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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 <stdint.h>
#include <iosfwd>
#include <limits>
#include <string>
#include <android-base/logging.h>
#include "base/bit_utils.h"
#include "base/locks.h"
#include "base/macros.h"
#include "base/mem_map.h"
#include "gc_root.h"
#include "obj_ptr.h"
#include "offsets.h"
#include "read_barrier_option.h"
namespace art {
class RootInfo;
namespace mirror {
class Object;
} // namespace mirror
// Maintain a table of indirect references. Used for local/global JNI references.
// The table contains object references, where the strong (local/global) references are part of the
// GC root set (but not the weak global references). 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 references are performed on upcalls and downcalls, 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).
// Only SynchronizedGet is 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 (potentially) large table index and a 2-bit reference type (global, local, weak
// global). We also reserve some bits to be used to detect stale indirect references: we put a
// serial number in the extra bits, and keep a copy of the serial number in the table. This requires
// more memory and additional memory accesses on add/get, but is moving-GC safe. 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;
// Indirect reference kind, used as the two low bits of IndirectRef.
// For convenience these match up with enum jobjectRefType from jni.h.
enum IndirectRefKind {
kHandleScopeOrInvalid = 0, // <<stack indirect reference table or invalid reference>>
kLocal = 1, // <<local reference>>
kGlobal = 2, // <<global reference>>
kWeakGlobal = 3, // <<weak global reference>>
kLastKind = kWeakGlobal
std::ostream& operator<<(std::ostream& os, const IndirectRefKind& rhs);
const char* GetIndirectRefKindString(const IndirectRefKind& kind);
// 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 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 "top_index" 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.
// 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.
// Holes are being locally cached for the segment. Otherwise we'd have to pass bottom index and
// number of holes, which restricts us to 16 bits for the top index. The value is cached within the
// table. To avoid code in generated JNI transitions, which implicitly form segments, the code for
// adding and removing references needs to detect the change of a segment. Helper fields are used
// for this detection.
// 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 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.
// The state of the current segment. We only store the index. Splitting it for index and hole
// count restricts the range too much.
struct IRTSegmentState {
uint32_t top_index;
// Use as initial value for "cookie", and when table has only one segment.
static constexpr IRTSegmentState kIRTFirstSegment = { 0 };
// Try to choose kIRTPrevCount so that sizeof(IrtEntry) is a power of 2.
// Contains multiple entries but only one active one, this helps us detect use after free errors
// since the serial stored in the indirect ref wont match.
static constexpr size_t kIRTPrevCount = kIsDebugBuild ? 7 : 3;
class IrtEntry {
void Add(ObjPtr<mirror::Object> obj) REQUIRES_SHARED(Locks::mutator_lock_);
GcRoot<mirror::Object>* GetReference() {
DCHECK_LT(serial_, kIRTPrevCount);
return &references_[serial_];
const GcRoot<mirror::Object>* GetReference() const {
DCHECK_LT(serial_, kIRTPrevCount);
return &references_[serial_];
uint32_t GetSerial() const {
return serial_;
void SetReference(ObjPtr<mirror::Object> obj) REQUIRES_SHARED(Locks::mutator_lock_);
uint32_t serial_;
GcRoot<mirror::Object> references_[kIRTPrevCount];
static_assert(sizeof(IrtEntry) == (1 + kIRTPrevCount) * sizeof(uint32_t),
"Unexpected sizeof(IrtEntry)");
static_assert(IsPowerOfTwo(sizeof(IrtEntry)), "Unexpected sizeof(IrtEntry)");
class IrtIterator {
IrtIterator(IrtEntry* table, size_t i, size_t capacity) REQUIRES_SHARED(Locks::mutator_lock_)
: table_(table), i_(i), capacity_(capacity) {
// capacity_ is used in some target; has warning with unused attribute.
