8382332: Large regression (~50%) in SPECjbb2015 when using JFR after JEP 500

src/hotspot/share/classfile/javaClasses.cpp

@@ -3588,6 +3588,7 @@ int java_lang_reflect_Field::_modifiers_offset;
 int java_lang_reflect_Field::_trusted_final_offset;
 int java_lang_reflect_Field::_signature_offset;
 int java_lang_reflect_Field::_annotations_offset;
+int java_lang_reflect_Field::slot_mask = (1 << 17) - 1;
 
 #define FIELD_FIELDS_DO(macro) \
   macro(_clazz_offset,     k, vmSymbols::clazz_name(),     class_signature,  false); \
@@ -3645,10 +3646,13 @@ void java_lang_reflect_Field::set_type(oop field, oop value) {
 }
 
 int java_lang_reflect_Field::slot(oop reflect) {
-  return reflect->int_field(_slot_offset);
+  // We use a mask because JFR is encoding
+  // information in the most significant bits.
+  return reflect->int_field(_slot_offset) & slot_mask;
 }
 
 void java_lang_reflect_Field::set_slot(oop reflect, int value) {
+  assert(value <= slot_mask, "slot value is too big");
   reflect->int_field_put(_slot_offset, value);
 }
 

src/hotspot/share/classfile/javaClasses.hpp

@@ -835,6 +835,8 @@ class java_lang_reflect_Field : public java_lang_reflect_AccessibleObject {
   static int _signature_offset;
   static int _annotations_offset;
 
+  static int slot_mask;
+
   static void compute_offsets();
 
  public:

src/hotspot/share/include/jvm.h

@@ -329,6 +329,9 @@ JVM_FindScopedValueBindings(JNIEnv *env, jclass threadClass);
 JNIEXPORT jlong JNICALL
 JVM_GetNextThreadIdOffset(JNIEnv *env, jclass threadClass);
 
+JNIEXPORT jlong JNICALL
+JVM_GetJfrEpochGenerationOffset(JNIEnv* env, jclass clazz);
+
 /*
  * jdk.internal.vm.Continuation
  */

src/hotspot/share/jfr/jfr.cpp

@@ -28,6 +28,7 @@
 #include "jfr/jni/jfrJavaSupport.hpp"
 #include "jfr/leakprofiler/leakProfiler.hpp"
 #include "jfr/recorder/checkpoint/jfrCheckpointManager.hpp"
+#include "jfr/recorder/checkpoint/types/traceid/jfrTraceIdEpoch.hpp"
 #include "jfr/recorder/jfrRecorder.hpp"
 #include "jfr/recorder/repository/jfrEmergencyDump.hpp"
 #include "jfr/recorder/repository/jfrRepository.hpp"
@@ -181,6 +182,10 @@ void Jfr::on_report_java_out_of_memory() {
   }
 }
 
+int64_t Jfr::epoch_generation_offset() {
+  return reinterpret_cast<int64_t>(JfrTraceIdEpoch::epoch_generation_address());
+}
+
 #if INCLUDE_CDS
 void Jfr::on_restoration(const Klass* k, JavaThread* jt) {
   assert(k != nullptr, "invariant");

src/hotspot/share/jfr/jfr.hpp

@@ -80,6 +80,7 @@ class Jfr : AllStatic {
   static bool has_sample_request(JavaThread* jt);
   static void check_and_process_sample_request(JavaThread* jt);
   static void on_report_java_out_of_memory();
+  static int64_t epoch_generation_offset();
   CDS_ONLY(static void on_restoration(const Klass* k, JavaThread* jt);)
 };
 

src/hotspot/share/prims/jvm.cpp

@@ -2890,6 +2890,14 @@ JVM_ENTRY(jlong, JVM_GetNextThreadIdOffset(JNIEnv* env, jclass threadClass))
   return ThreadIdentifier::unsafe_offset();
 JVM_END
 
+JVM_ENTRY(jlong, JVM_GetJfrEpochGenerationOffset(JNIEnv* env, jclass clazz))
+#if INCLUDE_JFR
+  return Jfr::epoch_generation_offset();
+#else
+  return 0;
+#endif
+JVM_END
+
 JVM_ENTRY(void, JVM_Interrupt(JNIEnv* env, jobject jthread))
   ThreadsListHandle tlh(thread);
   JavaThread* receiver = nullptr;

src/java.base/share/classes/java/lang/reflect/Field.java

@@ -1537,8 +1537,7 @@ private void postSetFinal(Class<?> caller, boolean unreflect) {
             VM.initialErr().println(sb);
         }
 
-        // record JFR event
-        FinalFieldMutationEvent.offer(getDeclaringClass(), getName());
+        FinalFieldMutationEvent.offer(root, root.slot);
     }
 
     /**

src/java.base/share/classes/jdk/internal/event/FinalFieldMutationEvent.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2025, 2026, 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
@@ -24,22 +24,51 @@
  */
 package jdk.internal.event;
 
