8335059: Consider renaming ClassLoaderData::keep_alive

Reviewed-by: dholmes, stefank

src/hotspot/cpu/aarch64/gc/shared/barrierSetAssembler_aarch64.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2018, 2024, 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
@@ -375,7 +375,7 @@ void BarrierSetAssembler::c2i_entry_barrier(MacroAssembler* masm) {
   __ load_method_holder_cld(rscratch1, rmethod);
 
   // Is it a strong CLD?
-  __ ldrw(rscratch2, Address(rscratch1, ClassLoaderData::keep_alive_offset()));
+  __ ldrw(rscratch2, Address(rscratch1, ClassLoaderData::keep_alive_ref_count_offset()));
   __ cbnz(rscratch2, method_live);
 
   // Is it a weak but alive CLD?

src/hotspot/cpu/ppc/gc/shared/barrierSetAssembler_ppc.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2018, 2024, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2018, 2022 SAP SE. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
@@ -238,7 +238,7 @@ void BarrierSetAssembler::c2i_entry_barrier(MacroAssembler *masm, Register tmp1,
   __ ld(tmp1_class_loader_data, in_bytes(InstanceKlass::class_loader_data_offset()), tmp1);
 
   // Fast path: If class loader is strong, the holder cannot be unloaded.
-  __ lwz(tmp2, in_bytes(ClassLoaderData::keep_alive_offset()), tmp1_class_loader_data);
+  __ lwz(tmp2, in_bytes(ClassLoaderData::keep_alive_ref_count_offset()), tmp1_class_loader_data);
   __ cmpdi(CCR0, tmp2, 0);
   __ bne(CCR0, skip_barrier);
 

src/hotspot/cpu/riscv/gc/shared/barrierSetAssembler_riscv.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2018, 2024, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2020, 2023, Huawei Technologies Co., Ltd. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
@@ -326,7 +326,7 @@ void BarrierSetAssembler::c2i_entry_barrier(MacroAssembler* masm) {
   __ load_method_holder_cld(t0, xmethod);
 
   // Is it a strong CLD?
-  __ lwu(t1, Address(t0, ClassLoaderData::keep_alive_offset()));
+  __ lwu(t1, Address(t0, ClassLoaderData::keep_alive_ref_count_offset()));
   __ bnez(t1, method_live);
 
   // Is it a weak but alive CLD?

src/hotspot/cpu/x86/gc/shared/barrierSetAssembler_x86.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2018, 2024, 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
@@ -423,7 +423,7 @@ void BarrierSetAssembler::c2i_entry_barrier(MacroAssembler* masm) {
   __ load_method_holder_cld(tmp1, rbx);
 
    // Is it a strong CLD?
-  __ cmpl(Address(tmp1, ClassLoaderData::keep_alive_offset()), 0);
+  __ cmpl(Address(tmp1, ClassLoaderData::keep_alive_ref_count_offset()), 0);
   __ jcc(Assembler::greater, method_live);
 
    // Is it a weak but alive CLD?

src/hotspot/share/classfile/classLoaderData.cpp

@@ -141,7 +141,7 @@ ClassLoaderData::ClassLoaderData(Handle h_class_loader, bool has_class_mirror_ho
   // A non-strong hidden class loader data doesn't have anything to keep
   // it from being unloaded during parsing of the non-strong hidden class.
   // The null-class-loader should always be kept alive.
-  _keep_alive((has_class_mirror_holder || h_class_loader.is_null()) ? 1 : 0),
+  _keep_alive_ref_count((has_class_mirror_holder || h_class_loader.is_null()) ? 1 : 0),
   _claim(0),
   _handles(),
   _klasses(nullptr), _packages(nullptr), _modules(nullptr), _unnamed_module(nullptr), _dictionary(nullptr),
@@ -345,26 +345,26 @@ void ClassLoaderData::demote_strong_roots() {
 // while the class is being parsed, and if the class appears on the module fixup list.
 // Due to the uniqueness that no other class shares the hidden class' name or
 // ClassLoaderData, no other non-GC thread has knowledge of the hidden class while
-// it is being defined, therefore _keep_alive is not volatile or atomic.
-void ClassLoaderData::inc_keep_alive() {
+// it is being defined, therefore _keep_alive_ref_count is not volatile or atomic.
+void ClassLoaderData::inc_keep_alive_ref_count() {
   if (has_class_mirror_holder()) {
-    assert(_keep_alive > 0, "Invalid keep alive increment count");
-    _keep_alive++;
+    assert(_keep_alive_ref_count > 0, "Invalid keep alive increment count");
+    _keep_alive_ref_count++;
   }
 }
 
