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

	PR middle-end/81897
	* tree-ssa-uninit.c (compute_control_dep_chain): Do not bail on
	basic blocks with a small number of successors.
	(convert_control_dep_chain_into_preds): Improve handling of
	forwarder blocks.
	(dump_predicates): Split apart into...
	(dump_pred_chain): ...here...
	(dump_pred_info): ...and here.
	(can_one_predicate_be_invalidated_p): Add debugging printfs.
	(can_chain_union_be_invalidated_p): Improve check for invalidation
	of paths.
	(uninit_uses_cannot_happen): Avoid unnecessary if
	convert_control_dep_chain_into_preds yielded nothing.

	PR middle-end/81897
	* gcc.dg/uninit-pr81897.c: New test.

From-SVN: r256320

gcc/ChangeLog

+2018-01-06  Aldy Hernandez  <aldyh@redhat.com>
+
+	PR middle-end/81897
+	* tree-ssa-uninit.c (compute_control_dep_chain): Do not bail on
+	basic blocks with a small number of successors.
+	(convert_control_dep_chain_into_preds): Improve handling of
+	forwarder blocks.
+	(dump_predicates): Split apart into...
+	(dump_pred_chain): ...here...
+	(dump_pred_info): ...and here.
+	(can_one_predicate_be_invalidated_p): Add debugging printfs.
+	(can_chain_union_be_invalidated_p): Improve check for invalidation
+	of paths.
+	(uninit_uses_cannot_happen): Avoid unnecessary if
+	convert_control_dep_chain_into_preds yielded nothing.
+
 2018-01-06  Martin Sebor  <msebor@redhat.com>
 
 	PR tree-optimization/83640

gcc/testsuite/ChangeLog

+2018-01-06  Aldy Hernandez  <aldyh@redhat.com>
+
+	PR middle-end/81897
+	* gcc.dg/uninit-pr81897.c: New test.
+
 2018-01-06  Martin Sebor  <msebor@redhat.com>
 
 	PR tree-optimization/83640

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

+/* { dg-do compile }  */
+/* { dg-options "-O2 -Wuninitialized" } */
+
+int f(void);
+static inline void rcu_read_unlock(void)
+{
+        static _Bool __warned;
+        if (f() && !__warned && !f()) {
+                __warned = 1;
+        }
+}
+int inet6_rtm_getroute(void)
+{
+        int dst;
+        int fibmatch = f();
+
+        if (!fibmatch)
+                dst = f();
+        rcu_read_unlock();
+        if (fibmatch)
+                dst = 0;
+
+        return dst;
+}

gcc/tree-ssa-uninit.c

@@ -33,6 +33,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "tree-ssa.h"
 #include "params.h"
 #include "tree-cfg.h"
+#include "cfghooks.h"
 
 /* This implements the pass that does predicate aware warning on uses of
    possibly uninitialized variables.  The pass first collects the set of
@@ -543,9 +544,6 @@ compute_control_dep_chain (basic_block bb, basic_block dep_bb,
   bool found_cd_chain = false;
   size_t cur_chain_len = 0;
 
-  if (EDGE_COUNT (bb->succs) < 2)
-    return false;
-
   if (*num_calls > PARAM_VALUE (PARAM_UNINIT_CONTROL_DEP_ATTEMPTS))
     return false;
   ++*num_calls;
@@ -671,11 +669,9 @@ 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);
-	  if (gsi_end_p (gsi))
-	    {
-	      has_valid_pred = false;
-	      break;
-	    }
+	  /* Ignore empty BBs as they're basically forwarder blocks.  */
+	  if (empty_block_p (guard_bb) && single_succ_p (guard_bb))
+	    continue;
 	  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.  */
@@ -916,38 +912,49 @@ find_def_preds (pred_chain_union *preds, gphi *phi)
   return has_valid_pred;
 }
 
+/* Dump a pred_info.  */
+
+static void
+dump_pred_info (pred_info one_pred)
+{
+  if (one_pred.invert)
+    fprintf (dump_file, " (.NOT.) ");
+  print_generic_expr (dump_file, one_pred.pred_lhs);
+  fprintf (dump_file, " %s ", op_symbol_code (one_pred.cond_code));
+  print_generic_expr (dump_file, one_pred.pred_rhs);
+}
+
+/* Dump a pred_chain.  */
+
+static void
+dump_pred_chain (pred_chain one_pred_chain)
+{
+  size_t np = one_pred_chain.length ();
+  for (size_t j = 0; j < np; j++)
+    {
+      dump_pred_info (one_pred_chain[j]);
+      if (j < np - 1)
+	fprintf (dump_file, " (.AND.) ");
+      else
+	fprintf (dump_file, "\n");
+    }
+}
+
 /* Dumps the predicates (PREDS) for USESTMT.  */
 
