Re: [PATCH] S/390 DWARF-2 CFI frame support

[Ulrich Weigand <weigand@i1.informatik.uni-erlangen.de>]

Richard Henderson wrote:

> On Thu, Dec 04, 2003 at 09:09:12PM +0100, Ulrich Weigand wrote:
> >   To fix this, I suggest the following.  What GCC assumes to happen
> >   when it leaves a register unspecified in the CFI depends on whether
> >   the register is call-saved or call-clobbered according to the 
> >   target's ABI.  If it is call-saved (and unspecified), the function
> >   doesn't save/restore it because it does not in fact ever modify it.
> >   Thus, in this case the debugger should copy the value from the 
> >   inner frame.  If it is call-clobbered (those will always be left
> >   unspecified), it should be assumed undefined.
> 
> This is wrong.  The debugger should just assume *all* registers
> that are not explicitly saved are preserved.  In the case of
> call-clobbered registers, you just won't *know* that they are
> actually dead.  But so what?  This is no worse than not having
> location list information that tells you that a value is dead
> after its register gets re-used for something else.

I have no strong opinion about that; if the gdb maintainers
accept treating all unspecified registers as preserved, that
is fine with me as well.   (I just don't want to see that
annoying warning all the time ;-/)

However, there is one point that requires special consideration
in any case: what if the stack pointer is unspecified?  On s390,
treating it as preserved from the inner frame is correct, but
on i386 and other platforms I guess this would be wrong -- it
needs to be set to the CFA there (which is wrong on s390 due
to the CFA bias we have).

I'm not familiar enough with gdb internals to decide what the
cleanest way of signaling that difference would be -- but there
needs to be *some* way or s390 will not work.   (My reggroup
suggestion would provide a way to solve this problem as well.)

Bye,
Ulrich

-- 
  Dr. Ulrich Weigand
  weigand@informatik.uni-erlangen.de