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

Ingo Molnar
2009-03-24T22:15:42Z

* 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