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Date: Wed, 16 Jan 2008 15:19:28 +0100
From: "Bernhard Michael" <michael.bernhard@domain.hid>
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Subject: Re: [Xenomai-help] Xenomai problems on pxa
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To: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Cc: xenomai@xenomai.org

Gilles Chanteperdrix wrote:
> I do not know how to solve this. From my point of view, this is a
> compiler bug: knowing that it uses instructions which needs 8 bytes
> alignment, the compiler should align stack frames on 8 bytes
> boundaries, period.

Digging a bit in the mailing lists about stack alignment and EABI I find
that gcc *realy* should handle proper stack alignment.

See http://gcc.gnu.org/ml/gcc/2006-07/msg00030.html
or http://www.codesourcery.com/archives/arm-gnu/msg01467.html

Now, looking at the assembly code for the SWI handler, I see that ipipe
introduces flowing code:

	#ifdef CONFIG_IPIPE
		stmfd	sp!, {r0-r3, ip}
		add	r1, sp, #S_OFF
		add	r1, r1, #20
		mov	r0, scno
		bl	__ipipe_syscall_root
		cmp	r0, #0
		ldmfd	sp!, {r0-r3, ip}
		blt	__ipipe_ret_fast_syscall
		bgt	__ipipe_fast_exit_syscall
	#endif /* CONFIG_IPIPE */

Assuming that the stack was properly aligned before this code snipped
the first instruction moves 20 bytes onto the stack. 20 modulo 8 = 4 which
means, the stack is not 8-byte aligned any more. This could be the reason
why the strd/ldrd instructions are failing later.

I can't verify this on hardware now but I'll do it for sure.

--
Michael