1 /*
2 * Copyright (c) 1997, 2024, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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5 * This code is free software; you can redistribute it and/or modify it
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7 * published by the Free Software Foundation.
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11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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13 * accompanied this code).
14 *
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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 #include "utilities/vmEnums.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 field in
49 // fieldDescriptor or symbol in a constant pool) 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 assignment
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 0xffff
102 #endif
103
104 class Symbol : public MetaspaceObj {
105 friend class VMStructs;
106 friend class SymbolTable;
107 friend class vmSymbols;
108 friend class JVMCIVMStructs;
109
110 private:
111
112 // This is an int because it needs atomic operation on the refcount. Mask hash
113 // in high half word. length is the number of UTF8 characters in the symbol
114 volatile uint32_t _hash_and_refcount;
115 u2 _length;
116 u1 _body[2];
117
118 static Symbol* _vm_symbols[];
119
120 enum {
121 max_symbol_length = 0xffff
122 };
123
124 static int byte_size(int length) {
125 // minimum number of bytes needed to hold these bits (no non-heap version)
126 return (int)(sizeof(Symbol) + (length > 2 ? length - 2 : 0));
127 }
128 static int size(int length) {
129 // minimum number of natural words needed to hold these bits (no non-heap version)
130 return (int)heap_word_size(byte_size(length));
131 }
132
133 // Constructor is private for use only by SymbolTable.
134 Symbol(const u1* name, int length, int refcount);
135
136 static short extract_hash(uint32_t value) { return (short)(value >> 16); }
137 static int extract_refcount(uint32_t value) { return value & 0xffff; }
138 static uint32_t pack_hash_and_refcount(short hash, int refcount);
139
140 int length() const { return _length; }
141
142 public:
143 Symbol(const Symbol& s1);
144
145 // Low-level access (used with care, since not GC-safe)
146 const u1* base() const { return &_body[0]; }
147
148 int size() const { return size(utf8_length()); }
149 int byte_size() const { return byte_size(utf8_length()); };
150
151 // Symbols should be stored in the read-only region of CDS archive.
152 static bool is_read_only_by_default() { return true; }
153
154 // Returns the largest size symbol we can safely hold.
155 static int max_length() { return max_symbol_length; }
156 unsigned identity_hash() const {
157 unsigned addr_bits = (unsigned)((uintptr_t)this >> LogBytesPerWord);
158 return ((unsigned)extract_hash(_hash_and_refcount) & 0xffff) |
159 ((addr_bits ^ (length() << 8) ^ (( _body[0] << 8) | _body[1])) << 16);
160 }
161
162 // Reference counting. See comments above this class for when to use.
163 int refcount() const { return extract_refcount(_hash_and_refcount); }
164 bool try_increment_refcount();
165 void increment_refcount();
166 void decrement_refcount();
167 bool is_permanent() const {
168 return (refcount() == PERM_REFCOUNT);
169 }
170 void update_identity_hash() NOT_CDS_RETURN;
171 void set_permanent() NOT_CDS_RETURN;
172 void make_permanent();
173
174 static void maybe_increment_refcount(Symbol* s) {
175 if (s != nullptr) {
176 s->increment_refcount();
177 }
178 }
179 static void maybe_decrement_refcount(Symbol* s) {
180 if (s != nullptr) {
181 s->decrement_refcount();
182 }
183 }
184 // Function char_at() returns the Symbol's selected u1 byte as a char type.
185 //
186 // Note that all multi-byte chars have the sign bit set on all their bytes.
187 // No single byte chars have their sign bit set.
188 char char_at(int index) const {
189 assert(index >=0 && index < length(), "symbol index overflow");
190 return (char)base()[index];
191 }
192
193 const u1* bytes() const { return base(); }
194
195 int utf8_length() const { return length(); }
196
197 // Compares the symbol with a string.
198 bool equals(const char* str, int len) const {
199 int l = utf8_length();
200 if (l != len) return false;
201 return contains_utf8_at(0, str, len);
202 }
203 bool equals(const char* str) const { return equals(str, (int) strlen(str)); }
204 bool is_star_match(const char* pattern) const;
205
206 // Tests if the symbol starts with the given prefix.
207 bool starts_with(const char* prefix, int len) const {
208 return contains_utf8_at(0, prefix, len);
209 }
210 bool starts_with(const char* prefix) const {
211 return starts_with(prefix, (int) strlen(prefix));
212 }
213 bool starts_with(char prefix_char) const {
214 return contains_byte_at(0, prefix_char);
215 }
216 // Tests if the symbol ends with the given suffix.
