runtime/objc-weak.h

   1 /*
   2  * Copyright (c) 2010-2011 Apple Inc. All rights reserved.
   3  *
   4  * @APPLE_LICENSE_HEADER_START@
   5  *
   6  * This file contains Original Code and/or Modifications of Original Code
   7  * as defined in and that are subject to the Apple Public Source License
   8  * Version 2.0 (the 'License'). You may not use this file except in
   9  * compliance with the License. Please obtain a copy of the License at
  10  * http://www.opensource.apple.com/apsl/ and read it before using this
  11  * file.
  12  *
  13  * The Original Code and all software distributed under the License are
  14  * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
  15  * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
  16  * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
  17  * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
  18  * Please see the License for the specific language governing rights and
  19  * limitations under the License.
  20  *
  21  * @APPLE_LICENSE_HEADER_END@
  22  */
  23
  24 #ifndef _OBJC_WEAK_H_
  25 #define _OBJC_WEAK_H_
  26
  27 #include <objc/objc.h>
  28 #include "objc-config.h"
  29
  30 __BEGIN_DECLS
  31
  32 /*
  33 The weak table is a hash table governed by a single spin lock.
  34 An allocated blob of memory, most often an object, but under GC any such
  35 allocation, may have its address stored in a __weak marked storage location
  36 through use of compiler generated write-barriers or hand coded uses of the
  37 register weak primitive. Associated with the registration can be a callback
  38 block for the case when one of the allocated chunks of memory is reclaimed.
  39 The table is hashed on the address of the allocated memory.  When __weak
  40 marked memory changes its reference, we count on the fact that we can still
  41 see its previous reference.
  42
  43 So, in the hash table, indexed by the weakly referenced item, is a list of
  44 all locations where this address is currently being stored.
  45
  46 For ARR, we also keep track of whether an arbitrary object is being
  47 deallocated by briefly placing it in the table just prior to invoking
  48 dealloc, and removing it via objc_clear_deallocating just prior to memory
  49 reclamation.
  50
  51 */
  52
  53 /// The address of a __weak object reference
  54 typedef objc_object ** weak_referrer_t;
  55
  56 #if __LP64__
  57 #define PTR_MINUS_1 63
  58 #else
  59 #define PTR_MINUS_1 31
  60 #endif
  61
  62 /**
  63  * The internal structure stored in the weak references table.
  64  * It maintains and stores
  65  * a hash set of weak references pointing to an object.
  66  * If out_of_line==0, the set is instead a small inline array.
  67  */
  68 #define WEAK_INLINE_COUNT 4
  69 struct weak_entry_t {
  70     DisguisedPtr<objc_object> referent;
  71     union {
  72         struct {
  73             weak_referrer_t *referrers;
  74             uintptr_t        out_of_line : 1;
  75             uintptr_t        num_refs : PTR_MINUS_1;
  76             uintptr_t        mask;
  77             uintptr_t        max_hash_displacement;
  78         };
  79         struct {
  80             // out_of_line=0 is LSB of one of these (don't care which)
  81             weak_referrer_t  inline_referrers[WEAK_INLINE_COUNT];
  82         };
  83     };
  84 };
  85
  86 /**
  87  * The global weak references table. Stores object ids as keys,
  88  * and weak_entry_t structs as their values.
  89  */
  90 struct weak_table_t {
  91     weak_entry_t *weak_entries;
  92     size_t    num_entries;
  93     uintptr_t mask;
  94     uintptr_t max_hash_displacement;
  95 };
  96
  97 /// Adds an (object, weak pointer) pair to the weak table.
  98 id weak_register_no_lock(weak_table_t *weak_table, id referent, id *referrer);
  99
 100 /// Removes an (object, weak pointer) pair from the weak table.
 101 void weak_unregister_no_lock(weak_table_t *weak_table, id referent, id *referrer);
 102
 103 #if !NDEBUG
 104 /// Returns true if an object is weakly referenced somewhere.
 105 bool weak_is_registered_no_lock(weak_table_t *weak_table, id referent);
 106 #endif
 107
 108 /// Assert a weak pointer is valid and retain the object during its use.
 109 id weak_read_no_lock(weak_table_t *weak_table, id *referrer);
 110
 111 /// Called on object destruction. Sets all remaining weak pointers to nil.
 112 void weak_clear_no_lock(weak_table_t *weak_table, id referent);
 113
 114 __END_DECLS
 115
 116 #endif /* _OBJC_WEAK_H_ */