Re: [PATCH] ARM: Do not allow unaligned accesses whenCONFIG_ALIGNMENT_TRAP

[Catalin Marinas]

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.

-- 
Catalin

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/