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Date: Fri, 20 Dec 2002 02:08:48 -0800
From: Ulrich Drepper <drepper@redhat.com>
Organization: Red Hat, Inc.
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To: Linus Torvalds <torvalds@transmeta.com>
CC: bart@etpmod.phys.tue.nl, davej@codemonkey.org.uk,
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Subject: Re: Intel P6 vs P7 system call performance
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Linus Torvalds wrote:

> For _zero_ gain.  The jump to the library address has to be indirect 
> anyway, and glibc has several places to put the information without any 
> mmap's or anything like that.

Correct.  The current implementation is optimal.

It is necessary to have indirection since the target address can change.

I'm never going to use self-modifying code.

And it's a simple, one-instruction change.

  int $0x80  ->  call *%gs:0x18


That's it.  It's all implemented and tested.  The results are in the
latest NPTL source drop.  The code won't be available in LinuxThreads
since it requires a kernel with TLS support.

As far as I'm concerned the discussion is over.  I'm happy with what I
have now.  The additional overhead for the case when AT_SYSINFO is not
available is neglegable (and can be compiled-out completely if one
really wants), and in case AT_SYSINFO is available the code really is
the fatest possible given the constraints mentioned above.

-- 
--------------.                        ,-.            444 Castro Street
Ulrich Drepper \    ,-----------------'   \ Mountain View, CA 94041 USA
Red Hat         `--' drepper at redhat.com `---------------------------

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