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

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Message-ID: <20070220135046.GE3945@ucw.cz>
Date:	Tue, 20 Feb 2007 13:50:46 +0000
From:	Pavel Machek <pavel@....cz>
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.

Hi!

> > > 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?
> > 
> > A quick grep for __attribute__((packed)) and __packed find around 900 hits,
> > I'd probably find more if I'd look for syntactical variations.  Some hits
> > are in arch/{i386,x86_64,ia64}.  At a glance it seems hard to configure a
> > useful x86 kernel that doesn't involve any packed attribute.  I take that
> > as statistical proof that icc either has doesn't really work for building
> > the kernel or groks packing.  Any compiler not implementing gcc extensions
> > is lost at building the kernel but that's old news.
> > 
> 
> 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.

Well, icc should be gcc compatible. If it is not, it is icc bug.

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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