Re: BUG: GCC-4.4.x changes the function frame on some functions

[Frederic Weisbecker <fweisbec@gmail.com>]

On Thu, Nov 19, 2009 at 03:05:41PM -0500, Steven Rostedt wrote:
> 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)
> 


Yeah right, we need at least such very tiny prologue for
archs that store return addresses in a reg.

	
> > 
> > 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.


Sure, the purpose is not to change the existing -mcount thing.
What I meant is that we could have -mcount and -real-ra-before-fp
at the same time to guarantee fp + 4 is really what we want while
using -mcount.

The __fentry__ idea is more neat, but the guarantee of a real pointer
to the return address is still something that lacks.


> > 
> > 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"?


I guess it has been first created for the single purpose of counting
specific functions but then it has been used for wider, unpredicted uses :)