re PR middle-end/81897 (spurious -Wmaybe-uninitialized warning)

	PR middle-end/81897
	* tree-ssa-uninit.c (convert_control_dep_chain_into_preds): Skip
	empty blocks.

From-SVN: r256456

gcc/ChangeLog

+2018-01-10  Aldy Hernandez  <aldyh@redhat.com>
+
+	PR middle-end/81897
+	* tree-ssa-uninit.c (convert_control_dep_chain_into_preds): Skip
+	empty blocks.
+
 2018-01-10  Eric Botcazou  <ebotcazou@adacore.com>
 
 	* dwarf2out.c (dwarf2out_var_location): Do not pass NULL to fprintf.

gcc/testsuite/gcc.dg/uninit-pr81897-2.c

+/* { dg-do compile }  */
+/* { dg-options "-O1 -fno-tree-ccp -Wmaybe-uninitialized" } */
+
+int oo;
+
+void
+pc (int *tt)
+{
+  int cf = 0;
+
+  if (*tt != 0)
+    {
+      if (0)
+        {
+          int *qg;
+          int uj = 0;
+
+ t6:
+          tt = &cf;
+          if (oo != 0)
+            {
+              ++uj; /* { dg-warning "may be used uninit" } */
+              *qg = !!oo && !!uj; /* { dg-warning "may be used uninit" } */
+            }
+        }
+      cf = 0;
+      goto t6;
+    }
+
+  if (oo != 0)
+    {
+      *tt = 1;
+      goto t6;
+    }
+}

gcc/tree-ssa-uninit.c

@@ -669,9 +669,16 @@ convert_control_dep_chain_into_preds (vec<edge> *dep_chains,
 	  e = one_cd_chain[j];
 	  guard_bb = e->src;
 	  gsi = gsi_last_bb (guard_bb);
-	  /* Ignore empty BBs as they're basically forwarder blocks.  */
+	  /* Ignore empty forwarder blocks.  */
 	  if (empty_block_p (guard_bb) && single_succ_p (guard_bb))
 	    continue;
+	  /* An empty basic block here is likely a PHI, and is not one
+	     of the cases we handle below.  */
+	  if (gsi_end_p (gsi))
+	    {
+	      has_valid_pred = false;
+	      break;
+	    }
 	  cond_stmt = gsi_stmt (gsi);
 	  if (is_gimple_call (cond_stmt) && EDGE_COUNT (e->src->succs) >= 2)
 	    /* Ignore EH edge.  Can add assertion on the other edge's flag.  */