/* GNU Objective C Runtime class related functions Copyright (C) 1993, 1995, 1996, 1997, 2001, 2002 Free Software Foundation, Inc. Contributed by Kresten Krab Thorup and Dennis Glatting. Lock-free class table code designed and written from scratch by Nicola Pero, 2001. This file is part of GCC. GCC is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version. GCC 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 for more details. You should have received a copy of the GNU General Public License along with GCC; see the file COPYING. If not, write to the Free Software Foundation, 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */ /* As a special exception, if you link this library with files compiled with GCC to produce an executable, this does not cause the resulting executable to be covered by the GNU General Public License. This exception does not however invalidate any other reasons why the executable file might be covered by the GNU General Public License. */ /* The code in this file critically affects class method invocation speed. This long preamble comment explains why, and the issues involved. One of the traditional weaknesses of the GNU Objective-C runtime is that class method invocations are slow. The reason is that when you write array = [NSArray new]; this gets basically compiled into the equivalent of array = [(objc_get_class ("NSArray")) new]; objc_get_class returns the class pointer corresponding to the string `NSArray'; and because of the lookup, the operation is more complicated and slow than a simple instance method invocation. Most high performance Objective-C code (using the GNU Objc runtime) I had the opportunity to read (or write) work around this problem by caching the class pointer: Class arrayClass = [NSArray class]; ... later on ... array = [arrayClass new]; array = [arrayClass new]; array = [arrayClass new]; In this case, you always perform a class lookup (the first one), but then all the [arrayClass new] methods run exactly as fast as an instance method invocation. It helps if you have many class method invocations to the same class. The long-term solution to this problem would be to modify the compiler to output tables of class pointers corresponding to all the class method invocations, and to add code to the runtime to update these tables - that should in the end allow class method invocations to perform precisely as fast as instance method invocations, because no class lookup would be involved. I think the Apple Objective-C runtime uses this technique. Doing this involves synchronized modifications in the runtime and in the compiler. As a first medicine to the problem, I [NP] have redesigned and rewritten the way the runtime is performing class lookup. This doesn't give as much speed as the other (definitive) approach, but at least a class method invocation now takes approximately 4.5 times an instance method invocation on my machine (it would take approx 12 times before the rewriting), which is a lot better. One of the main reason the new class lookup is so faster is because I implemented it in a way that can safely run multithreaded without using locks - a so-called `lock-free' data structure. The atomic operation is pointer assignment. The reason why in this problem lock-free data structures work so well is that you never remove classes from the table - and the difficult thing with lock-free data structures is freeing data when is removed from the structures. */ #include "objc/runtime.h" /* the kitchen sink */ #include "objc/sarray.h" #include "objc/objc.h" #include "objc/objc-api.h" #include "objc/thr.h" /* We use a table which maps a class name to the corresponding class * pointer. The first part of this file defines this table, and * functions to do basic operations on the table. The second part of * the file implements some higher level Objective-C functionality for * classes by using the functions provided in the first part to manage * the table. */ /** ** Class Table Internals **/ /* A node holding a class */ typedef struct class_node { struct class_node *next; /* Pointer to next entry on the list. NULL indicates end of list. */ const char *name; /* The class name string */ int length; /* The class name string length */ Class pointer; /* The Class pointer */ } *class_node_ptr; /* A