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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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