src/gc/card_table-inl.h

/*
 * 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
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#ifndef ART_SRC_GC_CARDTABLE_INL_H_
#define ART_SRC_GC_CARDTABLE_INL_H_

#include "base/logging.h"
#include "card_table.h"
#include "cutils/atomic-inline.h"
#include "space_bitmap.h"
#include "utils.h"

namespace art {

static inline bool byte_cas(byte old_value, byte new_value, byte* address) {
  // Little endian means most significant byte is on the left.
  const size_t shift = reinterpret_cast<uintptr_t>(address) % sizeof(uintptr_t);
  // Align the address down.
  address -= shift;
  int32_t* word_address = reinterpret_cast<int32_t*>(address);
  // Word with the byte we are trying to cas cleared.
  const int32_t cur_word = *word_address & ~(0xFF << shift);
  const int32_t old_word = cur_word | (static_cast<int32_t>(old_value) << shift);
  const int32_t new_word = cur_word | (static_cast<int32_t>(new_value) << shift);
  bool success = android_atomic_cas(old_word, new_word, word_address) == 0;
  return success;
}

template <typename Visitor, typename FingerVisitor>
inline void CardTable::Scan(SpaceBitmap* bitmap, byte* scan_begin, byte* scan_end,
                            const Visitor& visitor, const FingerVisitor& finger_visitor,
                            const byte minimum_age) const {
  DCHECK(bitmap->HasAddress(scan_begin));
  DCHECK(bitmap->HasAddress(scan_end - 1));  // scan_end is the byte after the last byte we scan.
  byte* card_cur = CardFromAddr(scan_begin);
  byte* card_end = CardFromAddr(scan_end);
  CheckCardValid(card_cur);
  CheckCardValid(card_end);

  // Handle any unaligned cards at the start.
  while (!IsAligned<sizeof(word)>(card_cur) && card_cur < card_end) {
    if (*card_cur >= minimum_age) {
      uintptr_t start = reinterpret_cast<uintptr_t>(AddrFromCard(card_cur));
      uintptr_t end = start + kCardSize;
      bitmap->VisitMarkedRange(start, end, visitor, finger_visitor);
    }
    ++card_cur;
  }

  byte* aligned_end = card_end -
      (reinterpret_cast<uintptr_t>(card_end) & (sizeof(uintptr_t) - 1));

  // Now we have the words, we can send these to be processed in parallel.
  uintptr_t* word_cur = reinterpret_cast<uintptr_t*>(card_cur);
  uintptr_t* word_end = reinterpret_cast<uintptr_t*>(aligned_end);

  // TODO: Parallelize
  while (word_cur < word_end) {
    // Find the first dirty card.
    while (*word_cur == 0 && word_cur < word_end) {
      word_cur++;
    }
    if (word_cur >= word_end) {
      break;
    }
    uintptr_t start_word = *word_cur;
    for (size_t i = 0; i < sizeof(uintptr_t); ++i) {
      if ((start_word & 0xFF) >= minimum_age) {
        byte* card = reinterpret_cast<byte*>(word_cur) + i;
        const byte card_byte = *card;
        DCHECK(card_byte == (start_word & 0xFF) || card_byte == kCardDirty)
        << "card " << static_cast<size_t>(card_byte) << " word " << (start_word & 0xFF);
        uintptr_t start = reinterpret_cast<uintptr_t>(AddrFromCard(card));
        uintptr_t end = start + kCardSize;
        bitmap->VisitMarkedRange(start, end, visitor, finger_visitor);
      }
      start_word >>= 8;
    }
    ++word_cur;
  }

  // Handle any unaligned cards at the end.
  card_cur = reinterpret_cast<byte*>(word_end);
  while (card_cur < card_end) {
    if (*card_cur >= minimum_age) {
      uintptr_t start = reinterpret_cast<uintptr_t>(AddrFromCard(card_cur));
      uintptr_t end = start + kCardSize;
      bitmap->VisitMarkedRange(start, end, visitor, finger_visitor);
    }
    ++card_cur;
  }
}

