hotspot/src/share/vm/oops/symbol.hpp
author coleenp
Sat, 01 Sep 2012 13:25:18 -0400
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6964458: Reimplement class meta-data storage to use native memory Summary: Remove PermGen, allocate meta-data in metaspace linked to class loaders, rewrite GC walking, rewrite and rename metadata to be C++ classes Reviewed-by: jmasa, stefank, never, coleenp, kvn, brutisso, mgerdin, dholmes, jrose, twisti, roland Contributed-by: jmasa <jon.masamitsu@oracle.com>, stefank <stefan.karlsson@oracle.com>, mgerdin <mikael.gerdin@oracle.com>, never <tom.rodriguez@oracle.com>
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/*
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 * Copyright (c) 1997, 2012, Oracle and/or its affiliates. All rights reserved.
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 *
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 * This code is free software; you can redistribute it and/or modify it
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 * under the terms of the GNU General Public License version 2 only, as
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 * published by the Free Software Foundation.
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 *
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 * This code is distributed in the hope that it will be useful, but WITHOUT
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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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 * version 2 for more details (a copy is included in the LICENSE file that
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 * accompanied this code).
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 *
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 * You should have received a copy of the GNU General Public License version
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 * 2 along with this work; if not, write to the Free Software Foundation,
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 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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 *
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 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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 * or visit www.oracle.com if you need additional information or have any
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 * questions.
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 *
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 */
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#ifndef SHARE_VM_OOPS_SYMBOL_HPP
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#define SHARE_VM_OOPS_SYMBOL_HPP
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#include "utilities/utf8.hpp"
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#include "memory/allocation.hpp"
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#include "runtime/atomic.hpp"
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// A Symbol is a canonicalized string.
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// All Symbols reside in global SymbolTable and are reference counted.
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// Reference counting
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//
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// All Symbols are allocated and added to the SymbolTable.
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// When a class is unloaded, the reference counts of the Symbol pointers in
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// the ConstantPool and in InstanceKlass (see release_C_heap_structures) are
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// decremented.  When the reference count for a Symbol goes to 0, the garbage
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// collector can free the Symbol and remove it from the SymbolTable.
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//
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// 0) Symbols need to be reference counted when a pointer to the Symbol is
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// saved in persistent storage.  This does not include the pointer
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// in the SymbolTable bucket (the _literal field in HashtableEntry)
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// that points to the Symbol.  All other stores of a Symbol*
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// to a field of a persistent variable (e.g., the _name filed in
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// FieldAccessInfo or _ptr in a CPSlot) is reference counted.
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//
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// 1) The lookup of a "name" in the SymbolTable either creates a Symbol F for
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// "name" and returns a pointer to F or finds a pre-existing Symbol F for
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// "name" and returns a pointer to it. In both cases the reference count for F
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// is incremented under the assumption that a pointer to F will be created from
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// the return value. Thus the increment of the reference count is on the lookup
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// and not on the assignment to the new Symbol*.  That is
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//    Symbol* G = lookup()
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//                ^ increment on lookup()
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// and not
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//    Symbol* G = lookup()
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//              ^ increment on assignmnet
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// The reference count must be decremented manually when the copy of the
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// pointer G is destroyed.
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//
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// 2) For a local Symbol* A that is a copy of an existing Symbol* B, the
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// reference counting is elided when the scope of B is greater than the scope
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// of A.  For example, in the code fragment
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// below "klass" is passed as a parameter to the method.  Symbol* "kn"
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// is a copy of the name in "klass".
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//
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//   Symbol*  kn = klass->name();
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//   unsigned int d_hash = dictionary()->compute_hash(kn, class_loader);
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//
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// The scope of "klass" is greater than the scope of "kn" so the reference
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// counting for "kn" is elided.
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//
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// Symbol* copied from ConstantPool entries are good candidates for reference
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// counting elision.  The ConstantPool entries for a class C exist until C is
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// unloaded.  If a Symbol* is copied out of the ConstantPool into Symbol* X,
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// the Symbol* in the ConstantPool will in general out live X so the reference
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// counting on X can be elided.
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//
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// For cases where the scope of A is not greater than the scope of B,
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// the reference counting is explicitly done.  See ciSymbol,
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// ResolutionErrorEntry and ClassVerifier for examples.
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//
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// 3) When a Symbol K is created for temporary use, generally for substrings of
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// an existing symbol or to create a new symbol, assign it to a
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// TempNewSymbol. The SymbolTable methods new_symbol(), lookup()
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// and probe() all potentially return a pointer to a new Symbol.
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// The allocation (or lookup) of K increments the reference count for K
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// and the destructor decrements the reference count.
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//
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// Another example of TempNewSymbol usage is parsed_name used in
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// ClassFileParser::parseClassFile() where parsed_name is used in the cleanup
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// after a failed attempt to load a class.  Here parsed_name is a
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// TempNewSymbol (passed in as a parameter) so the reference count on its symbol
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// will be decremented when it goes out of scope.
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// This cannot be inherited from ResourceObj because it cannot have a vtable.
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// Since sometimes this is allocated from Metadata, pick a base allocation
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// type without virtual functions.
