[Bug libfortran/49024] New: REAL*16 ERFC_SCALED inaccuracy

[thenlich at users dot sourceforge.net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49024

           Summary: REAL*16 ERFC_SCALED inaccuracy
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libfortran
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: thenlich@users.sourceforge.net


The libfortran implementation of the ERFC_SCALED intrinsic function is not
accurate enough to be useful for REAL*16 arithmetics.

program test_erfc_scaled
    real(16), parameter :: p16 = 1.0_16
    real(16) :: v16 = p16

    print *, erfc_scaled(v16)
    print *, erfc_scaled(p16)
    print *, erfc(v16) * exp(v16 * v16)
end program test_erfc_scaled

==>
  0.427583576155807003967191251980322016      
  0.427583576155807004410750344490515188      
  0.427583576155807004410750344490515140      

This should be easy to fix by adding an erfc_scaled() function to libquadmath.

In fact, the error function code in libquadmath is based on the same code as
http://www.netlib.org/cephes/128bdoc.html and the latter already contains a
separate function for a scaled error function. In libquadmath it has been
merged into one (or maybe the other was split later).