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

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Message-ID: <45D4E740.9020504@goop.org>
Date:	Thu, 15 Feb 2007 15:05:36 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ralf Baechle <ralf@...ux-mips.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	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.

Ralf Baechle wrote:
> 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?

So that the compiler has to assume that if its accessing this __packed
structure, it may be embedded unaligned within something else? And
because the pointer is cast through (void *) it isn't allowed to use
alias analysis to notice that the pointer wasn't originally (apparently)
unaligned.

Seems sound to me.

    J
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