[Bug math/887] Math library function "logb" and "nextafter" inconsistent

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https://sourceware.org/bugzilla/show_bug.cgi?id=887

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=bc3753638a1c3c93dea071414909ce2729e3ca50

commit bc3753638a1c3c93dea071414909ce2729e3ca50
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Mon Oct 5 17:46:50 2015 +0000

    Work around powerpc32 integer 0 converting to -0 (bug 887, bug 19049, bug
19050).

    On powerpc32 hard-float, older processors (ones where fcfid is not
    available for 32-bit code), GCC generates conversions from integers to
    floating point that wrongly convert integer 0 to -0 instead of +0 in
    FE_DOWNWARD mode.  This in turn results in logb and a few other
    functions wrongly returning -0 when they should return +0.

    This patch works around this issue in glibc as I proposed in
    <https://sourceware.org/ml/libc-alpha/2015-09/msg00728.html>, so that
    the affected functions can be correct and the affected tests pass in
    the absence of a GCC fix for this longstanding issue (GCC bug 67771 -
    if fixed, of course we can put in GCC version conditionals, and
    eventually phase out the workarounds).  A new macro
    FIX_INT_FP_CONVERT_ZERO is added in a new sysdeps header
    fix-int-fp-convert-zero.h, and the powerpc32/fpu version of that
    header defines the macro based on the results of a configure test for
    whether such conversions use the fcfid instruction.

    Tested for x86_64 (that installed stripped shared libraries are
    unchanged by the patch) and powerpc (that HAVE_PPC_FCFID comes out to
    0 as expected and that the relevant tests are fixed).  Also tested a
    build with GCC configured for -mcpu=power4 and verified that
    HAVE_PPC_FCFID comes out to 1 in that case.

    There are still some other issues to fix to get test-float and
    test-double passing cleanly for older powerpc32 processors (apart from
    the need for an ulps regeneration for powerpc).  (test-ldouble will be
    harder to get passing cleanly, but with a combination of selected
    fixes to ldbl-128ibm code that don't involve significant performance
    issues, allowing spurious underflow and inexact exceptions for that
    format, and lots of XFAILing for the default case of unpatched libgcc,
    it should be doable.)

        [BZ #887]
        [BZ #19049]
        [BZ #19050]
        * sysdeps/generic/fix-int-fp-convert-zero.h: New file.
        * sysdeps/ieee754/dbl-64/e_log10.c: Include
        <fix-int-fp-convert-zero.h>.
        (__ieee754_log10): Adjust signs as needed if FIX_INT_FP_CONVERT_ZERO.
        * sysdeps/ieee754/dbl-64/e_log2.c: Include
        <fix-int-fp-convert-zero.h>.
        (__ieee754_log2): Adjust signs as needed if FIX_INT_FP_CONVERT_ZERO.
        * sysdeps/ieee754/dbl-64/s_erf.c: Include
        <fix-int-fp-convert-zero.h>.
        (__erfc): Adjust signs as needed if FIX_INT_FP_CONVERT_ZERO.
        * sysdeps/ieee754/dbl-64/s_logb.c: Include
        <fix-int-fp-convert-zero.h>.
        (__logb): Adjust signs as needed if FIX_INT_FP_CONVERT_ZERO.
        * sysdeps/ieee754/flt-32/e_log10f.c: Include
        <fix-int-fp-convert-zero.h>.
        (__ieee754_log10f): Adjust signs as needed if FIX_INT_FP_CONVERT_ZERO.
        * sysdeps/ieee754/flt-32/e_log2f.c: Include
        <fix-int-fp-convert-zero.h>.
        (__ieee754_log2f): Adjust signs as needed if FIX_INT_FP_CONVERT_ZERO.
        * sysdeps/ieee754/flt-32/s_erff.c: Include
        <fix-int-fp-convert-zero.h>.
        (__erfcf): Adjust signs as needed if FIX_INT_FP_CONVERT_ZERO.
        * sysdeps/ieee754/flt-32/s_logbf.c: Include
        <fix-int-fp-convert-zero.h>.
        (__logbf): Adjust signs as needed if FIX_INT_FP_CONVERT_ZERO.
        * sysdeps/ieee754/ldbl-128ibm/s_erfl.c: Include
        <fix-int-fp-convert-zero.h>.
        (__erfcl): Adjust signs as needed if FIX_INT_FP_CONVERT_ZERO.
        * sysdeps/ieee754/ldbl-128ibm/s_logbl.c: Include
        <fix-int-fp-convert-zero.h>.
        (__logbl): Adjust signs as needed if FIX_INT_FP_CONVERT_ZERO.
        * sysdeps/powerpc/powerpc32/fpu/configure.ac: New file.
        * sysdeps/powerpc/powerpc32/fpu/configure: New generated file.
        * sysdeps/powerpc/powerpc32/fpu/fix-int-fp-convert-zero.h: New
        file.
        * config.h.in [_LIBC] (HAVE_PPC_FCFID): New macro.

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

Summary of changes:
 ChangeLog                                          |   42 ++++++++++++++++++++
 NEWS                                               |   18 ++++----
 config.h.in                                        |    3 +
 sysdeps/generic/fix-int-fp-convert-zero.h          |   27 +++++++++++++
 sysdeps/ieee754/dbl-64/e_log10.c                   |    3 +
 sysdeps/ieee754/dbl-64/e_log2.c                    |    7 +++-
 sysdeps/ieee754/dbl-64/s_erf.c                     |    6 ++-
 sysdeps/ieee754/dbl-64/s_logb.c                    |    3 +
 sysdeps/ieee754/flt-32/e_log10f.c                  |    3 +
 sysdeps/ieee754/flt-32/e_log2f.c                   |    8 +++-
 sysdeps/ieee754/flt-32/s_erff.c                    |    6 ++-
 sysdeps/ieee754/flt-32/s_logbf.c                   |    3 +
 sysdeps/ieee754/ldbl-128ibm/s_erfl.c               |    6 ++-
 sysdeps/ieee754/ldbl-128ibm/s_logbl.c              |    3 +
 sysdeps/powerpc/powerpc32/fpu/configure            |   29 +++++++++++++
 sysdeps/powerpc/powerpc32/fpu/configure.ac         |   18 ++++++++
 .../powerpc32/fpu/fix-int-fp-convert-zero.h        |   28 +++++++++++++
 17 files changed, 199 insertions(+), 14 deletions(-)
 create mode 100644 sysdeps/generic/fix-int-fp-convert-zero.h
 create mode 100644 sysdeps/powerpc/powerpc32/fpu/configure
 create mode 100644 sysdeps/powerpc/powerpc32/fpu/configure.ac
 create mode 100644 sysdeps/powerpc/powerpc32/fpu/fix-int-fp-convert-zero.h

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