1 /*
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  3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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  6  * under the terms of the GNU General Public License version 2 only, as
  7  * published by the Free Software Foundation.
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  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).
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 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 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 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 natural words 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   Symbol(const u1* name, int length, int refcount);
134   void* operator new(size_t size, int len) throw();
135   void* operator new(size_t size, int len, Arena* arena) throw();
136 
137   void  operator delete(void* p);
138 
139   static short extract_hash(uint32_t value)   { return (short)(value >> 16); }
140   static int extract_refcount(uint32_t value) { return value & 0xffff; }
141   static uint32_t pack_hash_and_refcount(short hash, int refcount);
142 
143   int length() const   { return _length; }
144 
145  public:
146   // Low-level access (used with care, since not GC-safe)
147   const u1* base() const { return &_body[0]; }
148 
149   int size()                { return size(utf8_length()); }
150   int byte_size()           { return byte_size(utf8_length()); }
151 
152   // Symbols should be stored in the read-only region of CDS archive.
153   static bool is_read_only_by_default() { return true; }
154 
155   // Returns the largest size symbol we can safely hold.
156   static int max_length() { return max_symbol_length; }
157   unsigned identity_hash() const {
158     unsigned addr_bits = (unsigned)((uintptr_t)this >> (LogBytesPerWord + 3));
159     return ((unsigned)extract_hash(_hash_and_refcount) & 0xffff) |
160            ((addr_bits ^ (length() << 8) ^ (( _body[0] << 8) | _body[1])) << 16);
161   }
162 
163   // Reference counting.  See comments above this class for when to use.
164   int refcount() const { return extract_refcount(_hash_and_refcount); }
165   bool try_increment_refcount();
166   void increment_refcount();
167   void decrement_refcount();
168   bool is_permanent() const {
169     return (refcount() == PERM_REFCOUNT);
170   }
171   void update_identity_hash() NOT_CDS_RETURN;
172   void set_permanent() NOT_CDS_RETURN;
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     return contains_utf8_at(0, str, len);
193   }
194   bool equals(const char* str) const { return equals(str, (int) strlen(str)); }
195 
196   // Tests if the symbol starts with the given prefix.
197   bool starts_with(const char* prefix, int len) const {
198     return contains_utf8_at(0, prefix, len);
199   }
200   bool starts_with(const char* prefix) const {
201     return starts_with(prefix, (int) strlen(prefix));
202   }
203   bool starts_with(char prefix_char) const {
204     return contains_byte_at(0, prefix_char);
205   }
206   // Tests if the symbol ends with the given suffix.
207   bool ends_with(const char* suffix, int len) const {
208     return contains_utf8_at(utf8_length() - len, suffix, len);
209   }
210   bool ends_with(const char* suffix) const {
211     return ends_with(suffix, (int) strlen(suffix));
212   }
213   bool ends_with(int suffix_char) const {
214     return contains_byte_at(utf8_length() - 1, suffix_char);
215   }
216 
217   // Tests if the symbol contains the given utf8 substring
218   // at the given byte position.
219   bool contains_utf8_at(int position, const char* substring, int len) const {
220     assert(len >= 0 && substring != NULL, "substring must be valid");
221     if (position < 0)  return false;  // can happen with ends_with
222     if (position + len > utf8_length()) return false;
223     return (memcmp((char*)base() + position, substring, len) == 0);
224   }
225 
226   // Tests if the symbol contains the given byte at the given position.
227   bool contains_byte_at(int position, char code_byte) const {
228     if (position < 0)  return false;  // can happen with ends_with
229     if (position >= utf8_length()) return false;
230     return code_byte == char_at(position);
231   }
232 
233   // True if this is a descriptor for a method with void return.
234   // (Assumes it is a valid descriptor.)
235   bool is_void_method_signature() const {
236     return starts_with('(') && ends_with('V');
237   }
238 
239   bool is_Q_signature() const;
240   bool is_Q_array_signature() const;
241   bool is_Q_method_signature() const;
242   Symbol* fundamental_name(TRAPS);
243   bool is_same_fundamental_type(Symbol*) const;
244 
245   // Tests if the symbol starts with the given prefix.
246   int index_of_at(int i, const char* str, int len) const;
247 
248   // Three-way compare for sorting; returns -1/0/1 if receiver is </==/> than arg
249   // note that the ordering is not alfabetical
250   inline int fast_compare(const Symbol* other) const;
251 
252   // Returns receiver converted to null-terminated UTF-8 string; string is
253   // allocated in resource area, or in the char buffer provided by caller.
254   char* as_C_string() const;
255   char* as_C_string(char* buf, int size) const;
256 
257   // Returns an escaped form of a Java string.
258   char* as_quoted_ascii() const;
259 
260   // Returns a null terminated utf8 string in a resource array
261   char* as_utf8() const { return as_C_string(); }
262 
263   jchar* as_unicode(int& length) const;
264 
265   // Treating this symbol as a class name, returns the Java name for the class.
266   // String is allocated in resource area if buffer is not provided.
267   // See Klass::external_name()
268   const char* as_klass_external_name() const;
269   const char* as_klass_external_name(char* buf, int size) const;
270 
271   // Treating the symbol as a signature, print the return
272   // type to the outputStream. Prints external names as 'double' or
273   // 'java.lang.Object[][]'.
274   void print_as_signature_external_return_type(outputStream *os);
275   // Treating the symbol as a signature, print the parameter types
276   // seperated by ', ' to the outputStream.  Prints external names as
277   //  'double' or 'java.lang.Object[][]'.
278   void print_as_signature_external_parameters(outputStream *os);
279 
280   void metaspace_pointers_do(MetaspaceClosure* it);
281   MetaspaceObj::Type type() const { return SymbolType; }
282 
283   // Printing
284   void print_symbol_on(outputStream* st = NULL) const;
285   void print_utf8_on(outputStream* st) const;
286   void print_on(outputStream* st) const;         // First level print
287   void print_value_on(outputStream* st) const;   // Second level print.
288   void print_Qvalue_on(outputStream* st) const;  // Second level print for Q-types.
289 
290   // printing on default output stream
291   void print() const;
292   void print_value() const;
293 
294   static bool is_valid(Symbol* s);
295 
296   static bool is_valid_id(vmSymbolID vm_symbol_id) PRODUCT_RETURN_(return true;);
297 
298   static Symbol* vm_symbol_at(vmSymbolID vm_symbol_id) {
299     assert(is_valid_id(vm_symbol_id), "must be");
300     return _vm_symbols[static_cast<int>(vm_symbol_id)];
301   }
302 
303 #ifndef PRODUCT
304   // Empty constructor to create a dummy symbol object on stack
305   // only for getting its vtable pointer.
306   Symbol() { }
307 
308   static size_t _total_count;
309 #endif
310 };
311 
312 // Note: this comparison is used for vtable sorting only; it doesn't matter
313 // what order it defines, as long as it is a total, time-invariant order
314 // Since Symbol*s are in C_HEAP, their relative order in memory never changes,
315 // so use address comparison for speed
316 int Symbol::fast_compare(const Symbol* other) const {
317  return (((uintptr_t)this < (uintptr_t)other) ? -1
318    : ((uintptr_t)this == (uintptr_t) other) ? 0 : 1);
319 }
320 #endif // SHARE_OOPS_SYMBOL_HPP