[Bug math/14796] New: fma use of Dekker multiplication in directed rounding modes unsafe

[Bug math/14796] New: fma use of Dekker multiplication in directed rounding modes unsafe

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=14796

             Bug #: 14796
           Summary: fma use of Dekker multiplication in directed rounding
                    modes unsafe
           Product: glibc
           Version: 2.16
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
        AssignedTo: unassigned@sourceware.org
        ReportedBy: jsm28@gcc.gnu.org
    Classification: Unclassified


fma uses Dekker multiplication, but in directed rounding modes this is unsafe
(at least for odd mantissa lengths); the rounding mode should be set to
FE_TONEAREST before using Dekker multiplication / Knuth addition.  Example,
when using dbl-64/s_fma.c: fma (0x0.fffffffffffff8p0, 0x0.fffffffffffff8p0,
-0x0.fffffffffffffp0) returns 0x1p-1074 in FE_DOWNWARD mode; it should return
0x1p-106 in all rounding modes.

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[Bug math/14796] fma use of Dekker multiplication in directed rounding modes unsafe

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=14796

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED

--- Comment #1 from Joseph Myers <jsm28 at gcc dot gnu.org> 2012-11-03 19:50:54 UTC ---
Fixed for 2.17 by:

commit 5b5b04d6282df0364424c6f2c0462e5c1a4394b0
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Sat Nov 3 19:48:53 2012 +0000

    Make fma use of Dekker and Knuth algorithms use round-to-nearest (bug
14796).

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[Bug math/14796] fma use of Dekker multiplication in directed rounding modes unsafe

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=14796

Florian Weimer <fweimer at redhat dot com> changed:

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

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