Index: gcc/tree-ssa-sccvn.c
===================================================================
--- gcc/tree-ssa-sccvn.c (revision 185028)
+++ gcc/tree-ssa-sccvn.c (working copy)
@@ -2598,27 +2598,44 @@ visit_reference_op_call (tree lhs, gimpl
 {
   bool changed = false;
   struct vn_reference_s vr1;
-  tree result;
+  vn_reference_t vnresult = NULL;
   tree vuse = gimple_vuse (stmt);
+  tree vdef = gimple_vdef (stmt);
+
+  if (vdef)
+    VN_INFO (vdef)->use_processed = true;
 
   vr1.vuse = vuse ? SSA_VAL (vuse) : NULL_TREE;
   vr1.operands = valueize_shared_reference_ops_from_call (stmt);
   vr1.type = gimple_expr_type (stmt);
   vr1.set = 0;
   vr1.hashcode = vn_reference_compute_hash (&vr1);
-  result = vn_reference_lookup_1 (&vr1, NULL);
-  if (result)
+  vn_reference_lookup_1 (&vr1, &vnresult);
+
+  if (vnresult)
     {
-      changed = set_ssa_val_to (lhs, result);
-      if (TREE_CODE (result) == SSA_NAME
-	  && VN_INFO (result)->has_constants)
-	VN_INFO (lhs)->has_constants = true;
+      if (vnresult->vdef)
+	changed |= set_ssa_val_to (vdef, vnresult->vdef);
+
+      if (!vnresult->result && lhs)
+	vnresult->result = lhs;
+
+      if (vnresult->result && lhs)
+	{
+	  changed |= set_ssa_val_to (lhs, vnresult->result);
+
+	  if (VN_INFO (vnresult->result)->has_constants)
+	    VN_INFO (lhs)->has_constants = true;
+	}
     }
   else
     {
       void **slot;
       vn_reference_t vr2;
-      changed = set_ssa_val_to (lhs, lhs);
+      if (vdef)
+	changed |= set_ssa_val_to (vdef, vdef);
+      if (lhs)
+	changed |= set_ssa_val_to (lhs, lhs);
       vr2 = (vn_reference_t) pool_alloc (current_info->references_pool);
       vr2->vuse = vr1.vuse;
       vr2->operands = valueize_refs (create_reference_ops_from_call (stmt));
@@ -2626,6 +2643,7 @@ visit_reference_op_call (tree lhs, gimpl
       vr2->set = vr1.set;
       vr2->hashcode = vr1.hashcode;
       vr2->result = lhs;
+      vr2->vdef = vdef;
       slot = htab_find_slot_with_hash (current_info->references,
 				       vr2, vr2->hashcode, INSERT);
       if (*slot)
@@ -3186,8 +3204,7 @@ visit_use (tree use)
     {
       if (gimple_code (stmt) == GIMPLE_PHI)
 	changed = visit_phi (stmt);
-      else if (!gimple_has_lhs (stmt)
-	       || gimple_has_volatile_ops (stmt))
+      else if (gimple_has_volatile_ops (stmt))
 	changed = defs_to_varying (stmt);
       else if (is_gimple_assign (stmt))
 	{
@@ -3349,34 +3366,42 @@ visit_use (tree use)
 
 	  /* ???  We could try to simplify calls.  */
 
-	  if (stmt_has_constants (stmt)
-	      && TREE_CODE (lhs) == SSA_NAME)
-	    VN_INFO (lhs)->has_constants = true;
-	  else if (TREE_CODE (lhs) == SSA_NAME)
+	  if (lhs && TREE_CODE (lhs) == SSA_NAME)
 	    {
-	      /* We reset expr and constantness here because we may
-		 have been value numbering optimistically, and
-		 iterating. They may become non-constant in this case,
-		 even if they were optimistically constant. */
-	      VN_INFO (lhs)->has_constants = false;
-	      VN_INFO (lhs)->expr = NULL_TREE;
+	      if (stmt_has_constants (stmt))
+		VN_INFO (lhs)->has_constants = true;
+	      else
+		{
+		  /* We reset expr and constantness here because we may
+		     have been value numbering optimistically, and
+		     iterating.  They may become non-constant in this case,
+		     even if they were optimistically constant.  */
+		  VN_INFO (lhs)->has_constants = false;
+		  VN_INFO (lhs)->expr = NULL_TREE;
+		}
+
+	      if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (lhs))
+		{
+		  changed = defs_to_varying (stmt);
+		  goto done;
+		}
 	    }
 
