[SciPy-dev] Progress with Intel Fortran Compiler support

Jeff Layton jeffrey.b.layton at lmco.com
Mon Oct 21 08:39:33 CDT 2002

> > I'm not sure of the speed difference between icc and gcc on ATLAS (one
> > person said icc was slower, one said it was faster).
> Yes, both persons might be right: ATLAS is basically developed with gcc
> and optimized for that. On the other hand, icc, at least in principle,
> should be faster on intel processors as they know better...
> So, I would not recommend either of them until seeing some benchmark
> results.

It probably depends on your specific application (one was GEMM and
one was GETRF).

> > Here are the ootions I used with icc on ATLAS:
> >
> > -O3 -unroll -align -tpp6 -axK -Xa
> >
> > Typically I modify the makefile that ATLAS creates to make sure it
> > is correct.
> I think, -align is set default. -O3 -unroll are good. Other options are
> CPU specific, hence users choice.
> >    As for LAPACK, you have to use a couple of options to make sure the
> > precision is correct. In particular, '-mp -pc64 -fp_port'.
> -pc64 is default. Though, to get identical results with gcc, one should
> use -pc80 when it matters.
> -mp -fp_port
> I guess using these flags depends on applications. So, I would let users
> to deside about these flags. Sofar, I haven't seen any anomalies if these
> flags are missing.

I'm not totally sure about LAPACK (if these options are required).
However for ScaLAPACK I had to use these options or the tests
would not pass. Another person also mentioned these options (I think
it was Tim Prince on the Fortran newsgroup - I'll have to google for
his posting again).

> > There is a speed
> > lose compared to not using the flags, but I'd rather have correct answers
> > today :) The flags I used for ifc are :
> >
> > -O2 -tpp6 -prefetch -align -unroll -Vaxlib -mp -pc64 -fp_port -xK -axK
> >
> > BTW, these flags are for an Athlon or a PIII. Also, don't forget to use
> > the '-Vaxlib' flag during linking.
> Hmm, is '-Vaxlib' really necessary? Linkage works fine here without
> -Vaxlib when using ifc 6 and gcc 3.1.1. ifc -help says:
> -Vaxlib        link with portability library

You're right. It may have been just for ScaLAPACK and not LAPACK
nor ATLAS.

> I guess it doesn't matter when building locally. I don't think that we
> should distribute scipy binaries compiled with Intel compilers, at
> least not in near future. Also there might be some legal matters with
> respect distributing Intel binaries.

Most definitely.




Jeff Layton
Senior Engineer
Lockheed-Martin Aeronautical Company - Marietta
Aerodynamic & CFD
Phone: 770-494-8341
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email: jeffrey.b.layton at lmco.com

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