linux-kernel - Re: [RFC] Documentation about unaligned memory access

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Message-ID: <p73k5o9iuk6.fsf@bingen.suse.de>
Date:	Fri, 23 Nov 2007 03:07:05 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Robert Hancock <hancockr@...w.ca>
Cc:	Daniel Drake <dsd@...too.org>, linux-kernel@...r.kernel.org,
	davem@...emloft.net
Subject: Re: [RFC] Documentation about unaligned memory access

Robert Hancock <hancockr@...w.ca> writes:
>
> Also, x86 doesn't prohibit unaligned accesses,

That depends, e.g. for SSE2 they can be forbidden.

> but I believe they have
> a significant performance cost and are best avoided where possible.

On Opteron the typical cost of a misaligned access is a single cycle
and some possible penalty to load-store forwarding.

On Intel it is a bit worse, but not all that much. Unless you do 
a lot of accesses of it in a loop it's not really worth something
caring about too much.

-Andi
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