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[Bug math/17899] [powerpc] floorl returns negative zero with FE_DOWNWARD


https://sourceware.org/bugzilla/show_bug.cgi?id=17899

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=1833769e19236119d6e41041a53e0f8047156c24

commit 1833769e19236119d6e41041a53e0f8047156c24
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Thu Feb 18 21:31:10 2016 +0000

    Fix ldbl-128ibm floorl for non-default rounding modes (bug 17899).

    The ldbl-128ibm implementation of floorl is only correct in
    round-to-nearest mode (in other modes, there are incorrect results and
    overflow exceptions in some cases going beyond the incorrect signs of
    zero results noted in bug 17899).  It is also unnecessarily
    complicated, rounding both high and low parts to the nearest integer
    and then adjusting for the semantics of floor, when it seems more
    natural to take the floor of the high part (__floor optimized inline
    versions can be used), and that of the low part if the high part is an
    integer.  This patch makes it use that simpler approach, with a
    canonicalization that works in all rounding modes (given that the only
    way the result can be noncanonical is if taking the floor of a
    negative noninteger low part increased its exponent).

    Tested for powerpc, where over a thousand failures are removed from
    test-ldouble.out (floorl problems affect many powl tests).

        [BZ #17899]
        * sysdeps/ieee754/ldbl-128ibm/math_ldbl.h (ldbl_canonicalize_int):
        New function.
        * sysdeps/ieee754/ldbl-128ibm/s_floorl.c (__floorl): Use __floor
        on high and low parts then use ldbl_canonicalize_int if needed.

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

Summary of changes:
 ChangeLog                               |    8 +++++
 sysdeps/ieee754/ldbl-128ibm/math_ldbl.h |   33 ++++++++++++++++++++++
 sysdeps/ieee754/ldbl-128ibm/s_floorl.c  |   45 +++++++++++--------------------
 3 files changed, 57 insertions(+), 29 deletions(-)

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