Date: Thu, 19 Nov 2009 12:36:25 -0800 (PST)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
cc: Steven Rostedt <rostedt@goodmis.org>,
       David Daney <ddaney@caviumnetworks.com>, Andrew Haley <aph@redhat.com>,
       Richard Guenther <richard.guenther@gmail.com>,
       Thomas Gleixner <tglx@linutronix.de>, 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,
       Peter Zijlstra <peterz@infradead.org>, jakub@redhat.com,
       gcc@gcc.gnu.org
Subject: Re: BUG: GCC-4.4.x changes the function frame on some functions
In-Reply-To: <20091119202526.GG4967@nowhere>
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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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