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

[Thomas Schwinge <thomas at codesourcery dot com>]

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: Âpow (x, y) signals the
invalid operation exception for finite x < 0 and finite non-integer yÂ,
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?

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


GrÃÃe,
 Thomas

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