1 /*
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24
25 #include "cds/aotClassInitializer.hpp"
26 #include "cds/aotLinkedClassBulkLoader.hpp"
27 #include "cds/archiveBuilder.hpp"
28 #include "cds/cdsConfig.hpp"
29 #include "cds/heapShared.hpp"
30 #include "cds/regeneratedClasses.hpp"
31 #include "classfile/symbolTable.hpp"
32 #include "classfile/systemDictionaryShared.hpp"
33 #include "classfile/vmSymbols.hpp"
34 #include "oops/instanceKlass.inline.hpp"
35 #include "oops/symbol.hpp"
36 #include "runtime/java.hpp"
37 #include "runtime/javaCalls.hpp"
38
39 DEBUG_ONLY(InstanceKlass* _aot_init_class = nullptr;)
40
41 bool AOTClassInitializer::can_archive_initialized_mirror(InstanceKlass* ik) {
42 assert(!ArchiveBuilder::is_active() || !ArchiveBuilder::current()->is_in_buffer_space(ik), "must be source klass");
43 if (!CDSConfig::is_initing_classes_at_dump_time()) {
44 return false;
45 }
46
47 if (RegeneratedClasses::is_regenerated_object(ik)) {
48 ik = RegeneratedClasses::get_original_object(ik);
49 }
50
51 if (!ik->is_initialized() && !ik->is_being_initialized()) {
52 return false;
53 }
54
55 // About "static field that may hold a different value" errors:
56 //
57 // Automatic selection for aot-inited classes
58 // ==========================================
59 //
60 // When CDSConfig::is_initing_classes_at_dump_time is enabled,
61 // AOTArtifactFinder::find_artifacts() finds the classes of all
62 // heap objects that are reachable from HeapShared::_run_time_special_subgraph,
63 // and mark these classes as aot-inited. This preserves the initialized
64 // mirrors of these classes, and their <clinit> methods are NOT executed
65 // at runtime. See aotArtifactFinder.hpp for more info.
66 //
67 // For example, with -XX:+AOTInvokeDynamicLinking, _run_time_special_subgraph
68 // will contain some DirectMethodHandle objects. As a result, the DirectMethodHandle
69 // class is automatically marked as aot-inited.
70 //
71 // When a class is aot-inited, its static fields are already set up
72 // by executing the <clinit> method at AOT assembly time. Later on
73 // in the production run, when the class would normally be
74 // initialized, the VM performs guarding and synchronization as if
75 // it were going to run the <clinit> again, but instead it simply
76 // observes that that class was aot-inited. The VM assumes that, if
77 // it were to run <clinit> again, it would get a semantically
78 // equivalent set of final field values, so it just adopts the
79 // existing field values (from AOT assembly) and skips the call to
80 // <clinit>. There may at that point be fixups performed by ad hoc
81 // code, if the VM recognizes a request in the library.
82 //
83 // It is true that this is not generally correct for all possible
84 // Java code. A <clinit> method might have a side effect beyond
85 // initializing the static fields. It might send an email somewhere
86 // noting the current time of day. In that case, such an email
87 // would have been sent during the AOT assembly phase, and the email
88 // would NOT be sent again during production. This is clearly NOT
89 // what a user would want, if this were a general purpose facility.
90 // But in fact it is only for certain well-behaved classes, which
91 // are known NOT to have such side effects. We know this because
92 // the optimization (of skipping <clinit> for aot-init classes) is
93 // only applied to classes fully defined by the JDK.
94 //
95 // (A day may come when we figure out how to gracefully extend this
96 // optimization to untrusted third parties, but it is not this day.)
97 //
98 // Manual selection
99 // ================
100 //
101 // There are important cases where one aot-init class has a side
102 // effect on another aot-class, a side effect which is not captured
103 // in any static field value in either class. The simplest example
104 // is class A forces the initialization of class B. In that case,
105 // we need to aot-init either both classes or neither. From looking
106 // at the JDK state after AOT assembly is done, it is hard to tell
107 // that A "touched" B and B might escape our notice. Another common
108 // example is A copying a field value from B. We don't know where A
109 // got the value, but it would be wrong to re-initialize B at
110 // startup, while keeping the snapshot of the old B value in A. In
111 // general, if we aot-init A, we need to aot-init every class B that
112 // somehow contributed to A's initial state, and every class C that
113 // was somehow side-effected by A's initialization. We say that the
114 // aot-init of A is "init-coupled" to those of B and C.
