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