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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: Fri, 16 Feb 2007 01:59:34 +0000
Cc: Atsushi Nemoto <anemo@mba.ocn.ne.jp>, linux-mips@linux-mips.org, linux-kernel@vger.kernel.org
In-reply-to: <20070215172720.3e9ce464.akpm@linux-foundation.org>
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References: <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> <20070215153823.239fd616.akpm@linux-foundation.org> <20070216004317.GA18987@linux-mips.org> <20070215172720.3e9ce464.akpm@linux-foundation.org>
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On Thu, Feb 15, 2007 at 05:27:20PM -0800, Andrew Morton wrote:

> No, icc surely supports attribute(packed).  My point is that we shouldn't
> rely upon the gcc info file for this, because other compilers can (or
> could) be used to build the kernel.
> 
> So it would be safer if the C spec said (or could be interpreted to say)
> "members of packed structures are always copied bytewise".  So then we
> can be reasonably confident that this change won't break the use of
> those compilers.
> 
> But then, I don't even know if any C standard says anything about packing.

Memory layout and alignment of structures and members are implementation
defined according to the C standard; the standard provides no means to
influence these.  So it takes a compiler extension such as gcc's
__attribute__().

> Ho hum.  Why are we talking about this, anyway?  Does the patch make the
> code faster?  Or just nicer?

Smaller binary and from looking at the disassembly a tad faster also.

  Ralf

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