Re: [PATCH -v4 9/9] tracing: add function graph tracer support forMIPS

[David Daney]

Steven Rostedt wrote:
> On Wed, 2009-10-21 at 09:37 -0700, David Daney wrote:
>
> > There is no deterministic way to identify MIPS function prologs.  This 
> > is especially true for leaf functions, but also for functions with 
> > multiple return sites.
> >
> > For certain GCC versions there may be a set of command line options that 
> > would give good results, but in general it is not possible.  Attempts at 
> > fast backtrace generation using code inspection are not reliable and 
> > will invariably result in faults and panics when they fail.
>
> Thanks for the update.
>
> We can easily protect against panics, since we do fault protection
> within the code (although currently it will panic on fault, but we can
> fix that ;-). We can limit the search to a couple of 100 instructions,
> as well as fail on first panic.
>
> But are you sure that when compiled with -pg, that GCC does not give a
> reliable prologue. Things are different when GCC is compiled with -pg,
> it may indeed always have something that we can flag.
>
> We could also add other tests, like the subtraction of the stack too.

I have not used -pg, so I don't know for sure, I think all it does is 
add the calls to _mcount.  Someone could investigate 
-fno-omit-frame-pointer, with that you may be able to use:

   move    s8,sp

To identify function prologs, but it would still be ad hoc, as modern 
versions of GCC will reorder instructions in the prolog for better 
scheduling.

David Daney
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