Ensure that objects that support ARC will use ARC, even if they are

created without sending a message to the class.

Also ensure that protocols are always valid objects so that ARC doesn't
become confused.

Protocol2.m

@@ -20,6 +20,8 @@
 - (id)self { return self; }
 @end
 @implementation Protocol2 @end
+@interface __IncompleteProtocol : Protocol2 @end
+@implementation __IncompleteProtocol @end
 
 /**
  * This class exists for the sole reason that the legacy GNU ABI did not

dtable.c

@@ -91,6 +91,50 @@ static void checkARCAccessors(Class cls)
 	objc_set_class_flag(cls, objc_class_flag_fast_arc);
 }
 
+PRIVATE void checkARCAccessorsSlow(Class cls)
+{
+	if (cls->dtable != uninstalled_dtable)
+	{
+		return;
+	}
+	static SEL retain, release, autorelease, isARC;
+	if (NULL == retain)
+	{
+		retain = sel_registerName("retain");
+		release = sel_registerName("release");
+		autorelease = sel_registerName("autorelease");
+		isARC = sel_registerName("_ARCCompliantRetainRelease");
+	}
+	if (cls->super_class != Nil)
+	{
+		checkARCAccessorsSlow(cls->super_class);
+	}
+	BOOL superIsFast = objc_test_class_flag(cls, objc_class_flag_fast_arc);
+	BOOL selfImplementsRetainRelease = NO;
+	for (struct objc_method_list *l=cls->methods ; l != NULL ; l= l->next)
+	{
+		for (int i=0 ; i<l->count ; i++)
+		{
+			SEL s = l->methods[i].selector;
+			if (sel_isEqual(s, retain) ||
+			    sel_isEqual(s, release) ||
+			    sel_isEqual(s, autorelease))
+			{
+				selfImplementsRetainRelease = YES;
+			}
+			else if (sel_isEqual(s, isARC))
+			{
+				objc_set_class_flag(cls, objc_class_flag_fast_arc);
+				return;
+			}
+		}
+	}
+	if (superIsFast && ! selfImplementsRetainRelease)
+	{
+		objc_set_class_flag(cls, objc_class_flag_fast_arc);
+	}
+}
+
 static void collectMethodsForMethodListToSparseArray(
 		struct objc_method_list *list,
 		SparseArray *sarray,

dtable.h

@@ -126,3 +126,9 @@ void add_method_list_to_class(Class cls,
  * Destroys a dtable.
  */
 void free_dtable(dtable_t dtable);
+
+/**
+ * Checks whether the class supports ARC.  This can be used before the dtable
+ * is installed.
+ */
+void checkARCAccessorsSlow(Class cls);

protocol.c

@@ -45,6 +45,17 @@ struct objc_protocol2 *protocol_for_name(const char *name)
 
 static id ObjC2ProtocolClass = 0;
 
+static id incompleteProtocolClass(void)
+{
+	static id IncompleteProtocolClass = 0;
+	if (IncompleteProtocolClass == nil)
+	{
+		IncompleteProtocolClass = objc_getClass("__IncompleteProtocol");
+	}
+	return IncompleteProtocolClass;
+}
+
+
 static int isEmptyProtocol(struct objc_protocol2 *aProto)
 {
 	int isEmpty =
@@ -497,7 +508,7 @@ Protocol*__unsafe_unretained* objc_copyProtocolList(unsigned int *outCount)
 Protocol *objc_allocateProtocol(const char *name)
 {
 	if (objc_getProtocol(name) != NULL) { return NULL; }
-	Protocol *p = calloc(1, sizeof(Protocol2));
+	Protocol *p = (Protocol*)class_createInstance((Class)incompleteProtocolClass(), 0);
 	p->name = strdup(name);
 	return p;
 }
@@ -506,7 +517,7 @@ void objc_registerProtocol(Protocol *proto)
 	if (NULL == proto) { return; }
 	LOCK_RUNTIME_FOR_SCOPE();
 	if (objc_getProtocol(proto->name) != NULL) { return; }
-	if (nil != proto->isa) { return; }
+	if (incompleteProtocolClass() != proto->isa) { return; }
 	proto->isa = ObjC2ProtocolClass;
 	protocol_table_insert((struct objc_protocol2*)proto);
 }
@@ -517,7 +528,7 @@ void protocol_addMethodDescription(Protocol *aProtocol,
                                    BOOL isInstanceMethod)
 {
 	if ((NULL == aProtocol) || (NULL == name) || (NULL == types)) { return; }
-	if (nil != aProtocol->isa) { return; }
+	if (incompleteProtocolClass() != aProtocol->isa) { return; }
 	Protocol2 *proto = (Protocol2*)aProtocol;
 	struct objc_method_description_list **listPtr;
 	if (isInstanceMethod)
@@ -561,6 +572,7 @@ void protocol_addMethodDescription(Protocol *aProtocol,
 void protocol_addProtocol(Protocol *aProtocol, Protocol *addition)
 {
 	if ((NULL == aProtocol) || (NULL == addition)) { return; }
+	if (incompleteProtocolClass() != aProtocol->isa) { return; }
 	Protocol2 *proto = (Protocol2*)aProtocol;
 	if (NULL == proto->protocol_list)
 	{
@@ -584,7 +596,7 @@ void protocol_addProperty(Protocol *aProtocol,
                           BOOL isInstanceProperty)
 {
 	if ((NULL == aProtocol) || (NULL == name)) { return; }
-	if (nil != aProtocol->isa) { return; }
+	if (incompleteProtocolClass() != aProtocol->isa) { return; }
 	if (!isInstanceProperty) { return; }
 	Protocol2 *proto = (Protocol2*)aProtocol;
 	struct objc_property_list **listPtr;

runtime.c

@@ -348,6 +348,7 @@ id class_createInstance(Class cls, size_t extraBytes)
 	if (Nil == cls)	{ return nil; }
 	id obj = gc->allocate_class(cls, extraBytes);
 	obj->isa = cls;
+	checkARCAccessorsSlow(cls);
 	call_cxx_construct(obj);
 	return obj;
 }