v8/src/zone.cc

// Copyright 2012 the V8 project authors. All rights reserved.
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// modification, are permitted provided that the following conditions are
// met:
//
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#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)
    : allocation_size_(0),
      segment_bytes_allocated_(0),
      position_(0),
      limit_(0),
      segment_head_(NULL),
      isolate_(isolate) {
}


Zone::~Zone() {
#ifdef DEBUG
  // Constant byte value used for zapping dead memory in debug mode.
  static const unsigned char kZapDeadByte = 0xcd;
#endif

  // Traverse the chained list of segments, zapping
  // (in debug mode) and freeing every segment
  Segment* current = segment_head_;
  while (current != NULL) {
    Segment* next = current->next();
    int size = current->size();
#ifdef DEBUG
    // Zap the entire current segment (including the header).
    memset(current, kZapDeadByte, size);
#endif
    DeleteSegment(current, size);
    current = next;
  }

  // We must clear the position and limit to force
  // a new segment to be allocated on demand.
  position_ = limit_ = 0;

  // Update the head segment.
  segment_head_ = NULL;
}


// 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);
}


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(size > limit_ - position_);

  // 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_no_overhead = size + (old_size << 1);
  int new_size = kSegmentOverhead + new_size_no_overhead;
  // Guard against integer overflow.
  if (new_size_no_overhead < size || new_size < kSegmentOverhead) {
    V8::FatalProcessOutOfMemory("Zone");
    return NULL;
  }
  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 = NewSegment(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.
  if (position_ < result) {
    V8::FatalProcessOutOfMemory("Zone");
    return NULL;
  }
  limit_ = segment->end();
  ASSERT(position_ <= limit_);
  return result;
}


} }  // namespace v8::internal