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 /*

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

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  * 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

 // 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

 // 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.

 class Symbol : public ResourceObj {

   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

   };

   static int object_size(int length) {

     size_t size = heap_word_size(sizeof(Symbol) + (length > 0 ? length - 1 : 0));

     return align_object_size(size);

   }

   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]; }

   int object_size()         { return object_size(utf8_length()); }

   // 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