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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 "logging.h"
#include "mem_map.h"
#include "UniquePtr.h"
namespace art {
class Object;
#define GC_CARD_SHIFT 7
#define GC_CARD_CLEAN 0
#define GC_CARD_DIRTY 0x70
// 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 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* cardAddr = CardFromAddr(addr);
*cardAddr = GC_CARD_DIRTY;
// Is the object on a dirty card?
bool IsDirty(const Object* obj) const {
return *CardFromAddr(obj) == GC_CARD_DIRTY;
// 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_;
// For every dirty card between begin and end invoke the visitor with the specified argument
typedef void Callback(Object* obj, void* arg);
void Scan(byte* begin, byte* end, Callback* visitor, void* arg) const;
// Assertion used to check the given address is covered by the card table
void CheckAddrIsInCardTable(const byte* addr) const;
CardTable(MemMap* begin, byte* biased_begin, size_t offset) :
mem_map_(begin), biased_begin_(biased_begin), offset_(offset) {}
// Resets all of the bytes in the card table to clean.
void ClearCardTable();
// Returns the address of the relevant byte in the card table, given an address on the heap.
byte* CardFromAddr(const void *addr) const {
byte *cardAddr = biased_begin_ + ((uintptr_t)addr >> GC_CARD_SHIFT);
// Sanity check the caller was asking for address covered by the card table
DCHECK(IsValidCard(cardAddr)) << "addr: " << addr
<< " cardAddr: " << reinterpret_cast<void*>(cardAddr);
return cardAddr;
// Returns the first address in the heap which maps to this card.
void* AddrFromCard(const byte *cardAddr) const {
uintptr_t offset = cardAddr - biased_begin_;
return (void *)(offset << GC_CARD_SHIFT);
// Returns true iff the card table address is within the bounds of the card table.
bool IsValidCard(const byte* cardAddr) const {
byte* begin = mem_map_->Begin() + offset_;
byte* end = mem_map_->End();
return cardAddr >= begin && cardAddr < end;
// 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