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    SubjectRe: [PATCH] ARM: Do not allow unaligned accesses when CONFIG_ALIGNMENT_TRAP
    On Thu, May 26, 2011 at 10:51:01PM +0100, Russell King - ARM Linux wrote:
    > On Thu, May 26, 2011 at 05:03:39PM -0400, Nicolas Pitre wrote:
    > > It is possible that -fconserve-stack is still valuable on ARM given that
    > > it is also used with -mno-unaligned-access for other things than
    > > structure packing on the stack, and therefore its merits can be debated
    > > independently from the alignment issue at hand.
    > Catalin said in his mail "I haven't tried with -mno-unaligned-access, I
    > suspect the variables on the stack would be aligned.". So I don't think
    > we know enough to say whether -mno-unaligned-access avoids the stack
    > packing.

    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.


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