--- gcc/tree-ssa-loop-unswitch.c.jj	2010-06-07 11:25:05.000000000 +0200
+++ gcc/tree-ssa-loop-unswitch.c	2010-06-24 17:25:09.000000000 +0200
@@ -129,6 +129,12 @@ tree_may_unswitch_on (basic_block bb, st
   if (!stmt || gimple_code (stmt) != GIMPLE_COND)
     return NULL_TREE;
 
+  /* To keep the things simple, we do not directly remove the conditions,
+     but just replace tests with 0 != 0 resp. 1 != 0.  Prevent the infinite
+     loop where we would unswitch again on such a condition.  */
+  if (gimple_cond_true_p (stmt) || gimple_cond_false_p (stmt))
+    return NULL_TREE;
+
   /* Condition must be invariant.  */
   FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE)
     {
@@ -142,12 +148,6 @@ tree_may_unswitch_on (basic_block bb, st
   cond = build2 (gimple_cond_code (stmt), boolean_type_node,
 		 gimple_cond_lhs (stmt), gimple_cond_rhs (stmt));
 
-  /* To keep the things simple, we do not directly remove the conditions,
-     but just replace tests with 0/1.  Prevent the infinite loop where we
-     would unswitch again on such a condition.  */
-  if (integer_zerop (cond) || integer_nonzerop (cond))
-    return NULL_TREE;
-
   return cond;
 }
 
@@ -193,21 +193,14 @@ tree_unswitch_single_loop (struct loop *
 {
   basic_block *bbs;
   struct loop *nloop;
-  unsigned i;
+  unsigned i, found;
   tree cond = NULL_TREE;
   gimple stmt;
   bool changed = false;
 
-  /* Do not unswitch too much.  */
-  if (num > PARAM_VALUE (PARAM_MAX_UNSWITCH_LEVEL))
-    {
-      if (dump_file && (dump_flags & TDF_DETAILS))
-	fprintf (dump_file, ";; Not unswitching anymore, hit max level\n");
-      return false;
-    }
-
   i = 0;
   bbs = get_loop_body (loop);
+  found = loop->num_nodes;
 
   while (1)
     {
@@ -218,8 +211,17 @@ tree_unswitch_single_loop (struct loop *
 
       if (i == loop->num_nodes)
 	{
-	  free (bbs);
-	  return changed;
+	  if (dump_file
+	      && num > PARAM_VALUE (PARAM_MAX_UNSWITCH_LEVEL)
+	      && (dump_flags & TDF_DETAILS))
+	    fprintf (dump_file, ";; Not unswitching anymore, hit max level\n");
+
+	  if (found == loop->num_nodes)
+	    {
+	      free (bbs);
+	      return changed;
+	    }
+	  break;
 	}
 
       cond = simplify_using_entry_checks (loop, cond);
@@ -236,19 +238,107 @@ tree_unswitch_single_loop (struct loop *
 	  gimple_cond_set_condition_from_tree (stmt, boolean_false_node);
 	  changed = true;
 	}
+      /* Do not unswitch too much.  */
+      else if (num > PARAM_VALUE (PARAM_MAX_UNSWITCH_LEVEL))
+	{
+	  i++;
+	  continue;
+	}
+      /* In nested tree_unswitch_single_loop first optimize all conditions
+	 using entry checks, then discover still reachable blocks in the
+	 loop and find the condition only among those still reachable bbs.  */
+      else if (num != 0)
+	{
+	  if (found == loop->num_nodes)
+	    found = i;
+	  i++;
+	  continue;
+	}
       else
-	break;
+	{
+	  found = i;
+	  break;
+	}
 
