8350756: C2 SuperWord Multiversioning: remove useless slow loop when the fast loop disappears

Reviewed-by: kvn, chagedorn

src/hotspot/share/opto/loopnode.cpp

@@ -41,6 +41,7 @@
 #include "opto/movenode.hpp"
 #include "opto/mulnode.hpp"
 #include "opto/opaquenode.hpp"
+#include "opto/opcodes.hpp"
 #include "opto/predicates.hpp"
 #include "opto/rootnode.hpp"
 #include "opto/runtime.hpp"
@@ -2731,6 +2732,20 @@ Node* CountedLoopNode::match_incr_with_optional_truncation(Node* expr, Node** tr
   return nullptr;
 }
 
+IfNode* CountedLoopNode::find_multiversion_if_from_multiversion_fast_main_loop() {
+  assert(is_main_loop() && is_multiversion_fast_loop(), "must be multiversion fast main loop");
+  CountedLoopEndNode* pre_end = find_pre_loop_end();
+  if (pre_end == nullptr) { return nullptr; }
+  Node* pre_entry = pre_end->loopnode()->in(LoopNode::EntryControl);
+  const Predicates predicates(pre_entry);
+  IfTrueNode* before_predicates = predicates.entry()->isa_IfTrue();
+  if (before_predicates != nullptr &&
+      before_predicates->in(0)->in(1)->is_OpaqueMultiversioning()) {
+    return before_predicates->in(0)->as_If();
+  }
+  return nullptr;
+}
+
 LoopNode* CountedLoopNode::skip_strip_mined(int expect_skeleton) {
   if (is_strip_mined() && in(EntryControl) != nullptr && in(EntryControl)->is_OuterStripMinedLoop()) {
     verify_strip_mined(expect_skeleton);
@@ -4536,6 +4551,49 @@ void PhaseIdealLoop::eliminate_useless_zero_trip_guard() {
   }
 }
 
+void PhaseIdealLoop::eliminate_useless_multiversion_if() {
+  if (_multiversion_opaque_nodes.size() == 0) {
+    return;
+  }
+
+  ResourceMark rm;
+  Unique_Node_List useful_multiversioning_opaque_nodes;
+
+  // The OpaqueMultiversioning is only used from the fast main loop in AutoVectorization, to add
+  // speculative runtime-checks to the multiversion_if. Thus, a OpaqueMultiversioning is only
+  // useful if it can be found from a fast main loop. If it can not be found from a fast main loop,
+  // then we cannot ever use that multiversion_if to add more speculative runtime-checks, and hence
+  // it is useless. If it is still in delayed mode, i.e. has not yet had any runtime-checks added,
+  // then we can let it constant fold towards the fast loop.
+  for (LoopTreeIterator iter(_ltree_root); !iter.done(); iter.next()) {
+    IdealLoopTree* lpt = iter.current();
+    if (lpt->_child == nullptr && lpt->is_counted()) {
+      CountedLoopNode* head = lpt->_head->as_CountedLoop();
+      if (head->is_main_loop() && head->is_multiversion_fast_loop()) {
+        // There are fast_loop pre/main/post loops, but the finding traversal starts at the main
+        // loop, and traverses via the fast pre loop to the multiversion_if.
+        IfNode* multiversion_if = head->find_multiversion_if_from_multiversion_fast_main_loop();
+        if (multiversion_if != nullptr) {
+            useful_multiversioning_opaque_nodes.push(multiversion_if->in(1)->as_OpaqueMultiversioning());
+        }
+      }
+    }
+  }
+
+  for (uint i = 0; i < _multiversion_opaque_nodes.size(); i++) {
+    OpaqueMultiversioningNode* opaque = _multiversion_opaque_nodes.at(i)->as_OpaqueMultiversioning();
+    if (!useful_multiversioning_opaque_nodes.member(opaque)) {
+      if (opaque->is_delayed_slow_loop()) {
+        // We cannot hack the node directly, otherwise the slow_loop will complain that it cannot
+        // find the multiversioning opaque node. Instead, we mark the opaque node as useless, and
+        // it can be constant folded during IGVN.
+        opaque->mark_useless();
+        _igvn._worklist.push(opaque);
+      }
+    }
+  }
+}
+
 //------------------------process_expensive_nodes-----------------------------
 // Expensive nodes have their control input set to prevent the GVN
 // from commoning them and as a result forcing the resulting node to
@@ -4805,6 +4863,7 @@ void PhaseIdealLoop::build_and_optimize() {
   }
 
