Fix PR63476

Index: gcc/tree-ssa-pre.c
===================================================================
--- gcc/tree-ssa-pre.c	(revision 216037)
+++ gcc/tree-ssa-pre.c	(working copy)
@@ -423,6 +423,9 @@  typedef struct bb_bitmap_sets
   /* A cache for value_dies_in_block_x.  */
   bitmap expr_dies;
 
+  /* The live virtual operand on successor edges.  */
+  tree vop_on_exit;
+
   /* True if we have visited this block during ANTIC calculation.  */
   unsigned int visited : 1;
 
@@ -440,6 +443,7 @@  typedef struct bb_bitmap_sets
 #define EXPR_DIES(BB)	((bb_value_sets_t) ((BB)->aux))->expr_dies
 #define BB_VISITED(BB)	((bb_value_sets_t) ((BB)->aux))->visited
 #define BB_MAY_NOTRETURN(BB) ((bb_value_sets_t) ((BB)->aux))->contains_may_not_return_call
+#define BB_LIVE_VOP_ON_EXIT(BB) ((bb_value_sets_t) ((BB)->aux))->vop_on_exit
 
 
 /* Basic block list in postorder.  */
@@ -2886,12 +2890,15 @@  create_expression_by_pieces (basic_block
 	      bitmap_value_replace_in_set (NEW_SETS (block), nameexpr);
 	      bitmap_value_replace_in_set (AVAIL_OUT (block), nameexpr);
 	    }
+
+	  gimple_set_vuse (stmt, BB_LIVE_VOP_ON_EXIT (block));
 	}
       gimple_seq_add_seq (stmts, forced_stmts);
     }
 
   name = make_temp_ssa_name (exprtype, NULL, "pretmp");
   newstmt = gimple_build_assign (name, folded);
+  gimple_set_vuse (newstmt, BB_LIVE_VOP_ON_EXIT (block));
   gimple_set_plf (newstmt, NECESSARY, false);
 
   gimple_seq_add_stmt (stmts, newstmt);
@@ -3593,6 +3623,9 @@  compute_avail (void)
        son = next_dom_son (CDI_DOMINATORS, son))
     worklist[sp++] = son;
 
+  BB_LIVE_VOP_ON_EXIT (ENTRY_BLOCK_PTR_FOR_FN (cfun))
+    = ssa_default_def (cfun, gimple_vop (cfun));
+
   /* Loop until the worklist is empty.  */
   while (sp)
     {
@@ -3607,7 +3640,10 @@  compute_avail (void)
 	 its immediate dominator.  */
       dom = get_immediate_dominator (CDI_DOMINATORS, block);
       if (dom)
-	bitmap_set_copy (AVAIL_OUT (block), AVAIL_OUT (dom));
+	{
+	  bitmap_set_copy (AVAIL_OUT (block), AVAIL_OUT (dom));
+	  BB_LIVE_VOP_ON_EXIT (block) = BB_LIVE_VOP_ON_EXIT (dom);
+	}
 
       /* Generate values for PHI nodes.  */
       for (gsi = gsi_start_phis (block); !gsi_end_p (gsi); gsi_next (&gsi))
@@ -3617,7 +3653,10 @@  compute_avail (void)
 	  /* We have no need for virtual phis, as they don't represent
 	     actual computations.  */
 	  if (virtual_operand_p (result))
-	    continue;
+	    {
+	      BB_LIVE_VOP_ON_EXIT (block) = result;
+	      continue;
+	    }
 
 	  pre_expr e = get_or_alloc_expr_for_name (result);
 	  add_to_value (get_expr_value_id (e), e);
@@ -3661,6 +3700,9 @@  compute_avail (void)
 	      bitmap_value_insert_into_set (AVAIL_OUT (block), e);
 	    }
 
+	  if (gimple_vdef (stmt))
+	    BB_LIVE_VOP_ON_EXIT (block) = gimple_vdef (stmt);
+
 	  if (gimple_has_side_effects (stmt)
 	      || stmt_could_throw_p (stmt)
 	      || is_gimple_debug (stmt))
@@ -4758,6 +4794,10 @@  pass_pre::execute (function *fun)
   remove_fake_exit_edges ();
   gsi_commit_edge_inserts ();
 
+  /* Eliminate folds statements which might (should not...) end up
+     not keeping virtual operands up-to-date.  */
+  gcc_assert (!need_ssa_update_p (fun));
+
   /* Remove all the redundant expressions.  */
   todo |= eliminate (true);
 
Index: gcc/testsuite/g++.dg/torture/pr63476.C
===================================================================
--- gcc/testsuite/g++.dg/torture/pr63476.C	(revision 0)
+++ gcc/testsuite/g++.dg/torture/pr63476.C	(working copy)
@@ -0,0 +1,39 @@ 
+// { dg-do compile }
+// { dg-additional-options "-std=gnu++11" }
+
+enum class nsresult;
+class A;
+class B
+{
+public:
+    B (int);
+    A *operator->();
+};
+class C
+{
+};
+class A
+{
+public:
+    virtual nsresult AddObserver (const char *, C *, bool) = 0;
+};
+class D : A
+{
+  nsresult
+      AddObserver (const char *p1, C *p2, bool p3)
+	{
+	  AddObserver (p1, p2, p3);
+	}
+};
+char *prefList[]{};
+class F : C
+{
+  nsresult Install ();
+};
+nsresult
+F::Install ()
+{
+  B branch = 0;
+  for (int i;;)
+    branch->AddObserver (prefList[i], this, false);
+}