linux-kernel - Re: [PATCH] ARM: Do not allow unaligned accesses when CONFIG_ALIGNMENT

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Message-ID: <1306490709.26257.4.camel@e102144-lin.cambridge.arm.com>
Date:	Fri, 27 May 2011 11:05:09 +0100
From:	Will Deacon <will.deacon@....com>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	Nicolas Pitre <nico@...xnic.net>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Måns Rullgård <mans@...sr.com>,
	linux-arm-kernel@...ts.infradead.org,
	lkml <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>, sam@...nborg.org
Subject: Re: [PATCH] ARM: Do not allow unaligned accesses when
 CONFIG_ALIGNMENT_TRAP

Hi Andi,

On Thu, 2011-05-26 at 22:10 +0100, Andi Kleen 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.
> 
> The big advantage of -fconserve-stack is that it throttles the inliner
> if the inlining
> would cause too much stack growth. This is something you likely want
> on ARM too, especially as code gets more and more complex.

Do you have any concrete examples of -fconserve-stack giving an overall
win that isn't in the noise? The fact that the GCC documentation
explicitly states that enabling the option can lead to `making the
program slower' does make me question why we're enabling it in the first
place.

>>From private conversation, the GCC guys don't seem to think this is a
bug so I'm reluctant to open a bugzilla ticket.

Will

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