[Bug math/16337] New: ldbl-128 lgammal overflows for small negative arguments

[Bug math/16337] New: ldbl-128 lgammal overflows for small negative arguments

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=16337

            Bug ID: 16337
           Summary: ldbl-128 lgammal overflows for small negative
                    arguments
           Product: glibc
           Version: 2.19
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
          Assignee: unassigned at sourceware dot org
          Reporter: jsm28 at gcc dot gnu.org

The ldbl-128 implementation of lgammal overflows for small negative arguments. 
For example:

Failure: lgamma (-0x1p-10000): errno set to 34, expected 0 (unchanged)
Failure: Test: lgamma (-0x1p-10000)
Result:
 is:          inf   inf
 should be:   6.93147180559945309417e+03   0x1.b1378c84073b880c67e5p+12

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[Bug math/16337] ldbl-128 lgammal overflows for small negative arguments

[cvs-commit at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=16337

--- Comment #1 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
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- Log -----------------------------------------------------------------
https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=4f40e4b30754c678b7f93cfb9a545deb534f7e4e

commit 4f40e4b30754c678b7f93cfb9a545deb534f7e4e
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Sun Dec 22 20:50:16 2013 +0000

    Fix ldbl-128 lgammal for small negative arguments (bug 16337).

    This patch fixes bug 16337, ldbl-128 lgammal spurious overflows for
    small negative arguments (the arguments in question are already in the
    testsuite).  The implementation uses the reflection formula to compute
    lgamma of negative x from lgamma of -x, effectively resulting in a
    calculation -log(x^2) + log(-x); cancellation isn't problematic in
    this case (bugs for problematic cancellation in lgamma are 2542, 2543,
    2558), but the x^2 calculation can underflow (in which case there is
    spurious logic to return an overflowing value - lgamma can only ever
    correctly overflow for large positive arguments, though tgamma can
    overflow for small arguments of either sign as well as large positive
    arguments).  The fix is simply to calculate the result directly with
    logl when the argument is a small enough negative number.

    Tested mips64.

        * sysdeps/ieee754/ldbl-128/e_lgammal_r.c (__ieee754_lgammal_r):
        Calculate results for small negative arguments directly rather
        than using reflection formula with special underflow handling.

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog                              |    5 +++++
 NEWS                                   |    2 +-
 sysdeps/ieee754/ldbl-128/e_lgammal_r.c |    4 ++--
 3 files changed, 8 insertions(+), 3 deletions(-)

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[Bug math/16337] ldbl-128 lgammal overflows for small negative arguments

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=16337

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

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

--- Comment #2 from Joseph Myers <jsm28 at gcc dot gnu.org> ---
Fixed for 2.19.

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[Bug math/16337] ldbl-128 lgammal overflows for small negative arguments

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=16337

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
              Flags|                            |security-

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