Re: kernel optimization

[<cutaway@bellsouth.net>]

----- Original Message ----- 
From: "Adrian Bunk" <bunk@stusta.de>
To: "christos gentsis" <christos_gentsis@yahoo.co.uk>
Cc: <linux-kernel@vger.kernel.org>
Sent: Friday, July 22, 2005 16:14
Subject: Re: kernel optimization
>
> It's completely untested.
> And since it's larger, it's also slower.

Larger does not always mean slower.  If it did, nobody would implement a
loop unrolling optimization.

ex. Look at how GCC generates jump tables for switch() when there's about
10-12 (or more) case's sparsely scattered in the rage from 0 through 255.
It generates a 256 element directly indexed jump table (obviously with many
duplicate entries).  This is faster than a cascaded if/else
construct(particularly for those that would have been on the end of the
if/else chain), but it is a very large construct.  You'll see some of these
"plump" switches generated in various SCSI drivers and in the VT102
emulation if you disassemble them.


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