[Numpy-discussion] Fast vectorized arithmetic with ~32 significant digits under Numpy

Eric Moore ewm at redtetrahedron.org
Fri Dec 11 15:17:57 EST 2015

I have a mostly complete wrapping of the double-double type from the QD
library (http://crd-legacy.lbl.gov/~dhbailey/mpdist/) into a numpy dtype.
The real problem is, as david pointed out, user dtypes aren't quite full
equivalents of the builtin dtypes.  I can post the code if there is

Something along the lines of what's being discussed here would be nice,
since the extended type is subject to such variation.


On Fri, Dec 11, 2015 at 12:51 PM, Charles R Harris <
charlesr.harris at gmail.com> wrote:

> On Fri, Dec 11, 2015 at 10:45 AM, Nathaniel Smith <njs at pobox.com> wrote:
>> On Dec 11, 2015 7:46 AM, "Charles R Harris" <charlesr.harris at gmail.com>
>> wrote:
>> >
>> >
>> >
>> > On Fri, Dec 11, 2015 at 6:25 AM, Thomas Baruchel <baruchel at gmx.com>
>> wrote:
>> >>
>> >> From time to time it is asked on forums how to extend precision of
>> computation on Numpy array. The most common answer
>> >> given to this question is: use the dtype=object with some arbitrary
>> precision module like mpmath or gmpy.
>> >> See
>> http://stackoverflow.com/questions/6876377/numpy-arbitrary-precision-linear-algebra
>> or
>> http://stackoverflow.com/questions/21165745/precision-loss-numpy-mpmath
>> or
>> http://stackoverflow.com/questions/15307589/numpy-array-with-mpz-mpfr-values
>> >>
>> >> While this is obviously the most relevant answer for many users
>> because it will allow them to use Numpy arrays exactly
>> >> as they would have used them with native types, the wrong thing is
>> that from some point of view "true" vectorization
>> >> will be lost.
>> >>
>> >> With years I got very familiar with the extended double-double type
>> which has (for usual architectures) about 32 accurate
>> >> digits with faster arithmetic than "arbitrary precision types". I even
>> used it for research purpose in number theory and
>> >> I got convinced that it is a very wonderful type as long as such
>> precision is suitable.
>> >>
>> >> I often implemented it partially under Numpy, most of the time by
>> trying to vectorize at a low-level the libqd library.
>> >>
>> >> But I recently thought that a very nice and portable way of
>> implementing it under Numpy would be to use the existing layer
>> >> of vectorization on floats for computing the arithmetic operations by
>> "columns containing half of the numbers" rather than
>> >> by "full numbers". As a proof of concept I wrote the following file:
>> https://gist.github.com/baruchel/c86ed748939534d8910d
>> >>
>> >> I converted and vectorized the Algol 60 codes from
>> http://szmoore.net/ipdf/documents/references/dekker1971afloating.pdf
>> >> (Dekker, 1971).
>> >>
>> >> A test is provided at the end; for inverting 100,000 numbers, my type
>> is about 3 or 4 times faster than GMPY and almost
>> >> 50 times faster than MPmath. It should be even faster for some other
>> operations since I had to create another np.ones
>> >> array for testing this type because inversion isn't implemented here
>> (which could of course be done). You can run this file by yourself
>> >> (maybe you will have to discard mpmath or gmpy if you don't have it).
>> >>
>> >> I would like to discuss about the way to make available something
>> related to that.
>> >>
>> >> a) Would it be relevant to include that in Numpy ? (I would think to
>> some "contribution"-tool rather than including it in
>> >> the core of Numpy because it would be painful to code all ufuncs; on
>> the other hand I am pretty sure that many would be happy
>> >> to perform several arithmetic operations by knowing that they can't
>> use cos/sin/etc. on this type; in other words, I am not
>> >> sure it would be a good idea to embed it as an every-day type but I
>> think it would be nice to have it quickly available
>> >> in some way). If you agree with that, in which way should I code it
>> (the current link only is a "proof of concept"; I would
>> >> be very happy to code it in some cleaner way)?
>> >>
>> >> b) Do you think such attempt should remain something external to Numpy
>> itself and be released on my Github account without being
>> >> integrated to Numpy?
>> >
>> >
>> > I think astropy does something similar for time and dates. There has
>> also been some talk of adding a user type for ieee 128 bit doubles. I've
>> looked once for relevant code for the latter and, IIRC, the available
>> packages were GPL :(.
>> You're probably thinking of the __float128 support in gcc, which relies
>> on a LGPL (not GPL) runtime support library. (LGPL = any patches to the
>> support library itself need to remain open source, but no restrictions are
>> imposed on code that merely uses it.)
>> Still, probably something that should be done outside of numpy itself for
>> now.
> No, there are several other software packages out there. I know of the gcc
> version, but was looking for something more portable.
> Chuck
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