8335393: C2: assert(!had_error) failed: bad dominance

Reviewed-by: thartmann, chagedorn

Author: rwestrel

src/hotspot/share/opto/loopTransform.cpp

@@ -2871,6 +2871,7 @@ void PhaseIdealLoop::do_range_check(IdealLoopTree *loop, Node_List &old_new) {
   // Must know if its a count-up or count-down loop
 
   int stride_con = cl->stride_con();
+  bool abs_stride_is_one = stride_con == 1 || stride_con == -1;
   Node* zero = _igvn.longcon(0);
   Node* one  = _igvn.longcon(1);
   // Use symmetrical int range [-max_jint,max_jint]
@@ -2882,6 +2883,15 @@ void PhaseIdealLoop::do_range_check(IdealLoopTree *loop, Node_List &old_new) {
   Node* loop_entry = cl->skip_strip_mined()->in(LoopNode::EntryControl);
   assert(loop_entry->is_Proj() && loop_entry->in(0)->is_If(), "if projection only");
 
+  // if abs(stride) == 1, an Assertion Predicate for the final iv value is added. We don't know the final iv value until
+  // we're done with range check elimination so use a place holder.
+  Node* final_iv_placeholder = nullptr;
+  if (abs_stride_is_one) {
+    final_iv_placeholder = new Node(1);
+    _igvn.set_type(final_iv_placeholder, TypeInt::INT);
+    final_iv_placeholder->init_req(0, loop_entry);
+  }
+
   // Check loop body for tests of trip-counter plus loop-invariant vs loop-variant.
   for (uint i = 0; i < loop->_body.size(); i++) {
     Node *iff = loop->_body[i];
@@ -2985,6 +2995,20 @@ void PhaseIdealLoop::do_range_check(IdealLoopTree *loop, Node_List &old_new) {
           Node* opaque_init = new OpaqueLoopInitNode(C, init);
           register_new_node(opaque_init, loop_entry);
 
+          if (abs_stride_is_one) {
+            // If the main loop becomes empty and the array access for this range check is sunk out of the loop, the index
+            // for the array access will be set to the index value of the final iteration which could be out of loop.
+            // Add an Assertion Predicate for that corner case. The final iv is computed from LoopLimit which is the
+            // LoopNode::limit() only if abs(stride) == 1 otherwise the computation depends on LoopNode::init_trip() as
+            // well. When LoopLimit only depends on LoopNode::limit(), there are cases where the zero trip guard for the
+            // main loop doesn't constant fold after range check elimination but, the array access for the final
+            // iteration of the main loop is out of bound and the index for that access is out of range for the range
+            // check CastII.
+            loop_entry = add_range_check_elimination_assertion_predicate(loop, loop_entry, scale_con, int_offset,
+                                                                         int_limit, stride_con, final_iv_placeholder, false);
+            assert(!assertion_predicate_has_loop_opaque_node(loop_entry->in(0)->as_If()), "unexpected");
+          }
+
           // Initialized Assertion Predicate for the value of the initial main-loop.
           loop_entry = add_range_check_elimination_assertion_predicate(loop, loop_entry, scale_con, int_offset,
                                                                        int_limit, stride_con, init, false);
@@ -3116,11 +3140,14 @@ void PhaseIdealLoop::do_range_check(IdealLoopTree *loop, Node_List &old_new) {
   register_new_node(main_limit, pre_ctrl);
   // Hack the now-private loop bounds
   _igvn.replace_input_of(main_cmp, 2, main_limit);
+  if (abs_stride_is_one) {
+    Node* final_iv = new SubINode(main_limit, cl->stride());
+    register_new_node(final_iv, loop_entry);
+    _igvn.replace_node(final_iv_placeholder, final_iv);
+  }
   // The OpaqueNode is unshared by design
   assert(opqzm->outcnt() == 1, "cannot hack shared node");
   _igvn.replace_input_of(opqzm, 1, main_limit);
-
-  return;
 }
 
 bool IdealLoopTree::compute_has_range_checks() const {

test/hotspot/jtreg/compiler/rangechecks/TestEmptyLoopDeadCast.java

@@ -0,0 +1,72 @@
+/*
+ * Copyright (c) 2024, Red Hat 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 8335393
+ * @summary C2: assert(!had_error) failed: bad dominance
+ * @requires vm.compiler2.enabled
+ * @run main/othervm -XX:-TieredCompilation -XX:-UseOnStackReplacement -XX:-BackgroundCompilation -XX:-UseLoopPredicate
+ *                   -XX:LoopMaxUnroll=0 TestEmptyLoopDeadCast
+ */
+
+public class TestEmptyLoopDeadCast {
+    public static void main(String[] args) {
+        int[] array = new int[100];
+        for (int i = 0; i < 20_000; i++) {
+            test1Helper(1, 101, array);
+            test1(0, array);
+            test2Helper(0, -101, array);
+            test2(0, array);
+        }
+    }
+
+    private static int test1(int start, int[] array) {
+        return test1Helper(start, 0, array);
+    }
+
+    private static int test1Helper(int start, int stop, int[] array) {
+        if (array == null) {
+        }
+        int v = 0;
+        for (int i = start; i < stop; i++) {
+            v = array[i - 1];
+        }
+        return v;
+    }
+
+    private static int test2(int start, int[] array) {
+        return test2Helper(start, -1, array);
+    }
+
+    private static int test2Helper(int start, int stop, int[] array) {
+        if (array == null) {
+        }
+        int v = 0;
+        for (int i = start-1; i > stop; i--) {
+            v = array[-1 - i];
+        }
+        return v;
+    }
+
+}