[Bug math/14868] New: ldbl-128ibm hypotl inaccurate for arguments differing by 2^60

[Bug math/14868] New: ldbl-128ibm hypotl inaccurate for arguments differing by 2^60

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=14868

             Bug #: 14868
           Summary: ldbl-128ibm hypotl inaccurate for arguments differing
                    by 2^60
           Product: glibc
           Version: 2.16
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
        AssignedTo: unassigned@sourceware.org
        ReportedBy: jsm28@gcc.gnu.org
    Classification: Unclassified
              Host: powerpc*


The ldbl-128ibm implementation of hypotl returns a+b if the absolute values of
the arguments have a ratio of over 2^60.  But while in such a case it's true
that the round-to-nearest result will be the larger absolute value of an
argument, a+b isn't the correct way to compute that result, and results in
large errors until the ratio is over 2^106.  For example, hypotl (1.0L,
0x1p-61L) has a 17592186044416 ulp error (as computed using libm-test.inc). 
This appears to be what's causing 16 ulp errors for csqrtl tests as well.

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[Bug math/14868] ldbl-128ibm hypotl inaccurate for arguments differing by 2^60

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=14868

Joseph Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED

--- Comment #1 from Joseph Myers <jsm28 at gcc dot gnu.org> 2012-11-22 15:03:25 UTC ---
Fixed for 2.17 by:

commit 8e27e3cc453849158e069dbed0eea7b8113ab8ff
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Thu Nov 22 14:57:22 2012 +0000

    Fix ldbl-128ibm hypotl inaccuracy for arguments with large ratio (bug
14868).

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[Bug math/14868] ldbl-128ibm hypotl inaccurate for arguments differing by 2^60

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=14868

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |fweimer at redhat dot com
              Flags|                            |security-

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[Bug math/14868] ldbl-128ibm hypotl inaccurate for arguments differing by 2^60

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=14868

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|fweimer at redhat dot com          |

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