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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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