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

[kargl at gcc dot gnu.org]

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

kargl at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kargl at gcc dot gnu.org

--- Comment #5 from kargl at gcc dot gnu.org 2012-08-02 15:23:51 UTC ---
(In reply to comment #4)
> I can confirm that if we fiddle the fp rounding mode using a bit of C (very
> similar to the one you just suggested) then the problem goes away for this
> example.
> 
> 
> With your clue and looking at the binary representation we can see the probable
> cause:
> 
> If we believe the output of http://babbage.cs.qc.cuny.edu/IEEE-754/
> 

You don't need to resort to that URL.

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 = 42246.3973278316589130554348230362000_qp
   qpreal2 = 42246.3973278316589130554348230361940_qp

   print '(A,Z32)', "qpreal1 = ", qpreal1
   print '(A,Z32)', "qpreal2 = ", qpreal2
   print *
   print '(A,Z16)', "trunc qpreal1 = ", real(qpreal1, kind=dp)
   print '(A,Z16)', "trunc qpreal2 = ", real(qpreal2, kind=dp)
end program quad_test

laptop:kargl[225] gfortran46 -o z -rpath /usr/local/lib/gcc46 a.f90
laptop:kargl[226] ./z
qpreal1 = 400E4A0CCB6E8DB58800000000000001
qpreal2 = 400E4A0CCB6E8DB58800000000000000

trunc qpreal1 = 40E4A0CCB6E8DB59
trunc qpreal2 = 40E4A0CCB6E8DB58