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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 15:38:23 -0800
Cc: Atsushi Nemoto <anemo@mba.ocn.ne.jp>, linux-mips@linux-mips.org, linux-kernel@vger.kernel.org
In-reply-to: <20070215221839.GA14103@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> <20070215135358.020781dd.akpm@linux-foundation.org> <20070215221839.GA14103@linux-mips.org>
Sender: linux-mips-bounce@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.

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