[Bug math/28358] f64xdivf128 and f64xmulf128 spurious underflows

["cvs-commit at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28358

--- Comment #1 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Joseph Myers <jsm28@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=1356f38df5be0776823eb2c40cc4e607b86b9680

commit 1356f38df5be0776823eb2c40cc4e607b86b9680
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Tue Sep 21 21:54:37 2021 +0000

    Fix f64xdivf128, f64xmulf128 spurious underflows (bug 28358)

    As described in bug 28358, the round-to-odd computations used in the
    libm functions that round their results to a narrower format can yield
    spurious underflow exceptions in the following circumstances: the
    narrowing only narrows the precision of the type and not the exponent
    range (i.e., it's narrowing _Float128 to _Float64x on x86_64, x86 or
    ia64), the architecture does after-rounding tininess detection (which
    applies to all those architectures), the result is inexact, tiny
    before rounding but not tiny after rounding (with the chosen rounding
    mode) for _Float64x (which is possible for narrowing mul, div and fma,
    not for narrowing add, sub or sqrt), so the underflow exception
    resulting from the toward-zero computation in _Float128 is spurious
    for _Float64x.

    Fixed by making ROUND_TO_ODD call feclearexcept (FE_UNDERFLOW) in the
    problem cases (as indicated by an extra argument to the macro); there
    is never any need to preserve underflow exceptions from this part of
    the computation, because the conversion of the round-to-odd value to
    the narrower type will underflow in exactly the cases in which the
    function should raise that exception, but it may be more efficient to
    avoid the extra manipulation of the floating-point environment when
    not needed.

    Tested for x86_64 and x86, and with build-many-glibcs.py.

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