8307927: C2: "malformed control flow" with irreducible loop

src/hotspot/share/opto/loopnode.cpp

@@ -3531,26 +3531,27 @@ bool IdealLoopTree::beautify_loops( PhaseIdealLoop *phase ) {
 }
 
 //------------------------------allpaths_check_safepts----------------------------
-// Allpaths backwards scan from loop tail, terminating each path at first safepoint
-// encountered.  Helper for check_safepts.
+// Allpaths backwards scan. Starting at the head, traversing all backedges, and the body. Terminating each path at first
+// safepoint encountered.  Helper for check_safepts.
 void IdealLoopTree::allpaths_check_safepts(VectorSet &visited, Node_List &stack) {
   assert(stack.size() == 0, "empty stack");
-  stack.push(_tail);
+  stack.push(_head);
   visited.clear();
-  visited.set(_tail->_idx);
+  visited.set(_head->_idx);
   while (stack.size() > 0) {
     Node* n = stack.pop();
     if (n->is_Call() && n->as_Call()->guaranteed_safepoint()) {
       // Terminate this path
     } else if (n->Opcode() == Op_SafePoint) {
       if (_phase->get_loop(n) != this) {
         if (_required_safept == nullptr) _required_safept = new Node_List();
-        _required_safept->push(n);  // save the one closest to the tail
+        // save the first we run into on that path: closest to the tail if the head has a single backedge
+        _required_safept->push(n);
       }
       // Terminate this path
     } else {
       uint start = n->is_Region() ? 1 : 0;
-      uint end   = n->is_Region() && !n->is_Loop() ? n->req() : start + 1;
+      uint end   = n->is_Region() && (!n->is_Loop() || n == _head) ? n->req() : start + 1;
       for (uint i = start; i < end; i++) {
         Node* in = n->in(i);
         assert(in->is_CFG(), "must be");
@@ -3622,53 +3623,55 @@ void IdealLoopTree::check_safepts(VectorSet &visited, Node_List &stack) {
   if (_child) _child->check_safepts(visited, stack);
   if (_next)  _next ->check_safepts(visited, stack);
 
-  if (!_head->is_CountedLoop() && !_has_sfpt && _parent != nullptr && !_irreducible) {
+  if (!_head->is_CountedLoop() && !_has_sfpt && _parent != nullptr) {
     bool  has_call         = false;    // call on dom-path
     bool  has_local_ncsfpt = false;    // ncsfpt on dom-path at this loop depth
     Node* nonlocal_ncsfpt  = nullptr;  // ncsfpt on dom-path at a deeper depth
-    // Scan the dom-path nodes from tail to head
-    for (Node* n = tail(); n != _head; n = _phase->idom(n)) {
-      if (n->is_Call() && n->as_Call()->guaranteed_safepoint()) {
-        has_call = true;
-        _has_sfpt = 1;          // Then no need for a safept!
-        break;
-      } else if (n->Opcode() == Op_SafePoint) {
-        if (_phase->get_loop(n) == this) {
-          has_local_ncsfpt = true;
+    if (!_irreducible) {
+      // Scan the dom-path nodes from tail to head
+      for (Node* n = tail(); n != _head; n = _phase->idom(n)) {
+        if (n->is_Call() && n->as_Call()->guaranteed_safepoint()) {
+          has_call = true;
+          _has_sfpt = 1;          // Then no need for a safept!
           break;
-        }
-        if (nonlocal_ncsfpt == nullptr) {
-          nonlocal_ncsfpt = n; // save the one closest to the tail
-        }
-      } else {
-        IdealLoopTree* nlpt = _phase->get_loop(n);
-        if (this != nlpt) {
-          // If at an inner loop tail, see if the inner loop has already
-          // recorded seeing a call on the dom-path (and stop.)  If not,
-          // jump to the head of the inner loop.
-          assert(is_member(nlpt), "nested loop");
-          Node* tail = nlpt->_tail;
-          if (tail->in(0)->is_If()) tail = tail->in(0);
-          if (n == tail) {
-            // If inner loop has call on dom-path, so does outer loop
-            if (nlpt->_has_sfpt) {
-              has_call = true;
-              _has_sfpt = 1;
-              break;
-            }
-            // Skip to head of inner loop
-            assert(_phase->is_dominator(_head, nlpt->_head), "inner head dominated by outer head");
-            n = nlpt->_head;
-            if (_head == n) {
-              // this and nlpt (inner loop) have the same loop head. This should not happen because
-              // during beautify_loops we call merge_many_backedges. However, infinite loops may not
-              // have been attached to the loop-tree during build_loop_tree before beautify_loops,
-              // but then attached in the build_loop_tree afterwards, and so still have unmerged
-              // backedges. Check if we are indeed in an infinite subgraph, and terminate the scan,
-              // since we have reached the loop head of this.
-              assert(_head->as_Region()->is_in_infinite_subgraph(),
-                     "only expect unmerged backedges in infinite loops");
-              break;
+        } else if (n->Opcode() == Op_SafePoint) {
+          if (_phase->get_loop(n) == this) {
+            has_local_ncsfpt = true;
+            break;
+          }
+          if (nonlocal_ncsfpt == nullptr) {
+            nonlocal_ncsfpt = n; // save the one closest to the tail
+          }
+        } else {
+          IdealLoopTree* nlpt = _phase->get_loop(n);
+          if (this != nlpt) {
+            // If at an inner loop tail, see if the inner loop has already
+            // recorded seeing a call on the dom-path (and stop.)  If not,
+            // jump to the head of the inner loop.
+            assert(is_member(nlpt), "nested loop");
+            Node* tail = nlpt->_tail;
+            if (tail->in(0)->is_If()) tail = tail->in(0);
+            if (n == tail) {
+              // If inner loop has call on dom-path, so does outer loop
+              if (nlpt->_has_sfpt) {
+                has_call = true;
+                _has_sfpt = 1;
+                break;
+              }
+              // Skip to head of inner loop
+              assert(_phase->is_dominator(_head, nlpt->_head), "inner head dominated by outer head");
+              n = nlpt->_head;
+              if (_head == n) {
+                // this and nlpt (inner loop) have the same loop head. This should not happen because
+                // during beautify_loops we call merge_many_backedges. However, infinite loops may not
+                // have been attached to the loop-tree during build_loop_tree before beautify_loops,
+                // but then attached in the build_loop_tree afterwards, and so still have unmerged
+                // backedges. Check if we are indeed in an infinite subgraph, and terminate the scan,
+                // since we have reached the loop head of this.
+                assert(_head->as_Region()->is_in_infinite_subgraph(),
+                       "only expect unmerged backedges in infinite loops");
+                break;
+              }
             }
           }
         }

