2 * Copyright (C) 2009 The Android Open Source Project
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
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17 #ifndef ART_RUNTIME_INDIRECT_REFERENCE_TABLE_H_
18 #define ART_RUNTIME_INDIRECT_REFERENCE_TABLE_H_
26 #include "base/bit_utils.h"
27 #include "base/logging.h"
28 #include "base/mutex.h"
31 #include "object_callbacks.h"
33 #include "read_barrier_option.h"
45 // Maintain a table of indirect references. Used for local/global JNI references.
47 // The table contains object references, where the strong (local/global) references are part of the
48 // GC root set (but not the weak global references). When an object is added we return an
49 // IndirectRef that is not a valid pointer but can be used to find the original value in O(1) time.
50 // Conversions to and from indirect references are performed on upcalls and downcalls, so they need
53 // To be efficient for JNI local variable storage, we need to provide operations that allow us to
54 // operate on segments of the table, where segments are pushed and popped as if on a stack. For
55 // example, deletion of an entry should only succeed if it appears in the current segment, and we
56 // want to be able to strip off the current segment quickly when a method returns. Additions to the
57 // table must be made in the current segment even if space is available in an earlier area.
59 // A new segment is created when we call into native code from interpreted code, or when we handle
60 // the JNI PushLocalFrame function.
62 // The GC must be able to scan the entire table quickly.
64 // In summary, these must be very fast:
65 // - adding or removing a segment
66 // - adding references to a new segment
67 // - converting an indirect reference back to an Object
68 // These can be a little slower, but must still be pretty quick:
69 // - adding references to a "mature" segment
70 // - removing individual references
71 // - scanning the entire table straight through
73 // If there's more than one segment, we don't guarantee that the table will fill completely before
74 // we fail due to lack of space. We do ensure that the current segment will pack tightly, which
75 // should satisfy JNI requirements (e.g. EnsureLocalCapacity).
77 // Only SynchronizedGet is synchronized.
79 // Indirect reference definition. This must be interchangeable with JNI's jobject, and it's
80 // convenient to let null be null, so we use void*.
82 // We need a (potentially) large table index and a 2-bit reference type (global, local, weak
83 // global). We also reserve some bits to be used to detect stale indirect references: we put a
84 // serial number in the extra bits, and keep a copy of the serial number in the table. This requires
85 // more memory and additional memory accesses on add/get, but is moving-GC safe. It will catch
86 // additional problems, e.g.: create iref1 for obj, delete iref1, create iref2 for same obj,
87 // lookup iref1. A pattern based on object bits will miss this.
88 typedef void* IndirectRef;
90 // Indirect reference kind, used as the two low bits of IndirectRef.
92 // For convenience these match up with enum jobjectRefType from jni.h.
93 enum IndirectRefKind {
94 kHandleScopeOrInvalid = 0, // <<stack indirect reference table or invalid reference>>
95 kLocal = 1, // <<local reference>>
96 kGlobal = 2, // <<global reference>>
97 kWeakGlobal = 3, // <<weak global reference>>
98 kLastKind = kWeakGlobal
100 std::ostream& operator<<(std::ostream& os, const IndirectRefKind& rhs);
101 const char* GetIndirectRefKindString(const IndirectRefKind& kind);
105 // For the global reference table, the expected common operations are adding a new entry and
106 // removing a recently-added entry (usually the most-recently-added entry). For JNI local
107 // references, the common operations are adding a new entry and removing an entire table segment.
109 // If we delete entries from the middle of the list, we will be left with "holes". We track the
110 // number of holes so that, when adding new elements, we can quickly decide to do a trivial append
111 // or go slot-hunting.
113 // When the top-most entry is removed, any holes immediately below it are also removed. Thus,
114 // deletion of an entry may reduce "top_index" by more than one.
116 // To get the desired behavior for JNI locals, we need to know the bottom and top of the current
117 // "segment". The top is managed internally, and the bottom is passed in as a function argument.
