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

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Message-ID: <20070215221839.GA14103@linux-mips.org>
Date:	Thu, 15 Feb 2007 22:18:39 +0000
From:	Ralf Baechle <ralf@...ux-mips.org>
To:	Andrew Morton <akpm@...ux-foundation.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, 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.
[...]

Qed?

  Ralf
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