+import java.lang.reflect.Field;
+
 public class FinalFieldMutationEvent extends Event {
+    /**
+     * We throttle FinalFieldMutation JFR events by generating only one event per field
+     * during each JFR epoch. This is accomplished by encoding the JFR epoch generation (15 bits)
+     * into the most significant bits of the root Field object's slot field, using a CAS operation.
+     *
+     * An event is emitted if enabled, the epoch encoded in the slot field differs from the current epoch,
+     * and a thread successfully updates the slot field's epoch to the current epoch.
+     *
+     * Borrowing the most significant 15 bits from the slot integer field relies on the premise
+     * that the maximum number of fields encoded in the class file format is 65,535.
+     * In addition, with 17 bits available for slot encoding, there remains considerable capacity
+     * for VM-injected fields, allowing for a theoretical maximum of 131,072 slots.
+     */
+    static final long SLOT_OFFSET = JfrEpochGeneration.getFieldOffset(Field.class, "slot");
+
     public Class<?> declaringClass;
     public String fieldName;
 
     /**
      * Commit a FinalFieldMutationEvent if enabled.
      */
-    public static void offer(Class<?> declaringClass, String fieldName) {
-        if (enabled()) {
+    public static void offer(Field root, int slot) {
+        if (shouldEmit(root, slot)) {
             var event = new FinalFieldMutationEvent();
-            event.declaringClass = declaringClass;
-            event.fieldName = fieldName;
+            event.declaringClass = root.getDeclaringClass();
+            event.fieldName = root.getName();
             event.commit();
         }
     }
 
+    private static boolean shouldEmit(Field root, int slot) {
+        if (enabled()) {
+            int currentEpoch = JfrEpochGeneration.getCurrentEpoch();
+            if (currentEpoch != (slot >> JfrEpochGeneration.EPOCH_SHIFT_INT)) {
+                int newRootFieldSlotEpoch =
+                    currentEpoch << JfrEpochGeneration.EPOCH_SHIFT_INT | (slot & JfrEpochGeneration.VALUE_MASK_INT);
+                return JfrEpochGeneration.compareAndExchange(root, SLOT_OFFSET, slot, newRootFieldSlotEpoch);
+            }
+        }
+        return false;
+    }
+
     public static boolean enabled() {
         // Generated by JFR
         return false;

src/java.base/share/classes/jdk/internal/event/JfrEpochGeneration.java

@@ -0,0 +1,87 @@
+/*
+ * Copyright (c) 2026, 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.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * 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 jdk.internal.event;
+
+import java.lang.reflect.Field;
+import jdk.internal.misc.Unsafe;
+
+class JfrEpochGeneration {
+    private static final Unsafe U;
+    private static final long EPOCH_GENERATION_OFFSET;
+
+    /**
+     * The JFR epoch generation is the range [1-32767],i.e. 15 bits.
+     *
+     * These constants are useful when operating on a field where
+     * the most significant bits are used to store a JFR epoch value.
+     */
+    static final int EPOCH_SHIFT_INT = 17;
+    static final int VALUE_MASK_INT = (1 << EPOCH_SHIFT_INT) - 1;
+    static final int EPOCH_SHIFT_LONG = 49;
+    static final long VALUE_MASK_LONG = (1L << EPOCH_SHIFT_LONG) - 1;
+
+    static {
+        U = Unsafe.getUnsafe();
+        EPOCH_GENERATION_OFFSET = getJfrEpochGenerationOffset();
+    }
+
+    private static native long getJfrEpochGenerationOffset();
+
+    /**
+     * The JFR epoch generation is the range [1-32767],i.e. 15 bits.
+     */
+    static short getCurrentEpoch() {
+        return U.getShort(EPOCH_GENERATION_OFFSET);
+    }
+
+    /**
+     * Get the address of a field to be used in cas operations.
+     */
+    static long getFieldOffset(Class<?> clazz, String fieldName) {
+        return U.objectFieldOffset(clazz, fieldName);
+    }
+
+    static long getFieldOffset(Field field) {
+        return U.objectFieldOffset(field);
+    }
+
+    static long getStaticFielddOffset(Field field) {
+        return U.staticFieldOffset(field);
+    }
+
+    /**
+     * An epoch generation can be stored atomically into a field,
+     * perhaps together with some other value (use the shift and mask constants).
+     * Can be useful sometimes when committing events, to ensure only
+     * the setter will send an event, so as to limit events per epoch.
+     */
+    static boolean compareAndExchange(Object o, long offset, int c, int v) {
+        return U.compareAndExchangeInt(o, offset, c, v) == c;
+    }
+
+    static boolean compareAndExchange(Object o, long offset, long c, long v) {
+        return U.compareAndExchangeLong(o, offset, c, v) == c;
+    }
+}

src/java.base/share/native/libjava/JfrEpochGeneration.c

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2026, 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.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * 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.
+ */
+
+#include "jni.h"
+#include "jvm.h"
+
+#include "jdk_internal_event_JfrEpochGeneration.h"
+
+JNIEXPORT jlong JNICALL
+Java_jdk_internal_event_JfrEpochGeneration_getJfrEpochGenerationOffset(JNIEnv* env, jclass jfrEpochGen) {
+  return JVM_GetJfrEpochGenerationOffset(env, jfrEpochGen);
+}