[Bug fortran/45175] New: In gfortran, the calculation of small differences seems to depend on optimization flags

["CycleTimeChart at yahoo dot com" <gcc-bugzilla@gcc.gnu.org>]

Just a little background:

I wrote a program that does high accuracy factorials.  It saves a given
factorial for later use as a double precision real between 1 and 2 and an
integer power of 2.  It does this by calculating the factorial to a practically
unlimited number of bits, rounding it to 53 bits only as the final step,
thereby avoiding the cumulative round off errors that occur.

I wanted to compare the accuracy of this method to a straight-forward way of
calculating factorials.  So I also scaled factorials calculated in a
straight-forward way the same way as above, that is as a double precision real
between 1 and 2 and an integer power of 2.

In the comparison, the integer power of 2 can be ignored, as it is practically
always the same.  As to the real portion, the difference between the two reals
should always be an integer multiple of 2**(-52).  The crux of the bug is that
when I compiled using the flag -O, this was true.  Using -O2, it was not. 
Stated another way, the calculation of small differences seems to depend on
optimization flags.


-- 
           Summary: In gfortran, the calculation of small differences seems
                    to depend on optimization flags
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: CycleTimeChart at yahoo dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45175