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[Bug math/22702] [powerpc-nofpu] nearbyintl traps with trapping "inexact"


https://sourceware.org/bugzilla/show_bug.cgi?id=22702

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=da09e6fa2f8194fbdd82281195145aec195144dc

commit da09e6fa2f8194fbdd82281195145aec195144dc
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Thu Jan 11 18:18:46 2018 +0000

    Make default libc_feholdsetround_noex_ctx use __feholdexcept (bug 22702).

    For soft-float powerpc, the math/test-nearbyint-except-2 test fails
    because nearbyintl traps when traps on "inexact" are enabled on entry
    (and an "inexact" exception is generated internally, though cleared
    for the final return).

    The problem is the default implementation of
    libc_feholdsetround_noex_ctx, which does not disable exception traps.
    There is some ambiguity about whether the *noex* interfaces are
    required to do so or only permitted to do so.  But given that we
    support fe* interfaces to enable and disable traps (on architectures
    with that functionality), functions that must not raise an exception
    (must not leave the flag set on exit if not set on entry) should also
    not trap on it when traps on that exception are enabled.  So it is
    appropriate to define these interfaces to have the feholdexcept effect
    of disabling exception traps; this patch updates the default
    implementation and comments accordingly.

    At least some architecture versions already disable traps; there are
    few uses of the *noex* interfaces at all, and while it's possible
    there are bugs on any architecture versions failing to disable traps
    that appear in the exp2 and remainder implementations, there are
    currently no tests, other than this one for nearbyintl (where only the
    ldbl-128ibm implementation uses SET_RESTORE_ROUND_NOEX), that would
    fail as a result of such a bug.  (Hard-float powerpc does disable
    traps here, hence the nearbyintl failure not appearing there.)

    Tested for powerpc (soft-float).  This brings that configuration to
    clean math/ test results, provided you build with GCC 8 to get the fix
    for GCC bug 64811.

        [BZ #22702]
        * sysdeps/generic/math_private.h (libc_feresetround_noex): Update
        comment to say exceptions are discarded.
        (libc_feholdsetround_noex_ctx): Use __feholdexcept instead of
        __fegetenv.
        (SET_RESTORE_ROUND_NOEX): Update comment to say non-stop mode must
        be enabled.

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

Summary of changes:
 ChangeLog                      |   10 ++++++++++
 sysdeps/generic/math_private.h |    9 +++++----
 2 files changed, 15 insertions(+), 4 deletions(-)

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