[Python-ideas] Fused multiply-add (FMA)

[David Mertz]

My understanding is that NumPy does NOT currently support a direct FMA
operation "natively."  However, higher-level routines like
`numpy.linalg.solve` that are linked to MKL or BLAS DO take advantage of
FMA within the underlying libraries.

On Mon, Jan 16, 2017 at 10:06 AM, Guido van Rossum <gvanrossum at gmail.com>
wrote:

> Does numpy support this?
>
> --Guido (mobile)
>
> On Jan 16, 2017 7:27 AM, "Stephan Houben" <stephanh42 at gmail.com> wrote:
>
>> Hi Steve,
>>
>> Very good!
>> Here is a version which also handles the nan's, infinities,
>> negative zeros properly.
>>
>> ===============
>> import math
>> from fractions import Fraction
>>
>> def fma2(x, y, z):
>>     if math.isfinite(x) and math.isfinite(y) and math.isfinite(z):
>>         result = float(Fraction(x)*Fraction(y) + Fraction(z))
>>         if not result and not z:
>>             result = math.copysign(result, x*y+z)
>>     else:
>>         result = x * y + z
>>         assert not math.isfinite(result)
>>     return result
>> ===========================
>>
>> Stephan
>>
>>
>> 2017-01-16 12:04 GMT+01:00 Steven D'Aprano <steve at pearwood.info>:
>>
>>> On Mon, Jan 16, 2017 at 11:01:23AM +0100, Stephan Houben wrote:
>>>
>>> [...]
>>> > So the following would not be a valid FMA fallback
>>> >
>>> > double bad_fma(double x, double y, double z) {
>>> >   return x*y + z;
>>> > }
>>> [...]
>>> > Upshot: if we want to provide a software fallback in the Python code,
>>> we
>>> > need to do something slow and complicated like musl does.
>>>
>>> I don't know about complicated. I think this is pretty simple:
>>>
>>> from fractions import Fraction
>>>
>>> def fma(x, y, z):
>>>     # Return x*y + z with only a single rounding.
>>>     return float(Fraction(x)*Fraction(y) + Fraction(z))
>>>
>>>
>>> When speed is not the number one priority and accuracy is important,
>>> its hard to beat the fractions module.
>>>
>>>
>>> --
>>> Steve
>>> _______________________________________________
>>> Python-ideas mailing list
>>> Python-ideas at python.org
>>> https://mail.python.org/mailman/listinfo/python-ideas
>>> Code of Conduct: http://python.org/psf/codeofconduct/
>>>
>>
>>
>> _______________________________________________
>> Python-ideas mailing list
>> Python-ideas at python.org
>> https://mail.python.org/mailman/listinfo/python-ideas
>> Code of Conduct: http://python.org/psf/codeofconduct/
>>
>
> _______________________________________________
> Python-ideas mailing list
> Python-ideas at python.org
> https://mail.python.org/mailman/listinfo/python-ideas
> Code of Conduct: http://python.org/psf/codeofconduct/
>



-- 
Keeping medicines from the bloodstreams of the sick; food
from the bellies of the hungry; books from the hands of the
uneducated; technology from the underdeveloped; and putting
advocates of freedom in prisons.  Intellectual property is
to the 21st century what the slave trade was to the 16th.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20170116/ad05ce3c/attachment.html>