217 bool ends_with(const char* suffix, int len) const {
218 return contains_utf8_at(utf8_length() - len, suffix, len);
219 }
220 bool ends_with(const char* suffix) const {
221 return ends_with(suffix, (int) strlen(suffix));
222 }
223 bool ends_with(char suffix_char) const {
224 return contains_byte_at(utf8_length() - 1, suffix_char);
225 }
226
227 // Tests if the symbol contains the given utf8 substring
228 // at the given byte position.
229 bool contains_utf8_at(int position, const char* substring, int len) const {
230 assert(len >= 0 && substring != nullptr, "substring must be valid");
231 if (position < 0) return false; // can happen with ends_with
232 if (position + len > utf8_length()) return false;
233 return (memcmp((char*)base() + position, substring, len) == 0);
234 }
235
236 // Tests if the symbol contains the given byte at the given position.
237 bool contains_byte_at(int position, char code_byte) const {
238 if (position < 0) return false; // can happen with ends_with
239 if (position >= utf8_length()) return false;
240 return code_byte == char_at(position);
241 }
242
243 // Test if the symbol has the give substring at or after the i-th char.
244 int index_of_at(int i, const char* substr, int substr_len) const;
245
246 // Three-way compare for sorting; returns -1/0/1 if receiver is </==/> than arg
247 // note that the ordering is not alfabetical
248 inline int fast_compare(const Symbol* other) const;
249
250 // Returns receiver converted to null-terminated UTF-8 string; string is
251 // allocated in resource area, or in the char buffer provided by caller.
252 char* as_C_string() const;
253 char* as_C_string(char* buf, int size) const;
254
255 // Returns an escaped form of a Java string.
256 char* as_quoted_ascii() const;
257
258 // Returns a null terminated utf8 string in a resource array
259 char* as_utf8() const { return as_C_string(); }
260
261 jchar* as_unicode(int& length) const;
262
263 // Treating this symbol as a class name, returns the Java name for the class.
264 // String is allocated in resource area if buffer is not provided.
265 // See Klass::external_name()
266 const char* as_klass_external_name() const;
267 const char* as_klass_external_name(char* buf, int size) const;
268
269 // Treating the symbol as a signature, print the return
270 // type to the outputStream. Prints external names as 'double' or
271 // 'java.lang.Object[][]'.
272 void print_as_signature_external_return_type(outputStream *os);
273 // Treating the symbol as a signature, print the parameter types
274 // separated by ', ' to the outputStream. Prints external names as
275 // 'double' or 'java.lang.Object[][]'.
276 void print_as_signature_external_parameters(outputStream *os);
277 void print_as_field_external_type(outputStream *os);
278
279 void metaspace_pointers_do(MetaspaceClosure* it);
280 MetaspaceObj::Type type() const { return SymbolType; }
281
282 // Printing
283 void print_symbol_on(outputStream* st = nullptr) const;
284 void print_utf8_on(outputStream* st) const;
285 void print_on(outputStream* st) const; // First level print
286 void print_value_on(outputStream* st) const; // Second level print.
287
288 // printing on default output stream
289 void print() const;
290 void print_value() const;
291
292 static bool is_valid(Symbol* s);
293
294 static bool is_valid_id(vmSymbolID vm_symbol_id) PRODUCT_RETURN_(return true;);
295
296 static Symbol* vm_symbol_at(vmSymbolID vm_symbol_id) {
297 assert(is_valid_id(vm_symbol_id), "must be");
298 return _vm_symbols[static_cast<int>(vm_symbol_id)];
299 }
300
301 static unsigned int compute_hash(const Symbol* const& name) {
302 return (unsigned int) name->identity_hash();
303 }
304
305 #ifndef PRODUCT
306 // Empty constructor to create a dummy symbol object on stack
307 // only for getting its vtable pointer.
308 Symbol() { }
309
310 static size_t _total_count;
311 #endif
312 };
313
314 // Note: this comparison is used for vtable sorting only; it doesn't matter
315 // what order it defines, as long as it is a total, time-invariant order
316 // Since Symbol*s are in C_HEAP, their relative order in memory never changes,
317 // so use address comparison for speed
318 int Symbol::fast_compare(const Symbol* other) const {
319 return (((uintptr_t)this < (uintptr_t)other) ? -1
320 : ((uintptr_t)this == (uintptr_t) other) ? 0 : 1);
321 }
322 #endif // SHARE_OOPS_SYMBOL_HPP