118 // When we call a native method or push a local frame, the current top index gets pushed on, and
119 // serves as the new bottom. When we pop a frame off, the value from the stack becomes the new top
120 // index, and the value stored in the previous frame becomes the new bottom.
122 // Holes are being locally cached for the segment. Otherwise we'd have to pass bottom index and
123 // number of holes, which restricts us to 16 bits for the top index. The value is cached within the
124 // table. To avoid code in generated JNI transitions, which implicitly form segments, the code for
125 // adding and removing references needs to detect the change of a segment. Helper fields are used
126 // for this detection.
128 // Common alternative implementation: make IndirectRef a pointer to the actual reference slot.
129 // Instead of getting a table and doing a lookup, the lookup can be done instantly. Operations like
130 // determining the type and deleting the reference are more expensive because the table must be
131 // hunted for (i.e. you have to do a pointer comparison to see which table it's in), you can't move
132 // the table when expanding it (so realloc() is out), and tricks like serial number checking to
133 // detect stale references aren't possible (though we may be able to get similar benefits with other
136 // TODO: consider a "lastDeleteIndex" for quick hole-filling when an add immediately follows a
137 // delete; must invalidate after segment pop might be worth only using it for JNI globals.
139 // TODO: may want completely different add/remove algorithms for global and local refs to improve
140 // performance. A large circular buffer might reduce the amortized cost of adding global
143 // The state of the current segment. We only store the index. Splitting it for index and hole
144 // count restricts the range too much.
145 struct IRTSegmentState {
149 // Use as initial value for "cookie", and when table has only one segment.
150 static constexpr IRTSegmentState kIRTFirstSegment = { 0 };
152 // Try to choose kIRTPrevCount so that sizeof(IrtEntry) is a power of 2.
153 // Contains multiple entries but only one active one, this helps us detect use after free errors
154 // since the serial stored in the indirect ref wont match.
155 static constexpr size_t kIRTPrevCount = kIsDebugBuild ? 7 : 3;
159 void Add(ObjPtr<mirror::Object> obj) REQUIRES_SHARED(Locks::mutator_lock_);
161 GcRoot<mirror::Object>* GetReference() {
162 DCHECK_LT(serial_, kIRTPrevCount);
163 return &references_[serial_];
166 const GcRoot<mirror::Object>* GetReference() const {
167 DCHECK_LT(serial_, kIRTPrevCount);
168 return &references_[serial_];
171 uint32_t GetSerial() const {
175 void SetReference(ObjPtr<mirror::Object> obj) REQUIRES_SHARED(Locks::mutator_lock_);
179 GcRoot<mirror::Object> references_[kIRTPrevCount];
181 static_assert(sizeof(IrtEntry) == (1 + kIRTPrevCount) * sizeof(uint32_t),
182 "Unexpected sizeof(IrtEntry)");
183 static_assert(IsPowerOfTwo(sizeof(IrtEntry)), "Unexpected sizeof(IrtEntry)");
187 IrtIterator(IrtEntry* table, size_t i, size_t capacity) REQUIRES_SHARED(Locks::mutator_lock_)
188 : table_(table), i_(i), capacity_(capacity) {
191 IrtIterator& operator++() REQUIRES_SHARED(Locks::mutator_lock_) {
196 GcRoot<mirror::Object>* operator*() REQUIRES_SHARED(Locks::mutator_lock_) {
197 // This does not have a read barrier as this is used to visit roots.
198 return table_[i_].GetReference();
201 bool equals(const IrtIterator& rhs) const {
202 return (i_ == rhs.i_ && table_ == rhs.table_);
206 IrtEntry* const table_;
208 const size_t capacity_;
211 bool inline operator==(const IrtIterator& lhs, const IrtIterator& rhs) {
212 return lhs.equals(rhs);
215 bool inline operator!=(const IrtIterator& lhs, const IrtIterator& rhs) {
216 return !lhs.equals(rhs);
219 class IndirectReferenceTable {
221 enum class ResizableCapacity {
226 // WARNING: Construction of the IndirectReferenceTable may fail.
