8293996: C2: fix and simplify IdealLoopTree::do_remove_empty_loop

Backport-of: dd51f7e0b75d3a16403608d89cd206ac0bedf882

src/hotspot/share/opto/loopTransform.cpp

@@ -3234,29 +3234,21 @@ bool IdealLoopTree::do_remove_empty_loop(PhaseIdealLoop *phase) {
   }
 
   // Replace the phi at loop head with the final value of the last
-  // iteration.  Then the CountedLoopEnd will collapse (backedge never
-  // taken) and all loop-invariant uses of the exit values will be correct.
-  Node *phi = cl->phi();
-  Node *exact_limit = phase->exact_limit(this);
-  if (exact_limit != cl->limit()) {
-    // We also need to replace the original limit to collapse loop exit.
-    Node* cmp = cl->loopexit()->cmp_node();
-    assert(cl->limit() == cmp->in(2), "sanity");
-    // Duplicate cmp node if it has other users
-    if (cmp->outcnt() > 1) {
-      cmp = cmp->clone();
-      cmp = phase->_igvn.register_new_node_with_optimizer(cmp);
-      BoolNode *bol = cl->loopexit()->in(CountedLoopEndNode::TestValue)->as_Bool();
-      phase->_igvn.replace_input_of(bol, 1, cmp); // put bol on worklist
-    }
-    phase->_igvn._worklist.push(cmp->in(2)); // put limit on worklist
-    phase->_igvn.replace_input_of(cmp, 2, exact_limit); // put cmp on worklist
-  }
+  // iteration (exact_limit - stride), to make sure the loop exit value
+  // is correct, for any users after the loop.
   // Note: the final value after increment should not overflow since
   // counted loop has limit check predicate.
-  Node *final = new SubINode(exact_limit, cl->stride());
-  phase->register_new_node(final,cl->in(LoopNode::EntryControl));
-  phase->_igvn.replace_node(phi,final);
+  Node* phi = cl->phi();
+  Node* exact_limit = phase->exact_limit(this);
+  Node* final_iv = new SubINode(exact_limit, cl->stride());
+  phase->register_new_node(final_iv, cl->in(LoopNode::EntryControl));
+  phase->_igvn.replace_node(phi, final_iv);
+
+  // Set loop-exit condition to false. Then the CountedLoopEnd will collapse,
+  // because the back edge is never taken.
+  Node* zero = phase->_igvn.intcon(0);
+  phase->_igvn.replace_input_of(cl->loopexit(), CountedLoopEndNode::TestValue, zero);
+
   phase->C->set_major_progress();
   return true;
 }

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

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2019, Huawei Technologies Co. Ltd. All rights reserved.
+ * 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
@@ -23,7 +24,7 @@
 
 /**
  * @test
- * @bug 8231988
+ * @bug 8231988 8293996
  * @summary Unexpected test result caused by C2 IdealLoopTree::do_remove_empty_loop
  *
  * @run main/othervm -XX:-TieredCompilation -XX:-BackgroundCompilation
@@ -34,22 +35,50 @@
 
 public class TestRemoveEmptyLoop {
 
-    public void test() {
+    public void test_cmp_helper() {
         int i = 34;
+        // The empty loop that collapses
         for (; i > 0; i -= 11);
+        // If uses same Cmp node as the loop condition
         if (i < 0) {
             // do nothing
         } else {
             throw new RuntimeException("Test failed.");
         }
     }
 
-    public static void main(String[] args) {
-        TestRemoveEmptyLoop _instance = new TestRemoveEmptyLoop();
+    public void test_cmp() {
+        // Loop is OSR compiled, and test_cmp_helper inlined
         for (int i = 0; i < 50000; i++) {
-            _instance.test();
+            test_cmp_helper();
+        }
+    }
+
+    void test_collapse_helper() {
+        int o = 11;
+        int e = 43542;
+        for (int i = 524; i < 19325; i += 1) {
+            // The empty loop that is supposed to collapse
+            for (int j = 0; j < 32767; j++) {
+                o++;
+            }
+            for (int k = 0; k < o; k++) {
+                e++;
+            }
+        }
+    }
+
+    public void test_collapse() {
+        // Loop is OSR compiled, and test_collapse_helper inlined
+        for (int i = 0; i < 50000; i++) {
+            test_collapse_helper();
         }
-        System.out.println("Test passed.");
     }
 
+    public static void main(String[] args) {
+        TestRemoveEmptyLoop _instance = new TestRemoveEmptyLoop();
+        _instance.test_cmp();
+        _instance.test_collapse();
+        System.out.println("Test passed.");
+    }
 }