On Thu, 2009-11-19 at 20:46 +0100, Frederic Weisbecker wrote:
> On Thu, Nov 19, 2009 at 02:28:06PM -0500, Steven Rostedt wrote:
> > <function>:
> > call __fentry__
> > [...]
> >
> >
> > -- Steve
>
>
> I would really like this. So that we can forget about other possible
> further suprises due to sophisticated function prologues beeing before
> the mcount call.
>
> And I guess that would fix it in every archs.
Well, other archs use a register to store the return address. But it
would also be easy to do (pseudo arch assembly):
<function>:
mov lr, (%sp)
add 8, %sp
blr __fentry__
sub 8, %sp
mov (%sp), lr
That way the lr would have the current function, and the parent would
still be at 8(%sp)
>
> That said, Linus had a good point about the fact there might other uses
> of mcount even more tricky than what does the function graph tracer,
> outside the kernel, and those may depend on the strict ABI assumption
> that 4(ebp) is always the _real_ return address, and that through all
> the previous stack call. This is even a concern that extrapolates the
> single mcount case.
As I am proposing a new call. This means that mcount stay as is for
legacy reasons. Yes I know there exists the -finstrument-functions but
that adds way too much bloat to the code. One single call to the
profiler is all I want.
>
> So I wonder that actually the real problem is the lack of something that
> could provide this guarantee. We may need a -real-ra-before-fp (yeah
> I suck in naming).
Don't worry, so do the C compiler folks, I mean, come on "mcount"?
-- Steve
