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Re: ATLAS degradation with -freduce-all-givs flag for PPC arch
- From: Toon Moene <toon at moene dot indiv dot nluug dot nl>
- To: Raúl de la Cruz <delacruz at ac dot upc dot es>
- Cc: gcc at gcc dot gnu dot org, Juan Jose Porta <porta at de dot ibm dot com>, "Jose M. Cela" <cela at ac dot upc dot es>
- Date: Thu, 15 Jul 2004 23:13:42 +0200
- Subject: Re: ATLAS degradation with -freduce-all-givs flag for PPC arch
- Organization: Moene Computational Physics, Maartensdijk, NL
- References: <40F6A5D6.3030804@ac.upc.es>
Raúl de la Cruz wrote:
We have got a big degradation of performance when we enable
'-freduce-all-givs' flag in GCC to compile ATLAS libraries. An example
of this is that performance drops from 6.5 GF for a SGEMM operation
(single precision matrix multiplication) of (800 x 800) size to 4.5 GF
when this flag enabled in GCC. Maybe this degradation is produced
because SGEMM kernel is written in PPC assembler and using AltiVec calls
and not is C coded.
I have read (in man gcc) that you are very interested to know when
application runs slower when this flag is enable. So here I am ;).
I thought up this flag, so I should probably confess here ;-)
The problem is *not* that the SGEMM kernel is written in PPC assembler,
because the flag only pertains to the languages gcc compiles (Fortran, C
and various other ones).
I have no easy explanation where this reduction in performance comes
from. I also do not have access to any PPC970 - so I have to remain
silent on this issue.
If you can produce a small test program that shows this behaviour, I'm
willing to try it against various processors.
Thanks !
--
Toon Moene - mailto:toon@moene.indiv.nluug.nl - phoneto: +31 346 214290
Saturnushof 14, 3738 XG Maartensdijk, The Netherlands
Maintainer, GNU Fortran 77: http://gcc.gnu.org/onlinedocs/g77_news.html
A maintainer of GNU Fortran 95: http://gcc.gnu.org/fortran/