1 /*
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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5 * This code is free software; you can redistribute it and/or modify it
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11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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24
25 #include "cds/aotReferenceObjSupport.hpp"
26 #include "cds/heapShared.hpp"
27 #include "classfile/javaClasses.hpp"
28 #include "classfile/symbolTable.hpp"
29 #include "classfile/systemDictionary.hpp"
30 #include "classfile/vmSymbols.hpp"
31 #include "logging/log.hpp"
32 #include "memory/resourceArea.hpp"
33 #include "memory/universe.hpp"
34 #include "oops/oop.inline.hpp"
35 #include "oops/oopHandle.inline.hpp"
36 #include "runtime/fieldDescriptor.inline.hpp"
37 #include "runtime/javaCalls.hpp"
38 #include "utilities/hashTable.hpp"
39
40 // Handling of java.lang.ref.Reference objects in the AOT cache
41 // ============================================================
42 //
43 // When AOTArtifactFinder finds an oop which is a instance of java.lang.ref.Reference:
44 //
45 // - We check if the oop is eligible to be stored in the AOT cache. If not, the AOT cache
46 // creation fails -- see AOTReferenceObjSupport::check_if_ref_obj()
47 //
48 // - Otherwise, we store the oop into the AOT cache, but we unconditionally reset its
49 // "next" and "discovered" fields to null. Otherwise, if AOTArtifactFinder follows these
50 // fields, it may found unrelated objects that we don't intend to cache.
51 //
52 // Eligibility
53 // ===========
54 //
55 // [1] A reference that does not require special clean up (i.e., Reference::queue == ReferenceQueue.NULL_QUEUE)
56 // is eligible.
57 //
58 // [2] A reference that REQUIRE specials clean up (i.e., Reference::queue != ReferenceQueue.NULL_QUEUE)
59 // is eligible ONLY if its referent is not null.
60 //
61 // As of this version, the only oops in group [2] that can be found by AOTArtifactFinder are
62 // the keys used by ReferencedKeyMap in the implementation of MethodType::internTable.
63 // stabilize_cached_reference_objects() ensures that all keys found by AOTArtifactFinder are eligible.
64 //
65 // The purpose of the error check in check_if_ref_obj() is to guard against changes in the JDK core
66 // libs that might introduce new types of oops in group [2] into the AOT cache.
67 //
68 // Reasons for the eligibility restrictions
69 // ========================================
70 //
71 // Reference handling is complex. In this version, we implement only enough functionality to support
72 // the use of Weak/Soft references used by java.lang.invoke.
73 //
74 // We intend to evolve the implementation in the future by
75 // -- implementing more assemblySetup() operations for other use cases, and/or
76 // -- relaxing the eligibility restrictions.
77 //
78 //
79 // null referents for group [1]
80 // ============================
81 //
82 // Any cached reference R1 of group [1] is allowed to have a null referent.
83 // This can happen in the following situations:
84 // (a) R1.clear() was called by Java code during the assembly phase.
85 // (b) The referent has been collected, and R1 is in the "pending" state.
86 // In case (b), the "next" and "discovered" fields of the cached copy of R1 will
87 // be set to null. During the production run:
88 // - It would appear to the Java program as if immediately during VM start-up, the referent
89 // was collected and ReferenceThread completed processing of R1.
90 // - It would appear to the GC as if immediately during VM start-up, the Java program called
91 // R1.clear().
92
93 #if INCLUDE_CDS_JAVA_HEAP
94
95 class KeepAliveObjectsTable : public HashTable<oop, bool,
96 36137, // prime number
97 AnyObj::C_HEAP,
98 mtClassShared,
99 HeapShared::oop_hash> {};
100
101 static KeepAliveObjectsTable* _keep_alive_objs_table;
102 static OopHandle _keep_alive_objs_array;
103 static OopHandle _null_queue;
104
105 bool AOTReferenceObjSupport::is_enabled() {
106 // For simplicity, AOTReferenceObjSupport is enabled only when dumping method handles.
107 // Otherwise we won't see Reference objects in the AOT cache. Let's be conservative now.
108 return CDSConfig::is_dumping_method_handles();
109 }
110
111 void AOTReferenceObjSupport::initialize(TRAPS) {
112 if (!AOTReferenceObjSupport::is_enabled()) {
113 return;
114 }
115
116 TempNewSymbol class_name = SymbolTable::new_symbol("java/lang/ref/ReferenceQueue");
117 Klass* k = SystemDictionary::resolve_or_fail(class_name, true, CHECK);
118 InstanceKlass* ik = InstanceKlass::cast(k);
119 ik->initialize(CHECK);
120
121 TempNewSymbol field_name = SymbolTable::new_symbol("NULL_QUEUE");
122 fieldDescriptor fd;
123 bool found = ik->find_local_field(field_name, vmSymbols::referencequeue_signature(), &fd);
124 precond(found);
125 precond(fd.is_static());
126
127 _null_queue = OopHandle(Universe::vm_global(), ik->java_mirror()->obj_field(fd.offset()));
128 }
129
130 // Ensure that all group [2] references found by AOTArtifactFinder are eligible.
131 void AOTReferenceObjSupport::stabilize_cached_reference_objects(TRAPS) {
132 if (AOTReferenceObjSupport::is_enabled()) {
133 // This assert means that the MethodType and MethodTypeForm tables won't be
134 // updated concurrently, so we can remove GC'ed entries ...
