8357671: JFR: Remove JfrTraceIdEpoch synchronizing

Reviewed-by: egahlin

src/hotspot/share/jfr/recorder/checkpoint/jfrCheckpointManager.cpp

@@ -497,15 +497,10 @@ typedef CompositeOperation<MutexedWriteOperation, ReleaseOperation> WriteRelease
 typedef VirtualThreadLocalCheckpointWriteOp<JfrCheckpointManager::Buffer> VirtualThreadLocalCheckpointOperation;
 typedef MutexedWriteOp<VirtualThreadLocalCheckpointOperation> VirtualThreadLocalWriteOperation;
 
-void JfrCheckpointManager::begin_epoch_shift() {
-  assert(SafepointSynchronize::is_at_safepoint(), "invariant");
-  JfrTraceIdEpoch::begin_epoch_shift();
-}
-
-void JfrCheckpointManager::end_epoch_shift() {
+void JfrCheckpointManager::shift_epoch() {
   assert(SafepointSynchronize::is_at_safepoint(), "invariant");
   DEBUG_ONLY(const u1 current_epoch = JfrTraceIdEpoch::current();)
-  JfrTraceIdEpoch::end_epoch_shift();
+  JfrTraceIdEpoch::shift_epoch();
   assert(current_epoch != JfrTraceIdEpoch::current(), "invariant");
   JfrStringPool::on_epoch_shift();
 }

src/hotspot/share/jfr/recorder/checkpoint/jfrCheckpointManager.hpp

@@ -96,8 +96,7 @@ class JfrCheckpointManager : public JfrCHeapObj {
   void clear_type_set();
   void write_type_set();
 
-  void begin_epoch_shift();
-  void end_epoch_shift();
+  void shift_epoch();
 
   static void on_unloading_classes();
   void on_rotation();

src/hotspot/share/jfr/recorder/checkpoint/types/traceid/jfrTraceIdEpoch.cpp

@@ -31,7 +31,7 @@
 /*
  * The epoch generation is the range [1-32767].
  *
- * When the epoch value is stored in a thread object,
+ * When the epoch value is stored in a vthread object,
  * the most significant bit of the u2 is used to denote
  * thread exclusion, i.e  1 << 15 == 32768 denotes exclusion.
 */
@@ -39,32 +39,17 @@ u2 JfrTraceIdEpoch::_generation = 0;
 JfrSignal JfrTraceIdEpoch::_tag_state;
 bool JfrTraceIdEpoch::_method_tracer_state = false;
 bool JfrTraceIdEpoch::_epoch_state = false;
-bool JfrTraceIdEpoch::_synchronizing = false;
 
 static constexpr const u2 epoch_generation_overflow = excluded_bit;
 
-void JfrTraceIdEpoch::begin_epoch_shift() {
+void JfrTraceIdEpoch::shift_epoch() {
   assert(SafepointSynchronize::is_at_safepoint(), "invariant");
-  _synchronizing = true;
-  OrderAccess::fence();
-}
-
-void JfrTraceIdEpoch::end_epoch_shift() {
-  assert(SafepointSynchronize::is_at_safepoint(), "invariant");
-  assert(_synchronizing, "invariant");
   _epoch_state = !_epoch_state;
-  ++_generation;
-  if (epoch_generation_overflow == _generation) {
+  if (++_generation == epoch_generation_overflow) {
     _generation = 1;
   }
   assert(_generation != 0, "invariant");
   assert(_generation < epoch_generation_overflow, "invariant");
-  OrderAccess::storestore();
-  _synchronizing = false;
-}
-
-bool JfrTraceIdEpoch::is_synchronizing() {
-  return Atomic::load_acquire(&_synchronizing);
 }
 
 void JfrTraceIdEpoch::set_method_tracer_tag_state() {

src/hotspot/share/jfr/recorder/checkpoint/types/traceid/jfrTraceIdEpoch.hpp

@@ -26,7 +26,6 @@
 #define SHARE_JFR_RECORDER_CHECKPOINT_TYPES_TRACEID_JFRTRACEIDEPOCH_HPP
 
 #include "jfr/utilities/jfrSignal.hpp"
-#include "jfr/utilities/jfrTypes.hpp"
 #include "memory/allStatic.hpp"
 
 #define BIT                                  1
@@ -41,16 +40,17 @@
 #define EPOCH_0_METHOD_AND_CLASS_BITS        (METHOD_AND_CLASS_BITS << EPOCH_0_SHIFT)
 #define EPOCH_1_METHOD_AND_CLASS_BITS        (METHOD_AND_CLASS_BITS << EPOCH_1_SHIFT)
 