   eliminate_useless_zero_trip_guard();
+  eliminate_useless_multiversion_if();
 
   if (stop_early) {
     assert(do_expensive_nodes, "why are we here?");
@@ -6596,6 +6655,9 @@ void PhaseIdealLoop::build_loop_late_post_work(Node *n, bool pinned) {
     _zero_trip_guard_opaque_nodes.push(n);
   }
 
+  if (!_verify_only && n->Opcode() == Op_OpaqueMultiversioning) {
+    _multiversion_opaque_nodes.push(n);
+  }
 }
 
 #ifdef ASSERT

src/hotspot/share/opto/loopnode.hpp

@@ -290,6 +290,8 @@ public:
   bool has_atomic_post_loop  () const { return (_loop_flags & HasAtomicPostLoop) == HasAtomicPostLoop; }
   void set_main_no_pre_loop() { _loop_flags |= MainHasNoPreLoop; }
 
+  IfNode* find_multiversion_if_from_multiversion_fast_main_loop();
+
   int main_idx() const { return _main_idx; }
 
 
@@ -932,6 +934,7 @@ private:
   // clear out dead code after build_loop_late
   Node_List _deadlist;
   Node_List _zero_trip_guard_opaque_nodes;
+  Node_List _multiversion_opaque_nodes;
 
   // Support for faster execution of get_late_ctrl()/dom_lca()
   // when a node has many uses and dominator depth is deep.
@@ -1453,6 +1456,7 @@ public:
   void eliminate_useless_template_assertion_predicates(Unique_Node_List& useful_predicates);
 
   void eliminate_useless_zero_trip_guard();
+  void eliminate_useless_multiversion_if();
 
  public:
   // Change the control input of expensive nodes to allow commoning by

src/hotspot/share/opto/loopopts.cpp

@@ -791,7 +791,10 @@ Node *PhaseIdealLoop::conditional_move( Node *region ) {
     // Ignore Template Assertion Predicates with OpaqueTemplateAssertionPredicate nodes.
     return nullptr;
   }
-  assert(bol->Opcode() == Op_Bool, "Unexpected node");
+  if (!bol->is_Bool()) {
+    assert(false, "Expected Bool, but got %s", NodeClassNames[bol->Opcode()]);
+    return nullptr;
+  }
   int cmp_op = bol->in(1)->Opcode();
   if (cmp_op == Op_SubTypeCheck) { // SubTypeCheck expansion expects an IfNode
     return nullptr;

src/hotspot/share/opto/opaquenode.cpp

@@ -82,6 +82,26 @@ IfNode* OpaqueZeroTripGuardNode::if_node() const {
   return iff->as_If();
 }
 
+Node* OpaqueMultiversioningNode::Identity(PhaseGVN* phase) {
+  // Constant fold the multiversion_if. Since the slow_loop is still delayed,
+  // i.e. we have not yet added any possibly failing condition, we can just
+  // take the true branch in all cases.
+  if (_useless) {
+    assert(_is_delayed_slow_loop, "the slow_loop should still be delayed");
+    return in(1);
+  }
+  return Opaque1Node::Identity(phase);
+}
+
+#ifndef PRODUCT
+void OpaqueMultiversioningNode::dump_spec(outputStream *st) const {
+  Opaque1Node::dump_spec(st);
+  if (_useless) {
+    st->print(" #useless");
+  }
+}
+#endif
+
 const Type* OpaqueNotNullNode::Value(PhaseGVN* phase) const {
   return phase->type(in(1));
 }