115 //
116 // So there are init-coupled classes that cannot be automatically discovered. For
117 // example, DirectMethodHandle::IMPL_NAMES points to MethodHandles::IMPL_NAMES,
118 // but the MethodHandles class is not automatically marked because there are
119 // no archived instances of the MethodHandles type.
120 //
121 // If we aot-initialize DirectMethodHandle, but allow MethodHandles to be
122 // initialized at runtime, MethodHandles::IMPL_NAMES will get a different
123 // value than DirectMethodHandle::IMPL_NAMES. This *may or may not* be a problem,
124 // but to ensure compatibility, we should try to preserve the identity equality
125 // of these two fields.
126 //
127 // To do that, we add MethodHandles to the indy_specs[] table below.
128 //
129 // Luckily we do not need to be all-knowing in order to choose which
130 // items to add to that table. We have tools to help detect couplings.
131 //
132 // Automatic validation
133 // ====================
134 //
135 // CDSHeapVerifier is used to detect potential problems with identity equality.
136 //
137 // A class B is assumed to be init-coupled to some aot-init class if
138 // B has a field which points to a live object X in the AOT heap.
139 // The live object X was created by some other class A which somehow
140 // used B's reference to X, perhaps with the help of an intermediate
141 // class Z. Or, B pulled the reference to X from some other class
142 // Y, and B obtained that reference from Y (or an intermediate Z).
143 // It is not certain how X got into the heap, nor whether B
144 // contributed it, but it is a good heuristic that B is init-coupled
145 // to X's class or some other aot-init class. In any case, B should
146 // be made an aot-init class as well, unless a manual inspection
147 // shows that would be a problem. If there is a problem, then the
148 // JDK code for B and/or X probably needs refactoring. If there is
149 // no problem, we add B to the list. Typically the same scan will
150 // find any other accomplices Y, Z, etc. One failure would be a
151 // class Q whose only initialization action is to scribble a special
152 // value into B, from which the value X is derived and then makes
153 // its way into the heap. In that case, the heuristic does not
154 // identify Q. It is (currently) a human responsibility, of JDK
155 // engineers, not to write such dirty JDK code, or to repair it if
156 // it crops up. Eventually we may have tools, or even a user mode
157 // with design rules and checks, that will vet our code base more
158 // automatically.
159 //
160 // To see how the tool detects the problem with MethodHandles::IMPL_NAMES:
161 //
162 // - Comment out all the lines in indy_specs[] except the {nullptr} line.
163 // - Rebuild the JDK
164 //
165 // Then run the following:
166 // java -XX:AOTMode=record -XX:AOTConfiguration=jc.aotconfig com.sun.tools.javac.Main
167 // java -XX:AOTMode=create -Xlog:aot -XX:AOTCache=jc.aot -XX:AOTConfiguration=jc.aotconfig
168 //
169 // You will see an error like this:
170 //
171 // Archive heap points to a static field that may hold a different value at runtime:
172 // Field: java/lang/invoke/MethodHandles::IMPL_NAMES
173 // Value: java.lang.invoke.MemberName$Factory
174 // {0x000000060e906ae8} - klass: 'java/lang/invoke/MemberName$Factory' - flags:
175 //
176 // - ---- fields (total size 2 words):
177 // --- trace begin ---
178 // [ 0] {0x000000060e8deeb0} java.lang.Class (java.lang.invoke.DirectMethodHandle::IMPL_NAMES)
179 // [ 1] {0x000000060e906ae8} java.lang.invoke.MemberName$Factory
180 // --- trace end ---
181 //
182 // Trouble-shooting
183 // ================
184 //
185 // If you see a "static field that may hold a different value" error, it's probably
186 // because you've made some changes in the JDK core libraries (most likely
187 // java.lang.invoke).
