text: /*
   * Copyright (c) 1997, 2019, 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
   * or visit www.oracle.com if you need additional information or have any
   * questions.
   *
   */
  
  #ifndef SHARE_OOPS_SYMBOL_HPP
  #define SHARE_OOPS_SYMBOL_HPP
  
  #include "memory/allocation.hpp"
  #include "utilities/exceptions.hpp"
  #include "utilities/macros.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
  // the ConstantPool and in InstanceKlass (see release_C_heap_structures) are
  // 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
  // fieldDescriptor 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.
  //
  // This cannot be inherited from ResourceObj because it cannot have a vtable.
  // Since sometimes this is allocated from Metadata, pick a base allocation
  // type without virtual functions.
  class ClassLoaderData;
  
  // Set _refcount to PERM_REFCOUNT to prevent the Symbol from being freed.
  #ifndef PERM_REFCOUNT
  #define PERM_REFCOUNT ((1 << 16) - 1)
  #endif
  
  class Symbol : public MetaspaceObj {
    friend class VMStructs;
    friend class SymbolTable;
  
   private:
  
    // This is an int because it needs atomic operation on the refcount.  Mask length
    // in high half word. length is the number of UTF8 characters in the symbol
    volatile uint32_t _length_and_refcount;
    short _identity_hash;
    u1 _body[2];
  
    enum {
      // max_symbol_length must fit into the top 16 bits of _length_and_refcount
      max_symbol_length = (1 << 16) -1
    };
  
    static int byte_size(int length) {
      // minimum number of natural words needed to hold these bits (no non-heap version)
      return (int)(sizeof(Symbol) + (length > 2 ? length - 2 : 0));
    }
    static int size(int length) {
      // minimum number of natural words needed to hold these bits (no non-heap version)
      return (int)heap_word_size(byte_size(length));
    }
  
    void byte_at_put(int index, u1 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) throw();
    void* operator new(size_t size, int len, Arena* arena) throw();
  
    void  operator delete(void* p);
  
    static int extract_length(uint32_t value)   { return value >> 16; }
    static int extract_refcount(uint32_t value) { return value & 0xffff; }
    static uint32_t pack_length_and_refcount(int length, int refcount);
  
    int length() const   { return extract_length(_length_and_refcount); }
  
   public:
    // Low-level access (used with care, since not GC-safe)
    const u1* base() const { return &_body[0]; }
  
    int size()                { return size(utf8_length()); }
    int byte_size()           { return byte_size(utf8_length()); }
  
    // Symbols should be stored in the read-only region of CDS archive.
    static bool is_read_only_by_default() { return true; }
  
    // Returns the largest size symbol we can safely hold.
    static int max_length() { return max_symbol_length; }
    unsigned identity_hash() const {
      unsigned addr_bits = (unsigned)((uintptr_t)this >> (LogMinObjAlignmentInBytes + 3));
      return ((unsigned)_identity_hash & 0xffff) |
             ((addr_bits ^ (length() << 8) ^ (( _body[0] << 8) | _body[1])) << 16);
    }
  
    // Reference counting.  See comments above this class for when to use.
    int refcount() const { return extract_refcount(_length_and_refcount); }
    bool try_increment_refcount();
    void increment_refcount();
    void decrement_refcount();
    bool is_permanent() {
      return (refcount() == PERM_REFCOUNT);
    }
    void set_permanent();
    void make_permanent();
  
    // Function char_at() returns the Symbol's selected u1 byte as a char type.
    //
    // Note that all multi-byte chars have the sign bit set on all their bytes.
    // No single byte chars have their sign bit set.
    char char_at(int index) const {
      assert(index >=0 && index < length(), "symbol index overflow");
      return (char)base()[index];
    }
  
    const u1* bytes() const { return base(); }
  
    int utf8_length() const { return length(); }
  
    // Compares the symbol with a string.
    bool equals(const char* str, int len) const {
      int l = utf8_length();
      if (l != len) return false;
      while (l-- > 0) {
        if (str[l] != char_at(l))
          return false;
      }
      assert(l == -1, "we should be at the beginning");
      return true;
    }
    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;
  
    // 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(const 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;
  
    // Returns an escaped form of a Java string.
    char* as_quoted_ascii() const;
  
    // Returns a null terminated utf8 string in a resource array
    char* as_utf8() const { return as_C_string(); }
  
    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;
  
    // Treating the symbol as a signature, print the return
    // type to the outputStream. Prints external names as 'double' or
    // 'java.lang.Object[][]'.
    void print_as_signature_external_return_type(outputStream *os);
    // Treating the symbol as a signature, print the parameter types
    // seperated by ', ' to the outputStream.  Prints external names as
    //  'double' or 'java.lang.Object[][]'.
    void print_as_signature_external_parameters(outputStream *os);
  
    void metaspace_pointers_do(MetaspaceClosure* it);
    MetaspaceObj::Type type() const { return SymbolType; }
  
    // Printing
    void print_symbol_on(outputStream* st = NULL) const;
    void print_utf8_on(outputStream* st) 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() const;
    void print_value() const;
  
    static bool is_valid(Symbol* s);
  
  #ifndef PRODUCT
    // Empty constructor to create a dummy symbol object on stack
    // only for getting its vtable pointer.
    Symbol() { }
  
    static size_t _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(const Symbol* other) const {
   return (((uintptr_t)this < (uintptr_t)other) ? -1
     : ((uintptr_t)this == (uintptr_t) other) ? 0 : 1);
  }
  #endif // SHARE_OOPS_SYMBOL_HPP
  