[Bug math/28358] New: f64xdivf128 and f64xmulf128 spurious underflows

["jsm28 at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28358

            Bug ID: 28358
           Summary: f64xdivf128 and f64xmulf128 spurious underflows
           Product: glibc
           Version: 2.34
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
          Assignee: unassigned at sourceware dot org
          Reporter: jsm28 at gcc dot gnu.org
  Target Milestone: ---
              Host: x86_64-* i?86-* ia64-*

The round-to-odd computations used in the libm functions that round their
results to a narrower format can yield spurious underflow exceptions in the
following circumstances: the narrowing only narrows the precision of the type
and not the exponent range (i.e., it's narrowing _Float128 to _Float64x on
x86_64, x86 or ia64), the architecture does after-rounding tininess detection
(which applies to all those architectures), the result is inexact, tiny before
rounding but not tiny after rounding (with the chosen rounding mode) for
_Float64x (which is possible for narrowing mul, div and fma, not for narrowing
add, sub or sqrt), so the underflow exception resulting from the toward-zero
computation in _Float128 is spurious for _Float64x.  For example, this applies
to the following test inputs if added to auto-libm-test-in:

div 0x1p-16382 0x1.00000000000000001p0
mul 0x0.ffffffffffffffff8p-16382 0x1.00000000000000001p0

I'm working on a fix for this issue, which showed up while working on
implementing the narrowing fma functions.

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