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

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

[jsm28 at gcc dot gnu.org]

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

            Bug ID: 16620
           Summary: [ldbl-128ibm] exp10l spurious overflows / bad directed
                    rounding results
           Product: glibc
           Version: 2.19
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
          Assignee: unassigned at sourceware dot org
          Reporter: jsm28 at gcc dot gnu.org
              Host: powerpc*-*-linux*

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 probably causing spurious
overflows from __ieee754_expl (exp_high) in cases close to the overflow
threshold (and definitely, given libgcc fixed for directed rounding modes,
results below LDBL_MAX for overflow when rounding downward / to zero just above
that threshold, which is the case where I found this problem).  The split
log(10) needs to be such that both parts are positive to avoid this issue.

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

[fweimer at redhat dot com]

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

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
              Flags|                            |security-

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

[cvs-commit at gcc dot gnu.org]

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

--- Comment #1 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
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The branch, master has been updated
       via  7cda516f5f23772fd37ca3a5e018fca5bf388435 (commit)
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- Log -----------------------------------------------------------------
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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[Bug math/16620] [ldbl-128ibm] exp10l spurious overflows / bad directed rounding results

[jsm28 at gcc dot gnu.org]

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

Joseph Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |2.23

--- Comment #2 from Joseph Myers <jsm28 at gcc dot gnu.org> ---
Fixed for 2.23.

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