[Bug math/14797] New: fma use of a * b + c on overflow incorrect in directed rounding modes

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=14797

             Bug #: 14797
           Summary: fma use of a * b + c on overflow incorrect in directed
                    rounding modes
           Product: glibc
           Version: 2.16
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
        AssignedTo: unassigned@sourceware.org
        ReportedBy: jsm28@gcc.gnu.org
    Classification: Unclassified


Various fma implementations fall back to a * b + c when a * b is known to
overflow so much that the final result would still overflow, whatever the value
of c.  But this is incorrect in directed rounding modes where a * b may not be
an infinity.  Example using dbl-64/s_fma.c: fma (DBL_MAX, DBL_MAX, -DBL_MIN)
should return DBL_MAX in FE_DOWNWARD mode but returns the next smaller
floating-point number instead.

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