227 // error_msg must not be null. If error_msg is set by the constructor, then
228 // construction has failed and the IndirectReferenceTable will be in an
229 // invalid state. Use IsValid to check whether the object is in an invalid
231 IndirectReferenceTable(size_t max_count,
232 IndirectRefKind kind,
233 ResizableCapacity resizable,
234 std::string* error_msg);
236 ~IndirectReferenceTable();
239 * Checks whether construction of the IndirectReferenceTable succeeded.
241 * This object must only be used if IsValid() returns true. It is safe to
242 * call IsValid from multiple threads without locking or other explicit
245 bool IsValid() const;
247 // Add a new entry. "obj" must be a valid non-null object reference. This function will
248 // abort if the table is full (max entries reached, or expansion failed).
249 IndirectRef Add(IRTSegmentState previous_state, ObjPtr<mirror::Object> obj)
250 REQUIRES_SHARED(Locks::mutator_lock_);
252 // Given an IndirectRef in the table, return the Object it refers to.
254 // This function may abort under error conditions.
255 template<ReadBarrierOption kReadBarrierOption = kWithReadBarrier>
256 ObjPtr<mirror::Object> Get(IndirectRef iref) const REQUIRES_SHARED(Locks::mutator_lock_)
259 // Synchronized get which reads a reference, acquiring a lock if necessary.
260 template<ReadBarrierOption kReadBarrierOption = kWithReadBarrier>
261 ObjPtr<mirror::Object> SynchronizedGet(IndirectRef iref) const
262 REQUIRES_SHARED(Locks::mutator_lock_) {
263 return Get<kReadBarrierOption>(iref);
266 // Updates an existing indirect reference to point to a new object.
267 void Update(IndirectRef iref, ObjPtr<mirror::Object> obj) REQUIRES_SHARED(Locks::mutator_lock_);
269 // Remove an existing entry.
271 // If the entry is not between the current top index and the bottom index
272 // specified by the cookie, we don't remove anything. This is the behavior
273 // required by JNI's DeleteLocalRef function.
275 // Returns "false" if nothing was removed.
276 bool Remove(IRTSegmentState previous_state, IndirectRef iref);
278 void AssertEmpty() REQUIRES_SHARED(Locks::mutator_lock_);
280 void Dump(std::ostream& os) const REQUIRES_SHARED(Locks::mutator_lock_);
282 // Return the #of entries in the entire table. This includes holes, and
283 // so may be larger than the actual number of "live" entries.
284 size_t Capacity() const {
285 return segment_state_.top_index;
288 // Note IrtIterator does not have a read barrier as it's used to visit roots.
289 IrtIterator begin() {
290 return IrtIterator(table_, 0, Capacity());
294 return IrtIterator(table_, Capacity(), Capacity());
297 void VisitRoots(RootVisitor* visitor, const RootInfo& root_info)
298 REQUIRES_SHARED(Locks::mutator_lock_);
300 IRTSegmentState GetSegmentState() const {
301 return segment_state_;
304 void SetSegmentState(IRTSegmentState new_state);
306 static Offset SegmentStateOffset(size_t pointer_size ATTRIBUTE_UNUSED) {
307 // Note: Currently segment_state_ is at offset 0. We're testing the expected value in
308 // jni_internal_test to make sure it stays correct. It is not OFFSETOF_MEMBER, as that
309 // is not pointer-size-safe.
313 // Release pages past the end of the table that may have previously held references.
314 void Trim() REQUIRES_SHARED(Locks::mutator_lock_);
316 // Determine what kind of indirect reference this is. Opposite of EncodeIndirectRefKind.
