# Re: CCL:ifc -O0 failure

*From*: Serguei Patchkovskii <ps..at..ned.sims.nrc.ca>
*Subject*: Re: CCL:ifc -O0 failure
*Date*: Mon, 9 Jun 2003 14:49:28 -0400 (EDT)

On Mon, 9 Jun 2003, caroline taylor wrote:
> -O0: 6.710886E+07 (21.960u 0.020s 0:22.19 99.0%)
> -O1: 1.718282E+08 (17.560u 0.010s 0:17.76 98.9%)
> The 0 level optimization is clearly inconsistent, and far larger than
> round off. This has been checked on multiple machines (all linux).
>
> Has anyone run into this before? Is there a known problem with this level,
> or is something else going on?
Caroline,
There is nothing wrong with ifc in this case - your program is badly
broken, and should not be relied upon to produce -any- definite
answer. You are trying to accumulate numbers, which deviate from
one in the 7-th decimal digit. However, you chose to use a type
(REAL), which is only accurate to 6-7 decimal digits. Obviously,
you should not expect to get any significant digits in the result.
With optimizations turned on, ifc carries the accumulation at
higher 80-bit precision (which is arguably allowed by the standard),
which -accidentally- leads to a result close to the mathematically
exact value.
If you change your SUM to DOUBLE PRECISION, you will get the answer of
171828183.705001 as an answer for ifc -O0 - which is reasonably close
to the formal result of 171828183.705045439197452888531281012...
Incidentally, if you try to use this loop for timing purposes, this
is not necessarily a good idea - a sufficiently smart compiler
could realise that the exact result is given by:
(E - 1) E**(1/100000000)
------------------------
E**(1/100000000) - 1
and completely eliminate the loop.
Serguei