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

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Message-Id: <200706161321.35940.arnd@arndb.de>
Date:	Sat, 16 Jun 2007 13:21:35 +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, Arnd Bergmann wrote:
> 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.

I just realized this was correct but slightly misleading. On powerpc, we
don't set the 'attribute((aligned(4)))' on compat_64, so there is never
a reason to handle the misalignment, even though it would work.

On x86_64, misaligned loads are always ok, so gcc never needs to
care about this, even attribute((packed)) does not cause byte access
here.

	Arnd <><
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