src/zone.h

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// Copyright 2006-2008 the V8 project authors. All rights reserved.
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#ifndef V8_ZONE_H_
#define V8_ZONE_H_

namespace v8 { namespace internal {


// Zone scopes are in one of two modes.  Either they delete the zone
// on exit or they do not.
enum ZoneScopeMode {
  DELETE_ON_EXIT,
  DONT_DELETE_ON_EXIT
};


// The Zone supports very fast allocation of small chunks of
// memory. The chunks cannot be deallocated individually, but instead
// the Zone supports deallocating all chunks in one fast
// operation. The Zone is used to hold temporary data structures like
// the abstract syntax tree, which is deallocated after compilation.

// Note: There is no need to initialize the Zone; the first time an
// allocation is attempted, a segment of memory will be requested
// through a call to malloc().

// Note: The implementation is inherently not thread safe. Do not use
// from multi-threaded code.

class Zone {
 public:
  // Allocate 'size' bytes of memory in the Zone; expands the Zone by
  // allocating new segments of memory on demand using malloc().
  static inline void* New(int size);

  // Delete all objects and free all memory allocated in the Zone.
  static void DeleteAll();

 private:
  // All pointers returned from New() have this alignment.
  static const int kAlignment = kPointerSize;

  // Never allocate segments smaller than this size in bytes.
  static const int kMinimumSegmentSize = 8 * KB;

  // Never keep segments larger than this size in bytes around.
  static const int kMaximumKeptSegmentSize = 64 * KB;


  // The Zone is intentionally a singleton; you should not try to
  // allocate instances of the class.
  Zone() { UNREACHABLE(); }


  // Expand the Zone to hold at least 'size' more bytes and allocate
  // the bytes. Returns the address of the newly allocated chunk of
  // memory in the Zone. Should only be called if there isn't enough
  // room in the Zone already.
  static Address NewExpand(int size);


  // The free region in the current (front) segment is represented as
  // the half-open interval [position, limit). The 'position' variable
  // is guaranteed to be aligned as dictated by kAlignment.
  static Address position_;
  static Address limit_;
};


// ZoneObject is an abstraction that helps define classes of objects
// allocated in the Zone. Use it as a base class; see ast.h.
class ZoneObject {
 public:
  // Allocate a new ZoneObject of 'size' bytes in the Zone.
  void* operator new(size_t size) { return Zone::New(size); }

  // Ideally, the delete operator should be private instead of
  // public, but unfortuately the compiler sometimes synthesizes
  // (unused) destructors for classes derived from ZoneObject, which
  // require the operator to be visible. MSVC requires the delete
  // operator to be public.

  // ZoneObjects should never be deleted individually; use
  // Zone::DeleteAll() to delete all zone objects in one go.
  void operator delete(void*, size_t) { UNREACHABLE(); }
};


class AssertNoZoneAllocation {
 public:
  AssertNoZoneAllocation() : prev_(allow_allocation_) {
    allow_allocation_ = false;
  }
  ~AssertNoZoneAllocation() { allow_allocation_ = prev_; }
  static bool allow_allocation() { return allow_allocation_; }
 private:
  bool prev_;
  static bool allow_allocation_;
};


// The ZoneListAllocationPolicy is used to specialize the GenericList
// implementation to allocate ZoneLists and their elements in the
// Zone.
class ZoneListAllocationPolicy {
 public:
  // Allocate 'size' bytes of memory in the zone.
  static void* New(int size) {  return Zone::New(size); }

  // De-allocation attempts are silently ignored.
  static void Delete(void* p) { }
};


// ZoneLists are growable lists with constant-time access to the
// elements. The list itself and all its elements are allocated in the
// Zone. ZoneLists cannot be deleted individually; you can delete all
// objects in the Zone by calling Zone::DeleteAll().
template<typename T>
class ZoneList: public List<T, ZoneListAllocationPolicy> {
 public:
  // Construct a new ZoneList with the given capacity; the length is
  // always zero. The capacity must be non-negative.
  explicit ZoneList(int capacity)
      : List<T, ZoneListAllocationPolicy>(capacity) { }
};


// ZoneScopes keep track of the current parsing and compilation
// nesting and cleans up generated ASTs in the Zone when exiting the
// outer-most scope.
class ZoneScope BASE_EMBEDDED {
 public:
  explicit ZoneScope(ZoneScopeMode mode) : mode_(mode) {
    nesting_++;
  }

  ~ZoneScope() {
    if (--nesting_ == 0 && mode_ == DELETE_ON_EXIT) Zone::DeleteAll();
  }

  // For ZoneScopes that do not delete on exit by default, call this
  // method to request deletion on exit.
  void DeleteOnExit() {
    mode_ = DELETE_ON_EXIT;
  }

 private:
  ZoneScopeMode mode_;
  static int nesting_;
};


} }  // namespace v8::internal

#endif  // V8_ZONE_H_