Re: Anyone working on ftrace function graph support on ARM?

[Ingo Molnar]

* Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:

> On Tue, Mar 24, 2009 at 10:48:46PM +0100, Ingo Molnar wrote:
> > 
> > * Frederic Weisbecker <fweisbec@xxxxxxxxx> 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
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