src/hotspot/share/opto/opaquenode.hpp

@@ -101,17 +101,30 @@ public:
 class OpaqueMultiversioningNode : public Opaque1Node {
 private:
   bool _is_delayed_slow_loop;
+  bool _useless;
 
 public:
   OpaqueMultiversioningNode(Compile* C, Node* n) :
-      Opaque1Node(C, n), _is_delayed_slow_loop(true)
+      Opaque1Node(C, n), _is_delayed_slow_loop(true), _useless(false)
   {
     init_class_id(Class_OpaqueMultiversioning);
   }
   virtual int Opcode() const;
   virtual const Type* bottom_type() const { return TypeInt::BOOL; }
   bool is_delayed_slow_loop() const { return _is_delayed_slow_loop; }
-  void notify_slow_loop_that_it_can_resume_optimizations() { _is_delayed_slow_loop = false; }
+
+  void notify_slow_loop_that_it_can_resume_optimizations() {
+    assert(!_useless, "must still be useful");
+    _is_delayed_slow_loop = false;
+  }
+
+  void mark_useless() {
+    assert(_is_delayed_slow_loop, "must still be delayed");
+    _useless = true;
+  }
+
+  virtual Node* Identity(PhaseGVN* phase);
+  NOT_PRODUCT(virtual void dump_spec(outputStream* st) const;)
 };
 
 // This node is used in the context of intrinsics. We sometimes implicitly know that an object is non-null even though

test/hotspot/jtreg/compiler/lib/ir_framework/IRNode.java

@@ -329,6 +329,11 @@ public class IRNode {
         superWordNodes(ADD_REDUCTION_VL, "AddReductionVL");
     }
 
+    public static final String OPAQUE_MULTIVERSIONING = PREFIX + "OPAQUE_MULTIVERSIONING" + POSTFIX;
+    static {
+        beforeMatchingNameRegex(OPAQUE_MULTIVERSIONING, "OpaqueMultiversioning");
+    }
+
     public static final String ADD_P_OF = COMPOSITE_PREFIX + "ADD_P_OF" + POSTFIX;
     static {
         String regex = START + "addP_" + IS_REPLACED + MID + ".*" + END;

test/hotspot/jtreg/compiler/loopopts/superword/TestMultiversionRemoveUselessSlowLoop.java

