1 /* GNU Objective C Runtime class related functions
2 Copyright (C) 1993, 1995, 1996, 1997, 2001, 2002, 2009
3 Free Software Foundation, Inc.
4 Contributed by Kresten Krab Thorup and Dennis Glatting.
6 Lock-free class table code designed and written from scratch by
9 This file is part of GCC.
11 GCC is free software; you can redistribute it and/or modify it under the
12 terms of the GNU General Public License as published by the Free Software
13 Foundation; either version 3, or (at your option) any later version.
15 GCC is distributed in the hope that it will be useful, but WITHOUT ANY
16 WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
17 FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
20 Under Section 7 of GPL version 3, you are granted additional
21 permissions described in the GCC Runtime Library Exception, version
22 3.1, as published by the Free Software Foundation.
24 You should have received a copy of the GNU General Public License and
25 a copy of the GCC Runtime Library Exception along with this program;
26 see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
27 <http://www.gnu.org/licenses/>. */
30 The code in this file critically affects class method invocation
31 speed. This long preamble comment explains why, and the issues
35 One of the traditional weaknesses of the GNU Objective-C runtime is
36 that class method invocations are slow. The reason is that when you
39 array = [NSArray new];
41 this gets basically compiled into the equivalent of
43 array = [(objc_get_class ("NSArray")) new];
45 objc_get_class returns the class pointer corresponding to the string
46 `NSArray'; and because of the lookup, the operation is more
47 complicated and slow than a simple instance method invocation.
49 Most high performance Objective-C code (using the GNU Objc runtime)
50 I had the opportunity to read (or write) work around this problem by
51 caching the class pointer:
53 Class arrayClass = [NSArray class];
57 array = [arrayClass new];
58 array = [arrayClass new];
59 array = [arrayClass new];
61 In this case, you always perform a class lookup (the first one), but
62 then all the [arrayClass new] methods run exactly as fast as an
63 instance method invocation. It helps if you have many class method
64 invocations to the same class.
66 The long-term solution to this problem would be to modify the
67 compiler to output tables of class pointers corresponding to all the
68 class method invocations, and to add code to the runtime to update
69 these tables - that should in the end allow class method invocations
70 to perform precisely as fast as instance method invocations, because
71 no class lookup would be involved. I think the Apple Objective-C
72 runtime uses this technique. Doing this involves synchronized
73 modifications in the runtime and in the compiler.
75 As a first medicine to the problem, I [NP] have redesigned and
76 rewritten the way the runtime is performing class lookup. This
77 doesn't give as much speed as the other (definitive) approach, but
78 at least a class method invocation now takes approximately 4.5 times
79 an instance method invocation on my machine (it would take approx 12
80 times before the rewriting), which is a lot better.
82 One of the main reason the new class lookup is so faster is because
83 I implemented it in a way that can safely run multithreaded without
84 using locks - a so-called `lock-free' data structure. The atomic
85 operation is pointer assignment. The reason why in this problem
86 lock-free data structures work so well is that you never remove
87 classes from the table - and the difficult thing with lock-free data
88 structures is freeing data when is removed from the structures. */
90 #include "objc-private/common.h"
91 #include "objc-private/error.h"
92 #include "objc/objc.h"
93 #include "objc/objc-api.h"
95 #include "objc-private/runtime.h" /* the kitchen sink */
96 #include <string.h> /* For memset */
98 /* We use a table which maps a class name to the corresponding class
99 * pointer. The first part of this file defines this table, and
100 * functions to do basic operations on the table. The second part of
101 * the file implements some higher level Objective-C functionality for
102 * classes by using the functions provided in the first part to manage
106 ** Class Table Internals
109 /* A node holding a class */
110 typedef struct class_node
112 struct class_node *next; /* Pointer to next entry on the list.
