[Numpy-discussion] Bug in 1.9?

[Charles R Harris]

On Wed, Oct 22, 2014 at 12:28 PM, Julian Taylor <
jtaylor.debian at googlemail.com> wrote:

> On 22.10.2014 20:00, Charles R Harris wrote:
> >
> >
> > On Wed, Oct 22, 2014 at 11:32 AM, Neil Girdhar <mistersheik at gmail.com
> > <mailto:mistersheik at gmail.com>> wrote:
> >
> >     Hello,
> >
> >     Is this desired behaviour or a regression or a bug?
> >
> >
> http://stackoverflow.com/questions/26497656/how-do-i-align-a-numpy-record-array-recarray
> >
> >     Thanks,
> >
> >
> > I'd guess that the definition of aligned may have become stricter,
> > that's the only thing I think has changed. Maybe Julian can comment on
> that.
> >
>
> structured dtypes have not really a well defined alignment, e.g. the
> stride of this is 12, so when element 0 is aligned element 1 is always
> unaligned.
>
> Before 1.9 structured dtype always had the aligned flag set, even if
> they were unaligned.
> Now we require a minimum alignment of 16 for strings and structured
> types so copying which sometimes works on the whole compound type
> instead of each item always works.
> This was the easiest way to get the testsuite running on sparc after
> fixing a couple of code paths not updating alignment information which
> forced some functions to always take super slow unaligned paths (e.g.
> ufunc.at)
> But the logic could certainly be improved.
>

The stackexchange example:

In [9]: a = np.zeros(4, dtype=dtype([('x', '<f8'), ('y', '<i4')],
align=False))

In [10]: a.data
Out[10]: <read-write buffer for 0x2f94440, size 48, offset 0 at 0x2f8caf0>

In [11]: a = np.zeros(4, dtype=dtype([('x', '<f8'), ('y', '<i4')],
align=True))
In [12]: a.data
Out[12]: <read-write buffer for 0x2f94030, size 64, offset 0 at 0x2f8c5b0>

Note that using an aligned dtype yields a different size on my 64 bit
system and 64 / 4 = 16.

Chuck
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