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

@@ -0,0 +1,91 @@
+/*
+ * Copyright (c) 2023, 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.
+ */
+
+super public class MalformedControlIrreducibleLoop
+	version 52:0
+{
+  public Method "<init>":"()V"
+	stack 1 locals 1
+  {
+		aload_0;
+		invokespecial	Method java/lang/Object."<init>":"()V";
+		return;
+  }
+  /* same as:
+      public static void actualTest(int flag) {
+        int i = 1;
+        int j = 1;
+        if (flag == 2) {
+            // goto second_entry;
+        }
+        loop:
+        for (;;) {
+            i = 1;
+            // second_entry:
+            do {
+                if (i == 1) {
+                    continue loop; // goto loop
+                }
+                i *= 2;
+                j *= 2;
+            } while (j < 2);
+            break;
+        }
+    }
+  */
+  public static Method actualTest:"(I)V"
+	stack 2 locals 3
+  {
+		iconst_1;
+		istore_1;
+		iconst_1;
+		istore_2;
+		iload_0;
+		iconst_2;
+		if_icmpne	L11;
+	L9:	stack_frame_type append;
+		locals_map int, int;
+		iconst_1;
+		istore_1;
+	L11:	stack_frame_type same;
+		iload_1;
+		iconst_1;
+		if_icmpeq	L9;
+	L19:	stack_frame_type same;
+		iload_1;
+		iconst_2;
+		imul;
+		istore_1;
+		iload_2;
+		iconst_2;
+		imul;
+		istore_2;
+		iload_2;
+		iconst_2;
+		if_icmplt	L11;
+		goto	L35;
+	L35:	stack_frame_type same;
+		return;
+  }
+
+} // end Class TestMalformedControlIrreducibleLoop

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

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2023, 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.
+ */
+
+/*
+ * @test
+ * @bug 8307927
+ * @summary C2: "malformed control flow" with irreducible loop
+ * @compile MalformedControlIrreducibleLoop.jasm
+ * @run main/othervm -Xcomp -XX:-TieredCompilation -XX:CompileOnly=TestMalformedControlIrreducibleLoop::test TestMalformedControlIrreducibleLoop
+ */
+
+public class TestMalformedControlIrreducibleLoop {
+    public static void main(String[] args) {
+        new MalformedControlIrreducibleLoop();
+        test(false);
+    }
+
+    private static void test(boolean flag) {
+        int i;
+        for (i = 1; i < 2; i *= 2) {
+        }
+        if (flag) {
+            MalformedControlIrreducibleLoop.actualTest(i);
+        }
+    }
+}