Slightly rework the implementation of objc_alloc_init.

Test/FastPathAlloc.m

@@ -21,6 +21,8 @@ @interface NoAlloc : Test @end
 @interface NoInit : Test @end
 @interface NoInit2 : NoInit @end
 
+@interface NilAlloc : NoInit @end
+
 @interface ShouldInitSubclassed : NoInit @end
 
 @implementation ShouldAlloc
@@ -100,6 +102,13 @@ + (instancetype) alloc
 }
 @end
 
+@implementation NilAlloc
++ (instancetype)alloc
+{
+	return nil;
+}
+@end
+
 Class getClassNamed(char *name)
 {
 	return nil;
@@ -147,6 +156,11 @@ int main(void)
 	// implementations can handle receivers that are nil
 	[getClassNamed("flibble") alloc];
 	[[getClassNamed("flibble") alloc] init];
+
+	// Check that we can handle fast-path init if alloc returns nil.
+	assert([[NilAlloc alloc] init] == nil);
+
+	return 0;
 }
 
 #endif

fast_paths.m

@@ -62,11 +62,23 @@ - (id)init;
 	{
 		return nil;
 	}
-	id instance = objc_alloc(cls);
-	// If +alloc was overwritten, it is not guaranteed that it returns
-	// an instance of cls.
-	cls = classForObject(instance);
-	if (objc_test_class_flag(cls, objc_class_flag_fast_alloc_init))
+	id instance;
+	if (LIKELY(objc_test_class_flag(cls->isa, objc_class_flag_fast_alloc_init)))
+	{
+		instance = class_createInstance(cls, 0);
+	}
+	else
+	{
+		instance = [cls alloc];
+		if (UNLIKELY(instance == nil))
+		{
+			return nil;
+		}
+		// If +alloc was overwritten, it is not guaranteed that it returns
+		// an instance of cls.
+		cls = classForObject(instance);
+	}
+	if (LIKELY(objc_test_class_flag(cls, objc_class_flag_fast_alloc_init)))
 	{
 		return instance;
 	}