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Re: [PATCH] Optimize generic get_unaligned / put_unaligned implementatio

To: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] Optimize generic get_unaligned / put_unaligned implementations.
From: Ralf Baechle <ralf@linux-mips.org>
Date: Thu, 15 Feb 2007 22:18:39 +0000
Cc: Atsushi Nemoto <anemo@mba.ocn.ne.jp>, linux-mips@linux-mips.org, linux-kernel@vger.kernel.org
In-reply-to: <20070215135358.020781dd.akpm@linux-foundation.org>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <20050830104056.GA4710@linux-mips.org> <20060306.203218.69025300.nemoto@toshiba-tops.co.jp> <20060306170552.0aab29c5.akpm@osdl.org> <20070214214226.GA17899@linux-mips.org> <20070214203903.8d013170.akpm@linux-foundation.org> <20070215143441.GA18155@linux-mips.org> <20070215135358.020781dd.akpm@linux-foundation.org>
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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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