jdk8-mips64-public/hotspot: src/share/vm/oops/symbol.hpp@e1162778c1c8 (annotated)

src/share/vm/oops/symbol.hpp@e1162778c1c8 (annotated)

src/share/vm/oops/symbol.hpp

Thu, 07 Apr 2011 09:53:20 -0700

author: johnc
date: Thu, 07 Apr 2011 09:53:20 -0700
changeset 2781: e1162778c1c8
parent 2497: 3582bf76420e
child 2708: 1d1603768966
permissions: -rw-r--r--

7009266: G1: assert(obj->is_oop_or_null(true )) failed: Error
Summary: A referent object that is only weakly reachable at the start of concurrent marking but is re-attached to the strongly reachable object graph during marking may not be marked as live. This can cause the reference object to be processed prematurely and leave dangling pointers to the referent object. Implement a read barrier for the java.lang.ref.Reference::referent field by intrinsifying the Reference.get() method, and intercepting accesses though JNI, reflection, and Unsafe, so that when a non-null referent object is read it is also logged in an SATB buffer.
Reviewed-by: kvn, iveresov, never, tonyp, dholmes

 /*
  * Copyright (c) 1997, 2009, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
  * under the terms of the GNU General Public License version 2 only, as
  * published by the Free Software Foundation.
  *
  * This code is distributed in the hope that it will be useful, but WITHOUT
  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  * version 2 for more details (a copy is included in the LICENSE file that
  * accompanied this code).
  *
  * You should have received a copy of the GNU General Public License version
  * 2 along with this work; if not, write to the Free Software Foundation,
  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  *
  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  * or visit www.oracle.com if you need additional information or have any
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 #ifndef SHARE_VM_OOPS_SYMBOL_HPP
 #define SHARE_VM_OOPS_SYMBOL_HPP
 #include "utilities/utf8.hpp"
 #include "memory/allocation.hpp"
 // A Symbol is a canonicalized string.
 // All Symbols reside in global SymbolTable and are reference counted.
 // Reference counting
 //
 // All Symbols are allocated and added to the SymbolTable.
 // When a class is unloaded, the reference counts of the Symbol pointers in
 // the ConstantPool and in instanceKlass (see release_C_heap_structures) are
 // decremented.  When the reference count for a Symbol goes to 0, the garbage
 // collector can free the Symbol and remove it from the SymbolTable.
 //
 // 0) Symbols need to be reference counted when a pointer to the Symbol is
 // saved in persistent storage.  This does not include the pointer
 // in the SymbolTable bucket (the _literal field in HashtableEntry)
 // that points to the Symbol.  All other stores of a Symbol*
 // to a field of a persistent variable (e.g., the _name filed in
 // FieldAccessInfo or _ptr in a CPSlot) is reference counted.
 //
 // 1) The lookup of a "name" in the SymbolTable either creates a Symbol F for
 // "name" and returns a pointer to F or finds a pre-existing Symbol F for
 // "name" and returns a pointer to it. In both cases the reference count for F
 // is incremented under the assumption that a pointer to F will be created from
 // the return value. Thus the increment of the reference count is on the lookup
 // and not on the assignment to the new Symbol*.  That is
 //    Symbol* G = lookup()
 //                ^ increment on lookup()
 // and not
 //    Symbol* G = lookup()
 //              ^ increment on assignmnet
 // The reference count must be decremented manually when the copy of the
 // pointer G is destroyed.
 //
 // 2) For a local Symbol* A that is a copy of an existing Symbol* B, the
 // reference counting is elided when the scope of B is greater than the scope
 // of A.  For example, in the code fragment
 // below "klass" is passed as a parameter to the method.  Symbol* "kn"
 // is a copy of the name in "klass".
 //
 //   Symbol*  kn = klass->name();
 //   unsigned int d_hash = dictionary()->compute_hash(kn, class_loader);
 //
 // The scope of "klass" is greater than the scope of "kn" so the reference
 // counting for "kn" is elided.
 //
 // Symbol* copied from ConstantPool entries are good candidates for reference
 // counting elision.  The ConstantPool entries for a class C exist until C is
 // unloaded.  If a Symbol* is copied out of the ConstantPool into Symbol* X,
 // the Symbol* in the ConstantPool will in general out live X so the reference
 // counting on X can be elided.
 //
 // For cases where the scope of A is not greater than the scope of B,
 // the reference counting is explicitly done.  See ciSymbol,
 // ResolutionErrorEntry and ClassVerifier for examples.
 //
 // 3) When a Symbol K is created for temporary use, generally for substrings of
 // an existing symbol or to create a new symbol, assign it to a
 // TempNewSymbol. The SymbolTable methods new_symbol(), lookup()
