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Re: Linux and aliasing?
Date: Sat, 05 Jun 1999 21:02:27 +0200
From: Toon Moene <toon@moene.indiv.nluug.nl>
I mean, if this sort of trickery permeates the Linux kernel, you
won't get any mileage out of the new optimization anyway, so you
could just as well disable it.
It is not believed that this is the case.
Networking will be used as an example.
The core of the fast paths in interrupt level processing consist of
parsing and verifying packet header data. These are the areas where
non-alias-friendly casts are used to speed up the header inspection
(to decrease the number of load instructions and also decrease the
number of comparisons executed).
Yet in the user side portion of networking, and to a decent amount in
the packet processing once we've obtained the header data, the bulk of
the work consists of updating state in the per-connection data
structures, where a plethora of alias analysis benefits exist.
So the situation here is quite the contrary to the assertion, one of
the most worrysome areas of the kernel, with respect to the
union'ization of data structures to remove non-alias-friendly casts,
is also the place where alias analysis would be highly beneficial.
Later,
David S. Miller
davem@redhat.com