annotate src/share/vm/oops/symbol.hpp @ 14909:4ca6dc0799b6

Backout jdk9 merge
author Gilles Duboscq <duboscq@ssw.jku.at>
date Tue, 01 Apr 2014 13:57:07 +0200
parents cd6b3f1a94ff
children 52b4284cb496
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1 /*
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2 * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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4 *
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5 * This code is free software; you can redistribute it and/or modify it
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6 * under the terms of the GNU General Public License version 2 only, as
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7 * published by the Free Software Foundation.
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8 *
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9 * This code is distributed in the hope that it will be useful, but WITHOUT
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10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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12 * version 2 for more details (a copy is included in the LICENSE file that
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13 * accompanied this code).
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14 *
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15 * You should have received a copy of the GNU General Public License version
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16 * 2 along with this work; if not, write to the Free Software Foundation,
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17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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18 *
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19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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20 * or visit www.oracle.com if you need additional information or have any
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21 * questions.
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22 *
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23 */
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24
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25 #ifndef SHARE_VM_OOPS_SYMBOL_HPP
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26 #define SHARE_VM_OOPS_SYMBOL_HPP
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27
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28 #include "utilities/utf8.hpp"
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29 #include "memory/allocation.hpp"
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30 #include "runtime/atomic.hpp"
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31
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32 // A Symbol is a canonicalized string.
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33 // All Symbols reside in global SymbolTable and are reference counted.
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34
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35 // Reference counting
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36 //
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37 // All Symbols are allocated and added to the SymbolTable.
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38 // When a class is unloaded, the reference counts of the Symbol pointers in
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39 // the ConstantPool and in InstanceKlass (see release_C_heap_structures) are
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40 // decremented. When the reference count for a Symbol goes to 0, the garbage
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41 // collector can free the Symbol and remove it from the SymbolTable.
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42 //
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43 // 0) Symbols need to be reference counted when a pointer to the Symbol is
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44 // saved in persistent storage. This does not include the pointer
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45 // in the SymbolTable bucket (the _literal field in HashtableEntry)
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46 // that points to the Symbol. All other stores of a Symbol*
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47 // to a field of a persistent variable (e.g., the _name filed in
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48 // fieldDescriptor or _ptr in a CPSlot) is reference counted.
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49 //
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50 // 1) The lookup of a "name" in the SymbolTable either creates a Symbol F for
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51 // "name" and returns a pointer to F or finds a pre-existing Symbol F for
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52 // "name" and returns a pointer to it. In both cases the reference count for F
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53 // is incremented under the assumption that a pointer to F will be created from
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54 // the return value. Thus the increment of the reference count is on the lookup
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55 // and not on the assignment to the new Symbol*. That is
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56 // Symbol* G = lookup()
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57 // ^ increment on lookup()
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58 // and not
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59 // Symbol* G = lookup()
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60 // ^ increment on assignmnet
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61 // The reference count must be decremented manually when the copy of the
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62 // pointer G is destroyed.
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63 //
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64 // 2) For a local Symbol* A that is a copy of an existing Symbol* B, the
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65 // reference counting is elided when the scope of B is greater than the scope
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66 // of A. For example, in the code fragment
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67 // below "klass" is passed as a parameter to the method. Symbol* "kn"
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68 // is a copy of the name in "klass".
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69 //
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70 // Symbol* kn = klass->name();
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71 // unsigned int d_hash = dictionary()->compute_hash(kn, class_loader);
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72 //
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73 // The scope of "klass" is greater than the scope of "kn" so the reference
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74 // counting for "kn" is elided.
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75 //
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76 // Symbol* copied from ConstantPool entries are good candidates for reference
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77 // counting elision. The ConstantPool entries for a class C exist until C is
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78 // unloaded. If a Symbol* is copied out of the ConstantPool into Symbol* X,
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79 // the Symbol* in the ConstantPool will in general out live X so the reference
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80 // counting on X can be elided.
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81 //
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82 // For cases where the scope of A is not greater than the scope of B,
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83 // the reference counting is explicitly done. See ciSymbol,
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84 // ResolutionErrorEntry and ClassVerifier for examples.
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85 //
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86 // 3) When a Symbol K is created for temporary use, generally for substrings of
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87 // an existing symbol or to create a new symbol, assign it to a
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88 // TempNewSymbol. The SymbolTable methods new_symbol(), lookup()
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89 // and probe() all potentially return a pointer to a new Symbol.
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90 // The allocation (or lookup) of K increments the reference count for K
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91 // and the destructor decrements the reference count.
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92 //
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93 // Another example of TempNewSymbol usage is parsed_name used in
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94 // ClassFileParser::parseClassFile() where parsed_name is used in the cleanup
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95 // after a failed attempt to load a class. Here parsed_name is a
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96 // TempNewSymbol (passed in as a parameter) so the reference count on its symbol
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97 // will be decremented when it goes out of scope.
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99
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100 // This cannot be inherited from ResourceObj because it cannot have a vtable.
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101 // Since sometimes this is allocated from Metadata, pick a base allocation
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102 // type without virtual functions.
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103 class ClassLoaderData;
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104
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105 // We separate the fields in SymbolBase from Symbol::_body so that
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106 // Symbol::size(int) can correctly calculate the space needed.
