8282592: C2: assert(false) failed: graph should be schedulable

Backport-of: 85628a871df3fdeec1b422d1c01c222abe45d0a8

src/hotspot/share/opto/loopTransform.cpp

@@ -1935,6 +1935,12 @@ bool IdealLoopTree::is_invariant(Node* n) const {
 }
 
 void PhaseIdealLoop::update_main_loop_skeleton_predicates(Node* ctrl, CountedLoopNode* loop_head, Node* init, int stride_con) {
+  if (init->Opcode() == Op_CastII) {
+    // skip over the cast added by PhaseIdealLoop::cast_incr_before_loop() when pre/post/main loops are created because
+    // it can get in the way of type propagation
+    assert(((CastIINode*)init)->carry_dependency() && loop_head->skip_predicates() == init->in(0), "casted iv phi from pre loop expected");
+    init = init->in(1);
+  }
   // Search for skeleton predicates and update them according to the new stride
   Node* entry = ctrl;
   Node* prev_proj = ctrl;

src/hotspot/share/opto/loopopts.cpp

@@ -3728,6 +3728,8 @@ void PhaseIdealLoop::reorg_offsets(IdealLoopTree *loop) {
       set_ctrl(neg_stride, C->root());
       Node *post = new AddINode(opaq, neg_stride);
       register_new_node(post, c);
+      post = new CastIINode(post, phi->bottom_type()); // preserve the iv phi's type
+      register_new_node(post, c);
       _igvn.rehash_node_delayed(use);
       for (uint j = 1; j < use->req(); j++) {
         if (use->in(j) == phi)

test/hotspot/jtreg/compiler/c2/irTests/TestSkeletonPredicates.java

@@ -0,0 +1,78 @@
+/*
+ * 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.
+ */
+
+package compiler.c2.irTests;
+
+import compiler.lib.ir_framework.*;
+import jdk.test.lib.Utils;
+import java.util.Random;
+
+/*
+ * @test
+ * @bug 8278228
+ * @summary C2: Improve identical back-to-back if elimination
+ * @library /test/lib /
+ * @run driver compiler.c2.irTests.TestSkeletonPredicates
+ */
+
+public class TestSkeletonPredicates {
+    public static void main(String[] args) {
+        TestFramework.runWithFlags("-XX:-UseLoopPredicate", "-XX:LoopUnrollLimit=240", "-XX:+StressIGVN", "-XX:StressSeed=255527877");
+        TestFramework.runWithFlags("-XX:-UseLoopPredicate", "-XX:LoopUnrollLimit=240", "-XX:+StressIGVN");
+    }
+
+    static volatile int barrier;
+
+    @ForceInline
+    static boolean test1_helper(int start, int stop, double[] array1, double[] array2) {
+        for (int i = start; i < stop; i++) {
+            if ((i % 2) == 0) {
+                array1[i] = 42.42;
+            } else {
+                barrier = 0x42;
+            }
+        }
+        return false;
+    }
+
+    @Test
+    @IR(counts = { IRNode.COUNTEDLOOP, "3" })
+    static double[] test1(int stop, double[] array2) {
+        double[] array1 = null;
+        array1 = new double[10];
+        for (int j = 0; j < stop; j++) {
+            if (test1_helper(8, j, array1, array2)) {
+                return null;
+            }
+        }
+        return array1;
+    }
+
+    @Run(test = "test1")
+    void test1_runner() {
+        double[] array2 = new double[10];
+        double[] array3 = new double[1000];
+        test1_helper(1, 1000, array3, array3);
+        test1(11, array3);
+    }
+}