1 /* 2 * Copyright (c) 1997, 2018, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. 8 * 9 * This code is distributed in the hope that it will be useful, but WITHOUT 10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 12 * version 2 for more details (a copy is included in the LICENSE file that 13 * accompanied this code). 14 * 15 * You should have received a copy of the GNU General Public License version 16 * 2 along with this work; if not, write to the Free Software Foundation, 17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 18 * 19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 20 * or visit www.oracle.com if you need additional information or have any 21 * questions. 22 * 23 */ 24 25 #ifndef SHARE_VM_OOPS_SYMBOL_HPP 26 #define SHARE_VM_OOPS_SYMBOL_HPP 27 28 #include "memory/allocation.hpp" 29 #include "utilities/exceptions.hpp" 30 #include "utilities/macros.hpp" 31 #include "utilities/utf8.hpp" 32 33 // A Symbol is a canonicalized string. 34 // All Symbols reside in global SymbolTable and are reference counted. 35 36 // Reference counting 37 // 38 // All Symbols are allocated and added to the SymbolTable. 39 // When a class is unloaded, the reference counts of the Symbol pointers in 40 // the ConstantPool and in InstanceKlass (see release_C_heap_structures) are 41 // decremented. When the reference count for a Symbol goes to 0, the garbage 42 // collector can free the Symbol and remove it from the SymbolTable. 43 // 44 // 0) Symbols need to be reference counted when a pointer to the Symbol is 45 // saved in persistent storage. This does not include the pointer 46 // in the SymbolTable bucket (the _literal field in HashtableEntry) 47 // that points to the Symbol. All other stores of a Symbol* 48 // to a field of a persistent variable (e.g., the _name filed in 49 // fieldDescriptor or _ptr in a CPSlot) is reference counted. 50 // 51 // 1) The lookup of a "name" in the SymbolTable either creates a Symbol F for 52 // "name" and returns a pointer to F or finds a pre-existing Symbol F for 53 // "name" and returns a pointer to it. In both cases the reference count for F 54 // is incremented under the assumption that a pointer to F will be created from 55 // the return value. Thus the increment of the reference count is on the lookup 56 // and not on the assignment to the new Symbol*. That is 57 // Symbol* G = lookup() 58 // ^ increment on lookup() 59 // and not 60 // Symbol* G = lookup() 61 // ^ increment on assignmnet 62 // The reference count must be decremented manually when the copy of the 63 // pointer G is destroyed. 64 // 65 // 2) For a local Symbol* A that is a copy of an existing Symbol* B, the 66 // reference counting is elided when the scope of B is greater than the scope 67 // of A. For example, in the code fragment 68 // below "klass" is passed as a parameter to the method. Symbol* "kn" 69 // is a copy of the name in "klass". 70 // 71 // Symbol* kn = klass->name(); 72 // unsigned int d_hash = dictionary()->compute_hash(kn, class_loader); 73 // 74 // The scope of "klass" is greater than the scope of "kn" so the reference 75 // counting for "kn" is elided. 76 // 77 // Symbol* copied from ConstantPool entries are good candidates for reference 78 // counting elision. The ConstantPool entries for a class C exist until C is 79 // unloaded. If a Symbol* is copied out of the ConstantPool into Symbol* X, 80 // the Symbol* in the ConstantPool will in general out live X so the reference 81 // counting on X can be elided. 82 // 83 // For cases where the scope of A is not greater than the scope of B, 84 // the reference counting is explicitly done. See ciSymbol, 85 // ResolutionErrorEntry and ClassVerifier for examples. 86 // 87 // 3) When a Symbol K is created for temporary use, generally for substrings of 88 // an existing symbol or to create a new symbol, assign it to a 89 // TempNewSymbol. The SymbolTable methods new_symbol(), lookup() 90 // and probe() all potentially return a pointer to a new Symbol. 91 // The allocation (or lookup) of K increments the reference count for K 92 // and the destructor decrements the reference count. 93 // 94 // This cannot be inherited from ResourceObj because it cannot have a vtable. 95 // Since sometimes this is allocated from Metadata, pick a base allocation 96 // type without virtual functions. 97 class ClassLoaderData; 98 99 // Set _refcount to PERM_REFCOUNT to prevent the Symbol from being freed. 100 #ifndef PERM_REFCOUNT 101 #define PERM_REFCOUNT ((1 << 16) - 1) 102 #endif 103 104 class Symbol : public MetaspaceObj { 105 friend class VMStructs; 106 friend class SymbolTable; 107 friend class MoveSymbols; 108 109 private: 110 111 // This is an int because it needs atomic operation on the refcount. Mask length 112 // in high half word. length is the number of UTF8 characters in the symbol 113 volatile uint32_t _length_and_refcount; 114 short _identity_hash; 115 jbyte _body[2]; 116 117 enum { 118 // max_symbol_length must fit into the top 16 bits of _length_and_refcount 119 max_symbol_length = (1 << 16) -1 120 }; 121 122 static int byte_size(int length) { 123 // minimum number of natural words needed to hold these bits (no non-heap version) 124 return (int)(sizeof(Symbol) + (length > 2 ? length - 2 : 0)); 125 } 126 static int size(int length) { 127 // minimum number of natural words needed to hold these bits (no non-heap version) 128 return (int)heap_word_size(byte_size(length)); 129 } 130 131 void byte_at_put(int