8297951: C2: Create skeleton predicates for all If nodes in loop predication

Reviewed-by: thartmann, kvn

src/hotspot/share/opto/loopPredicate.cpp

@@ -1337,13 +1337,11 @@ bool PhaseIdealLoop::loop_predication_impl_helper(IdealLoopTree *loop, ProjNode*
     upper_bound_iff->set_req(1, upper_bound_bol);
     if (TraceLoopPredicate) tty->print_cr("upper bound check if: %s %d ", negate ? " negated" : "", lower_bound_iff->_idx);
 
-    // Fall through into rest of the clean up code which will move
-    // any dependent nodes onto the upper bound test.
-    new_predicate_proj = upper_bound_proj;
-
-    if (iff->is_RangeCheck()) {
-      new_predicate_proj = insert_initial_skeleton_predicate(iff, loop, proj, predicate_proj, upper_bound_proj, scale, offset, init, limit, stride, rng, overflow, reason);
-    }
+    // Fall through into rest of the cleanup code which will move any dependent nodes to the skeleton predicates of the
+    // upper bound test. We always need to create skeleton predicates in order to properly remove dead loops when later
+    // splitting the predicated loop into (unreachable) sub-loops (i.e. done by unrolling, peeling, pre/main/post etc.).
+    new_predicate_proj = insert_initial_skeleton_predicate(iff, loop, proj, predicate_proj, upper_bound_proj, scale,
+                                                           offset, init, limit, stride, rng, overflow, reason);
 
 #ifndef PRODUCT
     if (TraceLoopOpts && !TraceLoopPredicate) {

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

@@ -0,0 +1,78 @@
+/*
+ * Copyright (c) 2022, Oracle and/or its affiliates. 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 8297951
+ * @summary Test that crashes because we do not emit skeleton predicates for normal If nodes for which a range check
+ *          predicate is created in loop predication.
+ * @requires vm.debug == true & vm.compiler2.enabled
+ * @run main/othervm -XX:-TieredCompilation -Xbatch -XX:-RangeCheckElimination -XX:+BailoutToInterpreterForThrows
+                     compiler.loopopts.TestMissingSkeletonPredicateForIfNode
+ */
+package compiler.loopopts;
+
+public class TestMissingSkeletonPredicateForIfNode {
+    static int iFld = 2, x;
+    static short limit = 10;
+
+    public static void main(String[] args) throws Exception {
+        for (int i = 0; i < 5000; i++) {
+            try {
+                test(i % 2 == 0, i % 3);
+            } catch (Exception e) {
+                // Expected
+            }
+        }
+    }
+
+    public static void test(boolean flag, int arg) throws Exception {
+        int sum = 1;
+        int[] iArr2 = new int[4];
+        RuntimeException r = new RuntimeException();
+
+        for (int i = 0; i < limit; i+=2) { // Pre/main/post + Unrolled once. This results in the following type for the iv phi i: [2..SHORT_MAX]
+            x = 5 / sum;
+            if (Integer.compareUnsigned(i, iArr2.length) < 0) { // (**) Loop predication creates a RC predicate for this check
+                // After unrolling, we have:
+                //
+                // iArr2[i]
+                // iArr2[i+2]
+                //
+                // The type of iArr2[i+2] is [4..SHORT_MAX+2] (we need limit to be short such that we do not have an integer overflow
+                // which would set the type to int). However, the type of the CastII node for the index i+2 is [0..3] because its size
+                // is only 4. Since the type of i+2 is outside the range of the CastII node, the CastII node is replaced by top and
+                // some of the data nodes and memory nodes die. We are left with a broken graph and later assert because of that.
+                iFld += iArr2[i]; // RangeCheck node is removed because it shares the same bool as the If (**).
+                sum += iFld;
+            } else {
+                // Emits an UCT with -XX:+BailoutToInterpreterForThrows and therefore the If (**) satisfies the condition of being a
+                // range check if with one of its blocks being an UCT.
+                throw r;
+            }
+            if (i > 50) {
+                break;
+            }
+        }
+    }
+}