Skip the C++ construct walk with a class flag

class.h

@@ -348,6 +348,13 @@ enum objc_class_flags
 	 * the underlying blocks runtime.
 	 */
 	objc_class_flag_is_block = (1 << 16),
+	/**
+	 * Neither this class nor any of its superclasses has a `.cxx_construct`
+	 * method, so constructing an instance of it has nothing to do.  Set when
+	 * the class's dispatch table is created, cleared if a `.cxx_construct` is
+	 * installed later.
+	 */
+	objc_class_flag_no_cxx_construct = (1<<17),
 };
 
 /**

dtable.c

@@ -373,17 +373,12 @@ static BOOL installMethodInDtable(Class class,
 	SparseArrayInsert(dtable, untyped_idx, method);
 #endif
 
-	static SEL cxx_construct, cxx_destruct;
-	if (NULL == cxx_construct)
-	{
-		cxx_construct = sel_registerName(".cxx_construct");
-		cxx_destruct = sel_registerName(".cxx_destruct");
-	}
-	if (selEqualUnTyped(method->selector, cxx_construct))
+	if (selEqualUnTyped(method->selector, cxx_construct_sel))
 	{
 		class->cxx_construct = method->imp;
+		objc_clear_class_flag(class, objc_class_flag_no_cxx_construct);
 	}
-	else if (selEqualUnTyped(method->selector, cxx_destruct))
+	else if (selEqualUnTyped(method->selector, cxx_destruct_sel))
 	{
 		class->cxx_destruct = method->imp;
 	}
@@ -604,6 +599,18 @@ PRIVATE dtable_t create_dtable_for_class(Class class, dtable_t root_dtable)
 		list = list->next;
 	}
 
+	// The superclass chain is fully initialised by this point, so a class
+	// whose superclass has nothing to construct and which has installed no
+	// `.cxx_construct` of its own has nothing to construct either.
+	if ((NULL == super) ||
+	    objc_test_class_flag(super, objc_class_flag_no_cxx_construct))
+	{
+		if (NULL == class->cxx_construct)
+		{
+			objc_set_class_flag(class, objc_class_flag_no_cxx_construct);
+		}
+	}
+
 	return dtable;
 }
 

loader.c

@@ -62,6 +62,10 @@ static void init_runtime(void)
 #if defined(EMBEDDED_BLOCKS_RUNTIME)
 		init_trampolines();
 #endif
+		// Registering a selector resizes the dispatch tables, which walks the
+		// class table, so this follows the tables it needs and precedes the
+		// first class whose methods are installed.
+		init_cxx_selectors();
 		init_builtin_classes();
 		first_run = NO;
 		if (getenv("LIBOBJC_MEMORY_PROFILE"))

loader.h

@@ -100,6 +100,11 @@ void init_protocol_table(void);
  */
 void init_selector_tables(void);
 
+/**
+ * Register the `.cxx_construct` and `.cxx_destruct` selectors.
+ */
+void init_cxx_selectors(void);
+
 /**
  * Initialise the trampolines for using blocks as methods.
  */

runtime.c

@@ -25,16 +25,20 @@ static inline void safe_remove_from_subclass_list(Class cls);
 PRIVATE BOOL objc_resolve_class(Class);
 void objc_send_initialize(id object);
 
+PRIVATE SEL cxx_construct_sel;
+PRIVATE SEL cxx_destruct_sel;
+
+PRIVATE void init_cxx_selectors(void)
+{
+	cxx_construct_sel = sel_registerName(".cxx_construct");
+	cxx_destruct_sel = sel_registerName(".cxx_destruct");
+}
+
 /**
  * Calls C++ destructors in the correct order.
  */
 PRIVATE void call_cxx_destruct(id obj)
 {
-	static SEL cxx_destruct;
-	if (NULL == cxx_destruct)
-	{
-		cxx_destruct = sel_registerName(".cxx_destruct");
-	}
 	// Don't call object_getClass(), because we want to get hidden classes too
 	Class cls = classForObject(obj);
 
@@ -46,32 +50,33 @@ PRIVATE void call_cxx_destruct(id obj)
 		cls = cls->super_class;
 		if (currentClass->cxx_destruct)
 		{
-			currentClass->cxx_destruct(obj, cxx_destruct);
+			currentClass->cxx_destruct(obj, cxx_destruct_sel);
 		}
 	}
 }
 
 static void call_cxx_construct_for_class(Class cls, id obj)
 {
-	static SEL cxx_construct;
-	if (NULL == cxx_construct)
-	{
-		cxx_construct = sel_registerName(".cxx_construct");
-	}
-
-	if (cls->super_class)
+	if (cls->super_class &&
+	    !objc_test_class_flag(cls->super_class, objc_class_flag_no_cxx_construct))
 	{
 		call_cxx_construct_for_class(cls->super_class, obj);
 	}
 	if (cls->cxx_construct)
 	{
-		cls->cxx_construct(obj, cxx_construct);
+		cls->cxx_construct(obj, cxx_construct_sel);
 	}
 }
 
 PRIVATE void call_cxx_construct(id obj)
 {
-	call_cxx_construct_for_class(classForObject(obj), obj);
+	Class cls = classForObject(obj);
+
+	if (objc_test_class_flag(cls, objc_class_flag_no_cxx_construct))
+	{
+		return;
+	}
+	call_cxx_construct_for_class(cls, obj);
 }
 
 /**

selector.h

@@ -2,6 +2,7 @@
 #define OBJC_SELECTOR_H_INCLUDED
 #include <stdint.h>
 #include "objc/runtime.h"
+#include "visibility.h"
 
 /**
  * Structure used to store selectors in the list.
@@ -56,6 +57,13 @@ extern "C"
  */
 SEL objc_register_selector(SEL aSel);
 
+/**
+ * The `.cxx_construct` and `.cxx_destruct` selectors, registered once during
+ * runtime initialisation by `init_cxx_selectors`.
+ */
+PRIVATE extern SEL cxx_construct_sel;
+PRIVATE extern SEL cxx_destruct_sel;
+
 #ifdef __cplusplus
 }
 #endif