8201367: assert(current != first_mem) failed: corrupted memory graph in superword code

Don't rely on the RPO order to find the "first" and "last" loads of the pack

Reviewed-by: kvn

src/hotspot/share/opto/superword.cpp

@@ -2144,8 +2144,21 @@ void SuperWord::co_locate_pack(Node_List* pk) {
     // we use the memory state of the last load. However, if any load could
     // not be moved down due to the dependence constraint, we use the memory
     // state of the first load.
-    Node* last_mem  = executed_last(pk)->in(MemNode::Memory);
-    Node* first_mem = executed_first(pk)->in(MemNode::Memory);
+    Node* first_mem = pk->at(0)->in(MemNode::Memory);
+    Node* last_mem = first_mem;
+    for (uint i = 1; i < pk->size(); i++) {
+      Node* ld = pk->at(i);
+      Node* mem = ld->in(MemNode::Memory);
+      assert(in_bb(first_mem) || in_bb(mem) || mem == first_mem, "2 different memory state from outside the loop?");
+      if (in_bb(mem)) {
+        if (in_bb(first_mem) && bb_idx(mem) < bb_idx(first_mem)) {
+          first_mem = mem;
+        }
+        if (!in_bb(last_mem) || bb_idx(mem) > bb_idx(last_mem)) {
+          last_mem = mem;
+        }
+      }
+    }
     bool schedule_last = true;
     for (uint i = 0; i < pk->size(); i++) {
       Node* ld = pk->at(i);

test/hotspot/jtreg/compiler/vectorization/TestUnexpectedLoadOrdering.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) 2018, Red Hat, Inc. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/**
+ * @test
+ * @bug 8201367
+ * @summary RPO walk of counted loop block doesn't properly order loads
+ *
+ * @run main/othervm -XX:-BackgroundCompilation -XX:-UseCountedLoopSafepoints TestUnexpectedLoadOrdering
+ *
+ */
+
+public class TestUnexpectedLoadOrdering {
+
+    public static void main(String[] args) {
+        double[] array1 = new double[1000];
+        double[] array2 = new double[1000];
+        for (int i = 0; i < 20_000; i++) {
+            test(array1, array2);
+        }
+    }
+
+    private static double test(double[] array1, double[] array2) {
+        double res = 0;
+        for (int i = 0; i < array1.length; i++) {
+            array2[i] = i;
+            res += array1[i];
+        }
+        return res;
+    }
+}