This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
Re: Fix powl inaccuracy for ldbl-128ibm (bug 14914)
On Tue, 4 Dec 2012, David Miller wrote:
> From: "Joseph S. Myers" <joseph@codesourcery.com>
> Date: Mon, 3 Dec 2012 22:42:26 +0000
>
> > Bug 14914 is inaccuracy of powl for ldbl-128ibm, which showed up in
> > validation for 2.17.
> >
> > In various places the code, based on that for ldbl-128, tries to split
> > numbers into high and low parts in order to carry out operations on
> > those parts separately, with at least some of the high-part operations
> > needing to be exact. However, the way the code extracts high parts is
> > to clear the low 47 bits of the mantissa. I've no idea how that
> > figure was chosen - the ldbl-128 code clears 59 bits, i.e. more than
> > half the mantissa - but as shown by the large errors for powl in the
> > testsuite, it doesn't work well. This patch changes the code to clear
> > the whole low double of the long double value. This eliminates all
> > the ulps seen for the tests recently added or enabled for long double;
> > no ulps at all are needed for pow for powerpc test-ldouble after this
> > patch. (Tested powerpc32, hard float; if this goes in then I'll
> > retest powerpc-nofpu to see if any pow ulps updates are needed there.)
>
> There aren't any major powl ULPs for sparc, so presumably ldbl-128 is
> OK?
Yes. The problem is with how the ldbl-128 code was adapated for
ldbl-128ibm.
--
Joseph S. Myers
joseph@codesourcery.com