[Bug fortran/54159] New: Fortran quad precision rounding seemingly nonsensical

[jonathan.hogg at stfc dot ac.uk]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54159

             Bug #: 54159
           Summary: Fortran quad precision rounding seemingly nonsensical
    Classification: Unclassified
           Product: gcc
           Version: 4.6.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: jonathan.hogg@stfc.ac.uk


program quad_test
   implicit none

   integer, parameter :: dp = kind(0d0)
   integer, parameter :: qp = selected_real_kind(32)

   real(qp) :: qpreal1, qpreal2

   ! Two quad prec numbers that differ by last 3 digits
   qpreal1 = 42246.397327831658913055434823036200_qp
   qpreal2 = 42246.397327831658913055434823036194_qp

   print *, "qpreal1 = ", qpreal1
   print *, "qpreal2 = ", qpreal2
   print *
   print *, "trunc qpreal1 = ", real(qpreal1, kind=dp)
   print *, "trunc qpreal2 = ", real(qpreal2, kind=dp)
end program quad_test

produces output

 qpreal1 =    42246.397327831658913055434823036200      
 qpreal2 =    42246.397327831658913055434823036194      

 trunc qpreal1 =    42246.397327831663     
 trunc qpreal2 =    42246.397327831655

the truncated output is incorrect in two ways:
1) Neither truncation is correct.
2) Both truncations are different but should be the same.

Being unable to rely on quad->double conversions makes quad precision
considerably less useful to us.