[gmx-users] ic vs gcc : strange results

Erik Lindahl lindahl at csb.stanford.edu
Fri Feb 20 00:39:01 CET 2004


The problem is not really the speed, which is nice as compared to other 
machines. Intel compilo gives me an additional 10-15% of speed. But 
what worries me most is that with intel my box explodes, and with gcc 
it doesn't. Which is right ??

I'd trust gcc more - although you could check out the test module from 
CVS and try it :-)

If nothing else, if you use gcc you are using the same compiler as all 
the libraries on your system was compiled with.

10-15% sounds way too much since the nonbonded interactions are 
identical (assembly is just fed on to the assembler). In most cases 
that's the whole fraction of the time spent outside the innerloops! In 
other words: it sounds as if the Intel compiler optimized away 
something it shouldn't :-)

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