From mboxrd@z Thu Jan  1 00:00:00 1970
From: will.deacon@arm.com (Will Deacon)
Date: Tue, 12 Feb 2013 16:36:45 +0000
Subject: [PATCH] [RFC] arm: fix memset-related crashes caused by recent
 GCC (4.7.2) optimizations
In-Reply-To: <20130212155801.GQ17833@n2100.arm.linux.org.uk>
References: <1359793988-6881-1-git-send-email-ivan.djelic@parrot.com>
 <20130211184114.GP9801@mudshark.cambridge.arm.com>
 <20130211194225.GK29329@parrot.com>
 <20130212140008.GB4123@mudshark.cambridge.arm.com>
 <20130212155801.GQ17833@n2100.arm.linux.org.uk>
Message-ID: <20130212163645.GI4123@mudshark.cambridge.arm.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On Tue, Feb 12, 2013 at 03:58:01PM +0000, Russell King - ARM Linux wrote:
> On Tue, Feb 12, 2013 at 02:00:08PM +0000, Will Deacon wrote:
> > Interesting... the GCC documentation also states that ffreestanding implies
> > fno-builtin, so memset and co shouldn't be targetted for this sort of
> > optimisation by GCC. Have you observed this problem even when passing this
> > option?
> 
> Rather than wondering whether we should be using -ffreestanding or not
> (which, x86 people have strongly resisted) I suggest that we just fix
> our memset() implementation to be compliant.
> 
> The reason it's not compliant is that I saw no reason for it to be
> compliant back in the gcc 2.7.x days, and it's persisted like that for
> the last 19-ish years.  If GCC is now making use of the return value,
> then we need to fix that and undo the "optimization" in our string.h.
> 
> So let's just bite the bullet, make the asm memset() compliant, and
> clean up string.h.

That would be the ideal thing to do, because it allows the compiler to
optimise around these functions, however it does mean we need to check/fix
*all* of the string functions at least (if we don't pass -fno-builtin then
any builtin function is up for optimisation, including strcpy etc).

Will