linux-kernel - Re: [PATCH] Introduce compat_u64 and compat

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Message-Id: <200706161226.40875.arnd@arndb.de>
Date:	Sat, 16 Jun 2007 12:26:40 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dave Airlie <airlied@...ux.ie>, linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Introduce compat_u64 and compat_s64 types

On Saturday 16 June 2007, David Woodhouse wrote:
> Will GCC know that it needs to emit code to handle that (mis)alignment?

I've tested this with gcc-4.0.3, and it does the right thing, which
is to split a 4 byte aligned 64 bit load/store into two 32 bit accesses,
if you pass -mstrict-align.

> Preliminary tests show that it does load the value bytewise if we use
> the 'packed' attribute structure on ppc64, but doesn't if we use
> compat_u64. But then, I don't think it actually _needs_ to handle it n
> ppc64 anyway, so maybe that's not such a good test.

Right. Note that the behaviour of compat_u64 is the same as when you
pass attribute((packed,aligned(4))) to the structure, where it also won't
do byte accesses, and split the 64 bit access only if you pass
-mstrict-align.

	Arnd <><
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