dead code in lgamma_r[f]?
Steve Kargl
sgk at troutmask.apl.washington.edu
Sat Dec 7 06:46:09 UTC 2013
On Fri, Dec 06, 2013 at 12:55:16PM +1100, Bruce Evans wrote:
> On Fri, 6 Dec 2013, Bruce Evans wrote:
>
> > On Thu, 5 Dec 2013, Steve Kargl wrote:
> > ...
> >> If we again look at the code from __ieee754_lgamma_r(), we see
> >> that sin_pi() is called if ix < 0x43300000, so by the time we
> >> arrive at the 'if(ix<0x43300000)' statement we already know that
> >> the condition is true.
> >
> > No, only for negative integers. hx<0 classifies negative values, and
> > then ix>=0x43300000 classifies numbers that are so large negative that
> > they must be integers, and the result is a sort of pole error. We
> > are just filtering out this case, perhaps as an optimization.
>
> Oops, sin_pi() is only called for negative integers, so your change
> seems to be correct. Just add a comment about the limited domain
> of sin_pi() (it already has one saying that "x is assumed negative".
>
I wish to retract my earlier statement that after 2 additional
years of reading fdlibm code that it was easier to work with.
I spent the better part of Friday giving myself a headache trying
to understand the algorithm for lgamma_r(). The code for x in
the interval (0,2) does not match any comment in lgamma_r().
I also think, but can't prove yet, that like erff() the
polynomial and rational approximations in lgammaf_r() have
too many terms.
--
Steve
More information about the freebsd-numerics
mailing list