Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!lll-winken!uunet!mcvax!inria!mirsa!huitema
From: huitema@mirsa.inria.fr (Christian Huitema)
Newsgroups: comp.arch
Subject: Re: Unaligned Accesses (was Re: How to use silicon)
Message-ID: <150@mirsa.inria.fr>
Date: 29 Mar 89 15:41:25 GMT
References: <844@bnr-rsc.UUCP>
Organization: INRIA, Sophia Antipolis. France
Lines: 18

From experience, I can say that not having to bother with alignment can 
make networking code much faster. Messages exchanged over nets include all
sort of variable size components, and padding is both frowned upon and
somewhat ineffective -- often, one cannot predict where a structure will
start. Thus, being able to simply define:

#define integer_value(mess,x) (*(int *)(mess + x))

rather than:

#define integer_value(mess,x) \
	(((((mess[x]<<8)|mess[x+1])<<8)|mess[x+2]<<8)|mess[x+3])

can speed up a lot the decoding of these messages. Indeed, one can object
the relative frequency of network operations vs computing, but the same holds
for compression programs, disk accesses, etc...

Christian Huitema