[Bug math/19602] [ldbl-128ibm] fmodl handling of equal arguments with low part zero incorrect

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https://sourceware.org/bugzilla/show_bug.cgi?id=19602

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=0fed79a8279bb4d4483895a742457946faf89594

commit 0fed79a8279bb4d4483895a742457946faf89594
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Thu Feb 18 22:54:07 2016 +0000

    Fix ldbl-128ibm fmodl handling of equal arguments with low part zero (bug
19602).

    The ldbl-128ibm implementation of fmodl has logic to detect when the
    first argument has absolute value less than or equal to the second.
    This logic is only correct for nonzero low parts; if the high parts
    are equal and the low parts are zero, then the signs of the low parts
    (which have no semantic effect on the value of the long double number)
    can result in equal values being wrongly treated as unequal, and an
    incorrect result being returned from fmodl.  This patch fixes this by
    checking for the case of zero low parts.

    Although this does show up in tests from libm-test.inc (both tests of
    fmodl, and, indirectly, of remainderl / dreml), the dependence on
    non-semantic zero low parts means that test shouldn't be expected to
    reproduce it reliably; thus, this patch adds a standalone test that
    sets up affected values using unions.

    Tested for powerpc.

        [BZ #19602]
        * sysdeps/ieee754/ldbl-128ibm/e_fmodl.c (__ieee754_fmodl): Handle
        equal high parts and both low parts zero specially.
        * sysdeps/ieee754/ldbl-128ibm/test-fmodl-ldbl-128ibm.c: New test.
        * sysdeps/ieee754/ldbl-128ibm/Makefile [$(subdir) = math] (tests):
        Add test-fmodl-ldbl-128ibm.

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

Summary of changes:
 ChangeLog                                          |    7 ++
 sysdeps/ieee754/ldbl-128ibm/Makefile               |    4 +
 sysdeps/ieee754/ldbl-128ibm/e_fmodl.c              |    7 ++
 .../ieee754/ldbl-128ibm/test-fmodl-ldbl-128ibm.c   |   77 ++++++++++++++++++++
 4 files changed, 95 insertions(+), 0 deletions(-)
 create mode 100644 sysdeps/ieee754/ldbl-128ibm/test-fmodl-ldbl-128ibm.c

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