Hello,
this patch adds support for DWARF-2 frame handling to the s390 backend.
However, the DWARF-2 frame common code makes a couple of assumptions
that are not valid on s390:
- The return address column is not unwound into the specified register,
only into the PC. On s390, it should be unwound into both.
The patch below fixes this by unwinding into both register
and PC if the return column corresponds to a GDB register.
What does the CFI output look like here? (I'm mainly curious but it always helps to have concrete examples).
For a variant of the PPC64 GCC I found:
- return column pointed at the FSCR
(which is stupid)
- return column same-value as "LR" was implied
(which is contrary to the DWARF2's example, and what you also encountered)
this means that we've now got the following initial states all mapping onto "unspecified":
- same value
- unknown
- (for SP_REGNUM) it's the CFA (but is it one frame out?)
- (for return column) it's the LR
I'm wondering if it would be easier, here to "give up". Throw the whole problem back at the architecture vis:
- initialize unwind table using per OSABI (?) method
- update the unwind table using the CFI initialize info
- update the unwind table using the PC's CFI unwind info
- if after all this a register is still unspecified, we complain
(btw, the compaint is only ment to appear once but apparently appears repeatedly?).
Thoughts for the moment on the theory? Mark?
It should also open the way for the elimination of SP_REGNUM and PC_REGNUM from this file (ya!).
As for an interface, the <ARCH>_gdbarch_init code could something like:
dwarf2_frame_add_sniffer (gdbarch, (*<ARCH>_dwarf2_cfi_init) (struct cfi_table *table));
(but with better names :-).
Andrew