[Bug math/14793] New: Further spurious underflows from fma when a * b is small

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=14793

             Bug #: 14793
           Summary: Further spurious underflows from fma when a * b is
                    small
           Product: glibc
           Version: 2.16
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
        AssignedTo: unassigned@sourceware.org
        ReportedBy: jsm28@gcc.gnu.org
    Classification: Unclassified


Given my patch http://sourceware.org/ml/libc-alpha/2012-11/msg00001.html for
bug 14784 there are still some underflows from fma (a, b, c) when a * b is
small, because the logic has a gap of two exponents between the cases when a *
b is so small that its exact value doesn't matter, and the cases where the
scaling-up for small a * b is enough to avoid underflow when computing the
Dekker multiplication.  Example, on x86 or x86_64: fmal
(0x1.0000000000000002p-16382L, 0x1.0000000000000002p-66L, 0x1p16383L) raises a
spurious underflow exception.

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