[gmx-users] CFLAGS

[Erik Lindahl]

Hi,

The intel compilers should support both -O3 (our default when icc is  
detected) and -O2. The difference isn't very large, though, since our  
assembly loops bypass the compiler on both ia32, X86-64, and ia64 -  
but then you need to use gromacs 3.3 or later!!! (3.2 only includes  
ia32 assembly loops).

If you're only having problems when compiling with MPI support the  
culprit is likely the mpi wrapper script for the compiler.

Cheers,

Erik

On Nov 16, 2005, at 2:53 PM, Joanne Hanna wrote:

> Hi
>
> I have recently compiled an mpi version of gromacs 3.2.1 with the  
> icc compiler. I ran into troubles with this error, at the ./ 
> configure stage:
> * Sorry, these optimization settings don't seem to work for       *
> * your C compiler. Use make CFLAGS=..., or edit the top Makefile. *
> I decided to use the cflags option -O2.
> Can anyone tell me whether this is a good option to use. I  
> understand that -O3 enables more aggressive optimisations such as  
> loop and  memory  access transformation. and is recommended for  
> applications that have loops with heavy use of float-ing point  
> calculations and process large data sets (as from the manual). But  
> i don't know which i should use.
> I have been told that optimising too aggressively can produce the  
> wrong numbers.
>
> Anu advice would be much appreciated.
>
> Thanks
> Jo
>
>
> Joanne Hanna
> Department of Chemistry
> University of Warwick
> Coventry
> CV4 7AL
>
> J.F.Hanna at warwick.ac.uk
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-----------------------------------------------------------
Erik Lindahl  <lindahl at sbc.su.se>
Assistant Professor, Stockholm Bioinformatics Center
Stockholm University, SE 106 91 Stockholm
Phone: +46 8 5537 8564     Fax: +46 8 5537 8214