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@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/