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.
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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
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
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16 * 2 along with this work; if not, write to the Free Software Foundation,
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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
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, TRAPS) throw();
136 void* operator new(size_t size, int len, Arena* arena, TRAPS) 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 make_permanent();
173
174 // Function char_at() returns the Symbol's selected u1 byte as a char type.
175 //
176 // Note that all multi-byte chars have the sign bit set on all their bytes.
177 // No single byte chars have their sign bit set.
178 char char_at(int index) const {
179 assert(index >=0 && index < length(), "symbol index overflow");
180 return (char)base()[index];
181 }
182
183 const u1* bytes() const { return base(); }
184
185 int utf8_length() const { return length(); }
186
187 // Compares the symbol with a string.
188 bool equals(const char* str, int len) const {
189 int l = utf8_length();
190 if (l != len) return false;
191 while (l-- > 0) {
192 if (str[l] != char_at(l))
193 return false;
194 }
195 assert(l == -1, "we should be at the beginning");
196 return true;
197 }
198 bool equals(const char* str) const { return equals(str, (int) strlen(str)); }
199
200 // Tests if the symbol starts with the given prefix.
201 bool starts_with(const char* prefix, int len) const {
202 return contains_utf8_at(0, prefix, len);
203 }
204 bool starts_with(const char* prefix) const {
205 return starts_with(prefix, (int) strlen(prefix));
206 }
207 bool starts_with(char prefix_char) const {
208 return contains_byte_at(0, prefix_char);
209 }
210 // Tests if the symbol ends with the given suffix.
211 bool ends_with(const char* suffix, int len) const {
212 return contains_utf8_at(utf8_length() - len, suffix, len);
213 }
214 bool ends_with(const char* suffix) const {
215 return ends_with(suffix, (int) strlen(suffix));
216 }
217 bool ends_with(char suffix_char) const {
218 return contains_byte_at(utf8_length()-1, suffix_char);
219 }
220 // Tests if the symbol contains the given utf8 substring
221 // or byte at the given byte position.
222 bool contains_utf8_at(int position, const char* substring, int len) const;
223 bool contains_byte_at(int position, char code_byte) const;
224
225 // True if this is a descriptor for a method with void return.
226 // (Assumes it is a valid descriptor.)
227 bool is_void_method_signature() const {
228 return starts_with('(') && ends_with('V');
229 }
230
231 bool is_Q_signature() const;
232 bool is_Q_array_signature() const;
233 Symbol* fundamental_name(TRAPS);
234 bool is_same_fundamental_type(Symbol*) const;
235
236 // Tests if the symbol starts with the given prefix.
237 int index_of_at(int i, const char* str, int len) const;
238
239 // Three-way compare for sorting; returns -1/0/1 if receiver is </==/> than arg
240 // note that the ordering is not alfabetical
241 inline int fast_compare(const Symbol* other) const;
242
243 // Returns receiver converted to null-terminated UTF-8 string; string is
244 // allocated in resource area, or in the char buffer provided by caller.
245 char* as_C_string() const;
246 char* as_C_string(char* buf, int size) const;
247
248 // Returns an escaped form of a Java string.
249 char* as_quoted_ascii() const;
250
251 // Returns a null terminated utf8 string in a resource array
252 char* as_utf8() const { return as_C_string(); }
253
254 jchar* as_unicode(int& length) const;
255
256 // Treating this symbol as a class name, returns the Java name for the class.
257 // String is allocated in resource area if buffer is not provided.
258 // See Klass::external_name()
259 const char* as_klass_external_name() const;
260 const char* as_klass_external_name(char* buf, int size) const;
261
262 // Treating the symbol as a signature, print the return
263 // type to the outputStream. Prints external names as 'double' or
264 // 'java.lang.Object[][]'.
265 void print_as_signature_external_return_type(outputStream *os);
266 // Treating the symbol as a signature, print the parameter types
267 // seperated by ', ' to the outputStream. Prints external names as
268 // 'double' or 'java.lang.Object[][]'.
269 void print_as_signature_external_parameters(outputStream *os);
270
271 void metaspace_pointers_do(MetaspaceClosure* it);
272 MetaspaceObj::Type type() const { return SymbolType; }
273
274 // Printing
275 void print_symbol_on(outputStream* st = NULL) const;
276 void print_utf8_on(outputStream* st) const;
277 void print_on(outputStream* st) const; // First level print
278 void print_value_on(outputStream* st) const; // Second level print.
279 void print_Qvalue_on(outputStream* st) const; // Second level print for Q-types.
280
281 // printing on default output stream
282 void print() { print_on(tty); }
283 void print_value() { print_value_on(tty); }
284
285 static bool is_valid(Symbol* s);
286
287 #ifndef PRODUCT
288 // Empty constructor to create a dummy symbol object on stack
289 // only for getting its vtable pointer.
290 Symbol() { }
291
292 static size_t _total_count;
293 #endif
294 };
295
296 // Note: this comparison is used for vtable sorting only; it doesn't matter
297 // what order it defines, as long as it is a total, time-invariant order
298 // Since Symbol*s are in C_HEAP, their relative order in memory never changes,
299 // so use address comparison for speed
300 int Symbol::fast_compare(const Symbol* other) const {
301 return (((uintptr_t)this < (uintptr_t)other) ? -1
302 : ((uintptr_t)this == (uintptr_t) other) ? 0 : 1);
303 }
304 #endif // SHARE_OOPS_SYMBOL_HPP