Small bug fixes:

- Don't call C++ constructors if they don't exist.
- Don't check the owner of retain / release methods if they are not implemented.
- Add arc.m as a file to ignore when checking for GC compatibility.

abi_version.c

@@ -72,7 +72,7 @@ static BOOL endsWith(const char *string, const char *suffix)
 
 PRIVATE BOOL objc_check_abi_version(struct objc_module_abi_8 *module)
 {
-	static int runtime_modules = 4;
+	static int runtime_modules = 5;
 	// As a quick and ugly hack, skip these three tests for the .m files in the
 	// runtime.  They should (in theory, at least) be aware of the GC mode and
 	// behave accordingly.
@@ -80,6 +80,7 @@ PRIVATE BOOL objc_check_abi_version(struct objc_module_abi_8 *module)
 	{
 		if (endsWith(module->name, "properties.m") ||
 		    endsWith(module->name, "associate.m") ||
+		    endsWith(module->name, "arc.m") ||
 		    endsWith(module->name, "blocks_runtime.m") ||
 		    endsWith(module->name, "Protocol2.m"))
 		{

dtable.c

@@ -52,19 +52,19 @@ static void checkARCAccessors(Class cls)
 		isARC = sel_registerName("_ARCCompliantRetainRelease");
 	}
 	struct objc_slot *slot = objc_get_slot(cls, retain);
-	if (!ownsMethod(slot->owner, isARC))
+	if ((NULL != slot) && !ownsMethod(slot->owner, isARC))
 	{
 		objc_clear_class_flag(cls, objc_class_flag_fast_arc);
 		return;
 	}
 	slot = objc_get_slot(cls, release);
-	if (!ownsMethod(slot->owner, isARC))
+	if ((NULL != slot) && !ownsMethod(slot->owner, isARC))
 	{
 		objc_clear_class_flag(cls, objc_class_flag_fast_arc);
 		return;
 	}
 	slot = objc_get_slot(cls, autorelease);
-	if (!ownsMethod(slot->owner, isARC))
+	if ((NULL != slot) && !ownsMethod(slot->owner, isARC))
 	{
 		objc_clear_class_flag(cls, objc_class_flag_fast_arc);
 		return;

runtime.c

@@ -55,12 +55,15 @@ static void call_cxx_construct_for_class(Class cls, id obj)
 		cxx_construct = sel_registerName(".cxx_contruct");
 	}
 	struct objc_slot *slot = objc_get_slot(cls, cxx_construct);
-	cls = slot->owner->super_class;
-	if (Nil != cls)
+	if (NULL != slot)
 	{
-		call_cxx_construct_for_class(cls, obj);
+		cls = slot->owner->super_class;
+		if (Nil != cls)
+		{
+			call_cxx_construct_for_class(cls, obj);
+		}
+		slot->method(obj, cxx_construct);
 	}
-	slot->method(obj, cxx_construct);
 }
 
 PRIVATE void call_cxx_construct(id obj)