[Bug math/19602] New: [ldbl-128ibm] fmodl handling of equal arguments with low part zero incorrect

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

https://sourceware.org/bugzilla/show_bug.cgi?id=19602

            Bug ID: 19602
           Summary: [ldbl-128ibm] fmodl handling of equal arguments with
                    low part zero incorrect
           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 fmodl has logic to detect when the first
argument has absolute value less than or equal to the second; this logic is
incorrect when the high parts are equal and the low parts are zero, resulting
in arguments being wrongly treated as unequal and incorrect results being
returned.  I have a patch for 2.24.

Failure: Test: fmod (-min_value, min_value)
Result:
 is:         -2.00416836000897277799610805135016e-292 
-0x1.000000000000000000000000000p-969
 should be:  -0.00000000000000000000000000000000e+00 
-0x0.000000000000000000000000000p+0
 difference:  2.00416836000897277799610805135016e-292  
0x1.000000000000000000000000000p-969
 ulp       :  40564819207303340847894502572032.0000
 max.ulp   :  1.0000

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