Compiling software with different compiler than cc or clang results in unusable output

[Andrew Brampton]

On 11 September 2010 10:28, O. Hartmann
<ohartman at mail.zedat.fu-berlin.de> wrote:
>
> Dear Sirs,
>
> you see me a kind of desperate. I wrote my own a small piece of  software in
> C, calculating the orbit and position of astronomical objects, astroids, in
> a heliocentric coordinate system from Keplerian orbital elements. So far.
> The software calculates the set of points of an ellipse based upon
> ephemeridal datas taken from the Minor Planet Cataloge. Again, so far,
> everything all right. The set of points of an orbit is all right and
> correct. But when it comes to positions at a specific time, then I loose
> hair!
>
> Compiling this piece of software with FreeBSD's gcc (V4.2) and clang (clang
> devel) on my private and lab's FreeBSD boxes (both most recent FreeBSD
> 8.1/amd64), this program does well, the calculated orbital positions are
> very close to professional applications or observational checks. But when
> compiling the sources with gcc44 or gcc45 (same source, same CFLAG setting,
> mostly no CFLAGS set), then there is a great discrepancy. Sometimes when
> plotting positions, the results plotted seconds before differs from the most
> recent. The ellipses are allways correct, but the position of a single point
> at a specific time isn't correct.
>
> I use the GNU autotools to build the package.
>
> I suspekt miscompilations in memory alloction or in some time- or
> mathematical functions like sin, cos.
>
> before I digg deeper I'd like to ask the community for some hints how to
> hunt down such a problem.
>
> regards,
> Oliver

Sounds a cool project. I suspect you are miss-using a feature of C or
are using uninitialised memory, and with gcc44/45's more aggressive
optimisations it is getting it wrong. I have three suggestions

1) Use valgrind to check if it finds anything wrong when running your
program. Check both the good and the bad builds.

2) If your program is made up of multiple C files, then try compiling
all of the C files with gcc42, but just one at a time with gcc44. This
way will help you track down exactly which C file has "the bug".

3) Finally do some printf debugging to find the first line of code
that is generating the wrong value.

I hope these suggestions help.
Andrew