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

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Date:	Fri, 27 May 2011 10:56:20 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Nicolas Pitre <nico@...xnic.net>,
	Will Deacon <Will.Deacon@....com>,
	Måns Rullgård <mans@...sr.com>,
	lkml <linux-kernel@...r.kernel.org>, ak@...ux.intel.com,
	Andrew Morton <akpm@...ux-foundation.org>, sam@...nborg.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] ARM: Do not allow unaligned accesses when
 CONFIG_ALIGNMENT_TRAP

On Fri, 2011-05-27 at 10:51 +0100, Catalin Marinas wrote:
> On Fri, May 27, 2011 at 09:54:14AM +0100, Russell King - ARM Linux wrote:
> > On Fri, May 27, 2011 at 09:38:08AM +0100, Catalin Marinas wrote:
> > > 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.
> > 
> > Ok, we need to check one last thing, and that's what the behaviour is
> > with -mno-unaligned-access and packed structures (such as the ethernet
> > header).  If it makes no difference, then I suggest we always build
> > with -mno-unaligned-access.
> 
> I tried some simple code below:
> 
> struct test {
> 	unsigned char a[6];
> 	unsigned long b;
> } __attribute__((packed));
> 
> void set(struct test *t, unsigned long v)
> {
> 	t->b = v;
> }
> 
> int main(void)
> {
> 	struct test t;
> 	
> 	set(&t, 10);
> 
> 	return 0;
> }
> 
> With -mno-unaligned-access in newer toolchains, the set() function looks
> like this (compiled with -march=armv7):
> 
> 00000000 <set>:
>    0:   e7e7c451        ubfx    ip, r1, #8, #8
>    4:   e7e72851        ubfx    r2, r1, #16, #8
>    8:   e1a03c21        lsr     r3, r1, #24
>    c:   e5c01006        strb    r1, [r0, #6]
>   10:   e5c0c007        strb    ip, [r0, #7]
>   14:   e5c02008        strb    r2, [r0, #8]
>   18:   e5c03009        strb    r3, [r0, #9]
>   1c:   e12fff1e        bx      lr
> 
> If I don't pass -mno-unaligned-access later toolchains use unaligned
> accesses by default and the set() function is more efficient:
> 
> 00000000 <set>:
>    0:   e5801006        str     r1, [r0, #6]
>    4:   e12fff1e        bx      lr

For completeness, I tried with "unsigned short b" in the structure above
hoping that the compiler would notice that it is 16-bit aligned.
Unfortunately, it doesn't. Code below with -mno-unaligned-access:

00000000 <set>:
   0:   e1a03421        lsr     r3, r1, #8
   4:   e5c01006        strb    r1, [r0, #6]
   8:   e5c03007        strb    r3, [r0, #7]
   c:   e12fff1e        bx      lr

-- 
Catalin


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