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

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Date:	Thu, 15 Feb 2007 16:13:41 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Ralf Baechle <ralf@...ux-mips.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.

Andrew Morton wrote:
> 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.

In Ralf's code, the structure is anonymous, and is used to declare a
pointer type, which is initialized from a void *.  So I think the
compiler isn't allowed to assume anything about its alignment.

    J
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