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comparison src/share/vm/oops/symbol.hpp @ 2177:3582bf76420e

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6990754: Use native memory and reference counting to implement SymbolTable Summary: move symbols from permgen into C heap and reference count them Reviewed-by: never, acorn, jmasa, stefank
author coleenp
date Thu, 27 Jan 2011 16:11:27 -0800
parents src/share/vm/oops/symbolOop.hpp@f95d63e2154a
children 1d1603768966
comparison
equal deleted inserted replaced
2176:27e4ea99855d 2177:3582bf76420e
1 /*
2 * Copyright (c) 1997, 2009, 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 "utilities/utf8.hpp"
29 #include "memory/allocation.hpp"
30
31 // A Symbol is a canonicalized string.
32 // All Symbols reside in global SymbolTable and are reference counted.
33
34 // Reference counting
35 //
36 // All Symbols are allocated and added to the SymbolTable.
37 // When a class is unloaded, the reference counts of the Symbol pointers in
38 // the ConstantPool and in instanceKlass (see release_C_heap_structures) are
39 // decremented. When the reference count for a Symbol goes to 0, the garbage
40 // collector can free the Symbol and remove it from the SymbolTable.
41 //
42 // 0) Symbols need to be reference counted when a pointer to the Symbol is
43 // saved in persistent storage. This does not include the pointer
44 // in the SymbolTable bucket (the _literal field in HashtableEntry)
45 // that points to the Symbol. All other stores of a Symbol*
46 // to a field of a persistent variable (e.g., the _name filed in
47 // FieldAccessInfo or _ptr in a CPSlot) is reference counted.
48 //
49 // 1) The lookup of a "name" in the SymbolTable either creates a Symbol F for
50 // "name" and returns a pointer to F or finds a pre-existing Symbol F for
51 // "name" and returns a pointer to it. In both cases the reference count for F
52 // is incremented under the assumption that a pointer to F will be created from
53 // the return value. Thus the increment of the reference count is on the lookup
54 // and not on the assignment to the new Symbol*. That is
55 // Symbol* G = lookup()
56 // ^ increment on lookup()
57 // and not
58 // Symbol* G = lookup()
59 // ^ increment on assignmnet
60 // The reference count must be decremented manually when the copy of the
61 // pointer G is destroyed.
62 //
63 // 2) For a local Symbol* A that is a copy of an existing Symbol* B, the
64 // reference counting is elided when the scope of B is greater than the scope
65 // of A. For example, in the code fragment
66 // below "klass" is passed as a parameter to the method. Symbol* "kn"
67 // is a copy of the name in "klass".
68 //
69 // Symbol* kn = klass->name();
70 // unsigned int d_hash = dictionary()->compute_hash(kn, class_loader);
71 //
72 // The scope of "klass" is greater than the scope of "kn" so the reference
73 // counting for "kn" is elided.
74 //
75 // Symbol* copied from ConstantPool entries are good candidates for reference
76 // counting elision. The ConstantPool entries for a class C exist until C is
77 // unloaded. If a Symbol* is copied out of the ConstantPool into Symbol* X,
78 // the Symbol* in the ConstantPool will in general out live X so the reference
79 // counting on X can be elided.
80 //
81 // For cases where the scope of A is not greater than the scope of B,
82 // the reference counting is explicitly done. See ciSymbol,
83 // ResolutionErrorEntry and ClassVerifier for examples.
84 //
85 // 3) When a Symbol K is created for temporary use, generally for substrings of
86 // an existing symbol or to create a new symbol, assign it to a
87 // TempNewSymbol. The SymbolTable methods new_symbol(), lookup()
88 // and probe() all potentially return a pointer to a new Symbol.
89 // The allocation (or lookup) of K increments the reference count for K
90 // and the destructor decrements the reference count.
91 //
92 // Another example of TempNewSymbol usage is parsed_name used in
93 // ClassFileParser::parseClassFile() where parsed_name is used in the cleanup
94 // after a failed attempt to load a class. Here parsed_name is a
95 // TempNewSymbol (passed in as a parameter) so the reference count on its symbol
96 // will be decremented when it goes out of scope.