IrtIterator& operator++() REQUIRES_SHARED(Locks::mutator_lock_) {
return *this;
GcRoot<mirror::Object>* operator*() REQUIRES_SHARED(Locks::mutator_lock_) {
// This does not have a read barrier as this is used to visit roots.
return table_[i_].GetReference();
bool equals(const IrtIterator& rhs) const {
return (i_ == rhs.i_ && table_ == rhs.table_);
IrtEntry* const table_;
size_t i_;
const size_t capacity_;
bool inline operator==(const IrtIterator& lhs, const IrtIterator& rhs) {
return lhs.equals(rhs);
bool inline operator!=(const IrtIterator& lhs, const IrtIterator& rhs) {
return !lhs.equals(rhs);
class IndirectReferenceTable {
enum class ResizableCapacity {
// WARNING: Construction of the IndirectReferenceTable may fail.
// error_msg must not be null. If error_msg is set by the constructor, then
// construction has failed and the IndirectReferenceTable will be in an
// invalid state. Use IsValid to check whether the object is in an invalid
// state.
IndirectReferenceTable(size_t max_count,
IndirectRefKind kind,
ResizableCapacity resizable,
std::string* error_msg);
* Checks whether construction of the IndirectReferenceTable succeeded.
* This object must only be used if IsValid() returns true. It is safe to
* call IsValid from multiple threads without locking or other explicit
* synchronization.
bool IsValid() const;
// Add a new entry. "obj" must be a valid non-null object reference. This function will
// return null if an error happened (with an appropriate error message set).
IndirectRef Add(IRTSegmentState previous_state,
ObjPtr<mirror::Object> obj,
std::string* error_msg)
// Given an IndirectRef in the table, return the Object it refers to.
// This function may abort under error conditions.
template<ReadBarrierOption kReadBarrierOption = kWithReadBarrier>
ObjPtr<mirror::Object> Get(IndirectRef iref) const REQUIRES_SHARED(Locks::mutator_lock_)
// Synchronized get which reads a reference, acquiring a lock if necessary.
template<ReadBarrierOption kReadBarrierOption = kWithReadBarrier>
ObjPtr<mirror::Object> SynchronizedGet(IndirectRef iref) const
REQUIRES_SHARED(Locks::mutator_lock_) {
return Get<kReadBarrierOption>(iref);
// Updates an existing indirect reference to point to a new object.
void Update(IndirectRef iref, ObjPtr<mirror::Object> obj) REQUIRES_SHARED(Locks::mutator_lock_);
// 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(IRTSegmentState previous_state, IndirectRef iref);
void AssertEmpty() REQUIRES_SHARED(Locks::mutator_lock_);
void Dump(std::ostream& os) 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 {
return segment_state_.top_index;
// Ensure that at least free_capacity elements are available, or return false.
bool EnsureFreeCapacity(size_t free_capacity, std::string* error_msg)
// See implementation of EnsureFreeCapacity. We'll only state here how much is trivially free,
// without recovering holes. Thus this is a conservative estimate.
size_t FreeCapacity() const;
// Note IrtIterator does not have a read barrier as it's used to visit roots.
IrtIterator begin() {
return IrtIterator(table_, 0, Capacity());
IrtIterator end() {
return IrtIterator(table_, Capacity(), Capacity());
void VisitRoots(RootVisitor* visitor, const RootInfo& root_info)
IRTSegmentState GetSegmentState() const {
return segment_state_;
void SetSegmentState(IRTSegmentState new_state);
static Offset SegmentStateOffset(size_t pointer_size ATTRIBUTE_UNUSED) {
// Note: Currently segment_state_ is at offset 0. We're testing the expected value in
// jni_internal_test to make sure it stays correct. It is not OFFSETOF_MEMBER, as that
// is not pointer-size-safe.
return Offset(0);
// Release pages past the end of the table that may have previously held references.
void Trim() REQUIRES_SHARED(Locks::mutator_lock_);
// Determine what kind of indirect reference this is. Opposite of EncodeIndirectRefKind.