table containing classes is a class_node_ptr (pointing to the first entry in the table - if it is NULL, then the table is empty). */ /* We have 1024 tables. Each table contains all class names which have the same hash (which is a number between 0 and 1023). To look up a class_name, we compute its hash, and get the corresponding table. Once we have the table, we simply compare strings directly till we find the one which we want (using the length first). The number of tables is quite big on purpose (a normal big application has less than 1000 classes), so that you shouldn't normally get any collisions, and get away with a single comparison (which we can't avoid since we need to know that you have got the right thing). */ #define CLASS_TABLE_SIZE 1024 #define CLASS_TABLE_MASK 1023 static class_node_ptr class_table_array[CLASS_TABLE_SIZE]; /* The table writing mutex - we lock on writing to avoid conflicts between different writers, but we read without locks. That is possible because we assume pointer assignment to be an atomic operation. */ static objc_mutex_t __class_table_lock = NULL; /* CLASS_TABLE_HASH is how we compute the hash of a class name. It is a macro - *not* a function - arguments *are* modified directly. INDEX should be a variable holding an int; HASH should be a variable holding an int; CLASS_NAME should be a variable holding a (char *) to the class_name. After the macro is executed, INDEX contains the length of the string, and HASH the computed hash of the string; CLASS_NAME is untouched. */ #define CLASS_TABLE_HASH(INDEX, HASH, CLASS_NAME) \ HASH = 0; \ for (INDEX = 0; CLASS_NAME[INDEX] != '\0'; INDEX++) \ { \ HASH = (HASH << 4) ^ (HASH >> 28) ^ CLASS_NAME[INDEX]; \ } \ \ HASH = (HASH ^ (HASH >> 10) ^ (HASH >> 20)) & CLASS_TABLE_MASK; /* Setup the table. */ static void class_table_setup (void) { /* Start - nothing in the table. */ memset (class_table_array, 0, sizeof (class_node_ptr) * CLASS_TABLE_SIZE); /* The table writing mutex. */ __class_table_lock = objc_mutex_allocate (); } /* Insert a class in the table (used when a new class is registered). */ static void class_table_insert (const char *class_name, Class class_pointer) { int hash, length; class_node_ptr new_node; /* Find out the class name's hash and length. */ CLASS_TABLE_HASH (length, hash, class_name); /* Prepare the new node holding the class. */ new_node = objc_malloc (sizeof (struct class_node)); new_node->name = class_name; new_node->length = length; new_node->pointer = class_pointer; /* Lock the table for modifications. */ objc_mutex_lock (__class_table_lock); /* Insert the new node in the table at the beginning of the table at class_table_array[hash]. */ new_node->next = class_table_array[hash]; class_table_array[hash] = new_node; objc_mutex_unlock (__class_table_lock); } /* Replace a class in the table (used only by poseAs:). */ static void class_table_replace (Class old_class_pointer, Class new_class_pointer) { int hash; class_node_ptr node; objc_mutex_lock (__class_table_lock); hash = 0; node = class_table_array[hash]; while (hash < CLASS_TABLE_SIZE) { if (node == NULL) { hash++; if (hash < CLASS_TABLE_SIZE) { node = class_table_array[hash]; } } else { Class class1 = node->pointer; if (class1 == old_class_pointer) { node->pointer = new_class_pointer; } node = node->next; } } objc_mutex_unlock (__class_table_lock); } /* Get a class from the table. This does not need mutex protection. Currently, this function is called each time you call a static method, this is why it must be very fast. */ static inline Class class_table_get_safe (const char *class_name) { class_node_ptr node; int length, hash; /* Compute length and hash. */ CLASS_TABLE_HASH (length, hash, class_name); node = class_table_array[hash]; if (node != NULL) { do { if (node->length == length) { /* Compare the class names. */ int i; for (i = 0; i < length; i++) { if ((node->name)[i] != class_name[i]) { break; } } if (i == length) { /* They are equal! */ return node->pointer; } } } while ((node = node->next) != NULL); } return Nil; } /* Enumerate over the class table. */ struct class_table_enumerator { int hash; class_node_ptr node; }; static Class class_table_next (struct class_table_enumerator **e) { struct class_table_enumerator *enumerator = *e; class_node_ptr next; if (enumerator == NULL) { *e = objc_malloc (sizeof (struct class_table_enumerator)); enumerator = *e; enumerator->hash = 0; enumerator->node = NULL; next = class_table_array[enumerator->hash]; } else { next = enumerator->node->next; } if (next != NULL) { enumerator->node = next; return enumerator->node->pointer; } else { enumerator->hash++; while (enumerator->hash < CLASS_TABLE_SIZE) { next = class_table_array[enumerator->hash]; if (next != NULL) { enumerator->node = next; return enumerator->node->pointer; } enumerator->hash++; } /* Ok - table finished - done. */ objc_free (enumerator); return Nil; } } #if 0 /* DEBUGGING FUNCTIONS */ /* Debugging function - print the class table. */ void class_table_print (void) { int i; for (i = 0; i < CLASS_TABLE_SIZE; i++) { class_node_ptr node; printf ("%d:\n", i); node = class_table_array[i]; while (node != NULL) { printf ("\t%s\n", node->name); node = node->next; } } } /* Debugging function - print an histogram of number of classes in function of hash key values. Useful to evaluate the hash function in real cases. */ void class_table_print_histogram (void) { int i, j; int counter = 0; for (i = 0; i < CLASS_TABLE_SIZE; i++) { class_node_ptr node; node = class_table_array[i]; while (node != NULL) { counter++; node = node->next; } if (((i + 1) % 50) == 0) { printf ("%4d:", i + 1); for (j = 0; j < counter; j++) { printf ("X"); } printf ("\n"); counter = 0; } } printf ("%4d:", i + 1); for (j = 0; j < counter; j++) { printf ("X"); } printf ("\n"); } #endif /* DEBUGGING FUNCTIONS */ /** ** Objective-C runtime functions **/ /* From now on, the only access to the class table data structure should be via the class_table_* functions. */ /* This is a hook which is called by objc_get_class and objc_lookup_class if the runtime is not able to find the class. This may e.g. try to load in the class using dynamic loading. */ Class (*_objc_lookup_class) (const char *name) = 0; /* !T:SAFE */ /* True when class links has been resolved. */ BOOL __objc_class_links_resolved = NO; /* !T:UNUSED */ void __objc_init_class_tables (void) { /* Allocate the class hash table. */ if (__class_table_lock) return; objc_mutex_lock (__objc_runtime_mutex); class_table_setup (); objc_mutex_unlock (__objc_runtime_mutex); } /* This function adds a class to the class hash table, and assigns the class a number, unless it's already known. */ void __objc_add_class_to_hash (Class class) { Class h_class; objc_mutex_lock (__objc_runtime_mutex); /* Make sure the table is there. */ assert (__class_table_lock); /* Make sure it's not a meta class. */ assert (CLS_ISCLASS (class)); /* Check to see if the class is already in the hash table. */ h_class = class_table_get_safe (class->name); if (! h_class) { /* The class isn't in the hash table. Add the class and assign a class number. */ static unsigned int class_number = 1; CLS_SETNUMBER (class, class_number); CLS_SETNUMBER (class->class_pointer, class_number); ++class_number; class_table_insert (class->name, class); } objc_mutex_unlock (__objc_runtime_mutex); } /* Get the class object for the class named NAME. If NAME does not identify a known class, the hook _objc_lookup_class is called. If this fails, nil is returned. */ Class objc_lookup_class (const char *name) { Class class; class = class_table_get_safe (name); if (class) return class; if (_objc_lookup_class) return (*_objc_lookup_class) (name); else return 0; } /* Get the class object for the class named NAME. If NAME does not identify a known class, the hook _objc_lookup_class is called. If this fails, an error message is issued and the system aborts. */ Class objc_get_class (const char *name) { Class class; class = class_table_get_safe (name); if (class) return class; if (_objc_lookup_class) class = (*_objc_lookup_class) (name); if (class) return class; objc_error (nil, OBJC_ERR_BAD_CLASS, "objc runtime: cannot find class %s\n", name); return 0; } MetaClass objc_get_meta_class (const char *name) { return objc_get_class (name)->class_pointer; } /* This function provides a way to enumerate all the classes in the executable. Pass *ENUM_STATE == NULL to start the enumeration. The function will return 0 when there are no more classes. For example: id class; void *es = NULL; while ((class = objc_next_class (&es))) ... do something with class; */ Class objc_next_class (void **enum_state) { Class class; objc_mutex_lock (__objc_runtime_mutex); /* Make sure the table is there. */ assert (__class_table_lock); class = class_table_next ((struct class_table_enumerator **) enum_state); objc_mutex_unlock (__objc_runtime_mutex); return class; } /* Resolve super/subclass links for all classes. The only thing we can be sure of is that the class_pointer for class objects point to the right meta class objects. */ void __objc_resolve_class_links (void) { struct class_table_enumerator *es = NULL; Class object_class = objc_get_class ("Object"); Class class1; assert (object_class); objc_mutex_lock (__objc_runtime_mutex); /* Assign subclass links. */ while ((class1 = class_table_next (&es))) { /* Make sure we have what we think we have. */ assert (CLS_ISCLASS (class1)); assert (CLS_ISMETA (class1->class_pointer)); /* The class_pointer of all meta classes point to Object's meta class. */ class1->class_pointer->class_pointer = object_class->class_pointer; if (! CLS_ISRESOLV (class1)) { CLS_SETRESOLV (class1); CLS_SETRESOLV (class1->class_pointer); if (class1->super_class) { Class a_super_class = objc_get_class ((char *) class1->super_class); assert (a_super_class); DEBUG_PRINTF ("making class connections for: %s\n", class1->name); /* Assign subclass links for superclass. */ class1->sibling_class = a_super_class->subclass_list; a_super_class->subclass_list = class1; /* Assign subclass links for meta class of superclass. */ if (a_super_class->class_pointer) { class1->class_pointer->sibling_class = a_super_class->class_pointer->subclass_list; a_super_class->class_pointer->subclass_list = class1->class_pointer; } } else /* A root class, make its meta object be a subclass of Object. */ { class1->class_pointer->sibling_class = object_class->subclass_list; object_class->subclass_list = class1->class_pointer; } } } /* Assign superclass links. */ es = NULL; while ((class1 = class_table_next (&es))) { Class sub_class; for (sub_class = class1->subclass_list; sub_class; sub_class = sub_class->sibling_class) { sub_class->super_class = class1; if (CLS_ISCLASS (sub_class)) sub_class->class_pointer->super_class = class1->class_pointer; } } objc_mutex_unlock (__objc_runtime_mutex); } #define CLASSOF(c) ((c)->class_pointer) Class class_pose_as (Class impostor, Class super_class) { if (! CLS_ISRESOLV (impostor)) __objc_resolve_class_links (); /* Preconditions */ assert (impostor); assert (super_class); assert (impostor->super_class == super_class); assert (CLS_ISCLASS (impostor)); assert (CLS_ISCLASS (super_class)); assert (impostor->instance_size == super_class->instance_size); { Class *subclass = &(super_class->subclass_list); /* Move subclasses of super_class to impostor. */ while (*subclass) { Class nextSub = (*subclass)->sibling_class; if (*subclass != impostor) { Class sub = *subclass; /* Classes */ sub->sibling_class = impostor->subclass_list; sub->super_class = impostor; impostor->subclass_list = sub; /* It will happen that SUB is not a class object if it is the top of the meta class hierarchy chain (root meta-class objects inherit their class object). If that is the case... don't mess with the meta-meta class. */ if (CLS_ISCLASS (sub)) { /* Meta classes */ CLASSOF (sub)->sibling_class = CLASSOF (impostor)->subclass_list; CLASSOF (sub)->super_class = CLASSOF (impostor); CLASSOF (impostor)->subclass_list = CLASSOF (sub); } } *subclass = nextSub; } /* Set subclasses of superclass to be impostor only. */ super_class->subclass_list = impostor; CLASSOF (super_class)->subclass_list = CLASSOF (impostor); /* Set impostor to have no sibling classes. */ impostor->sibling_class = 0; CLASSOF (impostor)->sibling_class = 0; } /* Check relationship of impostor and super_class is kept. */ assert (impostor->super_class == super_class); assert (CLASSOF (impostor)->super_class == CLASSOF (super_class)); /* This is how to update the lookup table. Regardless of what the keys of the hashtable is, change all values that are superclass into impostor. */ objc_mutex_lock (__objc_runtime_mutex); class_table_replace (super_class, impostor); objc_mutex_unlock (__objc_runtime_mutex); /* Next, we update the dispatch tables... */ __objc_update_dispatch_table_for_class (CLASSOF (impostor)); __objc_update_dispatch_table_for_class (impostor); return impostor; }