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.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
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11 * distributed under the License is distributed on an "AS IS" BASIS,
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13 * See the License for the specific language governing permissions and
14 * limitations under the License.
17 #ifndef _DALVIK_INDIRECTREFTABLE
18 #define _DALVIK_INDIRECTREFTABLE
20 * Maintain a table of indirect references. Used for local/global JNI
23 * The table contains object references that are part of the GC root set.
24 * When an object is added we return an IndirectRef that is not a valid
25 * pointer but can be used to find the original value in O(1) time.
26 * Conversions to and from indirect refs are performed on JNI method calls
27 * in and out of the VM, so they need to be very fast.
29 * To be efficient for JNI local variable storage, we need to provide
30 * operations that allow us to operate on segments of the table, where
31 * segments are pushed and popped as if on a stack. For example, deletion
32 * of an entry should only succeed if it appears in the current segment,
33 * and we want to be able to strip off the current segment quickly when
34 * a method returns. Additions to the table must be made in the current
35 * segment even if space is available in an earlier area.
37 * A new segment is created when we call into native code from interpreted
38 * code, or when we handle the JNI PushLocalFrame function.
40 * The GC must be able to scan the entire table quickly.
42 * In summary, these must be very fast:
43 * - adding or removing a segment
44 * - adding references to a new segment
45 * - converting an indirect reference back to an Object
46 * These can be a little slower, but must still be pretty quick:
47 * - adding references to a "mature" segment
48 * - removing individual references
49 * - scanning the entire table straight through
51 * If there's more than one segment, we don't guarantee that the table
52 * will fill completely before we fail due to lack of space. We do ensure
53 * that the current segment will pack tightly, which should satisfy JNI
54 * requirements (e.g. EnsureLocalCapacity).
56 * To make everything fit nicely in 32-bit integers, the maximum size of
57 * the table is capped at 64K.
59 * None of the table functions are synchronized.
63 * Indirect reference definition. This must be interchangeable with JNI's
64 * jobject, and it's convenient to let null be null, so we use void*.
66 * We need a 16-bit table index and a 2-bit reference type (global, local,
67 * weak global). Real object pointers will have zeroes in the low 2 or 3
68 * bits (4- or 8-byte alignment), so it's useful to put the ref type
69 * in the low bits and reserve zero as an invalid value.
71 * The remaining 14 bits can be used to detect stale indirect references.
72 * For example, if objects don't move, we can use a hash of the original
73 * Object* to make sure the entry hasn't been re-used. (If the Object*
74 * we find there doesn't match because of heap movement, we could do a
75 * secondary check on the preserved hash value; this implies that creating
76 * a global/local ref queries the hash value and forces it to be saved.)
77 * This is only done when CheckJNI is enabled.
79 * A more rigorous approach would be to put a serial number in the extra
80 * bits, and keep a copy of the serial number in a parallel table. This is
81 * easier when objects can move, but requires 2x the memory and additional
82 * memory accesses on add/get. It will catch additional problems, e.g.:
83 * create iref1 for obj, delete iref1, create iref2 for same obj, lookup
84 * iref1. A pattern based on object bits will miss this.
86 typedef void* IndirectRef;
89 * Indirect reference kind, used as the two low bits of IndirectRef.
91 * For convenience these match up with enum jobjectRefType from jni.h.
93 typedef enum IndirectRefKind {
94 kIndirectKindInvalid = 0,
95 kIndirectKindLocal = 1,
96 kIndirectKindGlobal = 2,
97 kIndirectKindWeakGlobal = 3
103 * For the global reference table, the expected common operations are
104 * adding a new entry and removing a recently-added entry (usually the
105 * most-recently-added entry). For JNI local references, the common
106 * operations are adding a new entry and removing an entire table segment.
108 * If "allocEntries" is not equal to "maxEntries", the table may expand
109 * when entries are added, which means the memory may move. If you want
110 * to keep pointers into "table" rather than offsets, you must use a
113 * If we delete entries from the middle of the list, we will be left with
114 * "holes". We track the number of holes so that, when adding new elements,
115 * we can quickly decide to do a trivial append or go slot-hunting.
