src/heap_bitmap.cc

/*
 * Copyright (C) 2008 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.
 */

#include "heap_bitmap.h"

#include <sys/mman.h>

#include "UniquePtr.h"
#include "logging.h"
#include "utils.h"

namespace art {

HeapBitmap* HeapBitmap::Create(const char* name, byte* heap_begin, size_t heap_capacity) {
  CHECK(heap_begin != NULL);
  size_t bitmap_size = HB_OFFSET_TO_INDEX(heap_capacity) * kWordSize;
  UniquePtr<MemMap> mem_map(MemMap::MapAnonymous(name, NULL, bitmap_size, PROT_READ | PROT_WRITE));
  if (mem_map.get() == NULL) {
    LOG(ERROR) << "Failed to allocate bitmap " << name;
    return NULL;
  }
  word* bitmap_begin = reinterpret_cast<word*>(mem_map->Begin());
  return new HeapBitmap(name, mem_map.release(), bitmap_begin, bitmap_size, heap_begin);
}

// Clean up any resources associated with the bitmap.
HeapBitmap::~HeapBitmap() {}

// Fill the bitmap with zeroes.  Returns the bitmap's memory to the
// system as a side-effect.
void HeapBitmap::Clear() {
  if (bitmap_begin_ != NULL) {
    // This returns the memory to the system.  Successive page faults
    // will return zeroed memory.
    int result = madvise(bitmap_begin_, bitmap_size_, MADV_DONTNEED);
    if (result == -1) {
      PLOG(WARNING) << "madvise failed";
    }
    heap_end_ = heap_begin_ - 1;
  }
}

// Return true iff <obj> is within the range of pointers that this bitmap could potentially cover,
// even if a bit has not been set for it.
bool HeapBitmap::HasAddress(const void* obj) const {
  if (obj != NULL) {
    const uintptr_t offset = (uintptr_t)obj - heap_begin_;
    const size_t index = HB_OFFSET_TO_INDEX(offset);
    return index < bitmap_size_ / kWordSize;
  }
  return false;
}

void HeapBitmap::VisitRange(uintptr_t visit_begin, uintptr_t visit_end, Callback* visitor, void* arg) const {
  size_t start = HB_OFFSET_TO_INDEX(visit_begin - heap_begin_);
  size_t end = HB_OFFSET_TO_INDEX(visit_end - heap_begin_ - 1);
  for (size_t i = start; i <= end; i++) {
    word w = bitmap_begin_[i];
    if (w != 0) {
      word high_bit = 1 << (kBitsPerWord - 1);
      uintptr_t ptr_base = HB_INDEX_TO_OFFSET(i) + heap_begin_;
      while (w != 0) {
        const int shift = CLZ(w);
        Object* obj = reinterpret_cast<Object*>(ptr_base + shift * kAlignment);
        (*visitor)(obj, arg);
        w &= ~(high_bit >> shift);
      }
    }
  }
}

// Visits set bits in address order.  The callback is not permitted to
// change the bitmap bits or max during the traversal.
void HeapBitmap::Walk(HeapBitmap::Callback* callback, void* arg) {
  CHECK(bitmap_begin_ != NULL);
  CHECK(callback != NULL);
  uintptr_t end = HB_OFFSET_TO_INDEX(heap_end_ - heap_begin_);
  for (uintptr_t i = 0; i <= end; ++i) {
    word w = bitmap_begin_[i];
    if (UNLIKELY(w != 0)) {
      word high_bit = 1 << (kBitsPerWord - 1);
      uintptr_t ptr_base = HB_INDEX_TO_OFFSET(i) + heap_begin_;
      while (w != 0) {
        const int shift = CLZ(w);
        Object* obj = reinterpret_cast<Object*>(ptr_base + shift * kAlignment);
        (*callback)(obj, arg);
        w &= ~(high_bit >> shift);
      }
    }
  }
}

