linux-kernel - Re: [PATCH] or51132.c: unaligned

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Message-ID: <alpine.LFD.1.10.0805201856200.3081@woody.linux-foundation.org>
Date:	Tue, 20 May 2008 19:02:08 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...IV.linux.org.uk>
cc:	Harvey Harrison <harvey.harrison@...il.com>,
	Al Viro <viro@....linux.org.uk>, mchehab@...radead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] or51132.c: unaligned

On Wed, 21 May 2008, Al Viro wrote:
> 
> FWIW, I wonder how they really compare on misaligned and whether it would
> make sense for gcc to try and generate a single load on targets that are
> known to allow that...

It would almost certainly help on x86. The cost of an unaligned integer 
access that doesn't cross a cache-fetch boundary (8 bytes on older CPU's, 
16 or 32 bytes on newer ones) is zero, last I saw. IOW, there are 
misaligned cases that have a higher cost, but they are pretty rare, and 
especially so with small data and modern CPU's.

So no disadvantage for 95% of all cases, and the advantage of doing just a 
single instruction, rather than four (2 zero-extending loads, a shift and 
an add/or, with data dependencies on most of them).

			Linus
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