- // Epoch alternation on each rotation allow for concurrent tagging.
- // The epoch shift happens only during a safepoint.
- //
- // _synchronizing is a transition state, the purpose of which is to
- // have JavaThreads that run _thread_in_native (i.e. Compiler threads)
- // respect the current epoch shift in-progress during the safepoint.
- //
- // _changed_tag_state == true signals an incremental modification to artifact tagging
- // (klasses, methods, CLDs, etc), purpose of which is to trigger collection of artifacts.
- //
+/*
+ * An epoch shift or alternation on each rotation enables concurrent tagging.
+ * The epoch shift happens only during a safepoint.
+ *
+ *   _generation - mainly used with virtual threads, but also for the generational string pool in Java.
+ *   _tag_state  - signals an incremental modification to artifact tagging (klasses, methods, CLDs, etc)
+ *                   purpose of which is to trigger a collection of artifacts.
+ *   _method_tracer_state - a special notification state only used with method timing and tracing.
+ *   _epoch_state - the fundamental binary epoch state that shifts on each rotation during a safepoint.
+ */
+
 class JfrTraceIdEpoch : AllStatic {
   friend class JfrCheckpointManager;
  private:
@@ -58,10 +58,8 @@ class JfrTraceIdEpoch : AllStatic {
   static JfrSignal _tag_state;
   static bool _method_tracer_state;
   static bool _epoch_state;
-  static bool _synchronizing;
 
-  static void begin_epoch_shift();
-  static void end_epoch_shift();
+  static void shift_epoch();
 
  public:
   static bool epoch() {

src/hotspot/share/jfr/recorder/service/jfrRecorderService.cpp

@@ -484,13 +484,12 @@ void JfrRecorderService::invoke_safepoint_clear() {
 
 void JfrRecorderService::safepoint_clear() {
   assert(SafepointSynchronize::is_at_safepoint(), "invariant");
-  _checkpoint_manager.begin_epoch_shift();
   _storage.clear();
   _checkpoint_manager.notify_threads();
   _chunkwriter.set_time_stamp();
   JfrDeprecationManager::on_safepoint_clear();
   JfrStackTraceRepository::clear();
-  _checkpoint_manager.end_epoch_shift();
+  _checkpoint_manager.shift_epoch();
 }
 
 void JfrRecorderService::post_safepoint_clear() {
@@ -593,14 +592,13 @@ void JfrRecorderService::invoke_safepoint_write() {
 
 void JfrRecorderService::safepoint_write() {
   assert(SafepointSynchronize::is_at_safepoint(), "invariant");
-  _checkpoint_manager.begin_epoch_shift();
   JfrStackTraceRepository::clear_leak_profiler();
   _checkpoint_manager.on_rotation();
   _storage.write_at_safepoint();
   _chunkwriter.set_time_stamp();
   JfrDeprecationManager::on_safepoint_write();
   write_stacktrace(_stack_trace_repository, _chunkwriter, true);
-  _checkpoint_manager.end_epoch_shift();
+  _checkpoint_manager.shift_epoch();
 }
 
 void JfrRecorderService::post_safepoint_write() {

src/hotspot/share/jfr/recorder/stringpool/jfrStringPool.cpp

@@ -44,8 +44,6 @@
 static int generation_offset = invalid_offset;
 static jobject string_pool = nullptr;
 
-static unsigned short generation = 0;
-
 static bool setup_string_pool_offsets(TRAPS) {
   const char class_name[] = "jdk/jfr/internal/StringPool";
   Symbol* const k_sym = SymbolTable::new_symbol(class_name);
@@ -281,9 +279,8 @@ void JfrStringPool::register_full(BufferPtr buffer, Thread* thread) {
 
 void JfrStringPool::on_epoch_shift() {
   assert(SafepointSynchronize::is_at_safepoint(), "invariant");
-  assert(!JfrTraceIdEpoch::is_synchronizing(), "invariant");
   assert(string_pool != nullptr, "invariant");
   oop mirror = JfrJavaSupport::resolve_non_null(string_pool);
   assert(mirror != nullptr, "invariant");
-  mirror->short_field_put(generation_offset, generation++);
+  mirror->short_field_put(generation_offset, JfrTraceIdEpoch::epoch_generation());
 }

src/jdk.jfr/share/classes/jdk/jfr/internal/StringPool.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016, 2024, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2016, 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
@@ -51,7 +51,7 @@ public final class StringPool {
     private static int preCacheOld = 0;
     /* max size bytes */
     private static long currentSizeUTF16;
-    /* string pool generation (0-65535) set by the JVM on epoch shift. Not private to avoid being optimized away. */
+    /* The string pool epoch generation is the range [1-32767] set by the JVM on epoch shift. Not private to avoid being optimized away. */
     static short generation = 0;
 
     /* internalSid is a composite id [48-bit externalSid][16-bit generation]. */