// Similar to Walk but the callback routine is permitted to change the bitmap bits and end during
// traversal.  Used by the the root marking scan exclusively.
//
// The callback is invoked with a finger argument.  The finger is a pointer to an address not yet
// visited by the traversal.  If the callback sets a bit for an address at or above the finger, this
// address will be visited by the traversal.  If the callback sets a bit for an address below the
// finger, this address will not be visited (typiscally such an address would be placed on the
// marking stack).
void HeapBitmap::ScanWalk(uintptr_t scan_begin, uintptr_t scan_end, ScanCallback* callback, void* arg) {
  CHECK(bitmap_begin_ != NULL);
  CHECK(callback != NULL);
  CHECK_LE(scan_begin, scan_end);
  CHECK_GE(scan_begin, heap_begin_);
  size_t start = HB_OFFSET_TO_INDEX(scan_begin - heap_begin_);
  if (scan_end < heap_end_) {
    // The end of the space we're looking at is before the current maximum bitmap PC, scan to that
    // and don't recompute end on each iteration
    size_t end = HB_OFFSET_TO_INDEX(scan_end - heap_begin_ - 1);
    for (size_t i = start; i <= end; i++) {
      word w = bitmap_begin_[i];
      if (UNLIKELY(w != 0)) {
        word high_bit = 1 << (kBitsPerWord - 1);
        uintptr_t ptr_base = HB_INDEX_TO_OFFSET(i) + heap_begin_;
        void* finger = reinterpret_cast<void*>(HB_INDEX_TO_OFFSET(i + 1) + heap_begin_);
        while (w != 0) {
          const int shift = CLZ(w);
          Object* obj = reinterpret_cast<Object*>(ptr_base + shift * kAlignment);
          (*callback)(obj, finger, arg);
          w &= ~(high_bit >> shift);
        }
      }
    }
  } else {
    size_t end = HB_OFFSET_TO_INDEX(heap_end_ - heap_begin_);
    for (size_t i = start; i <= end; i++) {
      word w = bitmap_begin_[i];
      if (UNLIKELY(w != 0)) {
        word high_bit = 1 << (kBitsPerWord - 1);
        uintptr_t ptr_base = HB_INDEX_TO_OFFSET(i) + heap_begin_;
        void* finger = reinterpret_cast<void*>(HB_INDEX_TO_OFFSET(i + 1) + heap_begin_);
        while (w != 0) {
          const int shift = CLZ(w);
          Object* obj = reinterpret_cast<Object*>(ptr_base + shift * kAlignment);
          (*callback)(obj, finger, arg);
          w &= ~(high_bit >> shift);
        }
      }
      // update 'end' in case callback modified bitmap
      end = HB_OFFSET_TO_INDEX(heap_end_ - heap_begin_);
    }
  }
}

// Walk through the bitmaps in increasing address order, and find the
// object pointers that correspond to garbage objects.  Call
// <callback> zero or more times with lists of these object pointers.
//
// The callback is not permitted to increase the max of either bitmap.
void HeapBitmap::SweepWalk(const HeapBitmap& live_bitmap,
                           const HeapBitmap& mark_bitmap,
                           uintptr_t sweep_begin, uintptr_t sweep_end,
                           HeapBitmap::SweepCallback* callback, void* arg) {
  CHECK(live_bitmap.bitmap_begin_ != NULL);
  CHECK(mark_bitmap.bitmap_begin_ != NULL);
  CHECK_EQ(live_bitmap.heap_begin_, mark_bitmap.heap_begin_);
  CHECK_EQ(live_bitmap.bitmap_size_, mark_bitmap.bitmap_size_);
  CHECK(callback != NULL);
  CHECK_LE(sweep_begin, sweep_end);
  CHECK_GE(sweep_begin, live_bitmap.heap_begin_);
  sweep_end = std::min(sweep_end - 1, live_bitmap.heap_end_);
  if (live_bitmap.heap_end_ < live_bitmap.heap_begin_) {
    // Easy case; both are obviously empty.
    // TODO: this should never happen
    return;
  }
  // TODO: rewrite the callbacks to accept a std::vector<Object*> rather than a Object**?
  std::vector<Object*> pointer_buf(4 * kBitsPerWord);
  Object** pb = &pointer_buf[0];
  size_t start = HB_OFFSET_TO_INDEX(sweep_begin - live_bitmap.heap_begin_);
  size_t end = HB_OFFSET_TO_INDEX(sweep_end - live_bitmap.heap_begin_);
  word* live = live_bitmap.bitmap_begin_;
  word* mark = mark_bitmap.bitmap_begin_;
  for (size_t i = start; i <= end; i++) {
    word garbage = live[i] & ~mark[i];
    if (UNLIKELY(garbage != 0)) {
      word high_bit = 1 << (kBitsPerWord - 1);
      uintptr_t ptr_base = HB_INDEX_TO_OFFSET(i) + live_bitmap.heap_begin_;
      while (garbage != 0) {
        int shift = CLZ(garbage);
        garbage &= ~(high_bit >> shift);
        *pb++ = reinterpret_cast<Object*>(ptr_base + shift * kAlignment);
      }
      // Make sure that there are always enough slots available for an
      // entire word of one bits.
      if (pb >= &pointer_buf[pointer_buf.size() - kBitsPerWord]) {
        (*callback)(pb - &pointer_buf[0], &pointer_buf[0], arg);
        pb = &pointer_buf[0];
      }
    }
  }
  if (pb > &pointer_buf[0]) {
    (*callback)(pb - &pointer_buf[0], &pointer_buf[0], arg);
  }
}

}  // namespace art