Tim Peters <[email protected]> added the comment:
Across millions of tries, same thing: Windows exp2 is off by at least 1 ulp
over a third of the time, and by over 2 ulp about 3 times per million. Still
haven't seen pow(2, x) off by as much as 0.52 ulp.
>From its behavior, it appears Windows implements exp2(x) like so:
i = floor(x)
x -= i # now 0 <= x < 1
return ldexp(exp2(x), i)
So it's apparently using some sub-state-of-the-art approximation to 2**x over
the domain [0, 1].
But a consequence is that it gets it exactly right whenever x is an integer, so
it's unlikely anyone will notice it's sloppy ;-)
I expect we should just live with it.
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<https://bugs.python.org/issue45917>
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