@@ -0,0 +1,124 @@
+/*
+ * Copyright (c) 2025, 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.
+ */
+
+package compiler.loopopts.superword;
+
+import compiler.lib.ir_framework.*;
+
+/*
+ * @test
+ * @bug 8350756
+ * @summary Test case where the multiversion fast_loop disappears, and we should
+ *          constant fold the multiversion_if, to remove the slow_loop.
+ * @library /test/lib /
+ * @run driver compiler.loopopts.superword.TestMultiversionRemoveUselessSlowLoop
+ */
+
+public class TestMultiversionRemoveUselessSlowLoop {
+
+    public static void main(String[] args) {
+        TestFramework framework = new TestFramework(TestMultiversionRemoveUselessSlowLoop.class);
+        // No traps means we cannot use the predicates version for SuperWord / AutoVectorization,
+        // and instead use multiversioning directly.
+        framework.addFlags("-XX:-TieredCompilation", "-XX:PerMethodTrapLimit=0");
+        framework.setDefaultWarmup(0); // simulates Xcomp
+        framework.start();
+    }
+
+    public static final int SIZE = 20;
+    public static final int[] a = new int[SIZE];
+    public static final int[] b = new int[SIZE];
+    public static final int SIZE2 = 10_000;
+    public static final int[] a2 = new int[SIZE2];
+    public static final int[] b2 = new int[SIZE2];
+
+    @Test
+    @IR(counts = {"pre .* multiversion_fast",  "= 2", // regular pre-main-post for both loops
+                  "main .* multiversion_fast", "= 2",
+                  "post .* multiversion_fast", "= 2",
+                  "multiversion_delayed_slow", "= 2", // both have the delayed slow_loop
+                  "multiversion",              "= 8", // nothing unexpected
+                  IRNode.OPAQUE_MULTIVERSIONING, "= 2"}, // Both multiversion_if are still here
+        applyIfPlatform = {"64-bit", "true"},
+        applyIfCPUFeatureOr = {"sse4.1", "true", "asimd", "true"},
+        phase = CompilePhase.PHASEIDEALLOOP1)
+    @IR(counts = {"pre .* multiversion_fast",  "= 2",
+                  "main .* multiversion_fast", "= 1", // The first main loop is fully unrolled
+                  "post .* multiversion_fast", "= 3", // the second loop is vectorized, and has a vectorized post loop
+                  "multiversion_delayed_slow", "= 1", // As a consequence of the first main loop being removed, we constant fold the multiversion_if
+                  "multiversion",              "= 7", // nothing unexpected
+                  IRNode.OPAQUE_MULTIVERSIONING, "= 1"}, // The multiversion_if of the first loop was constant folded, because the main loop disappeared.
+        applyIfPlatform = {"64-bit", "true"},
+        applyIfCPUFeatureOr = {"sse4.1", "true", "asimd", "true"},
+        phase = CompilePhase.PHASEIDEALLOOP_ITERATIONS)
+    @IR(counts = {"pre .* multiversion_fast.*", ">= 1", // In some cases, the pre loop of the first loop also disappears because it only has a single iteration
+                  "pre .* multiversion_fast.*", "<= 2", // but not in other cases the pre loop of the first loop remains.
+                  "main .* multiversion_fast", "= 1",
+                  "post .* multiversion_fast", "= 3",
+                  "multiversion_delayed_slow", "= 0", // The second loop's multiversion_if was also not used, so it is constant folded after loop opts.
+                  "multiversion",              ">= 5", // nothing unexpected
+                  "multiversion",              "<= 6", // nothing unexpected
+                  IRNode.OPAQUE_MULTIVERSIONING, "= 0"}, // After loop-opts, we also constant fold the multiversion_if of the second loop, as it is unused.
+        applyIfPlatform = {"64-bit", "true"},
+        applyIfCPUFeatureOr = {"sse4.1", "true", "asimd", "true"},
+        phase = CompilePhase.PRINT_IDEAL)
+    public static void testIR() {
+        // This loop is short, and the multiversion_fast main loop eventuall is fully unrolled.
+        for (int i = 0; i < SIZE; i++) {
+            a[i] = b[i];
+        }
+        // We take this second loop with a larger limit so that loop opts keeps going once the loop
+        // above is fully optimized. It also gives us a reference where the main loop of the
+        // multiverion fast_loop does not disappear.
+        for (int i = 0; i < SIZE2; i++) {
+            a2[i] = b2[i];
+        }
+    }
+
+    static long instanceCount;
+    static int iFld;
+    static int iFld1;
+
+    // The inner loop is Multiversioned, then PreMainPost and Unroll.
+    // Eventually, both the fast and slow loops (pre main and post) disappear,
+    // and leave us with a simple if-diamond using the multiversion_if.
+    //
+    // Verification code in PhaseIdealLoop::conditional_move finds this diamond
+    // and expects a Bool but gets an OpaqueMultiversioning instead.
+    //
+    // If we let the multiversion_if constant fold soon after the main fast loop
+    // disappears, then this issue does not occur any more.
+    @Test
+    public static void testCrash() {
+        boolean b2 = true;
+        for (int i = 0; i < 1000; i++) {
+            for (int i21 = 82; i21 > 9; --i21) {
+                if (b2)
+                    break;
+                iFld1 = iFld;
+                b2 = true;
+            }
+            instanceCount = iFld1;
+        }
+    }
+}