tree-optimization/107852 - missed optimization with PHIs

The following deals with the situation where we have

<bb 2> [local count: 1073741824]:
_5 = bytes.D.25336._M_impl.D.24643._M_start;
_6 = bytes.D.25336._M_impl.D.24643._M_finish;
pretmp_66 = bytes.D.25336._M_impl.D.24643._M_end_of_storage;
if (_5 != _6)
  goto <bb 3>; [70.00%]
else
  goto <bb 4>; [30.00%]

...

<bb 6> [local count: 329045359]:
_89 = operator new (4);
_43 = bytes.D.25336._M_impl.D.24643._M_start;
_Num_44 = _137 - _43;
if (_Num_44 != 0)

but fail to see that _137 is equal to _5 and thus eventually _Num_44
is zero if not operator new would possibly clobber the global
bytes variable.

The following resolves this in value-numbering by using the
predicated values for _5 == _6 recorded for the dominating
condition.

	PR tree-optimization/107852
	* tree-ssa-sccvn.cc (visit_phi): Use equivalences recorded
	as predicated values to elide more redundant PHIs.

	* gcc.dg/tree-ssa/ssa-fre-101.c: New testcase.

2 files changed, 97 insertions, 1 deletions

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-101.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-101.c
new file mode 100644
index 0000000..c67f211
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-101.c
@@ -0,0 +1,47 @@
+/* { dg-do compile } */
+/* { dg-options "-O -fdump-tree-fre1-details" } */
+
+int test1 (int i, int j)
+{
+  int k;
+  if (i != j)
+    k = i;
+  else
+    k = j;
+  return k;
+}
+
+int test2 (int i, int j)
+{
+  int k;
+  if (i != j)
+    k = j;
+  else
+    k = i;
+  return k;
+}
+
+int test3 (int i, int j)
+{
+  int k;
+  if (i == j)
+    k = j;
+  else
+    k = i;
+  return k;
+}
+
+int test4 (int i, int j)
+{
+  int k;
+  if (i == j)
+    k = i;
+  else
+    k = j;
+  return k;
+}
+
+/* We'd expect 4 hits but since we only keep one forwarder the
+   VN predication machinery cannot record something for the entry
+   block since it doesn't work on edges but on their source.  */
+/* { dg-final { scan-tree-dump-times "equal on edge" 2 "fre1" } } */
diff --git a/gcc/tree-ssa-sccvn.cc b/gcc/tree-ssa-sccvn.cc
index 3748440..1e7763b 100644
--- a/gcc/tree-ssa-sccvn.cc
+++ b/gcc/tree-ssa-sccvn.cc
@@ -5793,7 +5793,8 @@ visit_phi (gimple *phi, bool *inserted, bool backedges_varying_p)
 
   /* See if all non-TOP arguments have the same value.  TOP is
      equivalent to everything, so we can ignore it.  */
-  FOR_EACH_EDGE (e, ei, gimple_bb (phi)->preds)
+  basic_block bb = gimple_bb (phi);
+  FOR_EACH_EDGE (e, ei, bb->preds)
     if (e->flags & EDGE_EXECUTABLE)
       {
 	tree def = PHI_ARG_DEF_FROM_EDGE (phi, e);
@@ -5838,6 +5839,54 @@ visit_phi (gimple *phi, bool *inserted, bool backedges_varying_p)
 			 && known_eq (soff, doff))
 		  continue;
 	      }
+	    /* There's also the possibility to use equivalences.  */
+	    if (!FLOAT_TYPE_P (TREE_TYPE (def)))
+	      {
+		vn_nary_op_t vnresult;
+		tree ops[2];
+		ops[0] = def;
+		ops[1] = sameval;
+		tree val = vn_nary_op_lookup_pieces (2, EQ_EXPR,
+						     boolean_type_node,
+						     ops, &vnresult);
+		if (! val && vnresult && vnresult->predicated_values)
+		  {
+		    val = vn_nary_op_get_predicated_value (vnresult, e->src);
+		    if (val && integer_truep (val))
+		      {
+			if (dump_file && (dump_flags & TDF_DETAILS))
+			  {
+			    fprintf (dump_file, "Predication says ");
+			    print_generic_expr (dump_file, def, TDF_NONE);
+			    fprintf (dump_file, " and ");
+			    print_generic_expr (dump_file, sameval, TDF_NONE);
+			    fprintf (dump_file, " are equal on edge %d -> %d\n",
+				     e->src->index, e->dest->index);
+			  }
+			continue;
+		      }
+		    /* If on all previous edges the value was equal to def
+		       we can change sameval to def.  */
+		    if (EDGE_COUNT (bb->preds) == 2
+			&& (val = vn_nary_op_get_predicated_value
+				    (vnresult, EDGE_PRED (bb, 0)->src))
+			&& integer_truep (val))
+		      {
+			if (dump_file && (dump_flags & TDF_DETAILS))
+			  {
+			    fprintf (dump_file, "Predication says ");
+			    print_generic_expr (dump_file, def, TDF_NONE);
+			    fprintf (dump_file, " and ");
+			    print_generic_expr (dump_file, sameval, TDF_NONE);
+			    fprintf (dump_file, " are equal on edge %d -> %d\n",
+				     EDGE_PRED (bb, 0)->src->index,
+				     EDGE_PRED (bb, 0)->dest->index);
+			  }
+			sameval = def;
+			continue;
+		      }
+		  }
+	      }
 	    sameval = NULL_TREE;
 	    break;
 	  }