113 NULL indicates end of list. */
115 const char *name; /* The class name string */
116 int length; /* The class name string length */
117 Class pointer; /* The Class pointer */
121 /* A table containing classes is a class_node_ptr (pointing to the
122 first entry in the table - if it is NULL, then the table is
125 /* We have 1024 tables. Each table contains all class names which
126 have the same hash (which is a number between 0 and 1023). To look
127 up a class_name, we compute its hash, and get the corresponding
128 table. Once we have the table, we simply compare strings directly
129 till we find the one which we want (using the length first). The
130 number of tables is quite big on purpose (a normal big application
131 has less than 1000 classes), so that you shouldn't normally get any
132 collisions, and get away with a single comparison (which we can't
133 avoid since we need to know that you have got the right thing). */
134 #define CLASS_TABLE_SIZE 1024
135 #define CLASS_TABLE_MASK 1023
137 static class_node_ptr class_table_array[CLASS_TABLE_SIZE];
139 /* The table writing mutex - we lock on writing to avoid conflicts
140 between different writers, but we read without locks. That is
141 possible because we assume pointer assignment to be an atomic
143 static objc_mutex_t __class_table_lock = NULL;
145 /* CLASS_TABLE_HASH is how we compute the hash of a class name. It is
146 a macro - *not* a function - arguments *are* modified directly.
148 INDEX should be a variable holding an int;
149 HASH should be a variable holding an int;
150 CLASS_NAME should be a variable holding a (char *) to the class_name.
152 After the macro is executed, INDEX contains the length of the
153 string, and HASH the computed hash of the string; CLASS_NAME is
156 #define CLASS_TABLE_HASH(INDEX, HASH, CLASS_NAME) \
158 for (INDEX = 0; CLASS_NAME[INDEX] != '\0'; INDEX++) \
160 HASH = (HASH << 4) ^ (HASH >> 28) ^ CLASS_NAME[INDEX]; \
163 HASH = (HASH ^ (HASH >> 10) ^ (HASH >> 20)) & CLASS_TABLE_MASK;
165 /* Setup the table. */
167 class_table_setup (void)
169 /* Start - nothing in the table. */
170 memset (class_table_array, 0, sizeof (class_node_ptr) * CLASS_TABLE_SIZE);
172 /* The table writing mutex. */
173 __class_table_lock = objc_mutex_allocate ();
177 /* Insert a class in the table (used when a new class is registered). */
179 class_table_insert (const char *class_name, Class class_pointer)
182 class_node_ptr new_node;
184 /* Find out the class name's hash and length. */
185 CLASS_TABLE_HASH (length, hash, class_name);
187 /* Prepare the new node holding the class. */
188 new_node = objc_malloc (sizeof (struct class_node));
189 new_node->name = class_name;
190 new_node->length = length;
191 new_node->pointer = class_pointer;
193 /* Lock the table for modifications. */
194 objc_mutex_lock (__class_table_lock);
196 /* Insert the new node in the table at the beginning of the table at
197 class_table_array[hash]. */
198 new_node->next = class_table_array[hash];
199 class_table_array[hash] = new_node;
201 objc_mutex_unlock (__class_table_lock);
204 /* Replace a class in the table (used only by poseAs:). */
206 class_table_replace (Class old_class_pointer, Class new_class_pointer)
211 objc_mutex_lock (__class_table_lock);
214 node = class_table_array[hash];
216 while (hash < CLASS_TABLE_SIZE)
221 if (hash < CLASS_TABLE_SIZE)
223 node = class_table_array[hash];
228 Class class1 = node->pointer;
230 if (class1 == old_class_pointer)
232 node->pointer = new_class_pointer;
238 objc_mutex_unlock (__class_table_lock);
242 /* Get a class from the table. This does not need mutex protection.
