[rs6000] Fix PR 50906, eh_frame and other woes

[Alan Modra]

On Mon, Nov 07, 2011 at 04:10:38PM +0000, Joseph S. Myers wrote:
> FWIW, when I encountered such a problem in 4.4-based tools I found I 
> needed the following change to stack_tie patterns to fix it (applied to 
> and rediffed against trunk, but not tested there), in addition to a 
> DEFAULT_ABI == ABI_V4 check such as you added.  Given all the confusion 
> about this class of bugs and whether they should be addressed in each back 
> end or in a target-independent way, I didn't try to send this upstream at 
> the time.
> 
> 2011-04-11  Joseph Myers  <joseph@codesourcery.com>
> 
> 	* config/rs6000/rs6000.md (stack_tie): Use (mem:BLK (scratch)) as
> 	output of SET.
> 	(restore_stack_block, restore_stack_nonlocal): Update UNSPEC_TIE
> 	patterns.

OK, so you made the stack ties conflict with all memory.  I wondered
about that too, but didn't find a testcase where it mattered.  It
would certainly be safer to use the big hammer, but on the other hand
it can restrict scheduling unnecessarily.  For example

void byref (int *a)
{
  int x;

  asm ("#%0 set here" : "=r" (x) : : "r29", "r30");
  *a = x;
}

with the existing stack ties, the write to *a happens after stack
deallocation.

-- 
Alan Modra
Australia Development Lab, IBM