Re: [PATCH] ARM: Do not allow unaligned accesses when CONFIG_ALIGNMENT_TRAP

From: Catalin Marinas <catalin.marinas@arm.com>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: "Nicolas Pitre" <nico@fluxnic.net>,
	"Will Deacon" <Will.Deacon@arm.com>,
	"Måns Rullgård" <mans@mansr.com>,
	lkml <linux-kernel@vger.kernel.org>,
	ak@linux.intel.com, "Andrew Morton" <akpm@linux-foundation.org>,
	sam@ravnborg.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] ARM: Do not allow unaligned accesses when CONFIG_ALIGNMENT_TRAP
Date: Fri, 27 May 2011 09:38:08 +0100	[thread overview]
Message-ID: <20110527083806.GA21100@e102109-lin.cambridge.arm.com> (raw)
In-Reply-To: <20110526215101.GL24876@n2100.arm.linux.org.uk>

On Thu, May 26, 2011 at 10:51:01PM +0100, Russell King - ARM Linux wrote:
> On Thu, May 26, 2011 at 05:03:39PM -0400, Nicolas Pitre wrote:
> > It is possible that -fconserve-stack is still valuable on ARM given that
> > it is also used with -mno-unaligned-access for other things than
> > structure packing on the stack, and therefore its merits can be debated
> > independently from the alignment issue at hand.
> 
> Catalin said in his mail "I haven't tried with -mno-unaligned-access, I
> suspect the variables on the stack would be aligned.".  So I don't think
> we know enough to say whether -mno-unaligned-access avoids the stack
> packing.

OK, I tried this now:

-fconserve-stack: we get unaligned accesses on the stack because the
newer versions of gcc turned unaligned accesses on by default.

-fconserve-stack -mno-unaligned-access: the stack variables are aligned.
We probably get the benefit of -fconserve-stack as well.

So as per the initial post in this thread, we could have
-mno-unaligned-access on ARM always on (when CONFIG_ALIGNMENT_TRAP). As
Nicolas suggested, we could compile some files with -munaligned-access
(and maybe -fno-conserve-stack).

I raised this with the gcc guys so they are looking into it. But it
really doesn't look like a gcc bug as long as -mno-unaligned-access is
taken into account.

-- 
Catalin