[gentoo-science] Re: [Fwd: Re: [atlas-devel] 1) ATLAS shared libraries; 2) "ASM" -> "ASM VOLATILE"]

[Markus Dittrich]

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On Fri, 25 Aug 2006, Adam Pityszek wrote:

> Dear Markus, gentoo-science guys,
>
> Please find below the reply from Clint to my yesterday's email related to
> our work on ATLAS shared libraries in Gentoo.
>
> Markus, I think we can help with answering the questions (2) and (3). Of
> course, volunteers from gentoo-science are welcome as well.
>
> BR,
> /ediap
>
> (1) Is it true that the extra pointer may still be used if we restore it at
>    end of assembly routine?
> (2) Does throwing the -fpic or other required compiler flag changes change
>    the best cases (thus necessitating doubling the arch defaults)?
> (3) What is the overall performance affect when using .so?
>
> I've tried to answer (1) by looking at some docs, but never got convinced
> either way.  I've been meaning to write a resister stress-test to see if
> I can make gcc use the reserved register in a function w/o global data.
> Perhaps you know?
>
> You guys could help with (2) & (3) if you like.  You could build out-of-box
> to .a on whatever machines you can, and then build it to .so using your
> gentoo harness, and post some head-to-head timings . . .  If, as we suspect,
> the difference is essentially zero, that makes .so a lot more attractive . . .
>

Hi Adam,

Thanks for talking to upstream about this and Clint's response
sounds encouraging. We could definitely help out with 2) and 3);
it would be good to know anyway how well we do with our shared 
libs. In doing so we should also test the impact of using
the 387 floating point unit versus the sse instruction set. 
According to Clint, the former can give a significant performance
gain on some CPU's. If that is the case it might be worth a 
note in the ebuild to make our users aware of it.

We should get a hold of a nice benchmark suite for this purpose; 
Clint has posted one on this gcc bug
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27827
which we might be able to use. I'll have a look at it.

Best,
Markus


- -- 
Markus Dittrich (markusle)
Gentoo Linux Developer
Scientific applications
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