[Bug math/16620] [ldbl-128ibm] exp10l spurious overflows / bad directed rounding results

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https://sourceware.org/bugzilla/show_bug.cgi?id=16620

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=7cda516f5f23772fd37ca3a5e018fca5bf388435

commit 7cda516f5f23772fd37ca3a5e018fca5bf388435
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Wed Sep 30 23:34:59 2015 +0000

    Fix ldbl-128ibm exp10l spurious overflows (bug 16620).

    The ldbl-128ibm implementation of exp10l uses a version of log(10)
    split into high and low parts - but the low part is negative, so
    causing spurious overflows from __ieee754_expl (exp_high) in cases
    close to the overflow threshold (I added relevant tests close to the
    overflow threshold to the testsuite earlier today).  The same issue
    applies close to the underflow threshold as well (except that spurious
    underflows in IBM long double arithmetic are harder to fix than the
    other deficiencies, so we might end up permitting those for IBM long
    double in the libm testsuite, as permitted by ISO C).

    This patch fixes it to use a low part rounded downward to 48 bits
    instead.  (The choice of 48 instead of 53 bits is to make it more
    obviously safe even when the low part of the argument is negative.)

    Tested for powerpc.  (Note that because of libgcc bugs with
    multiplication very close to LDBL_MAX, libgcc also needs patching for
    all the problem cases to be fixed, but this patch is still safe and
    correct in the absence of such libgcc fixes.)

        [BZ #16620]
        * sysdeps/ieee754/ldbl-128ibm/e_exp10l.c (log10_high): Use value
        of log (10) rounded downward to 48 bits.
        (log10_low): Use corresponding low part of log (10).

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

Summary of changes:
 ChangeLog                              |    5 +++++
 NEWS                                   |   16 ++++++++--------
 sysdeps/ieee754/ldbl-128ibm/e_exp10l.c |    4 ++--
 3 files changed, 15 insertions(+), 10 deletions(-)

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