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Subject: Re: BUG: GCC-4.4.x changes the function frame on some functions
From: Steven Rostedt <rostedt@goodmis.org>
Reply-To: rostedt@goodmis.org
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: David Daney <ddaney@caviumnetworks.com>,
       Linus Torvalds <torvalds@linux-foundation.org>,
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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


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