       update_stmt (stmt);
       i++;
     }
 
+  if (num != 0)
+    {
+      basic_block *tos, *worklist;
+
+      /* When called recursively, first do a quick discovery
+	 of reachable bbs after the above changes and only
+	 consider conditions in still reachable bbs.  */
+      tos = worklist = XNEWVEC (basic_block, loop->num_nodes);
+
+      for (i = 0; i < loop->num_nodes; i++)
+	bbs[i]->flags &= ~BB_REACHABLE;
+
+      /* Start with marking header.  */
+      *tos++ = bbs[0];
+      bbs[0]->flags |= BB_REACHABLE;
+
+      /* Iterate: find everything reachable from what we've already seen
+	 within the same innermost loop.  Don't look through false edges
+	 if condition is always true or true edges if condition is
+	 always false.  */
+      while (tos != worklist)
+	{
+	  basic_block b = *--tos;
+	  edge e;
+	  edge_iterator ei;
+	  int flags = 0;
+
+	  if (EDGE_COUNT (b->succs) == 2)
+	    {
+	      gimple stmt = last_stmt (b);
+	      if (stmt
+		  && gimple_code (stmt) == GIMPLE_COND)
+		{
+		  if (gimple_cond_true_p (stmt))
+		    flags = EDGE_FALSE_VALUE;
+		  else if (gimple_cond_false_p (stmt))
+		    flags = EDGE_TRUE_VALUE;
+		}
+	    }
+
+	  FOR_EACH_EDGE (e, ei, b->succs)
+	    {
+	      basic_block dest = e->dest;
+
+	      if (dest->loop_father == loop
+		  && !(dest->flags & BB_REACHABLE)
+		  && !(e->flags & flags))
+		{
+		  *tos++ = dest;
+		  dest->flags |= BB_REACHABLE;
+		}
+	    }
+	}
+
+      free (worklist);
+
+      /* Find a bb to unswitch on.  */
+      for (; found < loop->num_nodes; found++)
+	if ((bbs[found]->flags & BB_REACHABLE)
+	    && (cond = tree_may_unswitch_on (bbs[found], loop)))
+	  break;
+
+      if (found == loop->num_nodes)
+	{
+	  free (bbs);
+	  return changed;
+	}
+    }
+
   if (dump_file && (dump_flags & TDF_DETAILS))
     fprintf (dump_file, ";; Unswitching loop\n");
 
   initialize_original_copy_tables ();
   /* Unswitch the loop on this condition.  */
-  nloop = tree_unswitch_loop (loop, bbs[i], cond);
+  nloop = tree_unswitch_loop (loop, bbs[found], cond);
   if (!nloop)
     {
       free_original_copy_tables ();
--- gcc/testsuite/gfortran.dg/pr43866.f90.jj	2010-06-25 09:51:40.000000000 +0200
+++ gcc/testsuite/gfortran.dg/pr43866.f90	2010-06-25 09:53:29.000000000 +0200
@@ -0,0 +1,43 @@
+! PR middle-end/43866
+! { dg-do run }
+! { dg-options "-funswitch-loops -fbounds-check" }
+
+MODULE PR43866
+  IMPLICIT NONE
+  TYPE TT
+    REAL(KIND=4), DIMENSION(:,:), POINTER :: A
+    REAL(KIND=8), DIMENSION(:,:), POINTER :: B
+  END TYPE
+CONTAINS
+  SUBROUTINE FOO(M,X,Y,T)
+    TYPE(TT), POINTER :: M
+    INTEGER, INTENT(IN) :: Y, X
+    INTEGER :: C, D
+    LOGICAL :: T
+    REAL(KIND = 4), DIMENSION(:,:), POINTER :: P
+    REAL(KIND = 8), DIMENSION(:,:), POINTER :: Q
+
+    Q => M%B
+    P => M%A
+    DO C=1,X
+      DO D=C+1,Y
+        IF (T) THEN
+          P(D,C)=P(C,D)
+        ELSE
+          Q(D,C)=Q(C,D)
+        ENDIF
+      ENDDO
+    ENDDO
+  END SUBROUTINE FOO
+END MODULE PR43866
+
+  USE PR43866
+  TYPE(TT), POINTER :: Q
+  INTEGER, PARAMETER :: N=17
+  ALLOCATE (Q)
+  ALLOCATE (Q%B(N,N))
+  Q%B=0
+  CALL FOO (Q,N,N,.FALSE.)
+END
+
+! { dg-final { cleanup-modules "pr43866" } }
