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

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Message-ID: <20091119202845.GH4967@nowhere>
Date:	Thu, 19 Nov 2009 21:28:48 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	David Daney <ddaney@...iumnetworks.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Haley <aph@...hat.com>,
	Richard Guenther <richard.guenther@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	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, Peter Zijlstra <peterz@...radead.org>,
	jakub@...hat.com, gcc@....gnu.org
Subject: Re: BUG: GCC-4.4.x changes the function frame on some functions

On Thu, Nov 19, 2009 at 03:17:16PM -0500, Steven Rostedt wrote:
> On Thu, 2009-11-19 at 15:05 -0500, Steven Rostedt wrote:
> 
> > Well, other archs use a register to store the return address. But it
> > would also be easy to do (pseudo arch assembly):
> > 
> > 	<function>:
> > 		mov lr, (%sp)
> > 		add 8, %sp
> > 		blr __fentry__
> 
> Should be bl __fentry__ for "branch and link".
> 
> > 		sub 8, %sp
> > 		mov (%sp), lr
> > 
> > 
> > That way the lr would have the current function, and the parent would
> > still be at 8(%sp)
> 
> Actually, if we add a new profiler and can make our own specification, I
> would say that the add and sub lines be the responsibility of
> __fentry__. Then we would have:
> 
> 	<function>:
> 		mov lr, (%sp)
> 		bl __fentry__
> 		mov (%sp), lr
> 
> If sp points to the current content, then replace (%sp) above with 
> -8(%sp).  Then the implementation of a nop __fentry__ would simply be:
> 
> 	__fentry__:
> 		blr


Good point!

 
> For anything more elaborate, __fentry__ would be responsible for all
> adjustments.


Yep. The more we control it from __fentry__, the less we fall
down into unexpected surprises.

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