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From: Andrew Morton <akpm@linux-foundation.org>
To: Ralf Baechle <ralf@linux-mips.org>
Cc: Atsushi Nemoto <anemo@mba.ocn.ne.jp>,
	linux-mips@linux-mips.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Optimize generic get_unaligned / put_unaligned implementations.
Date: Thu, 15 Feb 2007 15:38:23 -0800	[thread overview]
Message-ID: <20070215153823.239fd616.akpm@linux-foundation.org> (raw)
In-Reply-To: <20070215221839.GA14103@linux-mips.org>

On Thu, 15 Feb 2007 22:18:39 +0000
Ralf Baechle <ralf@linux-mips.org> wrote:

> On Thu, Feb 15, 2007 at 01:53:58PM -0800, Andrew Morton wrote:
> 
> > > 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?
> 
> Gcc info page says:
> 
> [...]
> `packed'
>      The `packed' attribute specifies that a variable or structure field
>      should have the smallest possible alignment--one byte for a
>      variable, and one bit for a field, unless you specify a larger
>      value with the `aligned' attribute.
> [...]
> 

hm.  So if I have

	struct bar {
		unsigned long b;
	} __attribute__((packed));

	struct foo {
		unsigned long u;
		struct bar b;
	};

then the compiler can see that foo.b.b is well-aligned, regardless of the
packedness.

Plus some crazy people compile the kernel with icc (or at least they used
to).  What happens there?

> Qed?

worried.

  parent reply	other threads:[~2007-02-15 23:41 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20050830104056.GA4710@linux-mips.org>
     [not found] ` <20060306.203218.69025300.nemoto@toshiba-tops.co.jp>
2006-03-07  1:05   ` [PATCH] 64bit unaligned access on 32bit kernel Andrew Morton
2006-03-07  2:03     ` Atsushi Nemoto
2006-03-07 18:09     ` Ralf Baechle
2006-03-08  4:58       ` Atsushi Nemoto
2006-03-08  5:12         ` Andrew Morton
2007-02-14 21:42     ` [PATCH] Optimize generic get_unaligned / put_unaligned implementations Ralf Baechle
2007-02-15  4:39       ` Andrew Morton
2007-02-15  8:35         ` Marcel Holtmann
2007-02-15 14:34         ` Ralf Baechle
2007-02-15 21:53           ` Andrew Morton
2007-02-15 22:18             ` Ralf Baechle
2007-02-15 23:05               ` Jeremy Fitzhardinge
2007-02-15 23:38               ` Andrew Morton [this message]
2007-02-16  0:13                 ` Jeremy Fitzhardinge
2007-02-16  0:43                 ` Ralf Baechle
2007-02-16  1:27                   ` Andrew Morton
2007-02-16  1:59                     ` Ralf Baechle
2007-02-20 13:50                     ` Pavel Machek

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