From: Pete Zaitcev <[email protected]>
Date: Mon, 11 Dec 2006 17:29:07 -0800
> On Mon, 11 Dec 2006 15:52:47 -0800, Matt Helsley <[email protected]> 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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