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 <st...@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@python.org
> https://mail.python.org/mailman/listinfo/python-ideas
> Code of Conduct: http://python.org/psf/codeofconduct/
>
_______________________________________________
Python-ideas mailing list
Python-ideas@python.org
https://mail.python.org/mailman/listinfo/python-ideas
Code of Conduct: http://python.org/psf/codeofconduct/
