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