-void ClassLoaderData::dec_keep_alive() {
+void ClassLoaderData::dec_keep_alive_ref_count() {
   if (has_class_mirror_holder()) {
-    assert(_keep_alive > 0, "Invalid keep alive decrement count");
-    if (_keep_alive == 1) {
-      // When the keep_alive counter is 1, the oop handle area is a strong root,
+    assert(_keep_alive_ref_count > 0, "Invalid keep alive decrement count");
+    if (_keep_alive_ref_count == 1) {
+      // When the keep_alive_ref_count counter is 1, the oop handle area is a strong root,
       // acting as input to the GC tracing. Such strong roots are part of the
       // snapshot-at-the-beginning, and can not just be pulled out from the
       // system when concurrent GCs are running at the same time, without
       // invoking the right barriers.
       demote_strong_roots();
     }
-    _keep_alive--;
+    _keep_alive_ref_count--;
   }
 }
 
@@ -680,8 +680,8 @@ oop ClassLoaderData::holder_no_keepalive() const {
 
 // Unloading support
 bool ClassLoaderData::is_alive() const {
-  bool alive = keep_alive()         // null class loader and incomplete non-strong hidden class.
-      || (_holder.peek() != nullptr);  // and not cleaned by the GC weak handle processing.
+  bool alive = (_keep_alive_ref_count > 0) // null class loader and incomplete non-strong hidden class.
+      || (_holder.peek() != nullptr);      // and not cleaned by the GC weak handle processing.
 
   return alive;
 }
@@ -1009,7 +1009,7 @@ void ClassLoaderData::print_on(outputStream* out) const {
   out->print_cr(" - unloading           %s", _unloading ? "true" : "false");
   out->print_cr(" - class mirror holder %s", _has_class_mirror_holder ? "true" : "false");
   out->print_cr(" - modified oops       %s", _modified_oops ? "true" : "false");
-  out->print_cr(" - keep alive          %d", _keep_alive);
+  out->print_cr(" - _keep_alive_ref_count %d", _keep_alive_ref_count);
   out->print   (" - claim               ");
   switch(_claim) {
     case _claim_none:                       out->print_cr("none"); break;

src/hotspot/share/classfile/classLoaderData.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2012, 2024, 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
@@ -125,10 +125,10 @@ class ClassLoaderData : public CHeapObj<mtClass> {
   // Remembered sets support for the oops in the class loader data.
   bool _modified_oops;     // Card Table Equivalent
 
-  int _keep_alive;         // if this CLD is kept alive.
-                           // Used for non-strong hidden classes and the
-                           // boot class loader. _keep_alive does not need to be volatile or
-                           // atomic since there is one unique CLD per non-strong hidden class.
+  int _keep_alive_ref_count; // if this CLD should not be considered eligible for unloading.
+                             // Used for non-strong hidden classes and the
+                             // boot class loader. _keep_alive_ref_count does not need to be volatile or
+                             // atomic since there is one unique CLD per non-strong hidden class.
 
   volatile int _claim; // non-zero if claimed, for example during GC traces.
                        // To avoid applying oop closure more than once.
@@ -205,12 +205,14 @@ private:
   bool has_modified_oops()               { return _modified_oops; }
 
   oop holder_no_keepalive() const;
+  // Resolving the holder keeps this CLD alive for the current GC cycle.
   oop holder() const;
+  void keep_alive() const { (void)holder(); }
 
   void classes_do(void f(Klass* const));
 
  private:
-  bool keep_alive() const       { return _keep_alive > 0; }
+  int keep_alive_ref_count() const { return _keep_alive_ref_count; }
 
   void loaded_classes_do(KlassClosure* klass_closure);
   void classes_do(void f(InstanceKlass*));
@@ -302,9 +304,10 @@ private:
     return _unloading;
   }
 
