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

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Message-ID: <yw1xvcwxfhwo.fsf@unicorn.mansr.com>
Date:	Thu, 26 May 2011 19:14:47 +0100
From:	Måns Rullgård <mans@...sr.com>
To:	Will Deacon <will.deacon@....com>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Måns Rullgård 
	<mans@...sr.com>, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	sam@...nborg.org, ak@...ux.intel.com
Subject: Re: [PATCH] ARM: Do not allow unaligned accesses when CONFIG_ALIGNMENT_TRAP

Will Deacon <will.deacon@....com> writes:

> This issue seems to be caused by passing -fconserve-stack to GCC. This
> was added in 8f7f5c9f ("kbuild: set -fconserve-stack option for gcc
> 4.5") and as you can see from the archive:
>
> http://lkml.org/lkml/2009/9/20/39
>
> it was thought to only have an impact on inlining decisions. Looking at
> the documentation for GCC 4.6:
>
> -fconserve-stack
>           Attempt to minimize stack usage. The compiler will attempt to
> use less stack space, even if that makes the program slower. This option
> implies setting the ‘large-stack-frame’ parameter to 100 and the
> ‘large-stack-frame-growth’ parameter to 400.
>
> So it sounds like we might not want to enable this blindly across all
> architectures. Indeed, on ARM, it encourages the compiler to pack
> variables on the stack which leads to the weird and wonderful alignment
> situation that has been encountered in this thread.
>
> Can we remove -fconserve-stack from the top-level Makefile (or at least
> make it conditional by architecture)?

Sounds like a good idea to me.

-- 
Måns Rullgård
mans@...sr.com
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