/*

 * Copyright (c) 1997, 2012, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 */

#ifndef SHARE_VM_OOPS_SYMBOL_HPP

#define SHARE_VM_OOPS_SYMBOL_HPP

#include "utilities/utf8.hpp"

#include "memory/allocation.hpp"

#include "runtime/atomic.hpp"

// A Symbol is a canonicalized string.

// All Symbols reside in global SymbolTable and are reference counted.

// Reference counting

//

// All Symbols are allocated and added to the SymbolTable.

// When a class is unloaded, the reference counts of the Symbol pointers in

// decremented.  When the reference count for a Symbol goes to 0, the garbage

// collector can free the Symbol and remove it from the SymbolTable.

//

// 0) Symbols need to be reference counted when a pointer to the Symbol is

// saved in persistent storage.  This does not include the pointer

// in the SymbolTable bucket (the _literal field in HashtableEntry)

// that points to the Symbol.  All other stores of a Symbol*

// to a field of a persistent variable (e.g., the _name filed in

// FieldAccessInfo or _ptr in a CPSlot) is reference counted.

//

// 1) The lookup of a "name" in the SymbolTable either creates a Symbol F for

// "name" and returns a pointer to F or finds a pre-existing Symbol F for

// "name" and returns a pointer to it. In both cases the reference count for F

// is incremented under the assumption that a pointer to F will be created from

// the return value. Thus the increment of the reference count is on the lookup

// and not on the assignment to the new Symbol*.  That is

//    Symbol* G = lookup()

//                ^ increment on lookup()

// and not

//    Symbol* G = lookup()

//              ^ increment on assignmnet

// The reference count must be decremented manually when the copy of the

// pointer G is destroyed.

//

// 2) For a local Symbol* A that is a copy of an existing Symbol* B, the

// reference counting is elided when the scope of B is greater than the scope

// of A.  For example, in the code fragment

// below "klass" is passed as a parameter to the method.  Symbol* "kn"

// is a copy of the name in "klass".

//

//   Symbol*  kn = klass->name();

//   unsigned int d_hash = dictionary()->compute_hash(kn, class_loader);

//

// The scope of "klass" is greater than the scope of "kn" so the reference

// counting for "kn" is elided.

//

// Symbol* copied from ConstantPool entries are good candidates for reference

// counting elision.  The ConstantPool entries for a class C exist until C is

// unloaded.  If a Symbol* is copied out of the ConstantPool into Symbol* X,

// the Symbol* in the ConstantPool will in general out live X so the reference

// counting on X can be elided.

//

// For cases where the scope of A is not greater than the scope of B,

// the reference counting is explicitly done.  See ciSymbol,

// ResolutionErrorEntry and ClassVerifier for examples.

//

// 3) When a Symbol K is created for temporary use, generally for substrings of

// an existing symbol or to create a new symbol, assign it to a

// TempNewSymbol. The SymbolTable methods new_symbol(), lookup()

// and probe() all potentially return a pointer to a new Symbol.

// The allocation (or lookup) of K increments the reference count for K

// and the destructor decrements the reference count.

//

// Another example of TempNewSymbol usage is parsed_name used in

// ClassFileParser::parseClassFile() where parsed_name is used in the cleanup

// after a failed attempt to load a class.  Here parsed_name is a

// TempNewSymbol (passed in as a parameter) so the reference count on its symbol

// will be decremented when it goes out of scope.

  friend class VMStructs;

  friend class SymbolTable;

  friend class MoveSymbols;

 private:

  volatile int   _refcount;

  int            _identity_hash;

  unsigned short _length; // number of UTF8 characters in the symbol

  jbyte _body[1];

  enum {

    // max_symbol_length is constrained by type of _length

    max_symbol_length = (1 << 16) -1

  };

  }

  void byte_at_put(int index, int value) {

    assert(index >=0 && index < _length, "symbol index overflow");

    _body[index] = value;