-  // Used to refcount a non-strong hidden class's s CLD in order to indicate their aliveness.
-  void inc_keep_alive();
-  void dec_keep_alive();
+  // Used to refcount a non-strong hidden class's CLD in order to force its aliveness during
+  // loading, when gc tracing may not find this CLD alive through the holder.
+  void inc_keep_alive_ref_count();
+  void dec_keep_alive_ref_count();
 
   void initialize_holder(Handle holder);
 
@@ -335,8 +338,8 @@ private:
   bool modules_defined() { return (_modules != nullptr); }
 
   // Offsets
-  static ByteSize holder_offset()     { return byte_offset_of(ClassLoaderData, _holder); }
-  static ByteSize keep_alive_offset() { return byte_offset_of(ClassLoaderData, _keep_alive); }
+  static ByteSize holder_offset() { return byte_offset_of(ClassLoaderData, _holder); }
+  static ByteSize keep_alive_ref_count_offset() { return byte_offset_of(ClassLoaderData, _keep_alive_ref_count); }
 
   // Loaded class dictionary
   Dictionary* dictionary() const { return _dictionary; }

src/hotspot/share/classfile/classLoaderDataGraph.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2018, 2024, 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
@@ -201,7 +201,7 @@ void ClassLoaderDataGraph::cld_do(CLDClosure* cl) {
 void ClassLoaderDataGraph::roots_cld_do(CLDClosure* strong, CLDClosure* weak) {
   assert_is_safepoint_or_gc();
   for (ClassLoaderData* cld = Atomic::load_acquire(&_head);  cld != nullptr; cld = cld->next()) {
-    CLDClosure* closure = cld->keep_alive() ? strong : weak;
+    CLDClosure* closure = (cld->keep_alive_ref_count() > 0) ? strong : weak;
     if (closure != nullptr) {
       closure->do_cld(cld);
     }
@@ -309,8 +309,7 @@ void ClassLoaderDataGraph::modules_do_keepalive(void f(ModuleEntry*)) {
   assert_locked_or_safepoint(Module_lock);
   ClassLoaderDataGraphIterator iter;
   while (ClassLoaderData* cld = iter.get_next()) {
-    // Keep the holder alive.
-    (void)cld->holder();
+    cld->keep_alive();
     cld->modules_do(f);
   }
 }
@@ -334,8 +333,7 @@ void ClassLoaderDataGraph::packages_do(void f(PackageEntry*)) {
 void ClassLoaderDataGraph::loaded_classes_do_keepalive(KlassClosure* klass_closure) {
   ClassLoaderDataGraphIterator iter;
   while (ClassLoaderData* cld = iter.get_next()) {
-    // Keep the holder alive.
-    (void)cld->holder();
+    cld->keep_alive();
     cld->loaded_classes_do(klass_closure);
   }
 }

src/hotspot/share/classfile/javaClasses.cpp

@@ -963,7 +963,7 @@ void java_lang_Class::set_mirror_module_field(JavaThread* current, Klass* k, Han
       // Keep list of classes needing java.base module fixup
       if (!ModuleEntryTable::javabase_defined()) {
         assert(k->java_mirror() != nullptr, "Class's mirror is null");
-        k->class_loader_data()->inc_keep_alive();
+        k->class_loader_data()->inc_keep_alive_ref_count();
         assert(fixup_module_field_list() != nullptr, "fixup_module_field_list not initialized");
         fixup_module_field_list()->push(k);
       } else {

src/hotspot/share/classfile/moduleEntry.cpp

@@ -705,7 +705,7 @@ void ModuleEntryTable::patch_javabase_entries(JavaThread* current, Handle module
     {
       java_lang_Class::fixup_module_field(k, module_handle);
     }
-    k->class_loader_data()->dec_keep_alive();
+    k->class_loader_data()->dec_keep_alive_ref_count();
   }
 
   delete java_lang_Class::fixup_module_field_list();

src/hotspot/share/prims/jvm.cpp

@@ -1032,7 +1032,7 @@ static jclass jvm_lookup_define_class(jclass lookup, const char *name,
     // The hidden class loader data has been artificially been kept alive to
     // this point. The mirror and any instances of this class have to keep
     // it alive afterwards.
-    ik->class_loader_data()->dec_keep_alive();
+    ik->class_loader_data()->dec_keep_alive_ref_count();
 
     if (is_nestmate && log_is_enabled(Debug, class, nestmates)) {
       ModuleEntry* module = ik->module();