8257575: C2: "failed: only phis" assert failure in loop strip mining verification

Reviewed-by: thartmann, neliasso

Author: rwestrel

src/hotspot/share/opto/loopnode.cpp

@@ -1792,12 +1792,28 @@ void LoopNode::verify_strip_mined(int expect_skeleton) const {
     } else {
       assert(expect_skeleton == 0 || expect_skeleton == -1, "no skeleton node?");
       uint phis = 0;
+      uint be_loads = 0;
+      Node* be = inner->in(LoopNode::LoopBackControl);
       for (DUIterator_Fast imax, i = inner->fast_outs(imax); i < imax; i++) {
         Node* u = inner->fast_out(i);
         if (u->is_Phi()) {
           phis++;
+          for (DUIterator_Fast jmax, j = be->fast_outs(jmax); j < jmax; j++) {
+            Node* n = be->fast_out(j);
+            if (n->is_Load()) {
+              assert(n->in(0) == be, "should be on the backedge");
+              do {
+                n = n->raw_out(0);
+              } while (!n->is_Phi());
+              if (n == u) {
+                be_loads++;
+                break;
+              }
+            }
+          }
         }
       }
+      assert(be_loads <= phis, "wrong number phis that depends on a pinned load");
       for (DUIterator_Fast imax, i = outer->fast_outs(imax); i < imax; i++) {
         Node* u = outer->fast_out(i);
         assert(u == outer || u == inner || u->is_Phi(), "nothing between inner and outer loop");
@@ -1809,7 +1825,9 @@ void LoopNode::verify_strip_mined(int expect_skeleton) const {
           stores++;
         }
       }
-      assert(outer->outcnt() >= phis + 2 && outer->outcnt() <= phis + 2 + stores + 1, "only phis");
+      // Late optimization of loads on backedge can cause Phi of outer loop to be eliminated but Phi of inner loop is
+      // not guaranteed to be optimized out.
+      assert(outer->outcnt() >= phis + 2 - be_loads && outer->outcnt() <= phis + 2 + stores + 1, "only phis");
     }
     assert(sfpt->outcnt() == 1, "no data node");
     assert(outer_tail->outcnt() == 1 || !has_skeleton, "no data node");

test/hotspot/jtreg/compiler/loopstripmining/TestEliminatedLoadPinnedOnBackedge.java

@@ -0,0 +1,156 @@
+/*
+ * Copyright (c) 2020, 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 8257575
+ * @summary C2: "failed: only phis" assert failure in loop strip mining verfication
+ *
+ * @run main/othervm -XX:LoopUnrollLimit=0 -XX:-UseOnStackReplacement -XX:-BackgroundCompilation -XX:CompileCommand=dontinline,TestEliminatedLoadPinnedOnBackedge::notInlined
+ *                    -XX:CompileCommand=inline,TestEliminatedLoadPinnedOnBackedge::inlined TestEliminatedLoadPinnedOnBackedge
+ *
+ */
+
+public class TestEliminatedLoadPinnedOnBackedge {
+    private static Object field2;
+
+    final static int iters = 2000;
+
+    public static void main(String[] args) {
+        boolean[] flags = new boolean[iters];
+        for (int i = 0; i < iters; i++) {
+            flags[i] = i < iters/2;
+        }
+        for (int i = 0; i < 20_000; i++) {
+            test1(flags);
+            test2(flags, 1);
+            test3(flags);
+            inlined(new Object(), 1);
+            inlined(new Object(), 4);
+            inlined2(42);
+            inlined2(0x42);
+        }
+    }
+
+    static int field;
+
+    private static int test1(boolean[] flags) {
+        int k = 2;
+        for (; k < 4; k *= 2) {
+        }
+        int[] array = new int[10];
+        notInlined(array);
+        // This load commons with the load on the backedge after the
+        // outer strip mined loop is expanded.
+        int v = array[0];
+        array[1] = 42;
+        // No use for o. Allocation removed at macro expansion time.
+        Object o = new Object();
+        inlined(o, k);
+        int i = 0;
+        for (; ; ) {
+            synchronized (array) {
+            }
+            if (i >= iters) {
+                break;
+            }
+            v = array[0]; // This load ends up on the backedge
+            if (flags[i]) {
+                inlined2(array[1]);
+            }
+            i++;
+        }
+        return v;
+    }
+
+    private static int test2(boolean[] flags, int d) {
+        int k = 2;
+        for (; k < 4; k *= 2) {
+        }
+        int[] array = new int[10];
+        notInlined(array);
+        int v = array[0];
+        array[1] = 42;
+        Object o = new Object();
+        inlined(o, k);
+        int i = 0;
+        for (; ; ) {
+            synchronized (array) {
+            }
+            if (d == 0) {}
+            if (i >= iters) {
+                break;
+            }
+            v = (array[0] + array[2]) / d;
+            if (flags[i]) {
+                inlined2(array[1]);
+            }
+            i++;
+        }
+        return v;
+    }
+
+    private static int test3(boolean[] flags) {
+        int k = 2;
+        for (; k < 4; k *= 2) {
+        }
+        int[] array = new int[10];
+        notInlined(array);
+        int v1 = array[0];
+        int v2 = array[2];
+        array[1] = 42;
+        Object o = new Object();
+        inlined(o, k);
+        int i = 0;
+        for (; ; ) {
+            synchronized (array) {
+            }
+            if (i >= iters) {
+                break;
+            }
+            v1 = array[0];
+            v2 = array[2];
+            if (flags[i]) {
+                inlined2(array[1]);
+            }
+            i++;
+        }
+        return v1 + v2;
+    }
+
+    private static void inlined2(int i) {
+        if (i != 42) {
+            field = 42;
+        }
+    }
+
+    private static void inlined(Object o, int i) {
+        if (i != 4) {
+            field2 = o;
+        }
+    }
+
+    private static void notInlined(int[] array) {
+        java.util.Arrays.fill(array, 1);
+    }
+}