linux-kernel - Re: [PATCH] Optimize generic get_unaligned / put

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Message-Id: <20070215135358.020781dd.akpm@linux-foundation.org>
Date:	Thu, 15 Feb 2007 13:53:58 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ralf Baechle <ralf@...ux-mips.org>
Cc:	Atsushi Nemoto <anemo@....ocn.ne.jp>, linux-mips@...ux-mips.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Optimize generic get_unaligned / put_unaligned
 implementations.

On Thu, 15 Feb 2007 14:34:41 +0000
Ralf Baechle <ralf@...ux-mips.org> wrote:

> On Wed, Feb 14, 2007 at 08:39:03PM -0800, Andrew Morton wrote:
> 
> > Can someone please tell us how this magic works?  (And it does appear to
> > work).
> > 
> > It seems to assuming that the compiler will assume that members of packed
> > structures can have arbitrary alignment, even if that alignment is obvious.
> > 
> > Which makes sense, but I'd like to see chapter-and-verse from the spec or
> > from the gcc docs so we can rely upon it working on all architectures and
> > compilers from now until ever more.
> > 
> > IOW: your changlogging sucks ;)
> 
> It was my entry for the next edition of the C Puzzle Book ;-)
> 
> The whole union thing was only needed to get rid of a warning but Marcel's
> solution does the same thing by attaching the packed keyword to the entire
> structure instead, so this patch is now using his macros but using __packed
> instead.

How do we know this trick will work as-designed across all versions of gcc
and icc (at least) and for all architectures and for all sets of compiler
options?

Basically, it has to be guaranteed by a C standard.  Is it?
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