[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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