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Date: Tue, 24 Mar 2009 23:54:58 +0100
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: Tim Bird <tim.bird@am.sony.com>,
       linux-arm-kernel <linux-arm-kernel@lists.arm.linux.org.uk>,
       linux kernel <linux-kernel@vger.kernel.org>,
       Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@redhat.com>,
       Abhishek Sagar <sagar.abhishek@gmail.com>,
       Russell King <rmk@arm.linux.org.uk>,
       Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= 
	<u.kleine-koenig@pengutronix.de>
Subject: Re: Anyone working on ftrace function graph support on ARM?
Message-ID: <20090324225458.GF5975@nowhere>
References: <49C936CA.8070800@am.sony.com> <20090324213618.GC5975@nowhere> <20090324214846.GB29509@elte.hu> <20090324215738.GD5975@nowhere> <20090324221439.GE29509@elte.hu>
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On Tue, Mar 24, 2009 at 11:14:39PM +0100, Ingo Molnar wrote:
> 
> * Frederic Weisbecker <fweisbec@gmail.com> wrote:
> 
> > On Tue, Mar 24, 2009 at 10:48:46PM +0100, Ingo Molnar wrote:
> > > 
> > > * Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > > 
> > > > Well it's a very naive listing, there are sometimes some problems. 
> > > > For example on x86-64, I had to save even some non-scratch 
> > > > registers before calling the return hook, I still don't know why.
> > > 
> > > btw., which are those registers?
> > > 
> > > 	Ingo
> > 
> > 
> > I would expect to only save rax,rdi,rsi,rdx,rcx,r8,r9 which are 
> > used for parameters.
> 
> > And I had some crashes until I append r10 and r11 which actually 
> > are scratch if I'm not wrong, but since they are scratch and are 
> > not used for arguments, I thought they didn't need to be saved.
> > 
> > Well, I think there were some code flow cases I was missing.
> 
> Correct, r10 and r11 are clobbered registers too - and you need to 
> save them too in mcount methods.
> 
> The reason is that mcount has a special calling convention - it's 
> not just about not destroying arguments - GCC can keep data in r10 
> or r11 scratch registers across function calls as well - for example 
> for relatively static functions that are in its local optimization 
> scope.
> 
> If GCC can prove that the local scope function itself does not 
> clobber r10/r11, it does not have to clobber them across the 
> function call. But the mcount() callback still gets inserted.
> 
> So the rule is: mcount must not destroy _any_ register state. 
> (beyond flags)
> 
> 	ngo


Aah, ok, understood!
Thanks.

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