135 assert(CDSConfig::allow_only_single_java_thread(), "Required");
136
137 {
138 TempNewSymbol method_name = SymbolTable::new_symbol("assemblySetup");
139 JavaValue result(T_VOID);
140 JavaCalls::call_static(&result, vmClasses::MethodType_klass(),
141 method_name,
142 vmSymbols::void_method_signature(),
143 CHECK);
144 }
145
146 {
147 Symbol* cds_name = vmSymbols::jdk_internal_misc_CDS();
148 Klass* cds_klass = SystemDictionary::resolve_or_fail(cds_name, true /*throw error*/, CHECK);
149 TempNewSymbol method_name = SymbolTable::new_symbol("getKeepAliveObjects");
150 TempNewSymbol method_sig = SymbolTable::new_symbol("()[Ljava/lang/Object;");
151 JavaValue result(T_OBJECT);
152 JavaCalls::call_static(&result, cds_klass, method_name, method_sig, CHECK);
153
154 _keep_alive_objs_array = OopHandle(Universe::vm_global(), result.get_oop());
155 }
156
157 // Trigger a GC to prune eligible referents that were not kept alive
158 Universe::heap()->collect(GCCause::_java_lang_system_gc);
159 }
160 }
161
162 void AOTReferenceObjSupport::init_keep_alive_objs_table() {
163 assert_at_safepoint(); // _keep_alive_objs_table uses raw oops
164 oop a = _keep_alive_objs_array.resolve();
165 if (a != nullptr) {
166 precond(a->is_objArray());
167 precond(AOTReferenceObjSupport::is_enabled());
168 objArrayOop array = objArrayOop(a);
169
170 _keep_alive_objs_table = new (mtClass)KeepAliveObjectsTable();
171 for (int i = 0; i < array->length(); i++) {
172 oop obj = array->obj_at(i);
173 _keep_alive_objs_table->put(obj, true); // The array may have duplicated entries but that's OK.
174 }
175 }
176 }
177
178 // Returns true IFF obj is an instance of java.lang.ref.Reference. If so, perform extra eligibility checks.
179 bool AOTReferenceObjSupport::check_if_ref_obj(oop obj) {
180 assert_at_safepoint(); // _keep_alive_objs_table uses raw oops
181
182 if (obj->klass()->is_subclass_of(vmClasses::Reference_klass())) {
183 // The following check works only if the java.lang.ref.Reference$ReferenceHandler thread
184 // is not running.
185 //
186 // This code is called on every object found by AOTArtifactFinder. When dumping the
187 // preimage archive, AOTArtifactFinder should not find any Reference objects.
188 precond(!CDSConfig::is_dumping_preimage_static_archive());
189 precond(CDSConfig::allow_only_single_java_thread());
190
191 precond(AOTReferenceObjSupport::is_enabled());
192 precond(JavaClasses::is_supported_for_archiving(obj));
193 precond(_keep_alive_objs_table != nullptr);
194
195 // GC needs to know about this load, It will keep referent alive until the current safepoint ends.
196 oop referent = HeapAccess<ON_UNKNOWN_OOP_REF>::oop_load_at(obj, java_lang_ref_Reference::referent_offset());
197
198 oop queue = obj->obj_field(java_lang_ref_Reference::queue_offset());
199 oop next = java_lang_ref_Reference::next(obj);
200 oop discovered = java_lang_ref_Reference::discovered(obj);
201 bool needs_special_cleanup = (queue != _null_queue.resolve());
202
203 // If you see the errors below, you probably modified the implementation of java.lang.invoke.
204 // Please check the comments at the top of this file.
205 if (needs_special_cleanup && (referent == nullptr || !_keep_alive_objs_table->contains(referent))) {
206 ResourceMark rm;
207
208 log_error(aot, heap)("Cannot archive reference object " PTR_FORMAT " of class %s",
209 p2i(obj), obj->klass()->external_name());
210 log_error(aot, heap)("referent = " PTR_FORMAT
211 ", queue = " PTR_FORMAT
212 ", next = " PTR_FORMAT
213 ", discovered = " PTR_FORMAT,
214 p2i(referent), p2i(queue), p2i(next), p2i(discovered));
215 log_error(aot, heap)("This object requires special clean up as its queue is not ReferenceQueue::N" "ULL ("
216 PTR_FORMAT ")", p2i(_null_queue.resolve()));
217 log_error(aot, heap)("%s", (referent == nullptr) ?
218 "referent cannot be null" : "referent is not registered with CDS.keepAlive()");
219 HeapShared::debug_trace();
220 AOTMetaspace::unrecoverable_writing_error();
221 }
222
223 if (log_is_enabled(Info, aot, ref)) {
224 ResourceMark rm;
225 log_info(aot, ref)("Reference obj:"
226 " r=" PTR_FORMAT
227 " q=" PTR_FORMAT
228 " n=" PTR_FORMAT
229 " d=" PTR_FORMAT
230 " %s",
231 p2i(referent),
232 p2i(queue),
233 p2i(next),
234 p2i(discovered),
235 obj->klass()->external_name());
236 }
237 return true;
238 } else {
239 return false;
240 }
241 }
242
243 bool AOTReferenceObjSupport::skip_field(int field_offset) {
244 return (field_offset == java_lang_ref_Reference::next_offset() ||
245 field_offset == java_lang_ref_Reference::discovered_offset());
246 }
247
248 #endif // INCLUDE_CDS_JAVA_HEAP