linux-kernel - Re: [PATCH] connector: Some fixes for ia64 unaligned access errors

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Date:	Mon, 11 Dec 2006 17:50:50 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	zaitcev@...hat.com
Cc:	matthltc@...ibm.com, erikj@....com, guillaume.thouvenin@...l.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] connector: Some fixes for ia64 unaligned access errors

From: Pete Zaitcev <zaitcev@...hat.com>
Date: Mon, 11 Dec 2006 17:29:07 -0800

> On Mon, 11 Dec 2006 15:52:47 -0800, Matt Helsley <matthltc@...ibm.com> wrote:
> 
> > 	I'm shocked memcpy() introduces 8-byte stores that violate architecture
> > alignment rules. Is there any chance this a bug in ia64's memcpy()
> > implementation? I've tried to read it but since I'm not familiar with
> > ia64 asm I can't make out significant parts of it in
> > arch/ia64/lib/memcpy.S.
> 
> The arch/ia64/lib/memcpy.S is probably fine, it must be gcc doing
> an inline substitution of a well-known function.
> 
> A commenter on my blog mentioned seeing the same thing in the past.
> (http://zaitcev.livejournal.com/107185.html?thread=128945#t128945)
> 
> It's possible that applying (void *) cast to the first argument of memcpy
> would disrupt this optimization. But since we have a well understood
> patch by Erik, which only adds a penalty of 32 bytes of stack waste
> and 32 bytes of memcpy, I thought it best not to bother with heaping
> workarounds.

Yes GCC can assume the object is aligned because of the type
of the argument to memcpy().

I tried myself some games with adding a "packed" attribute to the
pointer declaration (trying to tell it that "the thing pointed to"
might be unaligned), but to no avail.
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