linux-kernel - Re: BUG: GCC-4.4.x changes the function frame on some functions

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Date:	Mon, 23 Nov 2009 10:51:46 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jakub Jelinek <jakub@...hat.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Haley <aph@...hat.com>,
	Richard Guenther <richard.guenther@...il.com>,
	rostedt@...dmis.org, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	feng.tang@...el.com, Fr??d??ric Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>, gcc@....gnu.org
Subject: Re: BUG: GCC-4.4.x changes the function frame on some functions

On Mon, 23 Nov 2009, Jakub Jelinek wrote:

> On Thu, Nov 19, 2009 at 08:01:57PM +0100, Thomas Gleixner wrote:
> > Just compiled with -mincoming-stack-boundary=4 and the problem goes
> > away as gcc now thinks that the incoming stack is already 16 byte
> > aligned. But that might break code which actually uses SSE
> 
> Please don't do this, lying to the compiler is just going to result in
> wrong-code sooner or later, with the above switch gcc will assume the
> incoming stack is 16-byte aligned (which is not true in the ix86 kernel)
> and could very well e.g. optimize away code that looks at
> alignment of stack variables etc.

Right. I gave up the idea pretty fast. But in the current situation we
are forced to lie to the compiler in some way. Forcing -mtune=generic
when the function graph tracer is enabled seems to be a halfways sane
work around.

Thanks,

	tglx
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