This is the mail archive of the libc-alpha@sourceware.org mailing list for the glibc project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: [BZ #15335] Don't use hard-coded qNaN values in sysdeps/ieee754/dbl-64/e_pow.c:__ieee754_pow


On Thursday, April 04, 2013 16:17:45 Thomas Schwinge wrote:
> Hi!
>
> On Thu, 04 Apr 2013 09:38:58 +0200, Andreas Jaeger <aj@suse.com>
wrote:
> > On Wednesday, April 03, 2013 22:03:31 Thomas Schwinge wrote:
> > > @@ -139,7 +141,7 @@ __ieee754_pow(double x, double y) {
> > >
> > >        }
> > >        else if (qx == 0x7ff00000)
> > >
> > >  	return y < 0 ? 0.0 : INF.x;
> > >
> > > -      return NaNQ.x;                              /* y not
> > > integer and x<0 */ +      return (x - x) / (x - x);
> > >     /* y not integer and x<0 */>
> > I'm not convinced of this change, the rest is fine.
> >
> > The above raises an invalid exception - but the previous code did
> > not. Why is this change still fine?
>
> If my code and specification reading is correct, then this is a bug in
> the original code, for IEEE 754-2008 says: »pow (x, y) signals the
> invalid operation exception for finite x < 0 and finite non-integer
> y«, and/but this didn't matter in practice, as the wrapper code in
> math/w_pow.c:__pow would catch this case and handle it separately via
> sysdeps/ieee754/k_standard.c:__kernel_standard, case 24, raising an
> INVALID exception on its own.  Can you confirm my reasoning?

I wonder whether your change really does, what it should.

You want to pass the same NAN back that was passed in - but
kernel_standard always passes a new NAN out. So, shouldn't you change
k_standard.c accordingly as well?

> We could be adding the respective tests to libm-tests with
> <http://news.gmane.org/find-root.php?message_id=%3C871uaqzheb.fsf%40sc
> hwinge.name%3E> (or something similar) in place.

libm-tests already checks for the invalid exception,

Andreas
--
 Andreas Jaeger aj@{suse.com,opensuse.org} Twitter/Identica: jaegerandi
  SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany
   GF: Jeff Hawn,Jennifer Guild,Felix Imendörffer,HRB16746 (AG Nürnberg)
    GPG fingerprint = 93A3 365E CE47 B889 DF7F  FED1 389A 563C C272 A126

Attachment: signature.asc
Description: This is a digitally signed message part.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]