CSE2410 Fall 2014 Bounce

// Copyright 2006-2008 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 "v8.h"

#include "zone-inl.h"

namespace v8 { namespace internal {

Address Zone::position_ = 0;

Address Zone::limit_ = 0;

int Zone::zone_excess_limit_ = 256 * MB;

int Zone::segment_bytes_allocated_ = 0;

bool AssertNoZoneAllocation::allow_allocation_ = true;

int ZoneScope::nesting_ = 0;

// 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:

  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_); }

  static Segment* head() { return head_; }

  static void set_head(Segment* head) { head_ = head; }

  // Creates a new segment, sets it size, and pushes it to the front

  // of the segment chain. Returns the new segment.

  static Segment* New(int size) {

    Segment* result = reinterpret_cast<Segment*>(Malloced::New(size));

    Zone::adjust_segment_bytes_allocated(size);

    if (result != NULL) {

      result->next_ = head_;

      result->size_ = size;

      head_ = result;

    }

    return result;

  }

  // Deletes the given segment. Does not touch the segment chain.

  static void Delete(Segment* segment, int size) {

    Zone::adjust_segment_bytes_allocated(-size);

    Malloced::Delete(segment);

  }

  static int bytes_allocated() { return bytes_allocated_; }

 private:

  // Computes the address of the nth byte in this segment.

  Address address(int n) const {

    return Address(this) + n;

  }

  static Segment* head_;

  static int bytes_allocated_;

  Segment* next_;

  int size_;

};

Segment* Segment::head_ = NULL;

int Segment::bytes_allocated_ = 0;

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

      Segment::Delete(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::set_head(keep);

}

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(position_ + size > limit_);

  // 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();

  int old_size = (head == NULL) ? 0 : head->size();

  static const int kSegmentOverhead = sizeof(Segment) + kAlignment;

  int new_size = kSegmentOverhead + size + (old_size << 1);

  if (new_size < kMinimumSegmentSize) {

    new_size = kMinimumSegmentSize;

  } else if (new_size > 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(kSegmentOverhead + size, kMaximumSegmentSize);

  }

  Segment* segment = Segment::New(new_size);

  if (segment == NULL) V8::FatalProcessOutOfMemory("Zone");

  // Recompute 'top' and 'limit' based on the new segment.

  Address result = RoundUp(segment->start(), kAlignment);

  position_ = result + size;

  limit_ = segment->end();

  ASSERT(position_ <= limit_);

  return result;

}

} }  // namespace v8::internal