8261308: C2: assert(inner->is_valid_counted_loop(T_INT) && inner->is_strip_mined()) failed: OuterStripMinedLoop should have been removed

Reviewed-by: chagedorn, kvn

Author: rwestrel

src/hotspot/share/opto/loopTransform.cpp

@@ -124,7 +124,10 @@ void IdealLoopTree::compute_trip_count(PhaseIdealLoop* phase) {
     jlong limit_con = (stride_con > 0) ? limit_type->_hi : limit_type->_lo;
     int stride_m = stride_con - (stride_con > 0 ? 1 : -1);
     jlong trip_count = (limit_con - init_con + stride_m)/stride_con;
-    if (trip_count > 0 && (julong)trip_count < (julong)max_juint) {
+    // The loop body is always executed at least once even if init >= limit (for stride_con > 0) or
+    // init <= limit (for stride_con < 0).
+    trip_count = MAX2(trip_count, (jlong)1);
+    if (trip_count < (jlong)max_juint) {
       if (init_n->is_Con() && limit_n->is_Con()) {
         // Set exact trip count.
         cl->set_exact_trip_count((uint)trip_count);

test/hotspot/jtreg/compiler/loopopts/TestCountedLoopZeroIter.java

@@ -0,0 +1,64 @@
+/*
+ * Copyright (c) 2021, 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 8261308
+ * @summary C2: assert(inner->is_valid_counted_loop(T_INT) && inner->is_strip_mined()) failed: OuterStripMinedLoop should have been removed
+ *
+ * @run main/othervm -Xcomp -XX:CompileOnly=TestCountedLoopZeroIter TestCountedLoopZeroIter
+ *
+ */
+
+
+public class TestCountedLoopZeroIter {
+  static int N = 400;
+  static boolean b;
+  static long lArrFld[] = new long[N];
+  static int iArrFld[] = new int[N];
+
+  static void test() {
+    int i19 = 9, i21 = 8;
+    long l1;
+    do
+      if (!b) {
+        iArrFld[1] /= l1 = 1;
+        do {
+          int i22 = 1;
+          do {
+            iArrFld[0] = (int)l1;
+            iArrFld[i22 - 1] = i21;
+          } while (++i22 < 1);
+          lArrFld[1] = i21;
+        } while (++l1 < 145);
+      }
+    while (++i19 < 173);
+    System.out.println();
+  }
+
+  public static void main(String[] strArr) {
+      for (int i = 0; i < 10; i++) {
+        test();
+     }
+  }
+}