[gmx-developers] Turning off optimization

[David van der Spoel]

Erik Lindahl wrote:
> On Jul 1, 2009, at 10:25 AM, Mark Abraham wrote:
> 
>> David van der Spoel wrote:
>>> Hi,
>>> how do I turn off optimization when compiling gromacs, now that it 
>>> has become much more difficult to use CFLAGS, CXXFLAGS etc.?
>>> --disable-cpu-optimization drops the optimization from -03 to 02, but 
>>> I don't want any optimization (not just for debugging but because my 
>>> code does not compile otherwise).
> 
> That is compiler-dependent, so you'll have to create your own CFLAGS 
> string. I can't imagine it *that* difficult to cut-and-paste the default 
> CFLAGS you see for a compile and then add "-O0".
Ok then...
> 
> If your code doesn't compile with -O2, I would definitely say you need 
> debugging ;-)
> 
Debugging the compiler, indeed. It is just few thousand lines like
x = a + b -c and so on (note a few thousand lines in one expression).
Compiler goes out of memory and/or plain crashes with a SEGV trying to 
optimize that.

>> Sounds like whatever mechanism is being used by 
>> --disable-cpu-optimization should be used by a new option 
>> --disable-all-optimization as well. Why is CFLAGS now harder to use?
> 
> 
> Not all compilers support ISO C99 by default (not mandatory, but we try 
> to use it when possible), and there might be some code temporarily that 
> requires SSE intrinsics support on x86...
> 
> 
> --disable-cpu-optimization is quite a bit different; this option tries 
> to find optimizations for the actual *CPU* you are using by checking 
> cache sizes, architecture (e.g. Power6-specific, won't work on Power5+, 
> etc.). Disabling means we don't try this. There is no standard way to 
> disable all optimizations for all compilers, so I'd rather not give the 
> user the impression it is disabled just because they use such an option!
> 
> Cheers,
> 
> ERik
> 
> ------------
> Erik Lindahl   <lindahl at cbr.su.se>  Backup: <erik.lindahl at gmail.com>
> Associate Professor, Computational Structural Biology
> Center for Biomembrane Research, Dept. Biochemistry & Biophysics
> Stockholm University, SE-106 91 Stockholm, Sweden
> Tel: +46(0)8164675  Mobile: +46(0)703844534  Fax: mail a PDF instead
> 
> 
> 
> 
> 
> 
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-- 
David.
________________________________________________________________________
David van der Spoel, PhD, Professor of Biology
Dept. of Cell and Molecular Biology, Uppsala University.
Husargatan 3, Box 596,  	75124 Uppsala, Sweden
phone:	46 18 471 4205		fax: 46 18 511 755
spoel at xray.bmc.uu.se	spoel at gromacs.org   http://folding.bmc.uu.se
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