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

[catalin.marinas@arm.com (Catalin Marinas)]

2011/5/25 M?ns Rullg?rd <mans@mansr.com>:
> Dave Martin <dave.martin@linaro.org> writes:
>
>> On Wed, May 25, 2011 at 12:14:08PM +0100, Catalin Marinas wrote:
>>> 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.
>>
>> So, it looks like empty_str may be misaligned on the stack, and the compiler
>> is doing a misaligned store when initialising it.
>
> empty_str has type char[] so there are no alignment requirements.

I think the local variables after char empty_str[] are unaligned (int
alloc). Changing the array size to 16 solves the issue.

The gcc guys here in ARM will have a look and I'll get back to you.

-- 
Catalin