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[Bug math/18019] New: [ldbl-128ibm] acoshl inaccurate


https://sourceware.org/bugzilla/show_bug.cgi?id=18019

            Bug ID: 18019
           Summary: [ldbl-128ibm] acoshl inaccurate
           Product: glibc
           Version: 2.21
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
          Assignee: unassigned at sourceware dot org
          Reporter: jsm28 at gcc dot gnu.org

The ldbl-128ibm implementation of acoshl uses a cut-off of 0x1p28 to determine
when to use log(x) + log(2) as a formula.  That cut-off is too small for this
format, resulting in large errors, e.g.:

Failure: Test: acosh (0x1p+28)
Result:
 is:          2.01012682362384139731e+01   0x1.419ecb712c480c875dedp+4
 should be:   2.01012682362384139696e+01   0x1.419ecb712c480c835dedp+4
 difference:  3.46944695195361418882e-18   0x1.00000000000000000000p-58
 ulp       :  8796093022208.0000
 max.ulp   :  1.0000

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