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

From: catalin.marinas@arm.com (Catalin Marinas)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH] ARM: Do not allow unaligned accesses when CONFIG_ALIGNMENT_TRAP
Date: Wed, 25 May 2011 12:14:08 +0100	[thread overview]
Message-ID: <20110525111405.GA12010@e102109-lin.cambridge.arm.com> (raw)
In-Reply-To: <20110524171331.GA2941@arm.com>

On Tue, May 24, 2011 at 06:13:31PM +0100, Dave Martin wrote:
> On Tue, May 24, 2011 at 04:26:35PM +0100, Catalin Marinas wrote:
> > BTW, are we sure that the code generated with unaligned accesses is
> > better? AFAIK, while processors support unaligned accesses, they may
> > not always be optimal.
> 
> The code gcc generates to synthesise an unaligned access using aligned
> accesses is pretty simplistic:
...
> For code which natively needs to read unaligned fields from data structures,
> I sincerely doubt that the CPU will not beat the above code for efficiency...
> 
> So if there's code doing unaligned access to data structures for a good
> reason, building with unaligned access support turned on in the compiler
> seems a good idea, if that code might performance-critical for anything.

gcc generates unaligned accesses in the the pcpu_dump_alloc_info()
function. We have a local variable like below (9 bytes):

	char empty_str[] = "--------";

and it looks like other stack accesses are unaligned:

c0082ba0 <pcpu_dump_alloc_info>:
c0082ba0:   e92d4ff0    push    {r4, r5, r6, r7, r8, r9, sl, fp, lr}
c0082ba4:   e3074118    movw    r4, #28952  ; 0x7118
c0082ba8:   e24dd04c    sub sp, sp, #76 ; 0x4c
c0082bac:   e34c402a    movt    r4, #49194  ; 0xc02a
c0082bb0:   e58d1014    str r1, [sp, #20]
c0082bb4:   e58d0020    str r0, [sp, #32]
c0082bb8:   e8b40003    ldm r4!, {r0, r1}
c0082bbc:   e58d003f    str r0, [sp, #63]   <----------- !!!!!
c0082bc0:   e59d0014    ldr r0, [sp, #20]
c0082bc4:   e5d43000    ldrb    r3, [r4]

I haven't tried with -mno-unaligned-access, I suspect the variables on
the stack would be aligned.

-- 
Catalin