6869327: Add new C2 flag to keep safepoints in counted loops

Reviewed-by: kvn, shade

hotspot/src/share/vm/opto/c2_globals.hpp

@@ -213,6 +213,9 @@
   notproduct(bool, TraceProfileTripCount, false,                            \
           "Trace profile loop trip count information")                      \
                                                                             \
+  product(bool, UseCountedLoopSafepoints, false,                            \
+          "Force counted loops to keep a safepoint")                        \
+                                                                            \
   product(bool, UseLoopPredicate, true,                                     \
           "Generate a predicate to select fast/slow loop versions")         \
                                                                             \

hotspot/src/share/vm/opto/loopnode.cpp

@@ -690,14 +690,16 @@ bool PhaseIdealLoop::is_counted_loop( Node *x, IdealLoopTree *loop ) {
 
   } // LoopLimitCheck
 
-  // Check for SafePoint on backedge and remove
-  Node *sfpt = x->in(LoopNode::LoopBackControl);
-  if (sfpt->Opcode() == Op_SafePoint && is_deleteable_safept(sfpt)) {
-    lazy_replace( sfpt, iftrue );
-    if (loop->_safepts != NULL) {
-      loop->_safepts->yank(sfpt);
+  if (!UseCountedLoopSafepoints) {
+    // Check for SafePoint on backedge and remove
+    Node *sfpt = x->in(LoopNode::LoopBackControl);
+    if (sfpt->Opcode() == Op_SafePoint && is_deleteable_safept(sfpt)) {
+      lazy_replace( sfpt, iftrue );
+      if (loop->_safepts != NULL) {
+        loop->_safepts->yank(sfpt);
+      }
+      loop->_tail = iftrue;
     }
-    loop->_tail = iftrue;
   }
 
   // Build a canonical trip test.
@@ -786,12 +788,14 @@ bool PhaseIdealLoop::is_counted_loop( Node *x, IdealLoopTree *loop ) {
   lazy_replace( x, l );
   set_idom(l, init_control, dom_depth(x));
 
-  // Check for immediately preceding SafePoint and remove
-  Node *sfpt2 = le->in(0);
-  if (sfpt2->Opcode() == Op_SafePoint && is_deleteable_safept(sfpt2)) {
-    lazy_replace( sfpt2, sfpt2->in(TypeFunc::Control));
-    if (loop->_safepts != NULL) {
-      loop->_safepts->yank(sfpt2);
+  if (!UseCountedLoopSafepoints) {
+    // Check for immediately preceding SafePoint and remove
+    Node *sfpt2 = le->in(0);
+    if (sfpt2->Opcode() == Op_SafePoint && is_deleteable_safept(sfpt2)) {
+      lazy_replace( sfpt2, sfpt2->in(TypeFunc::Control));
+      if (loop->_safepts != NULL) {
+        loop->_safepts->yank(sfpt2);
+      }
     }
   }
 
@@ -1813,6 +1817,33 @@ void PhaseIdealLoop::replace_parallel_iv(IdealLoopTree *loop) {
   }
 }
 
+void IdealLoopTree::remove_safepoints(PhaseIdealLoop* phase, bool keep_one) {
+  // Look for a safepoint on the idom-path.
+  Node* keep = NULL;
+  if (keep_one) {
+    // Keep one if possible
+    for (Node* i = tail(); i != _head; i = phase->idom(i)) {
+      if (i->Opcode() == Op_SafePoint && phase->get_loop(i) == this) {
+        keep = i;
+        break; // Found one
+      }
+    }
+  }
+
+  // Delete other safepoints in this loop.
+  Node_List* sfpts = _safepts;
+  if (sfpts != NULL) {
+    assert(keep == NULL || keep->Opcode() == Op_SafePoint, "not safepoint");
+    for (uint i = 0; i < sfpts->size(); i++) {
+      Node* n = sfpts->at(i);
+      assert(phase->get_loop(n) == this, "");
+      if (n != keep && phase->is_deleteable_safept(n)) {
+        phase->lazy_replace(n, n->in(TypeFunc::Control));
+      }
+    }
+  }
+}
+
 //------------------------------counted_loop-----------------------------------
 // Convert to counted loops where possible
 void IdealLoopTree::counted_loop( PhaseIdealLoop *phase ) {
@@ -1824,42 +1855,23 @@ void IdealLoopTree::counted_loop( PhaseIdealLoop *phase ) {
 
   if (_head->is_CountedLoop() ||
       phase->is_counted_loop(_head, this)) {
-    _has_sfpt = 1;              // Indicate we do not need a safepoint here
 
