Re: [PATCH libquadmath/PR68686]

[Ed Smith-Rowland <3dw4rd at verizon dot net>]

On 11/5/18 1:19 PM, Joseph Myers wrote:
> On Sun, 4 Nov 2018, Ed Smith-Rowland wrote:
>
> > I looked in glibc.  Unfortunately, I see how they have the same mistake:
> > glibc/math/w_tgammal_compat.c:
> >      long double
> >      __tgammal(long double x)
> >      {
> >          int local_signgam;
> >          long double y = __ieee754_gammal_r(x,&local_signgam);
> >          ...
> >          return local_signgam < 0 ? - y : y;
> >      }
> > I'm very sure this is where tgammaq came from.
> > Ditto for glibc/math/w_tgamma_compat.c and glibc/math/w_tgammaf_compat.c.
> No, that's not a mistake.  __ieee754_gammal_r returns +/- the gamma
> function and sets the integer pointed to by the second argument to
> indicate whether to negate the result.  (This isn't a particularly good
> interface design for tgamma, as opposed to lgamma; unfortunately
> __gammal_r_finite, with this interface, is a public ABI.)
Excellent, I missed the replacement of  expq (lgammaq (x)) with the 
re-entrant lgamma with the good sign.
Thank you.  I'll let someone else check this off ;-)
FWIW, I'd like to see C++ at least return narrow types like
template<typename _Tp>
  struct lgamma_t
  {
    _Tp log_abs_gamma; // O something less verbose.
    int sign; // Maybe try for same size as _Tp.
  };
C could do something like also. That's a discussion for another forum.