-	  if (TREE_CODE (lhs) == SSA_NAME
-	      && SSA_NAME_OCCURS_IN_ABNORMAL_PHI (lhs))
-	    changed = defs_to_varying (stmt);
 	  /* ???  We should handle stores from calls.  */
-	  else if (TREE_CODE (lhs) == SSA_NAME)
-	    {
-	      if (!gimple_call_internal_p (stmt)
-		  && gimple_call_flags (stmt) & (ECF_PURE | ECF_CONST))
-		changed = visit_reference_op_call (lhs, stmt);
-	      else
-		changed = defs_to_varying (stmt);
-	    }
+	  if (!gimple_call_internal_p (stmt)
+	      && (gimple_call_flags (stmt) & (ECF_PURE | ECF_CONST)
+		  /* If the call has side effects, subsequent calls won't have
+		     the same incoming vuse, so it's save to assume
+		     equality.  */
+		  || gimple_has_side_effects (stmt))
+	      && ((lhs && TREE_CODE (lhs) == SSA_NAME)
+		  || (!lhs && gimple_vdef (stmt))))
+	    changed = visit_reference_op_call (lhs, stmt);
 	  else
 	    changed = defs_to_varying (stmt);
 	}
+      else
+	changed = defs_to_varying (stmt);
     }
  done:
   return changed;
Index: gcc/tree-ssa-sccvn.h
===================================================================
--- gcc/tree-ssa-sccvn.h (revision 185028)
+++ gcc/tree-ssa-sccvn.h (working copy)
@@ -110,6 +110,7 @@ typedef struct vn_reference_s
   tree type;
   VEC (vn_reference_op_s, heap) *operands;
   tree result;
+  tree vdef;
 } *vn_reference_t;
 typedef const struct vn_reference_s *const_vn_reference_t;
 
Index: gcc/testsuite/gcc.dg/pr51879-2.c
===================================================================
--- /dev/null (new file)
+++ gcc/testsuite/gcc.dg/pr51879-2.c (revision 0)
@@ -0,0 +1,19 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-pre" } */
+
+int bar (int);
+void baz (int);
+
+void
+foo (int y)
+{
+  int a;
+  if (y)
+    baz (bar (7) + 6);
+  else
+    baz (bar (7) + 6);
+}
+
+/* { dg-final { scan-tree-dump-times "bar \\(" 1 "pre"} } */
+/* { dg-final { scan-tree-dump-times "baz \\(" 1 "pre"} } */
+/* { dg-final { cleanup-tree-dump "pre" } } */
Index: gcc/testsuite/gcc.dg/pr51879-6.c
===================================================================
--- /dev/null (new file)
+++ gcc/testsuite/gcc.dg/pr51879-6.c (revision 0)
@@ -0,0 +1,27 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-pre" } */
+
+
+int bar (int);
+void baz (int);
+void bla (int);
+
+void
+foo (int y)
+{
+  int a;
+  if (y == 6)
+    {
+      bla (5);
+      a = bar (7);
+    }
+  else
+    {
+      bla (5);
+      a = bar (7);
+    }
+  baz (a);
+}
+
+/* { dg-final { scan-tree-dump-times "bar \\(" 1 "pre"} } */
+/* { dg-final { cleanup-tree-dump "pre" } } */
Index: gcc/testsuite/gcc.dg/pr51879.c
===================================================================
--- /dev/null (new file)
+++ gcc/testsuite/gcc.dg/pr51879.c (revision 0)
@@ -0,0 +1,19 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-pre" } */
+
+int bar (int);
+void baz (int);
+
+void
+foo (int y)
+{
+  int a;
+  if (y == 6)
+    a = bar (7);
+  else
+    a = bar (7);
+  baz (a);
+}
+
+/* { dg-final { scan-tree-dump-times "bar \\(" 1 "pre"} } */
+/* { dg-final { cleanup-tree-dump "pre" } } */
Index: gcc/testsuite/gcc.dg/pr51879-3.c
===================================================================
--- /dev/null (new file)
+++ gcc/testsuite/gcc.dg/pr51879-3.c (revision 0)
@@ -0,0 +1,19 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-pre" } */
+
+int bar (int);
+void baz (int);
+
+void
+foo (int y)
+{
+  int a;
+  if (y == 6)
+    a = bar (7) + 6;
+  else
+    a = bar (7) + 6;
+  baz (a);
+}
+
+/* { dg-final { scan-tree-dump-times "bar \\(" 1 "pre"} } */
+/* { dg-final { cleanup-tree-dump "pre" } } */
Index: gcc/testsuite/gcc.dg/pr51879-4.c
===================================================================
--- /dev/null (new file)
+++ gcc/testsuite/gcc.dg/pr51879-4.c (revision 0)
@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-pre" } */
+
+int bar (int);
+void baz (int);
+
+int foo (int y)
+{
+  int a, b;
+  a = bar (7) + 6;
+  b = bar (7) + 6;
+  return a + b;
+}
+
+/* { dg-final { scan-tree-dump-times "bar \\(" 2 "pre"} } */
+/* { dg-final { cleanup-tree-dump "pre" } } */