[Bug math/13932] dbl-64 pow unexpectedly slow for some inputs

[vincent-srcware at vinc17 dot net]

https://sourceware.org/bugzilla/show_bug.cgi?id=13932

--- Comment #16 from Vincent Lefèvre <vincent-srcware at vinc17 dot net> ---
(In reply to Wilco from comment #15)
> Removing buggy code is far better than fixing it...

The buggy code is in the fast path. You didn't remove it.

> (In reply to Vincent Lefèvre from comment #13)
> > For the example of slow case pow(0.562500, 1.5), the exact result is
> > 0.421875 = 27/64, so that cor should be much smaller than ulp(res) and a
> > positive value should be returned, which doesn't explain the behavior.
> 
> Whenever you're close to 0.5 ULP,

It is not close to 0.5 ulp. This is an *exact* case (27/64), so that the
correction term should be very small, much smaller than 0.5 ulp.

The correction term is close to 0.5 ulp (possibly above due to approximation
errors) when the exact result is a midpoint or close to a midpoint (cases where
the Table Maker's Dilemma occurs).

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