1 /* 2 * Copyright (C) 2009 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 #ifndef ART_RUNTIME_INDIRECT_REFERENCE_TABLE_H_ 18 #define ART_RUNTIME_INDIRECT_REFERENCE_TABLE_H_ 19 20 #include <stdint.h> 21 22 #include <iosfwd> 23 #include <string> 24 25 #include "base/logging.h" 26 #include "base/mutex.h" 27 #include "gc_root.h" 28 #include "mem_map.h" 29 #include "object_callbacks.h" 30 #include "offsets.h" 31 #include "read_barrier_option.h" 32 33 namespace art { 34 namespace mirror { 35 class Object; 36 } // namespace mirror 37 38 /* 39 * Maintain a table of indirect references. Used for local/global JNI 40 * references. 41 * 42 * The table contains object references that are part of the GC root set. 43 * When an object is added we return an IndirectRef that is not a valid 44 * pointer but can be used to find the original value in O(1) time. 45 * Conversions to and from indirect references are performed on upcalls 46 * and downcalls, so they need to be very fast. 47 * 48 * To be efficient for JNI local variable storage, we need to provide 49 * operations that allow us to operate on segments of the table, where 50 * segments are pushed and popped as if on a stack. For example, deletion 51 * of an entry should only succeed if it appears in the current segment, 52 * and we want to be able to strip off the current segment quickly when 53 * a method returns. Additions to the table must be made in the current 54 * segment even if space is available in an earlier area. 55 * 56 * A new segment is created when we call into native code from interpreted 57 * code, or when we handle the JNI PushLocalFrame function. 58 * 59 * The GC must be able to scan the entire table quickly. 60 * 61 * In summary, these must be very fast: 62 * - adding or removing a segment 63 * - adding references to a new segment 64 * - converting an indirect reference back to an Object 65 * These can be a little slower, but must still be pretty quick: 66 * - adding references to a "mature" segment 67 * - removing individual references 68 * - scanning the entire table straight through 69 * 70 * If there's more than one segment, we don't guarantee that the table 71 * will fill completely before we fail due to lack of space. We do ensure 72 * that the current segment will pack tightly, which should satisfy JNI 73 * requirements (e.g. EnsureLocalCapacity). 74 * 75 * To make everything fit nicely in 32-bit integers, the maximum size of 76 * the table is capped at 64K. 77 * 78 * Only SynchronizedGet is synchronized. 79 */ 80 81 /* 82 * Indirect reference definition. This must be interchangeable with JNI's 83 * jobject, and it's convenient to let null be null, so we use void*. 84 * 85 * We need a 16-bit table index and a 2-bit reference type (global, local, 86 * weak global). Real object pointers will have zeroes in the low 2 or 3 87 * bits (4- or 8-byte alignment), so it's useful to put the ref type 88 * in the low bits and reserve zero as an invalid value. 89 * 90 * The remaining 14 bits can be used to detect stale indirect references. 91 * For example, if objects don't move, we can use a hash of the original 92 * Object* to make sure the entry hasn't been re-used. (If the Object* 93 * we find there doesn't match because of heap movement, we could do a 94 * secondary check on the preserved hash value; this implies that creating 95 * a global/local ref queries the hash value and forces it to be saved.) 96 * 97 * A more rigorous approach would be to put a serial number in the extra 98 * bits, and keep a copy of the serial number in a parallel table. This is 99 * easier when objects can move, but requires 2x the memory and additional 100 * memory accesses on add/get. It will catch additional problems, e.g.: 101 * create iref1 for obj, delete iref1, create iref2 for same obj, lookup 102 * iref1. A pattern based on object bits will miss this. 103 */ 104 typedef void* IndirectRef; 105 106 // Magic failure values; must not pass Heap::ValidateObject() or Heap::IsHeapAddress(). 107 static mirror::Object* const kInvalidIndirectRefObject = reinterpret_cast<mirror::Object*>(0xdead4321); 108 static mirror::Object* const kClearedJniWeakGlobal = reinterpret_cast<mirror::Object*>(0xdead1234); 109 110 /* 111 * Indirect reference kind, used as the two low bits of IndirectRef. 112 * 113 * For convenience these match up with enum jobjectRefType from jni.h. 114 */ 115 enum IndirectRefKind { 116 kHandleScopeOrInvalid = 0, // <<stack indirect reference table or invalid reference>> 117 kLocal = 1, // <<local reference>> 118 kGlobal = 2, // <<global reference>> 119 kWeakGlobal = 3 // <<weak global reference>> 120 }; 121 std::ostream& operator<<(std::ostream& os, const IndirectRefKind& rhs); 122 123 /* 124 * Determine what kind of indirect reference this is. 125 */ 126 static inline IndirectRefKind GetIndirectRefKind(IndirectRef iref) { 127 return static_cast<IndirectRefKind>(reinterpret_cast<uintptr_t>(iref) & 0x03); 128 } 129 130 /* 131 * Extended debugging structure. We keep a parallel array of these, one 132 * per slot in the table. 