Re: [Valgrind-users] Callgrind results on ppc

[Julian S. <js...@ac...>]

Thanks Jeff for finding a small test case, and Josef for chasing it.
I have not much constructive to add except ...

> x86 with its explicit call/ret instructions is way easier to get right;
> on x86, the stack pointer always changes on call/ret. On PPC, this does
> not need to happen as the return address is stored in the link register.
> So: ideas for good heuristics welcome.

One difficulty on ppc is that the RA is not always in the link register,
not even for the innermost frame.  Suppose f is a leaf function.  Normally
RA would remain in lr and that would be OK; however suppose the compiler
wants to use lr for some other purpose - not calling a function, maybe
for an indirect jump.  Then it will have to store LR somewhere else inside
f.

I am not claiming to understand this fully.  I think studying the ppc32-ELF
ABI would help.

What I do know is that there is a nasty hack in m_stacktrace.c, the part
for unwinding the stack -- see VG_(get_StackTrace2) and specifically the
stuff for setting/using lr_is_first_RA.  This was from one of the IBM
linux guys, unfortunately moved on elsewhere now.

If you do come up with a good story on unwinding the ppc-linux stack
I would like to see it.  It may be that the logic for ppc in 
VG_(get_StackTrace2) is too complex or wrong or something, or maybe
it's exactly correct, I don't know.

J