ALWAYS_INLINE static inline IndirectRefKind GetIndirectRefKind(IndirectRef iref) {
return DecodeIndirectRefKind(reinterpret_cast<uintptr_t>(iref));
static constexpr size_t kSerialBits = MinimumBitsToStore(kIRTPrevCount);
static constexpr uint32_t kShiftedSerialMask = (1u << kSerialBits) - 1;
static constexpr size_t kKindBits = MinimumBitsToStore(
static constexpr uint32_t kKindMask = (1u << kKindBits) - 1;
static constexpr uintptr_t EncodeIndex(uint32_t table_index) {
static_assert(sizeof(IndirectRef) == sizeof(uintptr_t), "Unexpected IndirectRef size");
DCHECK_LE(MinimumBitsToStore(table_index), BitSizeOf<uintptr_t>() - kSerialBits - kKindBits);
return (static_cast<uintptr_t>(table_index) << kKindBits << kSerialBits);
static constexpr uint32_t DecodeIndex(uintptr_t uref) {
return static_cast<uint32_t>((uref >> kKindBits) >> kSerialBits);
static constexpr uintptr_t EncodeIndirectRefKind(IndirectRefKind kind) {
return static_cast<uintptr_t>(kind);
static constexpr IndirectRefKind DecodeIndirectRefKind(uintptr_t uref) {
return static_cast<IndirectRefKind>(uref & kKindMask);
static constexpr uintptr_t EncodeSerial(uint32_t serial) {
DCHECK_LE(MinimumBitsToStore(serial), kSerialBits);
return serial << kKindBits;
static constexpr uint32_t DecodeSerial(uintptr_t uref) {
return static_cast<uint32_t>(uref >> kKindBits) & kShiftedSerialMask;
constexpr uintptr_t EncodeIndirectRef(uint32_t table_index, uint32_t serial) const {
DCHECK_LT(table_index, max_entries_);
return EncodeIndex(table_index) | EncodeSerial(serial) | EncodeIndirectRefKind(kind_);
static void ConstexprChecks();
// Extract the table index from an indirect reference.
ALWAYS_INLINE static uint32_t ExtractIndex(IndirectRef iref) {
return DecodeIndex(reinterpret_cast<uintptr_t>(iref));
IndirectRef ToIndirectRef(uint32_t table_index) const {
DCHECK_LT(table_index, max_entries_);
uint32_t serial = table_[table_index].GetSerial();
return reinterpret_cast<IndirectRef>(EncodeIndirectRef(table_index, serial));
// Resize the backing table. Currently must be larger than the current size.
bool Resize(size_t new_size, std::string* error_msg);
void RecoverHoles(IRTSegmentState from);
// Abort if check_jni is not enabled. Otherwise, just log as an error.
static void AbortIfNoCheckJNI(const std::string& msg);
/* extra debugging checks */
bool GetChecked(IndirectRef) const REQUIRES_SHARED(Locks::mutator_lock_);
bool CheckEntry(const char*, IndirectRef, uint32_t) const;
/// semi-public - read/write by jni down calls.
IRTSegmentState segment_state_;
// Mem map where we store the indirect refs.
MemMap table_mem_map_;
// bottom of the stack. Do not directly access the object references
// in this as they are roots. Use Get() that has a read barrier.
IrtEntry* table_;
// bit mask, ORed into all irefs.
const IndirectRefKind kind_;
// max #of entries allowed (modulo resizing).
size_t max_entries_;
// Some values to retain old behavior with holes. Description of the algorithm is in the .cc
// file.
// TODO: Consider other data structures for compact tables, e.g., free lists.
size_t current_num_holes_;
IRTSegmentState last_known_previous_state_;
// Whether the table's capacity may be resized. As there are no locks used, it is the caller's
// responsibility to ensure thread-safety.
ResizableCapacity resizable_;
} // namespace art