6843752: missing code for an anti-dependent Phi in GCM

Don't place a load below anti-dependent PHI.

Reviewed-by: never, twisti

hotspot/src/share/vm/opto/gcm.cpp

@@ -617,6 +617,9 @@ Block* PhaseCFG::insert_anti_dependences(Block* LCA, Node* load, bool verify) {
             assert(!LCA_orig->dominates(pred_block) ||
                    early->dominates(pred_block), "early is high enough");
             must_raise_LCA = true;
+          } else {
+            // anti-dependent upon PHI pinned below 'early', no edge needed
+            LCA = early;             // but can not schedule below 'early'
           }
         }
       }

hotspot/test/compiler/6843752/Test.java

@@ -0,0 +1,119 @@
+/*
+ * Copyright 2009 Sun Microsystems, 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
+ * CA 95054 USA or visit www.sun.com if you need additional information or
+ * have any questions.
+ */
+
+/*
+ * @test
+ * @bug 6843752
+ * @summary missing code for an anti-dependent Phi in GCM
+ * @run main/othervm -Xbatch Test
+ */
+
+public class Test {
+
+    Item list;
+
+    static class Item {
+        public Item    next;
+        public Item    prev;
+        public boolean remove;
+
+        Item(boolean r) { remove = r; }
+    }
+
+    private void linkIn(Item item) {
+        Item head = list;
+        if (head == null) {
+            item.next = item;
+            item.prev = item;
+            list = item;
+        } else {
+            item.next = head;
+            item.prev = head.prev;
+            head.prev.next = item;
+            head.prev = item;
+        }
+    }
+
+    private void linkOut(Item item) {
+        Item head = list;
+        if (item.next == item) {
+            list = null;
+        } else {
+            item.prev.next = item.next;
+            item.next.prev = item.prev;
+            if (head == item) {
+                list = item.next;
+            }
+        }
+        item.next = null;
+        item.prev = null; // this is the null pointer we are seeing
+    }
+
+    private void removeItems(int numItems) {
+        Item item = list;
+        if (item == null) {
+            return;
+        }
+        Item last = item.prev;
+        boolean done = false;
+        while (!done && numItems > 1) {
+            // the original code "done = (item == last);" triggered an infinite loop
+            // and was changed slightly in order to produce an exception instead.
+            done = (item.next == last.next);
+            item = item.next;
+            if (item.prev.remove) {
+                linkOut(item.prev);
+            }
+        }
+    }
+
+    public void perform(int numItems) {
+        for (int i = 0; i < numItems; i++) {
+            linkIn(new Item(i == 0));
+        }
+        removeItems(numItems);
+        list = null;
+    }
+
+    static public void main(String[] args) {
+        int caseCnt = 0;
+        Test bj = new Test();
+        try {
+            for (; caseCnt < 500000;) {
+                int numItems = (++caseCnt % 2);
+                if ((caseCnt % 64) == 0) {
+                    numItems = 5;
+                }
+                bj.perform(numItems);
+                if ((caseCnt % 100000) == 0) {
+                    System.out.println("successfully performed " + caseCnt + " cases");
+                }
+            }
+        } catch (Exception e) {
+            System.out.println("ERROR: crashed during case " + caseCnt);
+            e.printStackTrace(System.out);
+            System.exit(97);
+        }
+    }
+}
+