/*
 * 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>
inline void CardTable::ModifyCardsAtomic(byte* scan_begin, byte* scan_end, const Visitor& visitor,
                                         const ModifiedVisitor& modified) {
  byte* card_cur = CardFromAddr(scan_begin);
  byte* card_end = CardFromAddr(scan_end);
  CheckCardValid(card_cur);
  CheckCardValid(card_end);

  // Handle any unaligned cards at the start.
  while (!IsAligned<sizeof(word)>(card_cur) && card_cur < card_end) {
    byte expected, new_value;
    do {
      expected = *card_cur;
      new_value = visitor(expected);
    } while (expected != new_value && UNLIKELY(!byte_cas(expected, new_value, card_cur)));
    if (expected != new_value) {
      modified(card_cur, expected, new_value);
    }
    ++card_cur;
  }

  // Handle unaligned cards at the end.
  while (!IsAligned<sizeof(word)>(card_end) && card_end > card_cur) {
    --card_end;
    byte expected, new_value;
    do {
      expected = *card_end;
      new_value = visitor(expected);
    } while (expected != new_value && UNLIKELY(!byte_cas(expected, new_value, card_end)));
    if (expected != new_value) {
      modified(card_cur, expected, new_value);
    }
  }

  // Now we have the words, we can process words in parallel.
  uintptr_t* word_cur = reinterpret_cast<uintptr_t*>(card_cur);
  uintptr_t* word_end = reinterpret_cast<uintptr_t*>(card_end);
  uintptr_t expected_word;
  uintptr_t new_word;

  // TODO: Parallelize.
  while (word_cur < word_end) {
    while ((expected_word = *word_cur) != 0) {
      new_word =
          (visitor((expected_word >> 0) & 0xFF) << 0) |
          (visitor((expected_word >> 8) & 0xFF) << 8) |
          (visitor((expected_word >> 16) & 0xFF) << 16) |
          (visitor((expected_word >> 24) & 0xFF) << 24);
      if (new_word == expected_word) {
        // No need to do a cas.
        break;
      }
      if (LIKELY(android_atomic_cas(expected_word, new_word,
                                    reinterpret_cast<int32_t*>(word_cur)) == 0)) {
        for (size_t i = 0; i < sizeof(uintptr_t); ++i) {
          const byte expected_byte = (expected_word >> (8 * i)) & 0xFF;
          const byte new_byte = (new_word >> (8 * i)) & 0xFF;
          if (expected_byte != new_byte) {
            modified(reinterpret_cast<byte*>(word_cur) + i, expected_byte, new_byte);
          }
        }
        break;
      }
    }
    ++word_cur;
  }
}

inline void* CardTable::AddrFromCard(const byte *card_addr) const {
  DCHECK(IsValidCard(card_addr))
    << " card_addr: " << reinterpret_cast<const void*>(card_addr)
    << " begin: " << reinterpret_cast<void*>(mem_map_->Begin() + offset_)
    << " end: " << reinterpret_cast<void*>(mem_map_->End());
  uintptr_t offset = card_addr - biased_begin_;
  return reinterpret_cast<void*>(offset << kCardShift);
}

inline byte* CardTable::CardFromAddr(const void *addr) const {
  byte *card_addr = biased_begin_ + (reinterpret_cast<uintptr_t>(addr) >> kCardShift);
  // Sanity check the caller was asking for address covered by the card table
  DCHECK(IsValidCard(card_addr)) << "addr: " << addr
      << " card_addr: " << reinterpret_cast<void*>(card_addr);
  return card_addr;
}

inline void CardTable::CheckCardValid(byte* card) const {
  DCHECK(IsValidCard(card))
      << " card_addr: " << reinterpret_cast<const void*>(card)
      << " begin: " << reinterpret_cast<void*>(mem_map_->Begin() + offset_)
      << " end: " << reinterpret_cast<void*>(mem_map_->End());
}

}  // namespace art

#endif  // ART_SRC_GC_CARDTABLE_INL_H_