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

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Date:	Thu, 19 Nov 2009 12:36:25 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	Steven Rostedt <rostedt@...dmis.org>,
	David Daney <ddaney@...iumnetworks.com>,
	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, 19 Nov 2009, Frederic Weisbecker wrote:
>
> > That way the lr would have the current function, and the parent would
> > still be at 8(%sp)
> 
> Yeah right, we need at least such very tiny prologue for
> archs that store return addresses in a reg.

Well, it will be architecture-dependent.

For example, alpha can store the return value in _any_ register if I 
recall correctly, so you can do the "call to __fentry__" by just picking 
another register than the default one as the return address.

And powerpc has two special registers: link and ctr, but iirc you can only 
load 'link' with a branch instruction. Which means that you could do 
something like 

	mflr 0
	bl __fentry__

in the caller (I forget if R0 is actually a call-trashed register or not), 
and then __fentry__ could do something like

	mflr 12		# save _new_ link
	mtlr 0		# restore original link
	mtctr 12	# move __fentry__ link to ctr
	.. do whatever ..
	bctr		# return to __fentry__ caller

to return with 'link' restored (but ctr and r0/r12 trashed - I don't 
recall the ppc calling conventions any more, but I think that is ok).

Saving to stack seems unnecessary and pointless.

            Linus
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