[Bug math/17042] New: log2 (1) incorrect in round-downward mode

[Bug math/17042] New: log2 (1) incorrect in round-downward mode

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=17042

            Bug ID: 17042
           Summary: log2 (1) incorrect in round-downward mode
           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

At least for long double on x86_64, possibly in other cases, log2 (1) returns
-0 in round-downward mode when it should return +0 in all rounding modes.

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[Bug math/17042] log2 (1) incorrect in round-downward mode

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=17042

--- Comment #1 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
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The branch, master has been updated
       via  f8ba1b5654d876c8db4c9a020e6fa976d434f988 (commit)
      from  2c3520d98d9773d9e092196c3c36150244eed1dd (commit)

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- Log -----------------------------------------------------------------
https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=f8ba1b5654d876c8db4c9a020e6fa976d434f988

commit f8ba1b5654d876c8db4c9a020e6fa976d434f988
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Tue Jun 10 12:07:15 2014 +0000

    Fix log2 (1) in round-downward mode (bug 17042).

    As with other issues of this kind, bug 17042 is log2 (1) wrongly
    returning -0 instead of +0 in round-downward mode because of
    implementations effectively in terms of log1p (x - 1).  This patch
    fixes the issue in the same way used for log and log10.

    Tested x86_64 and x86 and ulps updated accordingly.  Also tested for
    mips64 to confirm a fix was needed for ldbl-128 and to validate that
    fix (also applied to ldbl-128ibm since that version of log2l is
    essentially the same as the ldbl-128 one).

        [BZ #17042]
        * sysdeps/i386/fpu/e_log2.S (__ieee754_log2): Take absolete value
        when x - 1 is zero.
        * sysdeps/i386/fpu/e_log2f.S (__ieee754_log2f): Likewise.
        * sysdeps/i386/fpu/e_log2l.S (__ieee754_log2l): Likewise.
        * sysdeps/ieee754/ldbl-128/e_log2l.c (__ieee754_log2l): Return
        0.0L for an argument of 1.0L.
        * sysdeps/ieee754/ldbl-128ibm/e_log2l.c (__ieee754_log2l):
        Likewise.
        * sysdeps/x86_64/fpu/e_log2l.S (__ieee754_log2l): Take absolute
        value when x - 1 is zero.
        * math/libm-test.inc (log2_test): Use ALL_RM_TEST.
        * sysdeps/i386/fpu/libm-test-ulps: Update.
        * sysdeps/x86_64/fpu/libm-test-ulps: Likewise.

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

Summary of changes:
 ChangeLog                             |   17 +++++++++++++++++
 NEWS                                  |    2 +-
 math/libm-test.inc                    |    4 +---
 sysdeps/i386/fpu/e_log2.S             |    8 +++++++-
 sysdeps/i386/fpu/e_log2f.S            |    8 +++++++-
 sysdeps/i386/fpu/e_log2l.S            |    8 +++++++-
 sysdeps/i386/fpu/libm-test-ulps       |   16 ++++++++++++++++
 sysdeps/ieee754/ldbl-128/e_log2l.c    |    3 +++
 sysdeps/ieee754/ldbl-128ibm/e_log2l.c |    3 +++
 sysdeps/x86_64/fpu/e_log2l.S          |    8 +++++++-
 sysdeps/x86_64/fpu/libm-test-ulps     |   22 ++++++++++++++++++++++
 11 files changed, 91 insertions(+), 8 deletions(-)

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[Bug math/17042] log2 (1) incorrect in round-downward mode

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=17042

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 2.20.

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[Bug math/17042] log2 (1) incorrect in round-downward mode

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=17042

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/17042] log2 (1) incorrect in round-downward mode

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=17042

Florian Weimer <fweimer at redhat dot com> changed:

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

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