[Bug math/22146] C++ build issue with float128 on x86_64

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https://sourceware.org/bugzilla/show_bug.cgi?id=22146

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=d37c951fde57e8acb320a9a7d437ba50a1fc3c8a

commit d37c951fde57e8acb320a9a7d437ba50a1fc3c8a
Author: Gabriel F. T. Gomes <gabriel@inconstante.eti.br>
Date:   Wed Sep 20 15:10:26 2017 -0300

    Let fpclassify use the builtin when optimizing for size in C++ mode (bug
22146)

    When optimization for size is on (-Os), fpclassify does not use the
    type-generic __builtin_fpclassify builtin, instead it uses __MATH_TG.
    However, when library support for float128 is available, __MATH_TG uses
    __builtin_types_compatible_p, which is not available in C++ mode.

    On the other hand, libstdc++ undefines (in cmath) many macros from
    math.h, including fpclassify, so that it can provide its own functions.
    However, during its configure tests, libstdc++ just tests for the
    availability of the macros (it does not undefine them, nor does it
    provide its own functions).

    Finally, when libstdc++ is configured with optimization for size
    enabled, its configure tests include math.h and get the definition of
    fpclassify that uses __MATH_TG (and __builtin_types_compatible_p).
    Since libstdc++ does not undefine the macros during its configure tests,
    they fail.

    This patch lets fpclassify use the builtin in C++ mode, even when
    optimization for size is on.  This allows the configure test in
    libstdc++ to work.

    Tested for powerpc64le and x86_64.

        [BZ #22146]
        math/math.h: Let fpclassify use the builtin in C++ mode, even
        when optimazing for size.

    (cherry picked from commit c5c4a626098ec884b8527356abdf2a4bb7b6bf27)

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

Summary of changes:
 ChangeLog   |    6 ++++++
 NEWS        |    1 +
 math/math.h |    8 +++++++-
 3 files changed, 14 insertions(+), 1 deletions(-)

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