133 */ 134 static const size_t kIRTPrevCount = 4; 135 struct IndirectRefSlot { 136 uint32_t serial; 137 const mirror::Object* previous[kIRTPrevCount]; 138 }; 139 140 /* use as initial value for "cookie", and when table has only one segment */ 141 static const uint32_t IRT_FIRST_SEGMENT = 0; 142 143 /* 144 * Table definition. 145 * 146 * For the global reference table, the expected common operations are 147 * adding a new entry and removing a recently-added entry (usually the 148 * most-recently-added entry). For JNI local references, the common 149 * operations are adding a new entry and removing an entire table segment. 150 * 151 * If "alloc_entries_" is not equal to "max_entries_", the table may expand 152 * when entries are added, which means the memory may move. If you want 153 * to keep pointers into "table" rather than offsets, you must use a 154 * fixed-size table. 155 * 156 * If we delete entries from the middle of the list, we will be left with 157 * "holes". We track the number of holes so that, when adding new elements, 158 * we can quickly decide to do a trivial append or go slot-hunting. 159 * 160 * When the top-most entry is removed, any holes immediately below it are 161 * also removed. Thus, deletion of an entry may reduce "topIndex" by more 162 * than one. 163 * 164 * To get the desired behavior for JNI locals, we need to know the bottom 165 * and top of the current "segment". The top is managed internally, and 166 * the bottom is passed in as a function argument. When we call a native method or 167 * push a local frame, the current top index gets pushed on, and serves 168 * as the new bottom. When we pop a frame off, the value from the stack 169 * becomes the new top index, and the value stored in the previous frame 170 * becomes the new bottom. 171 * 172 * To avoid having to re-scan the table after a pop, we want to push the 173 * number of holes in the table onto the stack. Because of our 64K-entry 174 * cap, we can combine the two into a single unsigned 32-bit value. 175 * Instead of a "bottom" argument we take a "cookie", which includes the 176 * bottom index and the count of holes below the bottom. 177 * 178 * Common alternative implementation: make IndirectRef a pointer to the 179 * actual reference slot. Instead of getting a table and doing a lookup, 180 * the lookup can be done instantly. Operations like determining the 181 * type and deleting the reference are more expensive because the table 182 * must be hunted for (i.e. you have to do a pointer comparison to see 183 * which table it's in), you can't move the table when expanding it (so 184 * realloc() is out), and tricks like serial number checking to detect 185 * stale references aren't possible (though we may be able to get similar 186 * benefits with other approaches). 187 * 188 * TODO: consider a "lastDeleteIndex" for quick hole-filling when an 189 * add immediately follows a delete; must invalidate after segment pop 190 * (which could increase the cost/complexity of method call/return). 191 * Might be worth only using it for JNI globals. 192 * 193 * TODO: may want completely different add/remove algorithms for global 194 * and local refs to improve performance. A large circular buffer might 195 * reduce the amortized cost of adding global references. 196 * 197 */ 198 union IRTSegmentState { 199 uint32_t all; 200 struct { 201 uint32_t topIndex:16; /* index of first unused entry */ 202 uint32_t numHoles:16; /* #of holes in entire table */ 203 } parts; 204 }; 205 206 class IrtIterator { 207 public: 208 explicit IrtIterator(GcRoot<mirror::Object>* table, size_t i, size_t capacity) 209 SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) 210 : table_(table), i_(i), capacity_(capacity) { 211 SkipNullsAndTombstones(); 212 } 213 214 IrtIterator& operator++() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) { 215 ++i_; 216 SkipNullsAndTombstones(); 217 return *this; 218 } 219 220 mirror::Object** operator*() { 221 // This does not have a read barrier as this is used to visit roots. 222 return table_[i_].AddressWithoutBarrier(); 223 } 224 225 bool equals(const IrtIterator& rhs) const { 226 return (i_ == rhs.i_ && table_ == rhs.table_); 227 } 228 229 private: 230 void SkipNullsAndTombstones() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) { 231 // We skip NULLs and tombstones. Clients don't want to see implementation details. 232 while (i_ < capacity_ && 233 (table_[i_].IsNull() || 234 table_[i_].Read<kWithoutReadBarrier>() == kClearedJniWeakGlobal)) { 235 ++i_; 236 } 237 } 238 239 GcRoot<mirror::Object>* const table_; 240 size_t i_; 241 size_t capacity_; 242 }; 243 244 bool inline operator==(const IrtIterator& lhs, const IrtIterator& rhs) { 245 return lhs.equals(rhs); 246 } 247 248 bool inline operator!