1 /* 2 * Copyright (c) 1997, 2019, 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_OOPS_SYMBOL_HPP 26 #define SHARE_OOPS_SYMBOL_HPP 27 28 #include "memory/allocation.hpp" 29 #include "utilities/exceptions.hpp" 30 #include "utilities/macros.hpp" 31 32 // A Symbol is a canonicalized string. 33 // All Symbols reside in global SymbolTable and are reference counted. 34 35 // Reference counting 36 // 37 // All Symbols are allocated and added to the SymbolTable. 38 // When a class is unloaded, the reference counts of the Symbol pointers in 39 // the ConstantPool and in InstanceKlass (see release_C_heap_structures) are 40 // decremented. When the reference count for a Symbol goes to 0, the garbage 41 // collector can free the Symbol and remove it from the SymbolTable. 42 // 43 // 0) Symbols need to be reference counted when a pointer to the Symbol is 44 // saved in persistent storage. This does not include the pointer 45 // in the SymbolTable bucket (the _literal field in HashtableEntry) 46 // that points to the Symbol. All other stores of a Symbol* 47 // to a field of a persistent variable (e.g., the _name filed in 48 // fieldDescriptor or _ptr in a CPSlot) is reference counted. 49 // 50 // 1) The lookup of a "name" in the SymbolTable either creates a Symbol F for 51 // "name" and returns a pointer to F or finds a pre-existing Symbol F for 52 // "name" and returns a pointer to it. In both cases the reference count for F 53 // is incremented under the assumption that a pointer to F will be created from 54 // the return value. Thus the increment of the reference count is on the lookup 55 // and not on the assignment to the new Symbol*. That is 56 // Symbol* G = lookup() 57 // ^ increment on lookup() 58 // and not 59 // Symbol* G = lookup() 60 // ^ increment on assignmnet 61 // The reference count must be decremented manually when the copy of the 62 // pointer G is destroyed. 63 // 64 // 2) For a local Symbol* A that is a copy of an existing Symbol* B, the 65 // reference counting is elided when the scope of B is greater than the scope 66 // of A. For example, in the code fragment 67 // below "klass" is passed as a parameter to the method. Symbol* "kn" 68 // is a copy of the name in "klass". 69 // 70 // Symbol* kn = klass->name(); 71 // unsigned int d_hash = dictionary()->compute_hash(kn, class_loader); 72 // 73 // The scope of "klass" is greater than the scope of "kn" so the reference 74 // counting for "kn" is elided. 75 // 76 // Symbol* copied from ConstantPool entries are good candidates for reference 77 // counting elision. The ConstantPool entries for a class C exist until C is 78 // unloaded. If a Symbol* is copied out of the ConstantPool into Symbol* X, 79 // the Symbol* in the ConstantPool will in general out live X so the reference 80 // counting on X can be elided. 81 // 82 // For cases where the scope of A is not greater than the scope of B, 83 // the reference counting is explicitly done. See ciSymbol, 84 // ResolutionErrorEntry and ClassVerifier for examples. 85 // 86 // 3) When a Symbol K is created for temporary use, generally for substrings of 87 // an existing symbol or to create a new symbol, assign it to a 88 // TempNewSymbol. The SymbolTable methods new_symbol(), lookup() 89 // and probe() all potentially return a pointer to a new Symbol. 90 // The allocation (or lookup) of K increments the reference count for K 91 // and the destructor decrements the reference count. 92 // 93 // This cannot be inherited from ResourceObj because it cannot have a vtable. 94 // Since sometimes this is allocated from Metadata, pick a base allocation 95 // type without virtual functions. 96 class ClassLoaderData; 97 98 // Set _refcount to PERM_REFCOUNT to prevent the Symbol from being freed. 99 #ifndef PERM_REFCOUNT 100 #define PERM_REFCOUNT ((1 << 16) - 1) 101 #endif 102 103 class Symbol : public MetaspaceObj { 104 friend class VMStructs; 105 friend class SymbolTable; 106 107 private: 108 109 // This is an int because it needs atomic operation on the refcount. Mask length 110 // in high half word. length is the number of UTF8 characters in the symbol 111 volatile uint32_t _length_and_refcount; 112 short _identity_hash; 113 u1 _body[2]; 114 115 enum { 116 // max_symbol_length must fit into the top 16 bits of _length_and_refcount 117 max_symbol_length = (1 << 16) -1 118 }; 119 120 static int byte_size(int length) { 121 // minimum number of natural words needed to hold these bits (no non-heap version) 122 return (int)(sizeof(Symbol) + (length > 2 ? length - 2 : 0)); 123 } 124 static int size(int length) { 125 // minimum number of natural words needed to hold these bits (no non-heap version) 126 return (int)heap_word_size(byte_size(length)); 127 } 128 129 void byte_at_put(int index, u1 value) { 130 assert(index >=0 && index < length(), "symbol index overflow"); 131 _body[index] = value; 132 } 133 134 Symbol(const u1* name, int length, int refcount); 135 void* operator new(size_t size, int len) throw(); 136 void* operator new(size_t size, int len, Arena* arena) throw(); 137 138 void operator delete(void* p); 139 140 static int extract_length(uint32_t value) { return value >> 16; } 141 static int extract_refcount(uint32_t value) { return value & 0xffff; } 142 static uint32_t pack_length_and_refcount(int length, int refcount); 143 144 int length() const { return extract_length(_length_and_refcount); } 145 146 public: 147 // Low-level access (used with care, since not GC-safe) 148 const u1* base() const { return &_body[0]; } 149 150 int size() { return size(utf8_length()); } 151 int byte_size() { return byte_size(utf8_length()); } 152 153 // Symbols should be stored in the read-only region of CDS archive. 154 static bool is_read_only_by_default() { return true; } 155 156 // Returns the largest size symbol we can safely hold. 157 static int max_length() { return max_symbol_length; } 158 unsigned identity_hash() const { 159 unsigned addr_bits = (unsigned)((uintptr_t)this >> (LogMinObjAlignmentInBytes + 3)); 160 return ((unsigned)_identity_hash & 0xffff) | 161 ((addr_bits ^ (length() << 8) ^ (( _body[0] << 8) | _body[1])) << 16); 162 } 163 164 // Reference counting. See comments above this class for when to use. 165 int refcount() const { return extract_refcount(_length_and_refcount); } 166 bool try_increment_refcount(); 167 void increment_refcount(); 168 void decrement_refcount(); 169 bool is_permanent() { 170 return (refcount() == PERM_REFCOUNT); 171 } 172 void set_permanent(); 173 void make_permanent(); 174 175 // Function char_at() returns the Symbol's selected u1 byte as a char type. 176 // 177 // Note that all multi-byte chars have the sign bit set on all their bytes. 178 // No single byte chars have their sign bit set. 179 char char_at(int index) const { 180 assert(index >=0 && index < length(), "symbol index overflow"); 181 return (char)base()[index]; 182 } 183 184 const u1* 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_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; 209 210 // Three-way compare for sorting; returns -1/0/1 if receiver is </==/> than arg 211 // note that the ordering is not alfabetical 212 inline int fast_compare(const Symbol* other) const; 213 214 // Returns receiver converted to null-terminated UTF-8 string; string is 215 // allocated in resource area, or in the char buffer provided by caller. 216 char* as_C_string() const; 217 char* as_C_string(char* buf, int size) const; 218 219 // Returns an escaped form of a Java string. 220 char* as_quoted_ascii() const; 221 222 // Returns a null terminated utf8 string in a resource array 223 char* as_utf8() const { return as_C_string(); } 224 225 jchar* as_unicode(int& length) const; 226 227 // Treating this symbol as a class name, returns the Java name for the class. 228 // String is allocated in resource area if buffer is not provided. 229 // See Klass::external_name() 230 const char* as_klass_external_name() const; 231 const char* as_klass_external_name(char* buf, int size) const; 232 233 // Treating the symbol as a signature, print the return 234 // type to the outputStream. Prints external names as 'double' or 235 // 'java.lang.Object[][]'. 236 void print_as_signature_external_return_type(outputStream *os); 237 // Treating the symbol as a signature, print the parameter types 238 // seperated by ', ' to the outputStream. Prints external names as 239 // 'double' or 'java.lang.Object[][]'. 240 void print_as_signature_external_parameters(outputStream *os); 241 242 void metaspace_pointers_do(MetaspaceClosure* it); 243 MetaspaceObj::Type type() const { return SymbolType; } 244 245 // Printing 246 void print_symbol_on(outputStream* st = NULL) const; 247 void print_utf8_on(outputStream* st) const; 248 void print_on(outputStream* st) const; // First level print 249 void print_value_on(outputStream* st) const; // Second level print. 250 251 // printing on default output stream 252 void print() const; 253 void print_value() const; 254 255 static bool is_valid(Symbol* s); 256 257 #ifndef PRODUCT 258 // Empty constructor to create a dummy symbol object on stack 259 // only for getting its vtable pointer. 260 Symbol() { } 261 262 static size_t _total_count; 263 #endif 264 }; 265 266 // Note: this comparison is used for vtable sorting only; it doesn't matter 267 // what order it defines, as long as it is a total, time-invariant order 268 // Since Symbol*s are in C_HEAP, their relative order in memory never changes, 269 // so use address comparison for speed 270 int Symbol::fast_compare(const Symbol* other) const { 271 return (((uintptr_t)this < (uintptr_t)other) ? -1 272 : ((uintptr_t)this == (uintptr_t) other) ? 0 : 1); 273 } 274 #endif // SHARE_OOPS_SYMBOL_HPP