text: // Copyright 2012 the V8 project authors. All rights reserved.
  // Redistribution and use in source and binary forms, with or without
  // modification, are permitted provided that the following conditions are
  // met:
  //
  //     * Redistributions of source code must retain the above copyright
  //       notice, this list of conditions and the following disclaimer.
  //     * Redistributions in binary form must reproduce the above
  //       copyright notice, this list of conditions and the following
  //       disclaimer in the documentation and/or other materials provided
  //       with the distribution.
  //     * Neither the name of Google Inc. nor the names of its
  //       contributors may be used to endorse or promote products derived
  //       from this software without specific prior written permission.
  //
  // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  
  #include <string.h>
  
  #include "v8.h"
  #include "zone-inl.h"
  
  namespace v8 {
  namespace internal {
  
  
  // Segments represent chunks of memory: They have starting address
  // (encoded in the this pointer) and a size in bytes. Segments are
  // chained together forming a LIFO structure with the newest segment
  // available as segment_head_. Segments are allocated using malloc()
  // and de-allocated using free().
  
  class Segment {
   public:
    void Initialize(Segment* next, int size) {
      next_ = next;
      size_ = size;
    }
  
    Segment* next() const { return next_; }
    void clear_next() { next_ = NULL; }
  
    int size() const { return size_; }
    int capacity() const { return size_ - sizeof(Segment); }
  
    Address start() const { return address(sizeof(Segment)); }
    Address end() const { return address(size_); }
  
   private:
    // Computes the address of the nth byte in this segment.
    Address address(int n) const {
      return Address(this) + n;
    }
  
    Segment* next_;
    int size_;
  };
  
  
  Zone::Zone(Isolate* isolate)
      : zone_excess_limit_(256 * MB),
        segment_bytes_allocated_(0),
        position_(0),
        limit_(0),
        scope_nesting_(0),
        segment_head_(NULL),
        isolate_(isolate) {
  }
  unsigned Zone::allocation_size_ = 0;
  
  ZoneScope::~ZoneScope() {
    if (ShouldDeleteOnExit()) zone_->DeleteAll();
    zone_->scope_nesting_--;
  }
  
  
  // Creates a new segment, sets it size, and pushes it to the front
  // of the segment chain. Returns the new segment.
  Segment* Zone::NewSegment(int size) {
    Segment* result = reinterpret_cast<Segment*>(Malloced::New(size));
    adjust_segment_bytes_allocated(size);
    if (result != NULL) {
      result->Initialize(segment_head_, size);
      segment_head_ = result;
    }
    return result;
  }
  
  
  // Deletes the given segment. Does not touch the segment chain.
  void Zone::DeleteSegment(Segment* segment, int size) {
    adjust_segment_bytes_allocated(-size);
    Malloced::Delete(segment);
  }
  
  
  void Zone::DeleteAll() {
  #ifdef DEBUG
    // Constant byte value used for zapping dead memory in debug mode.
    static const unsigned char kZapDeadByte = 0xcd;
  #endif
  
    // Find a segment with a suitable size to keep around.
    Segment* keep = segment_head_;
    while (keep != NULL && keep->size() > kMaximumKeptSegmentSize) {
      keep = keep->next();
    }
  
    // Traverse the chained list of segments, zapping (in debug mode)
    // and freeing every segment except the one we wish to keep.
    Segment* current = segment_head_;
    while (current != NULL) {
      Segment* next = current->next();
      if (current == keep) {
        // Unlink the segment we wish to keep from the list.
        current->clear_next();
      } else {
        int size = current->size();
  #ifdef DEBUG
        // Zap the entire current segment (including the header).
        memset(current, kZapDeadByte, size);
  #endif
        DeleteSegment(current, size);
      }
      current = next;
    }
  
    // If we have found a segment we want to keep, we must recompute the
    // variables 'position' and 'limit' to prepare for future allocate
    // attempts. Otherwise, we must clear the position and limit to
    // force a new segment to be allocated on demand.
    if (keep != NULL) {
      Address start = keep->start();
      position_ = RoundUp(start, kAlignment);
      limit_ = keep->end();
  #ifdef DEBUG
      // Zap the contents of the kept segment (but not the header).
      memset(start, kZapDeadByte, keep->capacity());
  #endif
    } else {
      position_ = limit_ = 0;
    }
  
    // Update the head segment to be the kept segment (if any).
    segment_head_ = keep;
  }
  
  
  void Zone::DeleteKeptSegment() {
    if (segment_head_ != NULL) {
      DeleteSegment(segment_head_, segment_head_->size());
      segment_head_ = NULL;
    }
  }
  
  
  Address Zone::NewExpand(int size) {
    // Make sure the requested size is already properly aligned and that
    // there isn't enough room in the Zone to satisfy the request.
    ASSERT(size == RoundDown(size, kAlignment));
    ASSERT(limit_ < position_ ||
           reinterpret_cast<uintptr_t>(limit_) -
                   reinterpret_cast<uintptr_t>(position_) <
               size);
  
    // Compute the new segment size. We use a 'high water mark'
    // strategy, where we increase the segment size every time we expand
    // except that we employ a maximum segment size when we delete. This
    // is to avoid excessive malloc() and free() overhead.
    Segment* head = segment_head_;
    const size_t old_size = (head == NULL) ? 0 : head->size();
    static const size_t kSegmentOverhead = sizeof(Segment) + kAlignment;
    const size_t new_size_no_overhead = size + (old_size << 1);
    size_t new_size = kSegmentOverhead + new_size_no_overhead;
    const size_t min_new_size = kSegmentOverhead + static_cast<size_t>(size);
    // Guard against integer overflow.
    if (new_size_no_overhead < static_cast<size_t>(size) ||
        new_size < static_cast<size_t>(kSegmentOverhead)) {
      V8::FatalProcessOutOfMemory("Zone");
      return NULL;
    }
    if (new_size < static_cast<size_t>(kMinimumSegmentSize)) {
      new_size = kMinimumSegmentSize;
    } else if (new_size > static_cast<size_t>(kMaximumSegmentSize)) {
      // Limit the size of new segments to avoid growing the segment size
      // exponentially, thus putting pressure on contiguous virtual address space.
      // All the while making sure to allocate a segment large enough to hold the
      // requested size.
      new_size = Max(min_new_size, static_cast<size_t>(kMaximumSegmentSize));
    }
    if (new_size > INT_MAX) {
      V8::FatalProcessOutOfMemory("Zone");
      return NULL;
    }
    Segment* segment = NewSegment(static_cast<int>(new_size));
    if (segment == NULL) {
      V8::FatalProcessOutOfMemory("Zone");
      return NULL;
    }
  
    // Recompute 'top' and 'limit' based on the new segment.
    Address result = RoundUp(segment->start(), kAlignment);
    position_ = result + size;
    // Check for address overflow.
    // (Should not happen since the segment is guaranteed to accomodate
    // size bytes + header and alignment padding)
    if (reinterpret_cast<uintptr_t>(position_)
        < reinterpret_cast<uintptr_t>(result)) {
      V8::FatalProcessOutOfMemory("Zone");
      return NULL;
    }
    limit_ = segment->end();
    ASSERT(position_ <= limit_);
    return result;
  }
  
  
  } }  // namespace v8::internal
  