8286451: C2: assert(nb == 1) failed: only when the head is not shared

Reviewed-by: thartmann, chagedorn

Author: rwestrel

src/hotspot/share/ci/ciTypeFlow.cpp

@@ -2299,7 +2299,7 @@ ciTypeFlow::Block* ciTypeFlow::clone_loop_head(Loop* lp, StateVector* temp_vecto
   }
   // Have the most frequent ones branch to the clone instead
   int count = 0;
-  int nb = 0;
+  int loops_with_shared_head = 0;
   Block* latest_tail = tail;
   bool done = false;
   for (Loop* lp1 = lp; lp1 != NULL && !done; lp1 = lp1->parent()) {
@@ -2309,7 +2309,7 @@ ciTypeFlow::Block* ciTypeFlow::clone_loop_head(Loop* lp, StateVector* temp_vecto
         if (lp2->tail()->post_order() < latest_tail->post_order()) {
           latest_tail = lp2->tail();
         }
-        nb++;
+        loops_with_shared_head++;
         for (SuccIter iter(lp2->tail()); !iter.done(); iter.next()) {
           if (iter.succ() == head) {
             iter.set_succ(clone);
@@ -2319,29 +2319,28 @@ ciTypeFlow::Block* ciTypeFlow::clone_loop_head(Loop* lp, StateVector* temp_vecto
           }
         }
         flow_block(lp2->tail(), temp_vector, temp_set);
+        if (lp2->head() == lp2->tail()) {
+          // For self-loops, clone->head becomes clone->clone
+          flow_block(clone, temp_vector, temp_set);
+          for (SuccIter iter(clone); !iter.done(); iter.next()) {
+            if (iter.succ() == lp2->head()) {
+              iter.set_succ(clone);
+              // Update predecessor information
+              lp2->head()->predecessors()->remove(clone);
+              clone->predecessors()->append(clone);
+              break;
+            }
+          }
+        }
         if (total_count == 0 || count > (total_count * .9)) {
           done = true;
         }
       }
     }
   }
-  assert(nb >= 1, "at least one new");
+  assert(loops_with_shared_head >= 1, "at least one new");
   clone->set_rpo_next(latest_tail->rpo_next());
   latest_tail->set_rpo_next(clone);
-  if (head == tail) {
-    assert(nb == 1, "only when the head is not shared");
-    // For self-loops, clone->head becomes clone->clone
-    flow_block(clone, temp_vector, temp_set);
-    for (SuccIter iter(clone); !iter.done(); iter.next()) {
-      if (iter.succ() == head) {
-        iter.set_succ(clone);
-        // Update predecessor information
-        head->predecessors()->remove(clone);
-        clone->predecessors()->append(clone);
-        break;
-      }
-    }
-  }
   flow_block(clone, temp_vector, temp_set);
 
   return clone;

test/hotspot/jtreg/compiler/ciTypeFlow/TestSharedLoopHead.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2022, 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 8286451
+ * @summary C2: assert(nb == 1) failed: only when the head is not shared
+ * @run main/othervm -XX:-BackgroundCompilation TestSharedLoopHead
+ */
+
+public class TestSharedLoopHead {
+    public static void main(String[] args) {
+        for (int i = 0; i < 20_000; i++) {
+            test();
+        }
+    }
+
+    private static void test() {
+        int j = 0;
+        int i = 0;
+        do {
+            do {
+                i++;
+            } while (i < 2);
+            do {
+                i++;
+            } while (i < 2);
+            j++;
+            i = 0;
+        } while (j < 2);
+    }
+}