Re: Use DWARF CFI frame unwinder on powerpc

[Mark Kettenis <mark dot kettenis at xs4all dot nl>]

> Date: Mon, 15 May 2006 12:12:13 -0400
> From: Daniel Jacobowitz <drow@false.org>
> 
> Hi Andreas,
> 
> I know we tried to do this once before, and ran into hiccups based on
> the GCC numbering being inconsistent.  Do we know which versions of GCC
> this is likely to work with, and which are likely to make it fall down
> horribly?  I recall that GPRs were OK, but the numbering for LR was
> ambiguous.
> 
> It might have been this:
> 2004-01-21  Alan Modra  <amodra@bigpond.net.au>
> 
>         * config/rs6000/sysv4.h (DWARF2_FRAME_REG_OUT): Define.
>         * dwarf2out.c (output_cfi): Map regs using DWARF2_FRAME_REG_OUT.
>         * doc/tm.texi (DWARF_FRAME_REGNUM, DWARF2_FRAME_REG_OUT): Document.
> 
> Yep, here's Mark K. and Alan discussing it:
>   http://gcc.gnu.org/ml/gcc/2004-01/msg00025.html
> 
> I'm worried that since we don't currently use the "this is the unwind
> column" to indicate where lr lives in the table, although we could,
> we'll break on pre-3.4-ish(?) binaries.

Just verified that Andreas' patch breaks GDB on OpenBSD/macppc 3.9
which uses GCC 3.3.5 :(.  I'm afraid this means we can't
unconditionally enable the dwarf2 unwinder on all powerpc targets.
Selectively enabling them on platforms that are unlikely to have ever
used GCC versions before 3.4 should be fine.  I think all 64-bit
powerpc targets should be fine.

I'll have a look at what's needed to make things work for the old
unwind information.

Mark