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