8297642: PhaseIdealLoop::only_has_infinite_loops must detect all loops that never lead to termination

Reviewed-by: thartmann, roland

src/hotspot/share/opto/loopnode.cpp

@@ -4190,45 +4190,40 @@ bool PhaseIdealLoop::process_expensive_nodes() {
 }
 
 #ifdef ASSERT
-// Goes over all children of the root of the loop tree, collects all controls for the loop and its inner loops then
+// Goes over all children of the root of the loop tree. Check if any of them have a path
-// checks whether any control is a branch out of the loop and if it is, whether it's not a NeverBranch.
+// down to Root, that does not go via a NeverBranch exit.
 bool PhaseIdealLoop::only_has_infinite_loops() {
+  ResourceMark rm;
+  Unique_Node_List worklist;
+  // start traversal at all loop heads of first-level loops
   for (IdealLoopTree* l = _ltree_root->_child; l != NULL; l = l->_next) {
-    Unique_Node_List wq;
     Node* head = l->_head;
     assert(head->is_Region(), "");
-    for (uint i = 1; i < head->req(); ++i) {
+    worklist.push(head);
-      Node* in = head->in(i);
-      if (get_loop(in) != _ltree_root) {
-        wq.push(in);
   }
-    }
+  // BFS traversal down the CFG, except through NeverBranch exits
-    for (uint i = 0; i < wq.size(); ++i) {
+  for (uint i = 0; i < worklist.size(); ++i) {
-      Node* c = wq.at(i);
+    Node* n = worklist.at(i);
-      if (c == head) {
+    assert(n->is_CFG(), "only traverse CFG");
-        continue;
+    if (n->is_Root()) {
-      } else if (c->is_Region()) {
+      // Found root -> there was an exit!
-        for (uint j = 1; j < c->req(); ++j) {
-          wq.push(c->in(j));
-        }
-      } else {
-        wq.push(c->in(0));
-      }
-    }
-    assert(wq.member(head), "");
-    for (uint i = 0; i < wq.size(); ++i) {
-      Node* c = wq.at(i);
-      if (c->is_MultiBranch()) {
-        for (DUIterator_Fast jmax, j = c->fast_outs(jmax); j < jmax; j++) {
-          Node* u = c->fast_out(j);
-          assert(u->is_CFG(), "");
-          if (!wq.member(u) && c->Opcode() != Op_NeverBranch) {
       return false;
+    } else if (n->Opcode() == Op_NeverBranch) {
+      // Only follow the loop-internal projection, not the NeverBranch exit
+      ProjNode* proj = n->as_Multi()->proj_out_or_null(0);
+      assert(proj != nullptr, "must find loop-internal projection of NeverBranch");
+      worklist.push(proj);
+    } else {
+      // Traverse all CFG outputs
+      for (DUIterator_Fast imax, i = n->fast_outs(imax); i < imax; i++) {
+        Node* use = n->fast_out(i);
+        if (use->is_CFG()) {
+          worklist.push(use);
         }
       }
     }
   }
-  }
+  // No exit found for any loop -> all are infinite
   return true;
 }
 #endif

test/hotspot/jtreg/compiler/loopopts/TestOnlyInfiniteLoops.jasm

@@ -0,0 +1,96 @@
+/*
+ * 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.
+ */
+
+
+super public class TestOnlyInfiniteLoops
+{
+    public Method "<init>":"()V"
+    stack 2 locals 1
+    {
+        aload_0;
+        invokespecial  Method java/lang/Object."<init>":"()V";
+        return;
+    }
+
+    static Method test_simple:"(III)I"
+    stack 200 locals 10
+    {
+    // Nested infinite loop, where inner loop eventually
+    // looses exit to outer loop. Then, the inner loop
+    // floats outside the inner loop. The entry from
+    // outer to inner loop now becomes an exit for the
+    // outer loop, where it now enters the next loop, that
+    // used to be the inner loop.
+        iconst_0;
+        istore     9;
+
+        iload      0;
+        ifeq LEND; // skip
+
+    LOOP1:
+        iload      1;
+        ifeq LOOP1; // dominates
+    LOOP2:
+        // SKIP: prevent loop-exit from becoming zero-trip guard
+        iload      2;
+        ifeq SKIP;
+        iinc       9, 1;
+    SKIP:
+        iload      1;
+        ifeq LOOP1; // is dominated
+        goto LOOP2;
+
+    LEND:
+        iload      9;
+        ireturn;
+    }
+    static Method test_irreducible:"(IIII)V"
+    stack 200 locals 200
+    {
+        iload_0;
+        ifeq LEND; // skip
+
+    L1:
+        iload      1;
+        ifgt MERGE;
+    L2:
+        iload      2;
+        ifge MERGE;
+        goto L1;
+
+    MERGE:
+        nop;
+    LOOP:
+        iload      3;
+        ifle L2;
+        iconst_0; // always true
+        ifeq LOOP;
+        iconst_0; // always true
+        ifeq LOOP;
+    INFTY:
+        goto INFTY; // infinite loop
+
+    LEND:
+        return;
+    }
+}

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

@@ -0,0 +1,53 @@
+/*
+ * 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 8297642
+ * @compile TestOnlyInfiniteLoops.jasm
+ * @summary Nested irreducible loops, where the inner loop floats out of the outer
+ * @run main/othervm
+ *      -XX:CompileCommand=compileonly,TestOnlyInfiniteLoops::test*
+ *      -XX:-TieredCompilation -Xcomp
+ *      TestOnlyInfiniteLoopsMain
+ *
+ * @test
+ * @bug 8297642
+ * @compile TestOnlyInfiniteLoops.jasm
+ * @summary Nested irreducible loops, where the inner loop floats out of the outer
+ * @run main/othervm
+ *      -XX:CompileCommand=compileonly,TestOnlyInfiniteLoops::test*
+ *      -XX:-TieredCompilation -Xcomp
+ *      -XX:PerMethodTrapLimit=0
+ *      TestOnlyInfiniteLoopsMain
+*/
+
+public class TestOnlyInfiniteLoopsMain {
+    public static void main(String[] args) {
+        TestOnlyInfiniteLoops t = new TestOnlyInfiniteLoops();
+        System.out.println("test_simple");
+        t.test_simple(0, 0, 0);
+        System.out.println("test_irreducible");
+        t.test_irreducible(0, 0, 0, 0);
+    }
+}