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107 class SymbolBase : public MetaspaceObj {
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108 public:
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109 ATOMIC_SHORT_PAIR(
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110 volatile short _refcount, // needs atomic operation
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111 unsigned short _length // number of UTF8 characters in the symbol (does not need atomic op)
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112 );
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113 int _identity_hash;
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114 };
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115
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116 class Symbol : private SymbolBase {
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117 friend class VMStructs;
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118 friend class SymbolTable;
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119 friend class MoveSymbols;
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120 private:
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121 jbyte _body[1];
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122
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123 enum {
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124 // max_symbol_length is constrained by type of _length
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125 max_symbol_length = (1 << 16) -1
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126 };
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127
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128 static int size(int length) {
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129 size_t sz = heap_word_size(sizeof(SymbolBase) + (length > 0 ? length : 0));
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130 return align_object_size(sz);
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131 }
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132
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133 void byte_at_put(int index, int value) {
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134 assert(index >=0 && index < _length, "symbol index overflow");
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135 _body[index] = value;
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136 }
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137
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138 Symbol(const u1* name, int length, int refcount);
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139 void* operator new(size_t size, int len, TRAPS) throw();
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140 void* operator new(size_t size, int len, Arena* arena, TRAPS) throw();
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141 void* operator new(size_t size, int len, ClassLoaderData* loader_data, TRAPS) throw();
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142
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143 void operator delete(void* p);
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144
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145 public:
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146 // Low-level access (used with care, since not GC-safe)
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147 const jbyte* base() const { return &_body[0]; }
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148
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149 int size() { return size(utf8_length()); }
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150
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151 // Returns the largest size symbol we can safely hold.
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152 static int max_length() { return max_symbol_length; }
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153
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154 int identity_hash() { return _identity_hash; }
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155
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156 // For symbol table alternate hashing
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157 unsigned int new_hash(jint seed);
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158
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159 // Reference counting. See comments above this class for when to use.
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160 int refcount() const { return _refcount; }
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161 void increment_refcount();
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162 void decrement_refcount();
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163
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164 int byte_at(int index) const {
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165 assert(index >=0 && index < _length, "symbol index overflow");
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166 return base()[index];
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167 }
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168
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169 const jbyte* bytes() const { return base(); }
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170
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171 int utf8_length() const { return _length; }
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172
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173 // Compares the symbol with a string.
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174 bool equals(const char* str, int len) const;
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175 bool equals(const char* str) const { return equals(str, (int) strlen(str)); }
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176
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177 // Tests if the symbol starts with the given prefix.
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178 bool starts_with(const char* prefix, int len) const;
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179 bool starts_with(const char* prefix) const {
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180 return starts_with(prefix, (int) strlen(prefix));
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181 }
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182
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183 // Tests if the symbol starts with the given prefix.
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184 int index_of_at(int i, const char* str, int len) const;
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185 int index_of_at(int i, const char* str) const {
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186 return index_of_at(i, str, (int) strlen(str));
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187 }
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188
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189 // Three-way compare for sorting; returns -1/0/1 if receiver is </==/> than arg
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190 // note that the ordering is not alfabetical
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191 inline int fast_compare(Symbol* other) const;
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192
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193 // Returns receiver converted to null-terminated UTF-8 string; string is
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194 // allocated in resource area, or in the char buffer provided by caller.
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195 char* as_C_string() const;
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196 char* as_C_string(char* buf, int size) const;
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197 // Use buf if needed buffer length is <= size.
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198 char* as_C_string_flexible_buffer(Thread* t, char* buf, int size) const;
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199
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200 // Returns an escaped form of a Java string.
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201 char* as_quoted_ascii() const;
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202
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203 // Returns a null terminated utf8 string in a resource array
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204 char* as_utf8() const { return as_C_string(); }
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205 char* as_utf8_flexible_buffer(Thread* t, char* buf, int size) const {
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206 return as_C_string_flexible_buffer(t, buf, size);
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207 }
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208
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209 jchar* as_unicode(int& length) const;
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210
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211 // Treating this symbol as a class name, returns the Java name for the class.
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212 // String is allocated in resource area if buffer is not provided.
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213 // See Klass::external_name()
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214 const char* as_klass_external_name() const;
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215 const char* as_klass_external_name(char* buf, int size) const;
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216
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217 // Printing
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218 void print_symbol_on(outputStream* st = NULL) const;
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219 void print_on(outputStream* st) const; // First level print
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220 void print_value_on(outputStream* st) const; // Second level print.
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221
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222 // printing on default output stream
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223 void print() { print_on(tty); }
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224 void print_value() { print_value_on(tty); }
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225
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226 #ifndef PRODUCT
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227 // Empty constructor to create a dummy symbol object on stack
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228 // only for getting its vtable pointer.
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229 Symbol() { }
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230
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231 static int _total_count;
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232 #endif
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233 };
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234
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235 // Note: this comparison is used for vtable sorting only; it doesn't matter
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236 // what order it defines, as long as it is a total, time-invariant order
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237 // Since Symbol*s are in C_HEAP, their relative order in memory never changes,
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238 // so use address comparison for speed
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239 int Symbol::fast_compare(Symbol* other) const {
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240 return (((uintptr_t)this < (uintptr_t)other) ? -1
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241 : ((uintptr_t)this == (uintptr_t) other) ? 0 : 1);
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242 }
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243 #endif // SHARE_VM_OOPS_SYMBOL_HPP