-    // Look for safepoints to remove.
-    Node_List* sfpts = _safepts;
-    if (sfpts != NULL) {
-      for (uint i = 0; i < sfpts->size(); i++) {
-        Node* n = sfpts->at(i);
-        assert(phase->get_loop(n) == this, "");
-        if (phase->is_deleteable_safept(n)) {
-          phase->lazy_replace(n, n->in(TypeFunc::Control));
-        }
-      }
+    if (!UseCountedLoopSafepoints) {
+      // Indicate we do not need a safepoint here
+      _has_sfpt = 1;
     }
 
+    // Remove safepoints
+    bool keep_one_sfpt = !(_has_call || _has_sfpt);
+    remove_safepoints(phase, keep_one_sfpt);
+
     // Look for induction variables
     phase->replace_parallel_iv(this);
 
   } else if (_parent != NULL && !_irreducible) {
-    // Not a counted loop.
-    // Look for a safepoint on the idom-path.
-    Node* sfpt = tail();
-    for (; sfpt != _head; sfpt = phase->idom(sfpt)) {
-      if (sfpt->Opcode() == Op_SafePoint && phase->get_loop(sfpt) == this)
-        break; // Found one
-    }
-    // Delete other safepoints in this loop.
-    Node_List* sfpts = _safepts;
-    if (sfpts != NULL && sfpt != _head && sfpt->Opcode() == Op_SafePoint) {
-      for (uint i = 0; i < sfpts->size(); i++) {
-        Node* n = sfpts->at(i);
-        assert(phase->get_loop(n) == this, "");
-        if (n != sfpt && phase->is_deleteable_safept(n)) {
-          phase->lazy_replace(n, n->in(TypeFunc::Control));
-        }
-      }
-    }
+    // Not a counted loop. Keep one safepoint.
+    bool keep_one_sfpt = true;
+    remove_safepoints(phase, keep_one_sfpt);
   }
 
   // Recursively

hotspot/src/share/vm/opto/loopnode.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1998, 2013, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1998, 2015, 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
@@ -429,6 +429,9 @@ public:
   // encountered.
   void allpaths_check_safepts(VectorSet &visited, Node_List &stack);
 
+  // Remove safepoints from loop. Optionally keeping one.
+  void remove_safepoints(PhaseIdealLoop* phase, bool keep_one);
+
   // Convert to counted loops where possible
   void counted_loop( PhaseIdealLoop *phase );
 

hotspot/test/compiler/loopopts/UseCountedLoopSafepoints.java

@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) 2015, 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 6869327
+ * @summary Test that C2 flag UseCountedLoopSafepoints ensures a safepoint is kept in a CountedLoop
+ * @library /testlibrary
+ * @modules java.base
+ * @run main UseCountedLoopSafepoints
+ */
+
+import java.util.concurrent.atomic.AtomicLong;
+import jdk.test.lib.ProcessTools;
+import jdk.test.lib.OutputAnalyzer;
+
+public class UseCountedLoopSafepoints {
+    private static final AtomicLong _num = new AtomicLong(0);
+
+    // Uses the fact that an EnableBiasedLocking vmop will be started
+    // after 500ms, while we are still in the loop. If there is a
+    // safepoint in the counted loop, then we will reach safepoint
+    // very quickly. Otherwise SafepointTimeout will be hit.
+    public static void main (String args[]) throws Exception {
+        if (args.length == 1) {
+            final int loops = Integer.parseInt(args[0]);
+            for (int i = 0; i < loops; i++) {
+                _num.addAndGet(1);
+            }
+        } else {
+            ProcessBuilder pb = ProcessTools.createJavaProcessBuilder(
+                    "-XX:+IgnoreUnrecognizedVMOptions",
+                    "-XX:-TieredCompilation",
+                    "-XX:+UseBiasedLocking",
+                    "-XX:BiasedLockingStartupDelay=500",
+                    "-XX:+SafepointTimeout",
+                    "-XX:SafepointTimeoutDelay=2000",
+                    "-XX:+UseCountedLoopSafepoints",
+                    "UseCountedLoopSafepoints",
+                    "2000000000"
+                    );
+            OutputAnalyzer output = new OutputAnalyzer(pb.start());
+            output.shouldNotContain("Timeout detected");
+            output.shouldHaveExitValue(0);
+        }
+    }
+}