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* Copyright (C) 2009 The Android Open Source Project
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* See the License for the specific language governing permissions and
* limitations under the License.
#include "heap.h"
#include "logging.h"
#include <iosfwd>
#include <stdint.h>
#include <string>
namespace art {
class Object;
* Maintain a table of indirect references. Used for local/global JNI
* references.
* The table contains object references that are part of the GC root set.
* When an object is added we return an IndirectRef that is not a valid
* pointer but can be used to find the original value in O(1) time.
* Conversions to and from indirect refs are performed on JNI method calls
* in and out of the VM, so they need to be very fast.
* To be efficient for JNI local variable storage, we need to provide
* operations that allow us to operate on segments of the table, where
* segments are pushed and popped as if on a stack. For example, deletion
* of an entry should only succeed if it appears in the current segment,
* and we want to be able to strip off the current segment quickly when
* a method returns. Additions to the table must be made in the current
* segment even if space is available in an earlier area.
* A new segment is created when we call into native code from interpreted
* code, or when we handle the JNI PushLocalFrame function.
* The GC must be able to scan the entire table quickly.
* In summary, these must be very fast:
* - adding or removing a segment
* - adding references to a new segment
* - converting an indirect reference back to an Object
* These can be a little slower, but must still be pretty quick:
* - adding references to a "mature" segment
* - removing individual references
* - scanning the entire table straight through
* If there's more than one segment, we don't guarantee that the table
* will fill completely before we fail due to lack of space. We do ensure
* that the current segment will pack tightly, which should satisfy JNI
* requirements (e.g. EnsureLocalCapacity).
* To make everything fit nicely in 32-bit integers, the maximum size of
* the table is capped at 64K.
* None of the table functions are synchronized.
* Indirect reference definition. This must be interchangeable with JNI's
* jobject, and it's convenient to let null be null, so we use void*.
* We need a 16-bit table index and a 2-bit reference type (global, local,
* weak global). Real object pointers will have zeroes in the low 2 or 3
* bits (4- or 8-byte alignment), so it's useful to put the ref type
* in the low bits and reserve zero as an invalid value.
* The remaining 14 bits can be used to detect stale indirect references.
* For example, if objects don't move, we can use a hash of the original
* Object* to make sure the entry hasn't been re-used. (If the Object*
* we find there doesn't match because of heap movement, we could do a
* secondary check on the preserved hash value; this implies that creating
* a global/local ref queries the hash value and forces it to be saved.)
* A more rigorous approach would be to put a serial number in the extra
* bits, and keep a copy of the serial number in a parallel table. This is
* easier when objects can move, but requires 2x the memory and additional
* memory accesses on add/get. It will catch additional problems, e.g.:
* create iref1 for obj, delete iref1, create iref2 for same obj, lookup
* iref1. A pattern based on object bits will miss this.
typedef void* IndirectRef;
/* Magic failure values; must not pass Heap::ValidateObject() or Heap::IsHeapAddress(). */
static Object* const kInvalidIndirectRefObject = reinterpret_cast<Object*>(0xdead4321);
static Object* const kClearedJniWeakGlobal = reinterpret_cast<Object*>(0xdead1234);
* Indirect reference kind, used as the two low bits of IndirectRef.
* For convenience these match up with enum jobjectRefType from jni.h.
enum IndirectRefKind {
kSirtOrInvalid = 0,
kLocal = 1,
kGlobal = 2,
kWeakGlobal = 3
std::ostream& operator<<(std::ostream& os, IndirectRefKind rhs);
* Determine what kind of indirect reference this is.
static inline IndirectRefKind GetIndirectRefKind(IndirectRef iref) {
return static_cast<IndirectRefKind>(reinterpret_cast<uintptr_t>(iref) & 0x03);
* Extended debugging structure. We keep a parallel array of these, one
* per slot in the table.
static const size_t kIRTPrevCount = 4;
struct IndirectRefSlot {
uint32_t serial;
const Object* previous[kIRTPrevCount];
/* use as initial value for "cookie", and when table has only one segment */
static const uint32_t IRT_FIRST_SEGMENT = 0;
* Table definition.
* For the global reference table, the expected common operations are
* adding a new entry and removing a recently-added entry (usually the
* most-recently-added entry). For JNI local references, the common
* operations are adding a new entry and removing an entire table segment.
* If "alloc_entries_" is not equal to "max_entries_", the table may expand
* when entries are added, which means the memory may move. If you want
* to keep pointers into "table" rather than offsets, you must use a
* fixed-size table.
* If we delete entries from the middle of the list, we will be left with
* "holes". We track the number of holes so that, when adding new elements,
* we can quickly decide to do a trivial append or go slot-hunting.
* When the top-most entry is removed, any holes immediately below it are
* also removed. Thus, deletion of an entry may reduce "topIndex" by more
* than one.
* To get the desired behavior for JNI locals, we need to know the bottom
* and top of the current "segment". The top is managed internally, and
* the bottom is passed in as a function argument (the VM keeps it in a
* slot in the interpreted stack frame). When we call a native method or
* push a local frame, the current top index gets pushed on, and serves
* as the new bottom. When we pop a frame off, the value from the stack
* becomes the new top index, and the value stored in the previous frame
* becomes the new bottom.
* To avoid having to re-scan the table after a pop, we want to push the
* number of holes in the table onto the stack. Because of our 64K-entry
* cap, we can combine the two into a single unsigned 32-bit value.
* Instead of a "bottom" argument we take a "cookie", which includes the
* bottom index and the count of holes below the bottom.
