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Re: [PATCH] Optimize generic get_unaligned / put_unaligned implementatio

To: Ralf Baechle <ralf@linux-mips.org>
Subject: Re: [PATCH] Optimize generic get_unaligned / put_unaligned implementations.
From: Andrew Morton <akpm@linux-foundation.org>
Date: Thu, 15 Feb 2007 13:53:58 -0800
Cc: Atsushi Nemoto <anemo@mba.ocn.ne.jp>, linux-mips@linux-mips.org, linux-kernel@vger.kernel.org
In-reply-to: <20070215143441.GA18155@linux-mips.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>
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On Thu, 15 Feb 2007 14:34:41 +0000
Ralf Baechle <ralf@linux-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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