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* Copyright (C) 2011 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 "globals.h"
#include "locks.h"
#include "mem_map.h"
#include "UniquePtr.h"
namespace art {
namespace mirror {
class Object;
} // namespace mirror
class Heap;
class ContinuousSpace;
class SpaceBitmap;
// Maintain a card table from the the write barrier. All writes of
// non-NULL values to heap addresses should go through an entry in
// WriteBarrier, and from there to here.
class CardTable {
static const size_t kCardShift = 7;
static const size_t kCardSize = (1 << kCardShift);
static const uint8_t kCardClean = 0x0;
static const uint8_t kCardDirty = 0x70;
static CardTable* Create(const byte* heap_begin, size_t heap_capacity);
// Set the card associated with the given address to GC_CARD_DIRTY.
void MarkCard(const void *addr) {
byte* card_addr = CardFromAddr(addr);
*card_addr = kCardDirty;
// Is the object on a dirty card?
bool IsDirty(const mirror::Object* obj) const {
return GetCard(obj) == kCardDirty;
// Return the state of the card at an address.
byte GetCard(const mirror::Object* obj) const {
return *CardFromAddr(obj);
// Visit and clear cards within memory range, only visits dirty cards.
template <typename Visitor>
void VisitClear(const void* start, const void* end, const Visitor& visitor) {
byte* card_start = CardFromAddr(start);
byte* card_end = CardFromAddr(end);
for (byte* it = card_start; it != card_end; ++it) {
if (*it == kCardDirty) {
*it = kCardClean;
// Returns a value that when added to a heap address >> GC_CARD_SHIFT will address the appropriate
// card table byte. For convenience this value is cached in every Thread
byte* GetBiasedBegin() const {
return biased_begin_;
* Visitor is expected to take in a card and return the new value. When a value is modified, the
* modify visitor is called.
* visitor: The visitor which modifies the cards. Returns the new value for a card given an old
* value.
* modified: Whenever the visitor modifies a card, this visitor is called on the card. Enables
* us to know which cards got cleared.
template <typename Visitor, typename ModifiedVisitor>
void ModifyCardsAtomic(byte* scan_begin, byte* scan_end, const Visitor& visitor,
const ModifiedVisitor& modified);
// For every dirty at least minumum age between begin and end invoke the visitor with the
// specified argument.
template <typename Visitor, typename FingerVisitor>
void Scan(SpaceBitmap* bitmap, byte* scan_begin, byte* scan_end,
const Visitor& visitor, const FingerVisitor& finger_visitor,
const byte minimum_age = kCardDirty) const
// Assertion used to check the given address is covered by the card table
void CheckAddrIsInCardTable(const byte* addr) const;
// Resets all of the bytes in the card table to clean.
void ClearCardTable();
// Resets all of the bytes in the card table which do not map to the image space.
void ClearSpaceCards(ContinuousSpace* space);
// Returns the first address in the heap which maps to this card.
void* AddrFromCard(const byte *card_addr) const;
// Returns the address of the relevant byte in the card table, given an address on the heap.
byte* CardFromAddr(const void *addr) const;
bool AddrIsInCardTable(const void* addr) const;
CardTable(MemMap* begin, byte* biased_begin, size_t offset);
// Returns true iff the card table address is within the bounds of the card table.
bool IsValidCard(const byte* card_addr) const {
byte* begin = mem_map_->Begin() + offset_;
byte* end = mem_map_->End();
return card_addr >= begin && card_addr < end;
void CheckCardValid(byte* card) const;
// Verifies that all gray objects are on a dirty card.
void VerifyCardTable();
// Mmapped pages for the card table
UniquePtr<MemMap> mem_map_;
// Value used to compute card table addresses from object addresses, see GetBiasedBegin
byte* const biased_begin_;
// Card table doesn't begin at the beginning of the mem_map_, instead it is displaced by offset
// to allow the byte value of biased_begin_ to equal GC_CARD_DIRTY
const size_t offset_;
} // namespace art