Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!unmvax!polyslo!cquenel
From: cquenel@polyslo.CalPoly.EDU (46 more school days)
Newsgroups: comp.arch
Subject: Re: RISC vs unaligned data
Message-ID: <9931@polyslo.CalPoly.EDU>
Date: 3 Apr 89 22:26:58 GMT
References: <355@bnr-fos.UUCP> <13@microsoft.UUCP> <16058@cup.portal.com> <370@bnr-fos.UUCP> <11222@tekecs.GWD.TEK.COM> <16407@winchester.mips.COM>
Reply-To: cquenel@polyslo.CalPoly.EDU (46 more school days)
Organization: Blue Blaze Irregulars
Lines: 27

In article <16407@winchester.mips.COM> mash@mips.COM (John Mashey) writes:
>Of course, if N == 100 (if you were doing it with exceptions, perhaps),
>you now get + 5-15, i.e., 6-16X slower, which is clearly ungood, and
>only survivable for debugging.

	Wait a sec, aren't you comparing the proverbial
	mis-matched fruits here ?

	The hit you take from compiler generated "safe" code
	is paid whether the specific reference is actually
	aligned or not.  As you mention, in FORTRAN, things
	in a common block can have arbitrary alignment in
	THEORY, but in practice the words are often aligned.

	If you are "doing it with exceptions" then the exception
	is only taken when the word is, in fact, mis-aligned.

	Using exceptions to handle mis-aligned data isn't
	quite as bad as your (admittedly back-of-the-envelope)
	figure of 6-16X would suggest.

	--chris
-- 
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\. ./  | Chris Quenelle (The First Lab Rat) cquenel@polyslo.calpoly.edu |  \. ./
 \ /   |                + good; ++ good;     -- Annie Lennox            |   \ / 
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