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SubjectRe: [PATCH] ARM: Do not allow unaligned accesses when CONFIG_ALIGNMENT_TRAP
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Catalin Marinas <catalin.marinas@arm.com> writes:

> On Fri, May 27, 2011 at 09:54:14AM +0100, Russell King - ARM Linux wrote:
>>
>> 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

This is certainly something we should want. Although some people
expressed concerns over introducing unaligned accesses where there were
previously none, I don't see how this could pose a problem as long as we
make sure strict alignment checking is off. Some basic testing of code
paths known to use unaligned accesses should suffice IMO.

> The problem is that in addition to that we also get unaligned stack
> variables which are not really efficient. Either way we have a drawback
> somewhere. We could argue that -fconserve-stack is badly implemented on
> ARM.

Unless someone can demonstrate a clear win from -fconserve-stack, I
think it's pretty obvious that this flag does more harm than good on
ARM, especially in conjunction with unaligned accesses being allowed.

If the stack packing could be disabled while retaining the other
(presumably beneficial) effects of -fconserve-stack, it might be
reconsidered.

--
Måns Rullgård
mans@mansr.com
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