8352587: C2 SuperWord: we must avoid Multiversioning for PeelMainPost loops

Reviewed-by: chagedorn, kvn

Author: eme64

src/hotspot/share/opto/loopTransform.cpp

@@ -3429,10 +3429,15 @@ bool IdealLoopTree::iteration_split_impl(PhaseIdealLoop *phase, Node_List &old_n
         return false;
       }
 
-      // We are going to add pre-loop and post-loop.
-      // But should we also multi-version for auto-vectorization speculative
-      // checks, i.e. fast and slow-paths?
-      phase->maybe_multiversion_for_auto_vectorization_runtime_checks(this, old_new);
+      if (!peel_only) {
+        // We are going to add pre-loop and post-loop (PreMainPost).
+        // But should we also multiversion for auto-vectorization speculative
+        // checks, i.e. fast and slow-paths?
+        // Note: Just PeelMainPost is not sufficient, as we could never find the
+        //       multiversion_if again from the main loop: we need a nicely structured
+        //       pre-loop, a peeled iteration cannot easily be parsed through.
+        phase->maybe_multiversion_for_auto_vectorization_runtime_checks(this, old_new);
+      }
 
       phase->insert_pre_post_loops(this, old_new, peel_only);
     }

src/hotspot/share/opto/loopnode.cpp

@@ -4471,6 +4471,10 @@ void PhaseIdealLoop::eliminate_useless_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());
+        } else {
+          // We could not find the multiversion_if, and would never find it again. Remove the
+          // multiversion marking for consistency.
+          head->set_no_multiversion();
         }
       }
     }

src/hotspot/share/opto/loopopts.cpp

@@ -4484,7 +4484,7 @@ PhaseIdealLoop::auto_vectorize(IdealLoopTree* lpt, VSharedData &vshared) {
   return AutoVectorizeStatus::Success;
 }
 
-// Just before insert_pre_post_loops, we can multi-version the loop:
+// Just before insert_pre_post_loops, we can multiversion the loop:
 //
 //              multiversion_if
 //               |       |

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

@@ -0,0 +1,58 @@
+/*
+ * 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;
+
+/*
+ * @test
+ * @bug 8352587
+ * @summary Test case where we used to Multiversion a PeelMainPost loop,
+ *          which is useless and triggered an assert later on.
+ * @run main compiler.loopopts.superword.TestMultiversionWithPeelMainPost
+ * @run main/othervm -XX:CompileCommand=compileonly,compiler.loopopts.superword.TestMultiversionWithPeelMainPost::test
+ *                   -XX:-TieredCompilation -Xcomp
+ *                   -XX:PerMethodTrapLimit=0
+ *                   compiler.loopopts.superword.TestMultiversionWithPeelMainPost
+ */
+
+public class TestMultiversionWithPeelMainPost {
+    static byte byArr[] = new byte[2];
+
+    public static void main(String[] strArr) {
+        test();
+    }
+
+    static void test() {
+        int x = 2;
+        int i = 4;
+        while (i > 0) {
+            for (int j = 5; j < 56; j++) {
+                byArr[1] = 3;
+            }
+            for (int j = 256; j > 3; j -= 2) {
+                x += 3;
+            }
+            i--;
+        }
+    }
+}

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

@@ -0,0 +1,67 @@
+/*
+ * 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 8352587
+ * @summary IR test to ensure that PeelMainPost cases does not get Multiversioned.
+ * @library /test/lib /
+ * @run driver compiler.loopopts.superword.TestPeelMainPostNoMultiversioning
+ */
+
+public class TestPeelMainPostNoMultiversioning {
+
+    public static void main(String[] args) {
+        TestFramework framework = new TestFramework(TestPeelMainPostNoMultiversioning.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 long value = 1;
+    public static long multiplicator = 3;
+
+    @Test
+    @IR(counts = {".*multiversion.*", "= 0"},
+        phase = CompilePhase.PHASEIDEALLOOP1)
+    @IR(counts = {".*multiversion.*", "= 0"},
+        phase = CompilePhase.PHASEIDEALLOOP_ITERATIONS)
+    @IR(counts = {".*multiversion.*", "= 0"},
+        phase = CompilePhase.PRINT_IDEAL)
+    // We are checking it for a few phases, just to make sure we are not seeing multiversioning
+    // at any point.
+    public static void test() {
+        long x = value;
+        long y = multiplicator;
+        for (int i = 0; i < 10_000; i++) {
+            x *= y; // No memory load/store -> PeelMainPost
+        }
+        value = x;
+    }
+}