Hello,

On PPC targets, the debugger fails to unwind past some stack-check
sequences. The attached patch should fix that. I have double-checked
with Eric Botcazou to make sure that all possible cases are
caught... Roughly:

1) A small number of probes:
         stw 0, -<some immediate>(1) repeated any (small) number of times...

2) A probing loop:
         addi 12,1,-<some immediate>
         lis 0,-<some immediate>
         [possibly ori 0,0,<some immediate>]
         add 0,12,0
         cmpw 0,12,0
         beq 0,<disp>
         addi 12,12,-<some immediate>
         stw 0,0(12)
         b <disp>
         [possibly one last probe: stw 0,<some immediate>(12)]

3) No probe; instead, a comparizon between the stack size
     limit (saved in a run-time global variable) and the current stack
     pointer:

        addi 0,1,-<some immediate>
        lis 12,__gnat_stack_limit@ha
        lwz 12,__gnat_stack_limit@l(12)
        twllt 0,12


2008-09-01  Jerome Guitton  <guitton@adacore.com>

	* rs6000-tdep.c (rs6000_fetch_instruction)
	(rs6000_skip_stack_check): New functions.
	(skip_prologue): Skip stack check sequence.

Tested on AIX, no regression. I'll post my testcases in a minute. 
OK to apply?

Thanks,
- Jerome