 static void
 dump_predicates (gimple *usestmt, pred_chain_union preds, const char *msg)
 {
-  size_t i, j;
-  pred_chain one_pred_chain = vNULL;
   fprintf (dump_file, "%s", msg);
-  print_gimple_stmt (dump_file, usestmt, 0);
-  fprintf (dump_file, "is guarded by :\n\n");
+  if (usestmt)
+    {
+      print_gimple_stmt (dump_file, usestmt, 0);
+      fprintf (dump_file, "is guarded by :\n\n");
+    }
   size_t num_preds = preds.length ();
-  /* Do some dumping here:  */
-  for (i = 0; i < num_preds; i++)
+  for (size_t i = 0; i < num_preds; i++)
     {
-      size_t np;
-
-      one_pred_chain = preds[i];
-      np = one_pred_chain.length ();
-
-      for (j = 0; j < np; j++)
-	{
-	  pred_info one_pred = one_pred_chain[j];
-	  if (one_pred.invert)
-	    fprintf (dump_file, " (.NOT.) ");
-	  print_generic_expr (dump_file, one_pred.pred_lhs);
-	  fprintf (dump_file, " %s ", op_symbol_code (one_pred.cond_code));
-	  print_generic_expr (dump_file, one_pred.pred_rhs);
-	  if (j < np - 1)
-	    fprintf (dump_file, " (.AND.) ");
-	  else
-	    fprintf (dump_file, "\n");
-	}
+      dump_pred_chain (preds[i]);
       if (i < num_preds - 1)
 	fprintf (dump_file, "(.OR.)\n");
       else
@@ -2259,12 +2266,19 @@ normalize_preds (pred_chain_union preds, gimple *use_or_def, bool is_use)
 }
 
 /* Return TRUE if PREDICATE can be invalidated by any individual
-   predicate in WORKLIST.  */
+   predicate in USE_GUARD.  */
 
 static bool
 can_one_predicate_be_invalidated_p (pred_info predicate,
 				    pred_chain use_guard)
 {
+  if (dump_file && dump_flags & TDF_DETAILS)
+    {
+      fprintf (dump_file, "Testing if this predicate: ");
+      dump_pred_info (predicate);
+      fprintf (dump_file, "\n...can be invalidated by a USE guard of: ");
+      dump_pred_chain (use_guard);
+    }
   for (size_t i = 0; i < use_guard.length (); ++i)
     {
       /* NOTE: This is a very simple check, and only understands an
@@ -2273,7 +2287,15 @@ can_one_predicate_be_invalidated_p (pred_info predicate,
 	 invalidate with say [i > 5] or [i == 8].  There is certainly
 	 room for improvement here.  */
       if (pred_neg_p (predicate, use_guard[i]))
-	return true;
+	{
+	  if (dump_file && dump_flags & TDF_DETAILS)
+	    {
+	      fprintf (dump_file, "  Predicate was invalidated by: ");
+	      dump_pred_info (use_guard[i]);
+	      fputc ('\n', dump_file);
+	    }
+	  return true;
+	}
     }
   return false;
 }
@@ -2287,12 +2309,22 @@ can_chain_union_be_invalidated_p (pred_chain_union uninit_pred,
 {
   if (uninit_pred.is_empty ())
     return false;
+  if (dump_file && dump_flags & TDF_DETAILS)
+    dump_predicates (NULL, uninit_pred,
+		     "Testing if anything here can be invalidated: ");
   for (size_t i = 0; i < uninit_pred.length (); ++i)
     {
       pred_chain c = uninit_pred[i];
-      for (size_t j = 0; j < c.length (); ++j)
-	if (!can_one_predicate_be_invalidated_p (c[j], use_guard))
-	  return false;
+      size_t j;
+      for (j = 0; j < c.length (); ++j)
+	if (can_one_predicate_be_invalidated_p (c[j], use_guard))
+	  break;
+
+      /* If we were unable to invalidate any predicate in C, then there
+	 is a viable path from entry to the PHI where the PHI takes
+	 an uninitialized value and continues to a use of the PHI.  */
+      if (j == c.length ())
+	return false;
     }
   return true;
 }
@@ -2334,7 +2366,7 @@ uninit_uses_cannot_happen (gphi *phi, unsigned uninit_opnds,
 
       /* Build the control dependency chain for uninit operand `i'...  */
       uninit_preds = vNULL;
-      if (!compute_control_dep_chain (find_dom (e->src),
+      if (!compute_control_dep_chain (ENTRY_BLOCK_PTR_FOR_FN (cfun),
 				      e->src, dep_chains, &num_chains,
 				      &cur_chain, &num_calls))
 	{
@@ -2342,10 +2374,16 @@ uninit_uses_cannot_happen (gphi *phi, unsigned uninit_opnds,
 	  break;
 	}
       /* ...and convert it into a set of predicates.  */
-      convert_control_dep_chain_into_preds (dep_chains, num_chains,
-					    &uninit_preds);
+      bool has_valid_preds
+	= convert_control_dep_chain_into_preds (dep_chains, num_chains,
+						&uninit_preds);
       for (size_t j = 0; j < num_chains; ++j)
 	dep_chains[j].release ();
+      if (!has_valid_preds)
+	{
+	  ret = false;
+	  break;
+	}
       simplify_preds (&uninit_preds, NULL, false);
       uninit_preds = normalize_preds (uninit_preds, NULL, false);