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[Bug math/19790] [ldbl-128ibm] nearbyintl incorrect in non-default rounding modes


https://sourceware.org/bugzilla/show_bug.cgi?id=19790

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=613c92b3b59df6a06784cde1d4f410cef0b6da96

commit 613c92b3b59df6a06784cde1d4f410cef0b6da96
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Wed Mar 9 00:30:59 2016 +0000

    Fix ldbl-128ibm nearbyintl in non-default rounding modes (bug 19790).

    The ldbl-128ibm implementation of nearbyintl uses logic that only
    works in round-to-nearest mode.  This contrasts with rintl, which
    works in all rounding modes.

    Now, arguably nearbyintl could simply be aliased to rintl, given that
    spurious "inexact" is generally allowed for ldbl-128ibm, even for the
    underlying arithmetic operations.  But given that the only point of
    nearbyintl is to avoid "inexact", this patch follows the more
    conservative approach of adding conditionals to the rintl
    implementation to make it suitable for use to implement nearbyintl,
    then builds it for nearbyintl with USE_AS_NEARBYINTL defined.  The
    test test-nearbyint-except-2 shows up issues when traps on "inexact"
    are enabled, which turn out to be problems with the powerpc
    fenv_private.h implementation (two functions that should disable
    exception traps potentially failing to do so in some cases); this
    patch duly fixes that as well (I don't see any other existing cases
    where this would be user-visible; there isn't much use of *_NOEX,
    *hold* etc. in libm that requires exceptions to be discarded and not
    trapped on).

    Tested for powerpc.

        [BZ #19790]
        * sysdeps/ieee754/ldbl-128ibm/s_rintl.c [USE_AS_NEARBYINTL]
        (rintl): Define as macro.
        [USE_AS_NEARBYINTL] (__rintl): Likewise.
        (__rintl) [USE_AS_NEARBYINTL]: Use SET_RESTORE_ROUND_NOEX instead
        of fesetround.  Ensure results are evaluated before end of scope.
        * sysdeps/ieee754/ldbl-128ibm/s_nearbyintl.c: Define
        USE_AS_NEARBYINTL and include s_rintl.c.
        * sysdeps/powerpc/fpu/fenv_private.h (libc_feholdsetround_ppc):
        Disable exception traps in new environment.
        (libc_feholdsetround_ppc_ctx): Likewise.

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

Summary of changes:
 ChangeLog                                  |   14 ++++
 sysdeps/ieee754/ldbl-128ibm/s_nearbyintl.c |  109 +---------------------------
 sysdeps/ieee754/ldbl-128ibm/s_rintl.c      |   14 ++++
 sysdeps/powerpc/fpu/fenv_private.h         |    4 +-
 4 files changed, 32 insertions(+), 109 deletions(-)

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