[Bug math/18875] Excess precision leads incorrect libm

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https://sourceware.org/bugzilla/show_bug.cgi?id=18875

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=903af5af9a0404c1dfeb7b4db6d5a23a1e45593f

commit 903af5af9a0404c1dfeb7b4db6d5a23a1e45593f
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Mon Sep 14 22:00:12 2015 +0000

    Fix exp2 missing underflows (bug 16521).

    Various exp2 implementations in glibc can miss underflow exceptions
    when the scaling down part of the calculation is exact (or, in the x86
    case, when the conversion from extended precision to the target
    precision is exact).  This patch forces the exception in a similar way
    to previous fixes.

    The x86 exp2f changes may in fact not be needed for this purpose -
    it's likely to be the case that no argument of type float has an exp2
    result so close to an exact subnormal float value that it equals that
    value when rounded to 64 bits (even taking account of variation
    between different x86 implementations).  However, they are included
    for consistency with the changes to exp2 and so as to fix the exp2f
    part of bug 18875 by ensuring that excess range and precision is
    removed from underflowing return values.

    Tested for x86_64, x86 and mips64.

        [BZ #16521]
        [BZ #18875]
        * math/e_exp2l.c (__ieee754_exp2l): Force underflow exception for
        small results.
        * sysdeps/i386/fpu/e_exp2.S (dbl_min): New object.
        (MO): New macro.
        (__ieee754_exp2): For small results, force underflow exception and
        remove excess range and precision from return value.
        * sysdeps/i386/fpu/e_exp2f.S (flt_min): New object.
        (MO): New macro.
        (__ieee754_exp2f): For small results, force underflow exception
        and remove excess range and precision from return value.
        * sysdeps/i386/fpu/e_exp2l.S (ldbl_min): New object.
        (MO): New macro.
        (__ieee754_exp2l): Force underflow exception for small results.
        * sysdeps/ieee754/dbl-64/e_exp2.c (__ieee754_exp2): Likewise.
        * sysdeps/ieee754/flt-32/e_exp2f.c (__ieee754_exp2f): Likewise.
        * sysdeps/x86_64/fpu/e_exp2l.S (ldbl_min): New object.
        (MO): New macro.
        (__ieee754_exp2l): Force underflow exception for small results.
        * math/auto-libm-test-in: Add more tests or exp2.
        * math/auto-libm-test-out: Regenerated.

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

Summary of changes:
 ChangeLog                        |   25 ++
 NEWS                             |    8 +-
 math/auto-libm-test-in           |   23 ++
 math/auto-libm-test-out          |  575 ++++++++++++++++++++++++++++++++++++++
 math/e_exp2l.c                   |   12 +-
 sysdeps/i386/fpu/e_exp2.S        |   34 +++-
 sysdeps/i386/fpu/e_exp2f.S       |   34 +++-
 sysdeps/i386/fpu/e_exp2l.S       |   29 ++-
 sysdeps/ieee754/dbl-64/e_exp2.c  |   10 +-
 sysdeps/ieee754/flt-32/e_exp2f.c |   10 +-
 sysdeps/x86_64/fpu/e_exp2l.S     |   25 ++-
 11 files changed, 773 insertions(+), 12 deletions(-)

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