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

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Date:	Sat, 16 Jun 2007 12:34:11 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	Arnd Bergmann <arnd@...db.de>
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 Sat, 2007-06-16 at 13:21 +0200, Arnd Bergmann wrote:
> 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.

You're right. My question was probably not relevant -- all these 64-bit
architectures cope with misaligned loads anyway. If we ever have to deal
with 32-bit compat on a 64-bit architecture which can't handle
misalignment, I'm just going to hide under my desk and never come out.

> 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.

IA64 too, but it'll be handled there too -- either naturally or by
fixups; it doesn't matter.

-- 
dwmw2

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