index, int value) { 132 assert(index >=0 && index < length(), "symbol index overflow"); 133 _body[index] = value; 134 } 135 136 Symbol(const u1* name, int length, int refcount); 137 void* operator new(size_t size, int len, TRAPS) throw(); 138 void* operator new(size_t size, int len, Arena* arena, TRAPS) throw(); 139 void* operator new(size_t size, int len, ClassLoaderData* loader_data, TRAPS) throw(); 140 141 void operator delete(void* p); 142 143 static int extract_length(uint32_t value) { return value >> 16; } 144 static int extract_refcount(uint32_t value) { return value & 0xffff; } 145 static uint32_t pack_length_and_refcount(int length, int refcount); 146 147 int length() const { return extract_length(_length_and_refcount); } 148 149 public: 150 // Low-level access (used with care, since not GC-safe) 151 const jbyte* base() const { return &_body[0]; } 152 153 int size() { return size(utf8_length()); } 154 int byte_size() { return byte_size(utf8_length()); } 155 156 // Symbols should be stored in the read-only region of CDS archive. 157 static bool is_read_only_by_default() { return true; } 158 159 // Returns the largest size symbol we can safely hold. 160 static int max_length() { return max_symbol_length; } 161 unsigned identity_hash() const { 162 unsigned addr_bits = (unsigned)((uintptr_t)this >> (LogMinObjAlignmentInBytes + 3)); 163 return ((unsigned)_identity_hash & 0xffff) | 164 ((addr_bits ^ (length() << 8) ^ (( _body[0] << 8) | _body[1])) << 16); 165 } 166 167 // For symbol table alternate hashing 168 unsigned int new_hash(juint seed); 169 170 // Reference counting. See comments above this class for when to use. 171 int refcount() const { return extract_refcount(_length_and_refcount); } 172 bool try_increment_refcount(); 173 void increment_refcount(); 174 void decrement_refcount(); 175 bool is_permanent() { 176 return (refcount() == PERM_REFCOUNT); 177 } 178 179 int byte_at(int index) const { 180 assert(index >=0 && index < length(), "symbol index overflow"); 181 return base()[index]; 182 } 183 184 const jbyte* bytes() const { return base(); } 185 186 int utf8_length() const { return length(); } 187 188 // Compares the symbol with a string. 189 bool equals(const char* str, int len) const { 190 int l = utf8_length(); 191 if (l != len) return false; 192 while (l-- > 0) { 193 if (str[l] != (char) byte_at(l)) 194 return false; 195 } 196 assert(l == -1, "we should be at the beginning"); 197 return true; 198 } 199 bool equals(const char* str) const { return equals(str, (int) strlen(str)); } 200 201 // Tests if the symbol starts with the given prefix. 202 bool starts_with(const char* prefix, int len) const; 203 bool starts_with(const char* prefix) const { 204 return starts_with(prefix, (int) strlen(prefix)); 205 } 206 207 // Tests if the symbol starts with the given prefix. 208 int index_of_at(int i, const char* str, int len) const; 209 int index_of_at(int i, const char* str) const { 210 return index_of_at(i, str, (int) strlen(str)); 211 } 212 213 // Three-way compare for sorting; returns -1/0/1 if receiver is </==/> than arg 214 // note that the ordering is not alfabetical 215 inline int fast_compare(const Symbol* other) const; 216 217 // Returns receiver converted to null-terminated UTF-8 string; string is 218 // allocated in resource area, or in the char buffer provided by caller. 219 char* as_C_string() const; 220 char* as_C_string(char* buf, int size) const; 221 // Use buf if needed buffer length is <= size. 222 char* as_C_string_flexible_buffer(Thread* t, char* buf, int size) const; 223 224 // Returns an escaped form of a Java string. 225 char* as_quoted_ascii() const; 226 227 // Returns a null terminated utf8 string in a resource array 228 char* as_utf8() const { return as_C_string(); } 229 char* as_utf8_flexible_buffer(Thread* t, char* buf, int size) const { 230 return as_C_string_flexible_buffer(t, buf, size); 231 } 232 233 jchar* as_unicode(int& length) const; 234 235 // Treating this symbol as a class name, returns the Java name for the class. 236 // String is allocated in resource area if buffer is not provided. 237 // See Klass::external_name() 238 const char* as_klass_external_name() const; 239 const char* as_klass_external_name(char* buf, int size) const; 240 241 void metaspace_pointers_do(MetaspaceClosure* it); 242 MetaspaceObj::Type type() const { return SymbolType; } 243 244 // Printing 245 void print_symbol_on(outputStream* st = NULL) const; 246 void print_utf8_on(outputStream* st) const; 247 void print_on(outputStream* st) const; // First level print 248 void print_value_on(outputStream* st) const; // Second level print. 249 250 // printing on default output stream 251 void print() { print_on(tty); } 252 void print_value() { print_value_on(tty); } 253 254 #ifndef PRODUCT 255 // Empty constructor to create a dummy symbol object on stack 256 // only for getting its vtable pointer. 257 Symbol() { } 258 259 static int _total_count; 260 #endif 261 }; 262 263 // Note: this comparison is used for vtable sorting only; it doesn't matter 264 // what order it defines, as long as it is a total, time-invariant order 265 // Since Symbol*s are in C_HEAP, their relative order in memory never changes, 266 // so use address comparison for speed 267 int Symbol::fast_compare(const Symbol* other) const { 268 return (((uintptr_t)this < (uintptr_t)other) ? -1 269 : ((uintptr_t)this == (uintptr_t) other) ? 0 : 1); 270 } 271 #endif // SHARE_VM_OOPS_SYMBOL_HPP