[Bug math/19677] New: [ldbl-128ibm] remainderl equality test incorrect for zero low part

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

https://sourceware.org/bugzilla/show_bug.cgi?id=19677

            Bug ID: 19677
           Summary: [ldbl-128ibm] remainderl equality test incorrect for
                    zero low part
           Product: glibc
           Version: 2.23
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
          Assignee: unassigned at sourceware dot org
          Reporter: jsm28 at gcc dot gnu.org
  Target Milestone: ---

The ldbl-128ibm implementation of remainderl has logic resulting in incorrect
tests for equality of the absolute values of the arguments in the case of zero
low parts.  If the low parts are both zero but with different signs, this can
wrongly cause equal arguments to be treated as different, resulting in turn in
incorrect signs of zero result in nondefault rounding modes arising from the
subtractions done when the arguments are not equal.  E.g.:

Failure: Test: remainder_downward (min_value, -min_value)
Result:
 is:         -0.00000000000000000000000000000000e+00 
-0x0.000000000000000000000000000p+0
 should be:   0.00000000000000000000000000000000e+00  
0x0.000000000000000000000000000p+0
 difference:  0.00000000000000000000000000000000e+00  
0x0.000000000000000000000000000p+0
 ulp       :  0.0000
 max.ulp   :  0.0000

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