RFA: Fix bug in optimize_mode_switching

[Joern Rennecke <amylaar@spamcop.net>]

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I work on target with complex mode switching needs, so it can happen that
in some block, for an entity a mode is provided without the need for a set.
This causes the current optimize_mode_switching to crash when it later
dereferences a NULL seginfo pointer.
Fixed by using an actual flag to keep track if we have allocated any seginfo.

Bootstrappded on x86_64-unknown-linux-gnu.

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2011-07-08  Joern Rennecke <joern.rennecke@embecosm.com>

	* mode-switching.c (optimize_mode_switching): Fix bug in MODE_AFTER
	handling.

Index: mode-switching.c
===================================================================
--- mode-switching.c	(revision 1670)
+++ mode-switching.c	(revision 1671)
@@ -499,6 +499,7 @@ optimize_mode_switching (void)
 	{
 	  struct seginfo *ptr;
 	  int last_mode = no_mode;
+	  bool any_set_required = false;
 	  HARD_REG_SET live_now;
 
 	  REG_SET_TO_HARD_REG_SET (live_now, df_get_live_in (bb));
@@ -527,6 +528,7 @@ optimize_mode_switching (void)
 
 		  if (mode != no_mode && mode != last_mode)
 		    {
+		      any_set_required = true;
 		      last_mode = mode;
 		      ptr = new_seginfo (mode, insn, bb->index, live_now);
 		      add_seginfo (info + bb->index, ptr);
@@ -548,8 +550,10 @@ optimize_mode_switching (void)
 	    }
 
 	  info[bb->index].computing = last_mode;
-	  /* Check for blocks without ANY mode requirements.  */
-	  if (last_mode == no_mode)
+	  /* Check for blocks without ANY mode requirements.
+	     N.B. because of MODE_AFTER, last_mode might still be different
+	     from no_mode.  */
+	  if (!any_set_required)
 	    {
 	      ptr = new_seginfo (no_mode, BB_END (bb), bb->index, live_now);
 	      add_seginfo (info + bb->index, ptr);