  }

  Symbol(const u1* name, int length, int refcount);

  void* operator new(size_t size, int len, TRAPS);

  void* operator new(size_t size, int len, Arena* arena, TRAPS);

 public:

  // Low-level access (used with care, since not GC-safe)

  const jbyte* base() const { return &_body[0]; }

  // Returns the largest size symbol we can safely hold.

  static int max_length()   { return max_symbol_length; }

  int identity_hash()       { return _identity_hash; }

  // Reference counting.  See comments above this class for when to use.

  int refcount() const      { return _refcount; }

  inline void increment_refcount();

  inline void decrement_refcount();

  int byte_at(int index) const {

    assert(index >=0 && index < _length, "symbol index overflow");

    return base()[index];

  }

  const jbyte* bytes() const { return base(); }

  int utf8_length() const { return _length; }

  // Compares the symbol with a string.

  bool equals(const char* str, int len) const;

  bool equals(const char* str) const { return equals(str, (int) strlen(str)); }

  // Tests if the symbol starts with the given prefix.

  bool starts_with(const char* prefix, int len) const;

  bool starts_with(const char* prefix) const {

    return starts_with(prefix, (int) strlen(prefix));

  }

  // Tests if the symbol starts with the given prefix.

  int index_of_at(int i, const char* str, int len) const;

  int index_of_at(int i, const char* str) const {

    return index_of_at(i, str, (int) strlen(str));

  }

  // Three-way compare for sorting; returns -1/0/1 if receiver is </==/> than arg

  // note that the ordering is not alfabetical

  inline int fast_compare(Symbol* other) const;

  // Returns receiver converted to null-terminated UTF-8 string; string is

  // allocated in resource area, or in the char buffer provided by caller.

  char* as_C_string() const;

  char* as_C_string(char* buf, int size) const;

  // Use buf if needed buffer length is <= size.

  char* as_C_string_flexible_buffer(Thread* t, char* buf, int size) const;

  // Returns a null terminated utf8 string in a resource array

  char* as_utf8() const { return as_C_string(); }

  char* as_utf8_flexible_buffer(Thread* t, char* buf, int size) const {

    return as_C_string_flexible_buffer(t, buf, size);

  }

  jchar* as_unicode(int& length) const;

  // Treating this symbol as a class name, returns the Java name for the class.

  // String is allocated in resource area if buffer is not provided.

  // See Klass::external_name()

  const char* as_klass_external_name() const;

  const char* as_klass_external_name(char* buf, int size) const;

  // Printing

  void print_symbol_on(outputStream* st = NULL) const;

  void print_on(outputStream* st) const;         // First level print

  void print_value_on(outputStream* st) const;   // Second level print.

  // printing on default output stream

  void print()         { print_on(tty);       }

  void print_value()   { print_value_on(tty); }

#ifndef PRODUCT

  // Empty constructor to create a dummy symbol object on stack

  // only for getting its vtable pointer.

  Symbol() { }

  static int _total_count;

#endif

};

// Note: this comparison is used for vtable sorting only; it doesn't matter

// what order it defines, as long as it is a total, time-invariant order

// Since Symbol*s are in C_HEAP, their relative order in memory never changes,

// so use address comparison for speed

int Symbol::fast_compare(Symbol* other) const {

 return (((uintptr_t)this < (uintptr_t)other) ? -1

   : ((uintptr_t)this == (uintptr_t) other) ? 0 : 1);

}

inline void Symbol::increment_refcount() {

  // Only increment the refcount if positive.  If negative either

  // overflow has occurred or it is a permanent symbol in a read only

  // shared archive.

  if (_refcount >= 0) {

    Atomic::inc(&_refcount);

    NOT_PRODUCT(Atomic::inc(&_total_count);)

  }

}

inline void Symbol::decrement_refcount() {

  if (_refcount >= 0) {

    Atomic::dec(&_refcount);

#ifdef ASSERT

    if (_refcount < 0) {

      print();

      assert(false, "reference count underflow for symbol");

    }

#endif

  }

}

#endif // SHARE_VM_OOPS_SYMBOL_HPP