243 Currently, this function is called each time you call a static
244 method, this is why it must be very fast. */
246 class_table_get_safe (const char *class_name)
251 /* Compute length and hash. */
252 CLASS_TABLE_HASH (length, hash, class_name);
254 node = class_table_array[hash];
260 if (node->length == length)
262 /* Compare the class names. */
265 for (i = 0; i < length; i++)
267 if ((node->name)[i] != class_name[i])
275 /* They are equal! */
276 return node->pointer;
280 while ((node = node->next) != NULL);
286 /* Enumerate over the class table. */
287 struct class_table_enumerator
295 class_table_next (struct class_table_enumerator **e)
297 struct class_table_enumerator *enumerator = *e;
300 if (enumerator == NULL)
302 *e = objc_malloc (sizeof (struct class_table_enumerator));
304 enumerator->hash = 0;
305 enumerator->node = NULL;
307 next = class_table_array[enumerator->hash];
311 next = enumerator->node->next;
316 enumerator->node = next;
317 return enumerator->node->pointer;
323 while (enumerator->hash < CLASS_TABLE_SIZE)
325 next = class_table_array[enumerator->hash];
328 enumerator->node = next;
329 return enumerator->node->pointer;
334 /* Ok - table finished - done. */
335 objc_free (enumerator);
340 #if 0 /* DEBUGGING FUNCTIONS */
341 /* Debugging function - print the class table. */
343 class_table_print (void)
347 for (i = 0; i < CLASS_TABLE_SIZE; i++)
352 node = class_table_array[i];
356 printf ("\t%s\n", node->name);
362 /* Debugging function - print an histogram of number of classes in
363 function of hash key values. Useful to evaluate the hash function
366 class_table_print_histogram (void)
371 for (i = 0; i < CLASS_TABLE_SIZE; i++)
375 node = class_table_array[i];
382 if (((i + 1) % 50) == 0)
384 printf ("%4d:", i + 1);
385 for (j = 0; j < counter; j++)
393 printf ("%4d:", i + 1);
394 for (j = 0; j < counter; j++)
400 #endif /* DEBUGGING FUNCTIONS */
403 ** Objective-C runtime functions
406 /* From now on, the only access to the class table data structure
407 should be via the class_table_* functions. */
409 /* This is a hook which is called by objc_get_class and
410 objc_lookup_class if the runtime is not able to find the class.
411 This may e.g. try to load in the class using dynamic loading. */
412 Class (*_objc_lookup_class) (const char *name) = 0; /* !T:SAFE */
415 /* True when class links has been resolved. */
416 BOOL __objc_class_links_resolved = NO; /* !T:UNUSED */
420 __objc_init_class_tables (void)
422 /* Allocate the class hash table. */
424 if (__class_table_lock)
427 objc_mutex_lock (__objc_runtime_mutex);
429 class_table_setup ();
431 objc_mutex_unlock (__objc_runtime_mutex);
434 /* This function adds a class to the class hash table, and assigns the
435 class a number, unless it's already known. */
437 __objc_add_class_to_hash (Class class)
441 objc_mutex_lock (__objc_runtime_mutex);
443 /* Make sure the table is there. */
444 assert (__class_table_lock);
446 /* Make sure it's not a meta class. */
447 assert (CLS_ISCLASS (class));
449 /* Check to see if the class is already in the hash table. */
450 h_class = class_table_get_safe (class->name);
453 /* The class isn't in the hash table. Add the class and assign a class
455 static unsigned int class_number = 1;
457 CLS_SETNUMBER (class, class_number);
458 CLS_SETNUMBER (class->class_pointer, class_number);
461 class_table_insert (class->name, class);
464 objc_mutex_unlock (__objc_runtime_mutex);
467 /* Get the class object for the class named NAME. If NAME does not
468 identify a known class, the hook _objc_lookup_class is called. If
469 this fails, nil is returned. */
471 objc_lookup_class (const char *name)
475 class = class_table_get_safe (name);
480 if (_objc_lookup_class)
481 return (*_objc_lookup_class) (name);
486 /* Get the class object for the class named NAME. If NAME does not
487 identify a known class, the hook _objc_lookup_class is called. If
488 this fails, an error message is issued and the system aborts. */
490 objc_get_class (const char *name)
494 class = class_table_get_safe (name);
499 if (_objc_lookup_class)
500 class = (*_objc_lookup_class) (name);
505 /* FIXME: Should we abort the program here ? */
506 _objc_abort ("objc runtime: cannot find class %s\n", name);
512 objc_get_meta_class (const char *name)
514 return objc_get_class (name)->class_pointer;
517 /* This function provides a way to enumerate all the classes in the
518 executable. Pass *ENUM_STATE == NULL to start the enumeration. The
519 function will return 0 when there are no more classes.
