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[PATCH] Harden variably_modified_type_p

It seems with Ada one can have cycles in types via pointer types
and calling variably_modified_type_p on such type recurses indefinitely.
The following is an attempt to fix that (albeit I'm not 100% sure
there's no caller of the function using TREE_VISITED itself).

Cures c38102a.adb with LTO early debug patches for me.

Any comments?  (no further testing sofar)


Index: gcc/tree.c
--- gcc/tree.c	(revision 251141)
+++ gcc/tree.c	(working copy)
@@ -8597,8 +8601,16 @@ variably_modified_type_p (tree type, tre
     case POINTER_TYPE:
     case VECTOR_TYPE:
+      /* Ada can have pointer types refering to themselves indirectly.  */
+      if (TREE_VISITED (type))
+	return false;
+      TREE_VISITED (type) = true;
       if (variably_modified_type_p (TREE_TYPE (type), fn))
-	return true;
+	{
+	  TREE_VISITED (type) = false;
+	  return true;
+	}
+      TREE_VISITED (type) = false;
     case FUNCTION_TYPE:

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