[Bug math/16789] clog / clog10 sign of zero result incorrect in round-downward mode

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https://sourceware.org/bugzilla/show_bug.cgi?id=16789

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=6f05bafebac30a389807979f8efbb709f84b486f

commit 6f05bafebac30a389807979f8efbb709f84b486f
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Wed Apr 2 13:10:19 2014 +0000

    Fix clog / clog10 sign of zero result in round-downward mode (bug 16789).

    This patch fixes bug 16789, incorrect sign of (real part) zero result
    from clog and clog10 in round-downward mode, arising from that real
    part being computed as 0 - 0.  To ensure that an underflow exception
    occurred, the code used an underflowing value (the next term in the
    series for log1p) in arithmetic computing the real part of the result,
    yielding the problematic 0 - 0 computation in some cases even when the
    mathematical result would be small but positive.  The patch changes
    this code to use the math_force_eval approach to ensuring that an
    underflowing computation actually occurs.  Tests of clog and clog10
    are enabled in all rounding modes.

    Tested x86_64 and x86 and ulps updated accordingly.

        [BZ #16789]
        * math/s_clog.c (__clog): Use math_force_eval to ensure underflow
        instead of using underflowing value in computing result.
        * math/s_clog10.c (__clog10): Likewise.
        * math/s_clog10f.c (__clog10f): Likewise.
        * math/s_clog10l.c (__clog10l): Likewise.
        * math/s_clogf.c (__clogf): Likewise.
        * math/s_clogl.c (__clogl): Likewise.
        * math/libm-test.inc (clog_test): Use ALL_RM_TEST.
        (clog10_test): Likewise.
        * sysdeps/i386/fpu/libm-test-ulps: Update.
        * sysdeps/x86_64/fpu/libm-test-ulps: Likewise.

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

Summary of changes:
 ChangeLog                         |   15 ++++++
 NEWS                              |    3 +-
 math/libm-test.inc                |    8 +--
 math/s_clog.c                     |    9 +--
 math/s_clog10.c                   |    9 +--
 math/s_clog10f.c                  |   11 +---
 math/s_clog10l.c                  |    7 ++-
 math/s_clogf.c                    |    9 +--
 math/s_clogl.c                    |    6 ++-
 sysdeps/i386/fpu/libm-test-ulps   |   96 +++++++++++++++++++++++++++++++++++++
 sysdeps/x86_64/fpu/libm-test-ulps |   96 +++++++++++++++++++++++++++++++++++++
 11 files changed, 233 insertions(+), 36 deletions(-)

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