[Bug math/18220] New: lgammaf spurious underflows

[Bug math/18220] New: lgammaf spurious underflows

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18220

            Bug ID: 18220
           Summary: lgammaf spurious underflows
           Product: glibc
           Version: 2.21
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
          Assignee: unassigned at sourceware dot org
          Reporter: jsm28 at gcc dot gnu.org

The flt-32 implementation of lgammaf produces spurious underflow exceptions. 
For example, on x86_64, lgammaf (0x3.8p56f) produces such an exception.

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[Bug math/18220] lgammaf spurious underflows

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18220

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 2.22.

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[Bug math/18220] lgammaf spurious underflows

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18220

--- 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=ff069f024ae8cf15d53429e034d67ddcece0f67a

commit ff069f024ae8cf15d53429e034d67ddcece0f67a
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Fri May 15 17:21:08 2015 +0000

    Fix lgammaf spurious underflows (bug 18220).

    The flt-32 implementation of lgammaf produces spurious underflow
    exceptions for some large arguments, because of calculations involving
    x^-2 multiplied by small constants.  This patch fixes this by
    adjusting the threshold for a simpler computation to 2**26 (the error
    in the simpler computation is on the order of 0.5 * log (x), for a
    result on the order of x * log (x)).

    Tested for x86_64 and x86.

        [BZ #18220]
        * sysdeps/ieee754/flt-32/e_lgammaf_r.c (__ieee754_lgammaf_r): Use
        2**26 not 2**58 as threshold for returning x * (log (x) - 1).
        * math/auto-libm-test-in: Add another test of lgamma.
        * math/auto-libm-test-out: Regenerated.

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

Summary of changes:
 ChangeLog                            |    8 ++++++++
 NEWS                                 |    2 +-
 math/auto-libm-test-in               |    1 +
 math/auto-libm-test-out              |   25 +++++++++++++++++++++++++
 sysdeps/ieee754/flt-32/e_lgammaf_r.c |    6 +++---
 5 files changed, 38 insertions(+), 4 deletions(-)

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