Re: Saving syscall cycles on a Cyrix

Mike Jagdis (
Thu, 12 Jun 1997 12:23:24 +0100 (GMT/BST)

On Thu, 12 Jun 1997, Thomas Koenig wrote:

> prompted by a discussion on comp.sys.development.system, I had a look
> at entry.S, and found that this can be speeded up significantly for
> a Cyrix.
> The problem is that two pushl/popl instructions can't be paired in the
> Cyrix' X and Y pipelines because the esp register gets modified
> in both.
> [...]
> only takes four, because any two instructions can pair. The same
> goes for the reverse, i.e. the popl instructions. Cyrix patch
> maintainers, are you listening? :-)

Most modern x86 processors have dual pipelines and can benefit
from similar optimizations. The current gcc isn't particularly
smart about interleaving code paths to avoid "bubbling" in the
pipelines. I've been reading up on such tuning recently and, if
anyone is interested, have hand tuned the rc5 cracking code to
go from ~145K keys/s (on my machine) to ~205K keys/s (yes, you
can get a big difference!). Of course, this is at the assembler
level and the result is, ah, "not easily readable" but even
paying attention to the order you do things in C can show
reasonable gains. Whether such things are good for MIPS, Sparc,
Alpha, PPC etc. as well is another question :-).


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