* We need to minimize method call/return overhead. If we store the
* "cookie" externally, on the interpreted call stack, the VM can handle
* pushes and pops with a single 4-byte load and store. (We could also
* store it internally in a public structure, but the local JNI refs are
* logically tied to interpreted stack frames anyway.)
* Common alternative implementation: make IndirectRef a pointer to the
* actual reference slot. Instead of getting a table and doing a lookup,
* the lookup can be done instantly. Operations like determining the
* type and deleting the reference are more expensive because the table
* must be hunted for (i.e. you have to do a pointer comparison to see
* which table it's in), you can't move the table when expanding it (so
* realloc() is out), and tricks like serial number checking to detect
* stale references aren't possible (though we may be able to get similar
* benefits with other approaches).
* TODO: consider a "lastDeleteIndex" for quick hole-filling when an
* add immediately follows a delete; must invalidate after segment pop
* (which could increase the cost/complexity of method call/return).
* Might be worth only using it for JNI globals.
* TODO: may want completely different add/remove algorithms for global
* and local refs to improve performance. A large circular buffer might
* reduce the amortized cost of adding global references.
* TODO: if we can guarantee that the underlying storage doesn't move,
* e.g. by using oversized mmap regions to handle expanding tables, we may
* be able to avoid having to synchronize lookups. Might make sense to
* add a "synchronized lookup" call that takes the mutex as an argument,
* and either locks or doesn't lock based on internal details.
union IRTSegmentState {
uint32_t all;
struct {
uint32_t topIndex:16; /* index of first unused entry */
uint32_t numHoles:16; /* #of holes in entire table */
} parts;
class IrtIterator {
explicit IrtIterator(const Object** table, size_t i, size_t capacity)
: table_(table), i_(i), capacity_(capacity) {
IrtIterator& operator++() {
return *this;
const Object** operator*() {
return &table_[i_];
bool equals(const IrtIterator& rhs) const {
return (i_ == rhs.i_ && table_ == rhs.table_);
void SkipNullsAndTombstones() {
// We skip NULLs and tombstones. Clients don't want to see implementation details.
while (i_ < capacity_ && (table_[i_] == NULL || table_[i_] == kClearedJniWeakGlobal)) {
const Object** table_;
size_t i_;
size_t capacity_;
bool inline operator!=(const IrtIterator& lhs, const IrtIterator& rhs) {
return !lhs.equals(rhs);
class IndirectReferenceTable {
typedef IrtIterator iterator;
IndirectReferenceTable(size_t initialCount, size_t maxCount, IndirectRefKind kind);
* Add a new entry. "obj" must be a valid non-NULL object reference
* (though it's okay if it's not fully-formed, e.g. the result from
* dvmMalloc doesn't have obj->clazz set).
* Returns NULL if the table is full (max entries reached, or alloc
* failed during expansion).
IndirectRef Add(uint32_t cookie, const Object* obj);
* Given an IndirectRef in the table, return the Object it refers to.
* Returns kInvalidIndirectRefObject if iref is invalid.
const Object* Get(IndirectRef iref) const {
if (!GetChecked(iref)) {
return kInvalidIndirectRefObject;
return table_[ExtractIndex(iref)];
// TODO: remove when we remove work_around_app_jni_bugs support.
bool Contains(IndirectRef iref) const;
* Remove an existing entry.
* If the entry is not between the current top index and the bottom index
* specified by the cookie, we don't remove anything. This is the behavior
* required by JNI's DeleteLocalRef function.
* Returns "false" if nothing was removed.
bool Remove(uint32_t cookie, IndirectRef iref);
void Dump() const;
* Return the #of entries in the entire table. This includes holes, and
* so may be larger than the actual number of "live" entries.
size_t Capacity() const {
iterator begin() {
return iterator(table_, 0, Capacity());
iterator end() {
return iterator(table_, Capacity(), Capacity());
void VisitRoots(Heap::RootVisitor* visitor, void* arg);
static Offset SegmentStateOffset() {
return Offset(OFFSETOF_MEMBER(IndirectReferenceTable, segment_state_));
* Extract the table index from an indirect reference.
static uint32_t ExtractIndex(IndirectRef iref) {
uint32_t uref = (uint32_t) iref;
return (uref >> 2) & 0xffff;
* The object pointer itself is subject to relocation in some GC
* implementations, so we shouldn't really be using it here.
IndirectRef ToIndirectRef(const Object* obj, uint32_t tableIndex) const {
DCHECK_LT(tableIndex, 65536U);
uint32_t serialChunk = slot_data_[tableIndex].serial;
uint32_t uref = serialChunk << 20 | (tableIndex << 2) | kind_;
return (IndirectRef) uref;
* Update extended debug info when an entry is added.
* We advance the serial number, invalidating any outstanding references to
* this slot.
void UpdateSlotAdd(const Object* obj, int slot) {
if (slot_data_ != NULL) {
IndirectRefSlot* pSlot = &slot_data_[slot];
pSlot->previous[pSlot->serial % kIRTPrevCount] = obj;
/* extra debugging checks */
bool GetChecked(IndirectRef) const;
bool CheckEntry(const char*, IndirectRef, int) const;
/* semi-public - read/write by jni down calls */
IRTSegmentState segment_state_;
/* bottom of the stack */
const Object** table_;
/* bit mask, ORed into all irefs */
IndirectRefKind kind_;
/* extended debugging info */
IndirectRefSlot* slot_data_;
/* #of entries we have space for */
size_t alloc_entries_;
/* max #of entries allowed */
size_t max_entries_;
} // namespace art