97
98 class Symbol : public CHeapObj {
99 friend class VMStructs;
100 friend class SymbolTable;
101 friend class MoveSymbols;
102 private:
103 volatile int _refcount;
104 int _identity_hash;
105 unsigned short _length; // number of UTF8 characters in the symbol
106 jbyte _body[1];
107
108 enum {
109 // max_symbol_length is constrained by type of _length
110 max_symbol_length = (1 << 16) -1
111 };
112
113 static int object_size(int length) {
114 size_t size = heap_word_size(sizeof(Symbol) + length);
115 return align_object_size(size);
116 }
117
118 void byte_at_put(int index, int value) {
119 assert(index >=0 && index < _length, "symbol index overflow");
120 _body[index] = value;
121 }
122
123 Symbol(const u1* name, int length);
124 void* operator new(size_t size, int len);
125
126 public:
127 // Low-level access (used with care, since not GC-safe)
128 const jbyte* base() const { return &_body[0]; }
129
130 int object_size() { return object_size(utf8_length()); }
131
132 // Returns the largest size symbol we can safely hold.
133 static int max_length() {
134 return max_symbol_length;
135 }
136
137 int identity_hash() {
138 return _identity_hash;
139 }
140
141 // Reference counting. See comments above this class for when to use.
142 int refcount() const { return _refcount; }
143 void increment_refcount();
144 void decrement_refcount();
145
146 int byte_at(int index) const {
147 assert(index >=0 && index < _length, "symbol index overflow");
148 return base()[index];
149 }
150
151 const jbyte* bytes() const { return base(); }
152
153 int utf8_length() const { return _length; }
154
155 // Compares the symbol with a string.
156 bool equals(const char* str, int len) const;
157 bool equals(const char* str) const { return equals(str, (int) strlen(str)); }
158
159 // Tests if the symbol starts with the given prefix.
160 bool starts_with(const char* prefix, int len) const;
161 bool starts_with(const char* prefix) const {
162 return starts_with(prefix, (int) strlen(prefix));
163 }
164
165 // Tests if the symbol starts with the given prefix.
166 int index_of_at(int i, const char* str, int len) const;
167 int index_of_at(int i, const char* str) const {
168 return index_of_at(i, str, (int) strlen(str));
169 }
170
171 // Three-way compare for sorting; returns -1/0/1 if receiver is </==/> than arg
172 // note that the ordering is not alfabetical
173 inline int fast_compare(Symbol* other) const;
174
175 // Returns receiver converted to null-terminated UTF-8 string; string is
176 // allocated in resource area, or in the char buffer provided by caller.
177 char* as_C_string() const;
178 char* as_C_string(char* buf, int size) const;
179 // Use buf if needed buffer length is <= size.
180 char* as_C_string_flexible_buffer(Thread* t, char* buf, int size) const;
181
182
183 // Returns a null terminated utf8 string in a resource array
184 char* as_utf8() const { return as_C_string(); }
185 char* as_utf8_flexible_buffer(Thread* t, char* buf, int size) const {
186 return as_C_string_flexible_buffer(t, buf, size);
187 }
188
189 jchar* as_unicode(int& length) const;
190
191 // Treating this symbol as a class name, returns the Java name for the class.
192 // String is allocated in resource area if buffer is not provided.
193 // See Klass::external_name()
194 const char* as_klass_external_name() const;
195 const char* as_klass_external_name(char* buf, int size) const;
196
197 // Printing
198 void print_symbol_on(outputStream* st = NULL) const;
199 void print_on(outputStream* st) const; // First level print
200 void print_value_on(outputStream* st) const; // Second level print.
201
202 // printing on default output stream
203 void print() { print_on(tty); }
204 void print_value() { print_value_on(tty); }
205
206 #ifndef PRODUCT
207 // Empty constructor to create a dummy symbol object on stack
208 // only for getting its vtable pointer.
209 Symbol() { }
210
211 static int _total_count;
212 #endif
213 };
214
215 // Note: this comparison is used for vtable sorting only; it doesn't matter
216 // what order it defines, as long as it is a total, time-invariant order
217 // Since Symbol*s are in C_HEAP, their relative order in memory never changes,
218 // so use address comparison for speed
219 int Symbol::fast_compare(Symbol* other) const {
220 return (((uintptr_t)this < (uintptr_t)other) ? -1
221 : ((uintptr_t)this == (uintptr_t) other) ? 0 : 1);
222 }
223 #endif // SHARE_VM_OOPS_SYMBOL_HPP