[Bug math/18346] ldbl-128 roundl incorrect for exponents in [31, 47]

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https://sourceware.org/bugzilla/show_bug.cgi?id=18346

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=7d0b2575416aec2717e8665287d0ab77826a0ade

commit 7d0b2575416aec2717e8665287d0ab77826a0ade
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Tue Apr 28 17:27:02 2015 +0000

    Fix ldbl-128 roundl for exponents in [31, 47] (bug 18346).

    The implementation of roundl for ldbl-128 involves undefined behavior
    for arguments with exponents from 31 to 47 inclusive, from the shift:

          u_int64_t i = -1ULL >> (j0 - 48);

    For example, on mips64, this means roundl (0xffffffffffff.8p0L)
    wrongly returns its argument, which is not an integer.  A condition
    checking for exponents < 31 should actually be checking for exponents
    < 48, and this patch makes it do so.  (That condition is for whether
    the bit representing 0.5 is in the high 64-bit half of the
    floating-point number.  The value 31 might have arisen from an
    incorrect conversion of the ldbl-96 version to handle ldbl-128.)

    This was originally reported as a GCC libquadmath bug
    <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65757>.

    Tested for mips64; also tested for x86_64 and x86 to make sure the new
    tests pass there.

        [BZ #18346]
        * sysdeps/ieee754/ldbl-128/s_roundl.c (__roundl): Handle all
        exponents less than 48 as cases where high part of mantissa needs
        examining to determine whether argument is integral.
        * math/libm-test.inc (round_test_data): Add more tests.

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

Summary of changes:
 ChangeLog                           |    8 ++++++++
 NEWS                                |    2 +-
 math/libm-test.inc                  |   11 +++++++++++
 sysdeps/ieee754/ldbl-128/s_roundl.c |    2 +-
 4 files changed, 21 insertions(+), 2 deletions(-)

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