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

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


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