317 ALWAYS_INLINE static inline IndirectRefKind GetIndirectRefKind(IndirectRef iref) {
318 return DecodeIndirectRefKind(reinterpret_cast<uintptr_t>(iref));
322 static constexpr size_t kSerialBits = MinimumBitsToStore(kIRTPrevCount);
323 static constexpr uint32_t kShiftedSerialMask = (1u << kSerialBits) - 1;
325 static constexpr size_t kKindBits = MinimumBitsToStore(
326 static_cast<uint32_t>(IndirectRefKind::kLastKind));
327 static constexpr uint32_t kKindMask = (1u << kKindBits) - 1;
329 static constexpr uintptr_t EncodeIndex(uint32_t table_index) {
330 static_assert(sizeof(IndirectRef) == sizeof(uintptr_t), "Unexpected IndirectRef size");
331 DCHECK_LE(MinimumBitsToStore(table_index), BitSizeOf<uintptr_t>() - kSerialBits - kKindBits);
332 return (static_cast<uintptr_t>(table_index) << kKindBits << kSerialBits);
334 static constexpr uint32_t DecodeIndex(uintptr_t uref) {
335 return static_cast<uint32_t>((uref >> kKindBits) >> kSerialBits);
338 static constexpr uintptr_t EncodeIndirectRefKind(IndirectRefKind kind) {
339 return static_cast<uintptr_t>(kind);
341 static constexpr IndirectRefKind DecodeIndirectRefKind(uintptr_t uref) {
342 return static_cast<IndirectRefKind>(uref & kKindMask);
345 static constexpr uintptr_t EncodeSerial(uint32_t serial) {
346 DCHECK_LE(MinimumBitsToStore(serial), kSerialBits);
347 return serial << kKindBits;
349 static constexpr uint32_t DecodeSerial(uintptr_t uref) {
350 return static_cast<uint32_t>(uref >> kKindBits) & kShiftedSerialMask;
353 constexpr uintptr_t EncodeIndirectRef(uint32_t table_index, uint32_t serial) const {
354 DCHECK_LT(table_index, max_entries_);
355 return EncodeIndex(table_index) | EncodeSerial(serial) | EncodeIndirectRefKind(kind_);
358 static void ConstexprChecks();
360 // Extract the table index from an indirect reference.
361 ALWAYS_INLINE static uint32_t ExtractIndex(IndirectRef iref) {
362 return DecodeIndex(reinterpret_cast<uintptr_t>(iref));
365 IndirectRef ToIndirectRef(uint32_t table_index) const {
366 DCHECK_LT(table_index, max_entries_);
367 uint32_t serial = table_[table_index].GetSerial();
368 return reinterpret_cast<IndirectRef>(EncodeIndirectRef(table_index, serial));
371 // Resize the backing table. Currently must be larger than the current size.
372 bool Resize(size_t new_size, std::string* error_msg);
374 void RecoverHoles(IRTSegmentState from);
376 // Abort if check_jni is not enabled. Otherwise, just log as an error.
377 static void AbortIfNoCheckJNI(const std::string& msg);
379 /* extra debugging checks */
380 bool GetChecked(IndirectRef) const REQUIRES_SHARED(Locks::mutator_lock_);
381 bool CheckEntry(const char*, IndirectRef, uint32_t) const;
383 /// semi-public - read/write by jni down calls.
384 IRTSegmentState segment_state_;
386 // Mem map where we store the indirect refs.
387 std::unique_ptr<MemMap> table_mem_map_;
388 // bottom of the stack. Do not directly access the object references
389 // in this as they are roots. Use Get() that has a read barrier.
391 // bit mask, ORed into all irefs.
392 const IndirectRefKind kind_;
394 // max #of entries allowed (modulo resizing).
397 // Some values to retain old behavior with holes. Description of the algorithm is in the .cc
399 // TODO: Consider other data structures for compact tables, e.g., free lists.
400 size_t current_num_holes_;
401 IRTSegmentState last_known_previous_state_;
403 // Whether the table's capacity may be resized. As there are no locks used, it is the caller's
404 // responsibility to ensure thread-safety.
405 ResizableCapacity resizable_;
410 #endif // ART_RUNTIME_INDIRECT_REFERENCE_TABLE_H_