[Math-atlas-results] Abysmal performance with new gccs

[R C. W. <rw...@cs...>]

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Mon, 30 Jul 2001 19:38:48
Vers: All atlas versions

Guys,

Had a user report some performance problems, saying he suspected the compiler.
Told him gcc has been doing pretty good for a long while, thought he probably
misinstalled.  Tried the gcc given in RedHat 7.1 myself, got horrible results.
Same true of gcc 3.0.

You may have seen this as a feature of the new gcc 3.0:
    *  New x86 back-end, generating much improved code.
What this appears to translate to for ATLAS is a roughly 50% performance
drop, at least on the Athlons.  I don't have gcc 3.0 on a Pentium yet, but
the user reports a similar (though not quite as severe) degredation there.
I'd heard rumors about gcc 3.0 not doing as well for x86, but I certainly had
not imagined this . . .

By using different parameters, I managed to get code more like 65% of previous
performance.  Perhaps there's a magic flag to turn all these great new
features off, but somehow I doubt it.  I'm sending this out before I've got
all the facts in case others know something about it, or want to see if they
can figure it out.  In the meantime, if you want performance, stick with
the old stuff, and if you play with it, any info appreciated . . .

Clint
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