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

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

Mark Kettenis wrote:

> I've considered per-architecture initialization of the unwind table
> before.  However, the things Richard Henderson says about treating
> uninitialized columns as "same value" make sense.

However, I rather like to see 'value not available' instead of
an incorrect value in an 'info reg' display.  So if we do have
an arch-dependent callback, we might as well use ABI knowledge
to get this right.

> On the other hand, we should be able to introduce our own "rules" in
> addition to the ones given by the DWARF2/3 specification.  We could
> add the following two:
> 
> * REG_RETURN_ADDRESS: Set the particular register to the return
>   address of the function.
> 
> * REG_CFA: Set the particular register to the call frame address.
> 
> We could even allow for an offset, such that we could specify rules
> such as: set the ISA PC register to the return address plus an offset
> of 8 bytes, or set the ISA SP register to the call frame address plus
> minus an offset of 128 bytes.  I might need such a facility for SPARC.
> 
> How does that sound?

This would certainly solve the s390 situation.

> In the meantime, I'm going to try to remove some of the PC and
> SP-related hacks in dwarf2-frame.c and see what happens.

The only hack that cannot be replaced using the rules described
above (as far as I can see) is the 
  if (column == fs->retaddr_column)
    continue;
in dwarf2_frame_cache.  Does any platform rely on this behaviour?

Bye,
Ulrich

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