On Mon, 21 Oct 2002, Jeff Layton wrote:
I thought I would chip in with my sugestions for ATLAS and LAPACK since I've built and tested those with ifc/icc (version 6).
Your suggestions are welcome!
I would suggest building ATLAS with icc (the Intel C compiler). You can build it with gcc and link it to ifc code, but it makes life easier to use icc.
Hmm, actually what comes to using ATLAS in Scipy, I saw no downside of using gcc, in fact, I didn't even need to bother with this matter (I haven't installed icc either).
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.
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.
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 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.
Finally, after you build the LAPACK library it was recommended to me to go back and recompile xerbla.f, slamch.f, and dlamch.f using no optimization ('-O0').
Recompiling these files (didn't know that about xerbla.f before) with -O0 is absolutely required.
Then relink the libs (I erase the final libs, recompile these 3 routines, and then run 'make libs' again). So far this process allows ATLAS to pass all of the tests and for LAPACK to pass all the tests as well. Thanks for all of your hard work this weekend!
Thanks for your notes! I'll update INSTALL.txt accordingly. Pearu