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

[O. Hartmann]

On 09/11/10 11:43, Andrew Brampton wrote:
> 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

Hello Andrew.

Thanks for your comments, they are worth trying out. I will do so ...

item 2) oh, yes, a very good idea ...

item 3) I did already, the whole software is built up by those printf's.

The problem boiled down to be some problem in the UNIX time routines. I 
use localtime(3), time(3) and a strftime(3) and strptime(3).

I use a 'wikipedia'-algorithm converting the actual time string into an 
'epoch' used in astronomical calculations. Compiling this routine with 
gcc42 and clang everything is all right, compiling it with gcc44 or 
gcc45 it returns 10 times higher values. I use very 'primitive' cutoffs 
for casting a double value into an int - I need the integrale value, not 
the remainings after the decimal point. I will check this again and look 
forward for a cleaner solution. But isn't this a 'bug'?

I'll try the BETA of the new FreeBSD PathScale compiler if I get some.

Well, I'll report ...

Oliver