 // and probe() all potentially return a pointer to a new Symbol.
 // The allocation (or lookup) of K increments the reference count for K
 // and the destructor decrements the reference count.
 //
 // Another example of TempNewSymbol usage is parsed_name used in
 // ClassFileParser::parseClassFile() where parsed_name is used in the cleanup
 // after a failed attempt to load a class.  Here parsed_name is a
 // TempNewSymbol (passed in as a parameter) so the reference count on its symbol
 // will be decremented when it goes out of scope.
 class Symbol : public CHeapObj {
   friend class VMStructs;
   friend class SymbolTable;
   friend class MoveSymbols;
  private:
   volatile int   _refcount;
   int            _identity_hash;
   unsigned short _length; // number of UTF8 characters in the symbol
   jbyte _body[1];
   enum {
     // max_symbol_length is constrained by type of _length
     max_symbol_length = (1 << 16) -1
   };
   static int object_size(int length) {
     size_t size = heap_word_size(sizeof(Symbol) + length);
     return align_object_size(size);
   }
   void byte_at_put(int index, int value) {
     assert(index >=0 && index < _length, "symbol index overflow");
     _body[index] = value;
   }
   Symbol(const u1* name, int length);
   void* operator new(size_t size, int len);
  public:
   // Low-level access (used with care, since not GC-safe)
   const jbyte* base() const { return &_body[0]; }
   int object_size() { return object_size(utf8_length()); }
   // Returns the largest size symbol we can safely hold.
   static int max_length() {
     return max_symbol_length;
   }
   int identity_hash() {
     return _identity_hash;
   }
   // Reference counting.  See comments above this class for when to use.
   int refcount() const { return _refcount; }
   void increment_refcount();
   void decrement_refcount();
   int byte_at(int index) const {
     assert(index >=0 && index < _length, "symbol index overflow");
     return base()[index];
   }
   const jbyte* bytes() const { return base(); }
   int utf8_length() const { return _length; }
   // Compares the symbol with a string.
   bool equals(const char* str, int len) const;
   bool equals(const char* str) const { return equals(str, (int) strlen(str)); }
   // Tests if the symbol starts with the given prefix.
   bool starts_with(const char* prefix, int len) const;
   bool starts_with(const char* prefix) const {
     return starts_with(prefix, (int) strlen(prefix));
   }
   // Tests if the symbol starts with the given prefix.
   int index_of_at(int i, const char* str, int len) const;
   int index_of_at(int i, const char* str) const {
     return index_of_at(i, str, (int) strlen(str));
   }
   // Three-way compare for sorting; returns -1/0/1 if receiver is </==/> than arg
   // note that the ordering is not alfabetical
   inline int fast_compare(Symbol* other) const;
   // Returns receiver converted to null-terminated UTF-8 string; string is
   // allocated in resource area, or in the char buffer provided by caller.
   char* as_C_string() const;
   char* as_C_string(char* buf, int size) const;
   // Use buf if needed buffer length is <= size.
   char* as_C_string_flexible_buffer(Thread* t, char* buf, int size) const;
   // Returns a null terminated utf8 string in a resource array
   char* as_utf8() const { return as_C_string(); }
   char* as_utf8_flexible_buffer(Thread* t, char* buf, int size) const {
     return as_C_string_flexible_buffer(t, buf, size);
   }
   jchar* as_unicode(int& length) const;
   // Treating this symbol as a class name, returns the Java name for the class.
   // String is allocated in resource area if buffer is not provided.
   // See Klass::external_name()
   const char* as_klass_external_name() const;
   const char* as_klass_external_name(char* buf, int size) const;
   // Printing
   void print_symbol_on(outputStream* st = NULL) const;
   void print_on(outputStream* st) const;         // First level print
   void print_value_on(outputStream* st) const;   // Second level print.
   // printing on default output stream
   void print()         { print_on(tty);       }
   void print_value()   { print_value_on(tty); }
 #ifndef PRODUCT
   // Empty constructor to create a dummy symbol object on stack
   // only for getting its vtable pointer.
   Symbol() { }
   static int _total_count;
 #endif
 };
 // Note: this comparison is used for vtable sorting only; it doesn't matter
 // what order it defines, as long as it is a total, time-invariant order
 // Since Symbol*s are in C_HEAP, their relative order in memory never changes,
 // so use address comparison for speed
 int Symbol::fast_compare(Symbol* other) const {
  return (((uintptr_t)this < (uintptr_t)other) ? -1
    : ((uintptr_t)this == (uintptr_t) other) ? 0 : 1);
 }
 #endif // SHARE_VM_OOPS_SYMBOL_HPP

Mercurial > jdk8-mips64-public > hotspot / annotate

src/share/vm/oops/symbol.hpp@e1162778c1c8 (annotated)

src/share/vm/oops/symbol.hpp