523 while ((class = objc_next_class (&es)))
524 ... do something with class;
527 objc_next_class (void **enum_state)
531 objc_mutex_lock (__objc_runtime_mutex);
533 /* Make sure the table is there. */
534 assert (__class_table_lock);
536 class = class_table_next ((struct class_table_enumerator **) enum_state);
538 objc_mutex_unlock (__objc_runtime_mutex);
543 /* Resolve super/subclass links for all classes. The only thing we
544 can be sure of is that the class_pointer for class objects point to
545 the right meta class objects. */
547 __objc_resolve_class_links (void)
549 struct class_table_enumerator *es = NULL;
550 Class object_class = objc_get_class ("Object");
553 assert (object_class);
555 objc_mutex_lock (__objc_runtime_mutex);
557 /* Assign subclass links. */
558 while ((class1 = class_table_next (&es)))
560 /* Make sure we have what we think we have. */
561 assert (CLS_ISCLASS (class1));
562 assert (CLS_ISMETA (class1->class_pointer));
564 /* The class_pointer of all meta classes point to Object's meta
566 class1->class_pointer->class_pointer = object_class->class_pointer;
568 if (! CLS_ISRESOLV (class1))
570 CLS_SETRESOLV (class1);
571 CLS_SETRESOLV (class1->class_pointer);
573 if (class1->super_class)
576 = objc_get_class ((char *) class1->super_class);
578 assert (a_super_class);
580 DEBUG_PRINTF ("making class connections for: %s\n",
583 /* Assign subclass links for superclass. */
584 class1->sibling_class = a_super_class->subclass_list;
585 a_super_class->subclass_list = class1;
587 /* Assign subclass links for meta class of superclass. */
588 if (a_super_class->class_pointer)
590 class1->class_pointer->sibling_class
591 = a_super_class->class_pointer->subclass_list;
592 a_super_class->class_pointer->subclass_list
593 = class1->class_pointer;
596 else /* A root class, make its meta object be a subclass of
599 class1->class_pointer->sibling_class
600 = object_class->subclass_list;
601 object_class->subclass_list = class1->class_pointer;
606 /* Assign superclass links. */
608 while ((class1 = class_table_next (&es)))
611 for (sub_class = class1->subclass_list; sub_class;
612 sub_class = sub_class->sibling_class)
614 sub_class->super_class = class1;
615 if (CLS_ISCLASS (sub_class))
616 sub_class->class_pointer->super_class = class1->class_pointer;
620 objc_mutex_unlock (__objc_runtime_mutex);
625 #define CLASSOF(c) ((c)->class_pointer)
628 class_pose_as (Class impostor, Class super_class)
630 if (! CLS_ISRESOLV (impostor))
631 __objc_resolve_class_links ();
635 assert (super_class);
636 assert (impostor->super_class == super_class);
637 assert (CLS_ISCLASS (impostor));
638 assert (CLS_ISCLASS (super_class));
639 assert (impostor->instance_size == super_class->instance_size);
642 Class *subclass = &(super_class->subclass_list);
644 /* Move subclasses of super_class to impostor. */
647 Class nextSub = (*subclass)->sibling_class;
649 if (*subclass != impostor)
651 Class sub = *subclass;
654 sub->sibling_class = impostor->subclass_list;
655 sub->super_class = impostor;
656 impostor->subclass_list = sub;
658 /* It will happen that SUB is not a class object if it is
659 the top of the meta class hierarchy chain (root
660 meta-class objects inherit their class object). If
661 that is the case... don't mess with the meta-meta
663 if (CLS_ISCLASS (sub))
666 CLASSOF (sub)->sibling_class =
667 CLASSOF (impostor)->subclass_list;
668 CLASSOF (sub)->super_class = CLASSOF (impostor);
669 CLASSOF (impostor)->subclass_list = CLASSOF (sub);
676 /* Set subclasses of superclass to be impostor only. */
677 super_class->subclass_list = impostor;
678 CLASSOF (super_class)->subclass_list = CLASSOF (impostor);
680 /* Set impostor to have no sibling classes. */
681 impostor->sibling_class = 0;
682 CLASSOF (impostor)->sibling_class = 0;
685 /* Check relationship of impostor and super_class is kept. */
686 assert (impostor->super_class == super_class);
687 assert (CLASSOF (impostor)->super_class == CLASSOF (super_class));
689 /* This is how to update the lookup table. Regardless of what the
690 keys of the hashtable is, change all values that are superclass
693 objc_mutex_lock (__objc_runtime_mutex);
695 class_table_replace (super_class, impostor);
697 objc_mutex_unlock (__objc_runtime_mutex);
699 /* Next, we update the dispatch tables... */
700 __objc_update_dispatch_table_for_class (CLASSOF (impostor));
701 __objc_update_dispatch_table_for_class (impostor);