117 * When the top-most entry is removed, any holes immediately below it are
118 * also removed. Thus, deletion of an entry may reduce "topIndex" by more
121 * To get the desired behavior for JNI locals, we need to know the bottom
122 * and top of the current "segment". The top is managed internally, and
123 * the bottom is passed in as a function argument (the VM keeps it in a
124 * slot in the interpreted stack frame). When we call a native method or
125 * push a local frame, the current top index gets pushed on, and serves
126 * as the new bottom. When we pop a frame off, the value from the stack
127 * becomes the new top index, and the value stored in the previous frame
128 * becomes the new bottom.
130 * To avoid having to re-scan the table after a pop, we want to push the
131 * number of holes in the table onto the stack. Because of our 64K-entry
132 * cap, we can combine the two into a single unsigned 32-bit value.
133 * Instead of a "bottom" argument we take a "cookie", which includes the
134 * bottom index and the count of holes below the bottom.
136 * We need to minimize method call/return overhead. If we store the
137 * "cookie" externally, on the interpreted call stack, the VM can handle
138 * pushes and pops with a single 4-byte load and store. (We could also
139 * store it internally in a public structure, but the local JNI refs are
140 * logically tied to interpreted stack frames anyway.)
142 * Common alternative implementation: make IndirectRef a pointer to the
143 * actual reference slot. Instead of getting a table and doing a lookup,
144 * the lookup can be done instantly. Operations like determining the
145 * type and deleting the reference are more expensive because the table
146 * must be hunted for (i.e. you have to do a pointer comparison to see
147 * which table it's in), you can't move the table when expanding it (so
148 * realloc() is out), and tricks like serial number checking to detect
149 * stale references aren't possible (though we may be able to get similar
150 * benefits with other approaches).
152 * TODO: consider a "lastDeleteIndex" for quick hole-filling when an
153 * add immediately follows a delete; must invalidate after segment pop
154 * (which could increase the cost/complexity of method call/return).
155 * Might be worth only using it for JNI globals.
157 * TODO: may want completely different add/remove algorithms for global
158 * and local refs to improve performance. A large circular buffer might
159 * reduce the amortized cost of adding global references.
161 * TODO: if we can guarantee that the underlying storage doesn't move,
162 * e.g. by using oversized mmap regions to handle expanding tables, we may
163 * be able to avoid having to synchronize lookups. Might make sense to
164 * add a "synchronized lookup" call that takes the mutex as an argument,
165 * and either locks or doesn't lock based on internal details.
167 typedef union IRTSegmentState {
170 u4 topIndex:16; /* index of first unused entry */
171 u4 numHoles:16; /* #of holes in entire table */
174 typedef struct IndirectRefTable {
175 /* semi-public - read/write by interpreter in native call handler */
176 IRTSegmentState segmentState;
178 /* semi-public - read-only during GC scan; pointer must not be kept */
179 Object** table; /* bottom of the stack */
182 int allocEntries; /* #of entries we have space for */
183 int maxEntries; /* max #of entries allowed */
184 IndirectRefKind kind; /* bit mask, ORed into all irefs */
186 // TODO: want hole-filling stats (#of holes filled, total entries scanned)
187 // for performance evaluation.
190 /* use as initial value for "cookie", and when table has only one segment */
191 #define IRT_FIRST_SEGMENT 0
194 * (This is PRIVATE, but we want it inside other inlines in this header.)
196 * Indirectify the object.
198 * The object pointer itself is subject to relocation in some GC
199 * implementations, so we shouldn't really be using it here.
201 INLINE IndirectRef dvmObjectToIndirectRef(Object* obj, u4 tableIndex,
202 IndirectRefKind kind)
204 assert(tableIndex < 65536);
205 u4 objChunk = (((u4) obj >> 3) ^ ((u4) obj >> 19)) & 0x3fff;
206 u4 uref = objChunk << 18 | (tableIndex << 2) | kind;
207 return (IndirectRef) uref;
211 * (This is PRIVATE, but we want it inside other inlines in this header.)
213 * Extract the table index from an indirect reference.
215 INLINE u4 dvmIndirectRefToIndex(IndirectRef iref)
218 return (uref >> 2) & 0xffff;
222 * Determine what kind of indirect reference this is.
224 INLINE IndirectRefKind dvmGetIndirectRefType(IndirectRef iref)
226 return (u4) iref & 0x03;
230 * Initialize an IndirectRefTable.
232 * If "initialCount" != "maxCount", the table will expand as required.
234 * "kind" should be Local or Global. The Global table may also hold
237 * Returns "false" if table allocation fails.
