8369134: [lworld] We should preserve the markWord for all GCs

src/hotspot/share/oops/oop.inline.hpp

@@ -94,9 +94,11 @@ void oopDesc::init_mark() {
   set_mark(prototype_mark());
 }
 
-// This is for parallel gc, which doesn't always have the klass.
-// markWord::must_be_preserved preserves the original prototype header bits for EnableValhalla,
-// I don't know why serial gc doesn't work the same.
+// This is specifically for Parallel GC. The other collectors need klass()->prototype_header()
+// even without using Compact Object Headers. The issue is that this operation is unsafe
+// using Parallel, as there are multiple concurrent GC workers that could access it.
+// In practice, this has lead to relatively frequent crashes.
+// More work needs to be done in the future to consolidate reinit_mark with init_mark.
 void oopDesc::reinit_mark() {
   if (UseCompactObjectHeaders) {
     set_mark(klass()->prototype_header());

test/hotspot/jtreg/runtime/valhalla/inlinetypes/LarvalMarkWordTest.java

@@ -0,0 +1,125 @@
+/*
+ * 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.
+ */
+
+/**
+ * @test
+ * @summary ensure larval bit is kept by Serial
+ * @library /test/lib /
+ * @requires vm.flagless
+ * @enablePreview
+ * @modules java.base/jdk.internal.misc
+            java.management
+ * @build jdk.test.whitebox.WhiteBox
+ * @run driver jdk.test.lib.helpers.ClassFileInstaller jdk.test.whitebox.WhiteBox
+ * @run main/othervm -XX:+UseSerialGC -Xlog:gc*=info
+                     -Xbootclasspath/a:. -XX:+UnlockDiagnosticVMOptions -XX:+WhiteBoxAPI
+                     runtime.valhalla.inlinetypes.LarvalMarkWordTest
+ */
+
+/**
+ * @test
+ * @summary ensure larval bit is kept by Parallel
+ * @library /test/lib /
+ * @requires vm.flagless
+ * @enablePreview
+ * @modules java.base/jdk.internal.misc
+            java.management
+ * @build jdk.test.whitebox.WhiteBox
+ * @run driver jdk.test.lib.helpers.ClassFileInstaller jdk.test.whitebox.WhiteBox
+ * @run main/othervm -XX:+UseParallelGC -Xlog:gc*=info
+                     -Xbootclasspath/a:. -XX:+UnlockDiagnosticVMOptions -XX:+WhiteBoxAPI
+                     runtime.valhalla.inlinetypes.LarvalMarkWordTest
+ */
+
+/**
+ * @test
+ * @summary ensure larval bit is kept by G1
+ * @library /test/lib /
+ * @requires vm.flagless
+ * @enablePreview
+ * @modules java.base/jdk.internal.misc
+            java.management
+ * @build jdk.test.whitebox.WhiteBox
+ * @run driver jdk.test.lib.helpers.ClassFileInstaller jdk.test.whitebox.WhiteBox
+ * @run main/othervm -XX:+UseG1GC -Xlog:gc*=info
+                     -Xbootclasspath/a:. -XX:+UnlockDiagnosticVMOptions -XX:+WhiteBoxAPI
+                     runtime.valhalla.inlinetypes.LarvalMarkWordTest
+ */
+
+/**
+ * @test
+ * @summary ensure larval bit is kept by Z
+ * @library /test/lib /
+ * @requires vm.flagless
+ * @enablePreview
+ * @modules java.base/jdk.internal.misc
+            java.management
+ * @build jdk.test.whitebox.WhiteBox
+ * @run driver jdk.test.lib.helpers.ClassFileInstaller jdk.test.whitebox.WhiteBox
+ * @run main/othervm -XX:+UseZGC -Xlog:gc*=info
+                     -Xbootclasspath/a:. -XX:+UnlockDiagnosticVMOptions -XX:+WhiteBoxAPI
+                     runtime.valhalla.inlinetypes.LarvalMarkWordTest
+ */
+package runtime.valhalla.inlinetypes;
+
+import jdk.test.whitebox.WhiteBox;
+
+import jdk.internal.misc.Unsafe;
+
+public class LarvalMarkWordTest {
+  private static final Unsafe UNSAFE = Unsafe.getUnsafe();
+  private static final WhiteBox WB = WhiteBox.getWhiteBox();
+
+  // We want to ensure that the larval bit in the markWord is preserved
+  // accross GC events.
+  public static void main(String[] args) throws ReflectiveOperationException {
+    // Take a value class with one field and record its field offset.
+    MyValue val = new MyValue(0);
+    long offset = UNSAFE.objectFieldOffset(val.getClass().getDeclaredField("x"));
+    // Create a new larval value object.
+    val = UNSAFE.makePrivateBuffer(val);
+    // Write a new value to it.
+    UNSAFE.putLong(val, offset, 19);
+
+    // Explicitly cause a full GC.
+    // This will cause relocations to happen, which is what we want to test.
+    WB.fullGC();
+
+    // At this point, the GC's preservation should have seen that val was
+    // a larval, and if it did anything with val, preserved the markWord.
+    // There is a VM assertion that will fail here if the bit is lost.
+    val = UNSAFE.finishPrivateBuffer(val);
+    // Sanity check just in case.
+    if (val.x != 19) {
+      throw new IllegalStateException("something went wrong during larval construction");
+    }
+  }
+
+  private static value class MyValue {
+    private final int x;
+
+    private MyValue(int x) {
+      this.x = x;
+    }
+  }
+}