lgamma_r and lgammaf_r return the wrong sign for -0.f
enh
enh at google.com
Fri Sep 12 22:52:14 UTC 2014
On Fri, Sep 12, 2014 at 3:41 PM, Steve Kargl
<sgk at troutmask.apl.washington.edu> wrote:
> On Fri, Sep 12, 2014 at 03:17:02PM -0700, enh via freebsd-numerics wrote:
>> On Fri, Sep 12, 2014 at 3:05 PM, Steve Kargl
>> > F.9.5.3 The lgamma functions
>> > -- lgamma(1) returns +0.
>> > -- lgamma(2) returns +0.
>> > -- lgamma(x) returns +inf and raises the ``divide-by-zero'' floating-point
>> > exception for x a negative integer or zero.
>> > -- lgamma(-inf) returns +inf.
>> > -- lgamma(+inf) returns +inf.
>> >
>> > See the 3rd bullet. -0 is 0 and -0 is a negative integer.
>
> Of course, neither POSIX nor ISO C specify lgamma_r() only lgamma.
> I just spent too much time on the 'divide-by-zero' bug, and
> conflated that with signgam.
i think the reporter's feeling was that *signgamp should be -1 because
as x approaches 0 from the negative side, Gamma(x) approaches
-Infinity. (though i suspect they noticed this when porting code from
glibc.)
>> > POSIX appears to defer to ISO C. n1570.pdf (committe draft for C11)
>> > has (almost?) identical text.
>> >
>> >> patch below (whitespace mangled courtesy of gmail). i'd prefer to wait
>> >> for this to be fixed in FreeBSD and pull down the fix rather than just
>> >> fix it locally.
>> >
>> > I have a bigger patch coming with ld80 and ld128 version of lgammal
>> > and lgammal_r.
>>
>> sorry, i haven't even looked at the *l variants.
>
> I only just finished writing the *l variants. It took me
> a long time to unwind the comments in e_lgamma_r.c,
> so that I could write the long double versions.
is there a freebsd-numerics-commits or equivalent i could subscribe to?
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