239 bool dvmInitIndirectRefTable(IndirectRefTable* pRef, int initialCount,
240 int maxCount, IndirectRefKind kind);
243 * Clear out the contents, freeing allocated storage. Does not free "pRef".
245 * You must call dvmInitReferenceTable() before you can re-use this table.
247 void dvmClearIndirectRefTable(IndirectRefTable* pRef);
250 * Start a new segment at the top of the table.
252 * Returns an opaque 32-bit value that must be provided when the segment
255 * IMPORTANT: this is implemented as a single instruction in mterp, rather
256 * than a call here. You can add debugging aids for the C-language
257 * interpreters, but the basic implementation may not change.
259 INLINE u4 dvmPushIndirectRefTableSegment(IndirectRefTable* pRef)
261 return pRef->segmentState.all;
264 /* extra debugging checks */
265 bool dvmPopIndirectRefTableSegmentCheck(IndirectRefTable* pRef, u4 cookie);
268 * Remove one or more segments from the top. The table entry identified
269 * by "cookie" becomes the new top-most entry.
271 * IMPORTANT: this is implemented as a single instruction in mterp, rather
272 * than a call here. You can add debugging aids for the C-language
273 * interpreters, but the basic implementation may not change.
275 INLINE void dvmPopIndirectRefTableSegment(IndirectRefTable* pRef, u4 cookie)
277 dvmPopIndirectRefTableSegmentCheck(pRef, cookie);
278 pRef->segmentState.all = cookie;
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.
285 INLINE size_t dvmIndirectRefTableEntries(const IndirectRefTable* pRef)
287 return pRef->segmentState.parts.topIndex;
291 * Returns "true" if the table is full. The table is considered full if
292 * we would need to expand it to add another entry to the current segment.
294 INLINE size_t dvmIsIndirectRefTableFull(const IndirectRefTable* pRef)
296 return dvmIndirectRefTableEntries(pRef) == (size_t)pRef->allocEntries;
300 * Add a new entry. "obj" must be a valid non-NULL object reference
301 * (though it's okay if it's not fully-formed, e.g. the result from
302 * dvmMalloc doesn't have obj->clazz set).
304 * Returns NULL if the table is full (max entries reached, or alloc
305 * failed during expansion).
307 IndirectRef dvmAddToIndirectRefTable(IndirectRefTable* pRef, u4 cookie,
311 * Add a new entry at the end. Similar to Add but does not usually attempt
312 * to fill in holes. This is only appropriate to use right after a new
313 * segment has been pushed.
315 * (This is intended for use when calling into a native JNI method, so
316 * performance is critical.)
318 INLINE IndirectRef dvmAppendToIndirectRefTable(IndirectRefTable* pRef,
319 u4 cookie, Object* obj)
321 int topIndex = pRef->segmentState.parts.topIndex;
322 if (topIndex == pRef->allocEntries) {
323 /* up against alloc or max limit, call the fancy version */
324 return dvmAddToIndirectRefTable(pRef, cookie, obj);
326 IndirectRef result = dvmObjectToIndirectRef(obj, topIndex, pRef->kind);
327 pRef->table[topIndex++] = obj;
328 pRef->segmentState.parts.topIndex = topIndex;
333 /* extra debugging checks */
334 bool dvmGetFromIndirectRefTableCheck(IndirectRefTable* pRef, IndirectRef iref);
337 * Given an IndirectRef in the table, return the Object it refers to.
339 * Returns NULL if iref is invalid.
341 INLINE Object* dvmGetFromIndirectRefTable(IndirectRefTable* pRef,
344 if (!dvmGetFromIndirectRefTableCheck(pRef, iref))
347 int idx = dvmIndirectRefToIndex(iref);
348 return pRef->table[idx];
352 * Remove an existing entry.
354 * If the entry is not between the current top index and the bottom index
355 * specified by the cookie, we don't remove anything. This is the behavior
356 * required by JNI's DeleteLocalRef function.
358 * Returns "false" if nothing was removed.
360 bool dvmRemoveFromIndirectRefTable(IndirectRefTable* pRef, u4 cookie,
364 * Dump the contents of a reference table to the log file.
366 void dvmDumpIndirectRefTable(const IndirectRefTable* pRef, const char* descr);
368 #endif /*_DALVIK_INDIRECTREFTABLE*/