at cantab.net> wrote:
> > I've written a few simple cython routines for assisting in creating
> > byte-aligned numpy arrays. The point being for the arrays to work with
> > SSE/AVX code.
> >
> > https://github.com/hgomersall/pyFFTW/blob/master/pyfftw/utils.pxi
> >
> > The change recently has been to add a check on the CPU as to what flags
> > are supported (though it's not complete, I should make the default
> > return 0 or something).
> >
> > It occurred to me that this is something that (a) other people almost
> > certainly need and are solving themselves and (b) I lack the necessary
> > platforms to test all the possible CPU/OS combinations to make sure
> > something sensible happens in all cases.
> >
> > Is this something that can be rolled into Numpy (the feature, not my
> > particular implementation or interface - though I'd be happy for it to
> > be so)?
> >
> > Regarding (b), I've written a test case that works for Linux on x86-64
> > with GCC (my platform!). I can test it on 32-bit windows, but that's it.
> > Is ARM supported by Numpy? Neon would be great to include as well. What
> > other platforms might need this?
> Your code looks simple and portable to me (at least the alignment
> part). I can see a good argument for adding this sort of functionality
> directly to numpy with a nice interface, though, since these kind of
> requirements seem quite common these days. Maybe an interface like
>   a = np.asarray([1, 2, 3], base_alignment=32)  # should this be in
> bits or in bytes?
>   b = np.empty((10, 10), order="C", base_alignment=32)
>   # etc.
>   assert a.base_alignment == 32
> which underneath tries to use posix_memalign/_aligned_malloc when
> possible, or falls back on the overallocation trick otherwise?
There is a thread about this from several years back. IIRC, David
Cournapeau was interested in the same problem. At first glance, the
alignment keyword looks interesting. One possible concern is keeping
alignment for rows, views, etc., which is probably not possible in any
sensible way. But people who need this most likely know what they are doing
and just need memory allocated on the proper boundary.

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