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class ClassLoaderData;
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class Symbol : public MetaspaceObj {
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  friend class VMStructs;
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  friend class SymbolTable;
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  friend class MoveSymbols;
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 private:
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  volatile int   _refcount;
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  int            _identity_hash;
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  unsigned short _length; // number of UTF8 characters in the symbol
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  jbyte _body[1];
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  enum {
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    // max_symbol_length is constrained by type of _length
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    max_symbol_length = (1 << 16) -1
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  };
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  static int size(int length) {
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    size_t sz = heap_word_size(sizeof(Symbol) + (length > 0 ? length - 1 : 0));
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    return align_object_size(sz);
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  }
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  void byte_at_put(int index, int value) {
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    assert(index >=0 && index < _length, "symbol index overflow");
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    _body[index] = value;
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  }
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  Symbol(const u1* name, int length, int refcount);
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  void* operator new(size_t size, int len, TRAPS);
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  void* operator new(size_t size, int len, Arena* arena, TRAPS);
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  void* operator new(size_t size, int len, ClassLoaderData* loader_data, TRAPS);
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  void  operator delete(void* p);
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  // Low-level access (used with care, since not GC-safe)
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  const jbyte* base() const { return &_body[0]; }
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  int size()                { return size(utf8_length()); }
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  // Returns the largest size symbol we can safely hold.
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  static int max_length()   { return max_symbol_length; }
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  int identity_hash()       { return _identity_hash; }
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  // For symbol table alternate hashing
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  unsigned int new_hash(jint seed);
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  // Reference counting.  See comments above this class for when to use.
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  int refcount() const      { return _refcount; }
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  inline void increment_refcount();
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  inline void decrement_refcount();
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  int byte_at(int index) const {
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    assert(index >=0 && index < _length, "symbol index overflow");
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    return base()[index];
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  }
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  const jbyte* bytes() const { return base(); }
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  int utf8_length() const { return _length; }
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  // Compares the symbol with a string.
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  bool equals(const char* str, int len) const;
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  bool equals(const char* str) const { return equals(str, (int) strlen(str)); }
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  // Tests if the symbol starts with the given prefix.
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  bool starts_with(const char* prefix, int len) const;
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  bool starts_with(const char* prefix) const {
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    return starts_with(prefix, (int) strlen(prefix));
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  }
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  // Tests if the symbol starts with the given prefix.
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  int index_of_at(int i, const char* str, int len) const;
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  int index_of_at(int i, const char* str) const {
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    return index_of_at(i, str, (int) strlen(str));
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  }
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  // Three-way compare for sorting; returns -1/0/1 if receiver is </==/> than arg
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  // note that the ordering is not alfabetical
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  inline int fast_compare(Symbol* other) const;
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  // Returns receiver converted to null-terminated UTF-8 string; string is
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  // allocated in resource area, or in the char buffer provided by caller.
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  char* as_C_string() const;
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  char* as_C_string(char* buf, int size) const;
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  // Use buf if needed buffer length is <= size.
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  char* as_C_string_flexible_buffer(Thread* t, char* buf, int size) const;
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  // Returns a null terminated utf8 string in a resource array
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  char* as_utf8() const { return as_C_string(); }
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  char* as_utf8_flexible_buffer(Thread* t, char* buf, int size) const {
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    return as_C_string_flexible_buffer(t, buf, size);
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  }
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  jchar* as_unicode(int& length) const;
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  // Treating this symbol as a class name, returns the Java name for the class.
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  // String is allocated in resource area if buffer is not provided.
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  // See Klass::external_name()
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  const char* as_klass_external_name() const;
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  const char* as_klass_external_name(char* buf, int size) const;
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  // Printing
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  void print_symbol_on(outputStream* st = NULL) const;
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  void print_on(outputStream* st) const;         // First level print
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  void print_value_on(outputStream* st) const;   // Second level print.
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  // printing on default output stream
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  void print()         { print_on(tty);       }
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  void print_value()   { print_value_on(tty); }
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#ifndef PRODUCT
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  // Empty constructor to create a dummy symbol object on stack
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  // only for getting its vtable pointer.
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  Symbol() { }
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  static int _total_count;
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#endif
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};
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// Note: this comparison is used for vtable sorting only; it doesn't matter
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// what order it defines, as long as it is a total, time-invariant order
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// Since Symbol*s are in C_HEAP, their relative order in memory never changes,
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// so use address comparison for speed
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int Symbol::fast_compare(Symbol* other) const {
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 return (((uintptr_t)this < (uintptr_t)other) ? -1
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   : ((uintptr_t)this == (uintptr_t) other) ? 0 : 1);
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}
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inline void Symbol::increment_refcount() {
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  // Only increment the refcount if positive.  If negative either
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  // overflow has occurred or it is a permanent symbol in a read only
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  // shared archive.
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  if (_refcount >= 0) {
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    Atomic::inc(&_refcount);
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    NOT_PRODUCT(Atomic::inc(&_total_count);)
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  }
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}
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inline void Symbol::decrement_refcount() {
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  if (_refcount >= 0) {
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    Atomic::dec(&_refcount);
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#ifdef ASSERT
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    if (_refcount < 0) {
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      print();
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      assert(false, "reference count underflow for symbol");
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    }
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#endif
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  }
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}
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#endif // SHARE_VM_OOPS_SYMBOL_HPP