188 //
189 // - Did you add a new static field to a class that could be referenced by
190 // cached object instances of MethodType, MethodHandle, etc? You may need
191 // to add that class to indy_specs[].
192 // - Did you modify the <clinit> of the classes in java.lang.invoke such that
193 // a static field now points to an object that should not be cached (e.g.,
194 // a native resource such as a file descriptior, or a Thread)?
195 //
196 // Note that these potential problems only occur when one class gets
197 // the aot-init treatment, AND another class is init-coupled to it,
198 // AND the coupling is not detected. Currently there are a number
199 // classes that get the aot-init treatment, in java.lang.invoke
200 // because of invokedynamic. They are few enough for now to be
201 // manually tracked. There may be more in the future.
202
203 {
204 if (ik == vmClasses::Object_klass()) {
205 // everybody's favorite super
206 return true;
207 }
208 }
209
210 if (CDSConfig::is_dumping_method_handles()) {
211 // The minimal list of @AOTSafeClassInitializer was created with the help of CDSHeapVerifier.
212 // Also, some $Holder classes are needed. E.g., Invokers.<clinit> explicitly
213 // initializes Invokers$Holder. Since Invokers.<clinit> won't be executed
214 // at runtime, we need to make sure Invokers$Holder is also aot-inited.
215 if (ik->has_aot_safe_initializer()) {
216 return true;
217 }
218 }
219
220 #ifdef ASSERT
221 if (ik == _aot_init_class) {
222 return true;
223 }
224 #endif
225
226 return false;
227 }
228
229 void AOTClassInitializer::call_runtime_setup(JavaThread* current, InstanceKlass* ik) {
230 assert(ik->has_aot_initialized_mirror(), "sanity");
231 if (ik->is_runtime_setup_required()) {
232 if (log_is_enabled(Info, aot, init)) {
233 ResourceMark rm;
234 log_info(aot, init)("Calling %s::runtimeSetup()", ik->external_name());
235 }
236 JavaValue result(T_VOID);
237 JavaCalls::call_static(&result, ik,
238 vmSymbols::runtimeSetup(),
239 vmSymbols::void_method_signature(), current);
240 if (current->has_pending_exception()) {
241 // We cannot continue, as we might have cached instances of ik in the heap, but propagating the
242 // exception would cause ik to be in an error state.
243 AOTLinkedClassBulkLoader::exit_on_exception(current);
244 }
245 }
246 }
247
248 #ifdef ASSERT
249 void AOTClassInitializer::init_test_class(TRAPS) {
250 // -XX:AOTInitTestClass is used in regression tests for adding additional AOT-initialized classes
251 // and heap objects into the AOT cache. The tests must be carefully written to avoid including
252 // any classes that cannot be AOT-initialized.
253 //
254 // -XX:AOTInitTestClass is NOT a general mechanism for including user-defined objects into
255 // the AOT cache. Therefore, this option is NOT available in product JVM.
256 if (AOTInitTestClass != nullptr && CDSConfig::is_initing_classes_at_dump_time()) {
257 log_info(aot)("Debug build only: force initialization of AOTInitTestClass %s", AOTInitTestClass);
258 TempNewSymbol class_name = SymbolTable::new_symbol(AOTInitTestClass);
259 Handle app_loader(THREAD, SystemDictionary::java_system_loader());
260 Klass* k = SystemDictionary::resolve_or_null(class_name, app_loader, CHECK);
261 if (k == nullptr) {
262 vm_exit_during_initialization("AOTInitTestClass not found", AOTInitTestClass);
263 }
264 if (!k->is_instance_klass()) {
265 vm_exit_during_initialization("Invalid name for AOTInitTestClass", AOTInitTestClass);
266 }
267
268 _aot_init_class = InstanceKlass::cast(k);
269 _aot_init_class->initialize(CHECK);
270 }
271 }
272
273 bool AOTClassInitializer::has_test_class() {
274 return _aot_init_class != nullptr;
275 }
276 #endif