=(const IrtIterator& lhs, const IrtIterator& rhs) { 249 return !lhs.equals(rhs); 250 } 251 252 class IndirectReferenceTable { 253 public: 254 IndirectReferenceTable(size_t initialCount, size_t maxCount, IndirectRefKind kind); 255 256 ~IndirectReferenceTable(); 257 258 /* 259 * Add a new entry. "obj" must be a valid non-NULL object reference. 260 * 261 * Returns NULL if the table is full (max entries reached, or alloc 262 * failed during expansion). 263 */ 264 IndirectRef Add(uint32_t cookie, mirror::Object* obj) 265 SHARED_LOCKS_REQUIRED(Locks::mutator_lock_); 266 267 /* 268 * Given an IndirectRef in the table, return the Object it refers to. 269 * 270 * Returns kInvalidIndirectRefObject if iref is invalid. 271 */ 272 template<ReadBarrierOption kReadBarrierOption = kWithReadBarrier> 273 mirror::Object* Get(IndirectRef iref) const SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) 274 ALWAYS_INLINE; 275 276 // Synchronized get which reads a reference, acquiring a lock if necessary. 277 template<ReadBarrierOption kReadBarrierOption = kWithReadBarrier> 278 mirror::Object* SynchronizedGet(Thread* /*self*/, ReaderWriterMutex* /*mutex*/, 279 IndirectRef iref) const 280 SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) { 281 return Get<kReadBarrierOption>(iref); 282 } 283 284 /* 285 * Remove an existing entry. 286 * 287 * If the entry is not between the current top index and the bottom index 288 * specified by the cookie, we don't remove anything. This is the behavior 289 * required by JNI's DeleteLocalRef function. 290 * 291 * Returns "false" if nothing was removed. 292 */ 293 bool Remove(uint32_t cookie, IndirectRef iref); 294 295 void AssertEmpty(); 296 297 void Dump(std::ostream& os) const SHARED_LOCKS_REQUIRED(Locks::mutator_lock_); 298 299 /* 300 * Return the #of entries in the entire table. This includes holes, and 301 * so may be larger than the actual number of "live" entries. 302 */ 303 size_t Capacity() const { 304 return segment_state_.parts.topIndex; 305 } 306 307 // Note IrtIterator does not have a read barrier as it's used to visit roots. 308 IrtIterator begin() { 309 return IrtIterator(table_, 0, Capacity()); 310 } 311 312 IrtIterator end() { 313 return IrtIterator(table_, Capacity(), Capacity()); 314 } 315 316 void VisitRoots(RootCallback* callback, void* arg, uint32_t tid, RootType root_type) 317 SHARED_LOCKS_REQUIRED(Locks::mutator_lock_); 318 319 uint32_t GetSegmentState() const { 320 return segment_state_.all; 321 } 322 323 void SetSegmentState(uint32_t new_state) { 324 segment_state_.all = new_state; 325 } 326 327 static Offset SegmentStateOffset() { 328 return Offset(OFFSETOF_MEMBER(IndirectReferenceTable, segment_state_)); 329 } 330 331 private: 332 /* 333 * Extract the table index from an indirect reference. 334 */ 335 static uint32_t ExtractIndex(IndirectRef iref) { 336 uintptr_t uref = reinterpret_cast<uintptr_t>(iref); 337 return (uref >> 2) & 0xffff; 338 } 339 340 /* 341 * The object pointer itself is subject to relocation in some GC 342 * implementations, so we shouldn't really be using it here. 343 */ 344 IndirectRef ToIndirectRef(uint32_t tableIndex) const { 345 DCHECK_LT(tableIndex, 65536U); 346 uint32_t serialChunk = slot_data_[tableIndex].serial; 347 uintptr_t uref = serialChunk << 20 | (tableIndex << 2) | kind_; 348 return reinterpret_cast<IndirectRef>(uref); 349 } 350 351 /* 352 * Update extended debug info when an entry is added. 353 * 354 * We advance the serial number, invalidating any outstanding references to 355 * this slot. 356 */ 357 void UpdateSlotAdd(const mirror::Object* obj, int slot) { 358 if (slot_data_ != NULL) { 359 IndirectRefSlot* pSlot = &slot_data_[slot]; 360 pSlot->serial++; 361 pSlot->previous[pSlot->serial % kIRTPrevCount] = obj; 362 } 363 } 364 365 // Abort if check_jni is not enabled. 366 static void AbortIfNoCheckJNI(); 367 368 /* extra debugging checks */ 369 bool GetChecked(IndirectRef) const; 370 bool CheckEntry(const char*, IndirectRef, int) const; 371 372 /* semi-public - read/write by jni down calls */ 373 IRTSegmentState segment_state_; 374 375 // Mem map where we store the indirect refs. 376 std::unique_ptr<MemMap> table_mem_map_; 377 // Mem map where we store the extended debugging info. 378 std::unique_ptr<MemMap> slot_mem_map_; 379 // bottom of the stack. Do not directly access the object references 380 // in this as they are roots. Use Get() that has a read barrier. 381 GcRoot<mirror::Object>* table_; 382 /* bit mask, ORed into all irefs */ 383 IndirectRefKind kind_; 384 /* extended debugging info */ 385 IndirectRefSlot* slot_data_; 386 /* #of entries we have space for */ 387 size_t alloc_entries_; 388 /* max #of entries allowed */ 389 size_t max_entries_; 390 }; 391 392 } // namespace art 393 394 #endif // ART_RUNTIME_INDIRECT_REFERENCE_TABLE_H_ 395