Implement acyclic object tracking

Acyclic objects don't need to be added to the GC. We know an object is acyclic
if all properties are typed as acyclic, and the object does not have any dynamic
properties. This is possible to track at runtime with minimal overhead.

Fixes GH-17127

Author: iluuu1994

Zend/tests/gc/gc_045.phpt

@@ -11,6 +11,9 @@ class GlobalData
 
 class Value
 {
+    /* Force object to be added to GC, even though it is acyclic. */
+    public $dummy;
+
     public function __destruct()
     {
         new Bar();
@@ -19,6 +22,9 @@ class Value
 
 class Bar
 {
+    /* Force object to be added to GC, even though it is acyclic. */
+    public $dummy;
+
     public function __construct()
     {
         GlobalData::$bar = $this;

Zend/tests/weakrefs/gh10043-008.phpt

@@ -5,6 +5,9 @@ Self-referencing map entry GC - 008
 
 class Canary extends stdClass
 {
+    /* Force object to be added to GC, even though it is acyclic. */
+    public $dummy;
+
     public function __construct(public string $name)
     {
     }

Zend/zend_API.c

@@ -4494,6 +4494,27 @@ static zend_always_inline bool is_persistent_class(zend_class_entry *ce) {
 		&& ce->info.internal.module->type == MODULE_PERSISTENT;
 }
 
+static bool zend_type_may_be_cyclic(zend_type type)
+{
+	if (!ZEND_TYPE_IS_SET(type)) {
+		return true;
+	}
+
+	if (!ZEND_TYPE_IS_COMPLEX(type)) {
+		return ZEND_TYPE_PURE_MASK(type) & (MAY_BE_OBJECT|MAY_BE_ARRAY);
+	} else if (ZEND_TYPE_IS_UNION(type)) {
+		zend_type *list_type;
+		ZEND_TYPE_LIST_FOREACH(ZEND_TYPE_LIST(type), list_type) {
+			if (zend_type_may_be_cyclic(*list_type)) {
+				return true;
+			}
+		} ZEND_TYPE_LIST_FOREACH_END();
+		return false;
+	}
+
+	return true;
+}
+
 ZEND_API zend_property_info *zend_declare_typed_property(zend_class_entry *ce, zend_string *name, zval *property, int access_type, zend_string *doc_comment, zend_type type) /* {{{ */
 {
 	zend_property_info *property_info, *property_info_ptr;
@@ -4506,6 +4527,12 @@ ZEND_API zend_property_info *zend_declare_typed_property(zend_class_entry *ce, z
 		}
 	}
 
+	if (!(access_type & ZEND_ACC_STATIC)
+	 && !(ce->ce_flags & ZEND_ACC_MAY_BE_CYCLIC)
+	 && zend_type_may_be_cyclic(type)) {
+		ce->ce_flags |= ZEND_ACC_MAY_BE_CYCLIC;
+	}
+
 	if (ce->type == ZEND_INTERNAL_CLASS) {
 		property_info = pemalloc(sizeof(zend_property_info), 1);
 	} else {

Zend/zend_closures.c

@@ -705,6 +705,9 @@ void zend_register_closure_ce(void) /* {{{ */
 	zend_ce_closure = register_class_Closure();
 	zend_ce_closure->create_object = zend_closure_new;
 	zend_ce_closure->default_object_handlers = &closure_handlers;
+	/* FIXME: Potentially improve during construction of closure? static closures
+	 * not binding by references can't be cyclic. */
+	zend_ce_closure->ce_flags |= ZEND_ACC_MAY_BE_CYCLIC;
 
 	memcpy(&closure_handlers, &std_object_handlers, sizeof(zend_object_handlers));
 	closure_handlers.free_obj = zend_closure_free_storage;

Zend/zend_compile.h

@@ -267,7 +267,7 @@ typedef struct _zend_oparray_context {
 #define ZEND_ACC_PROTECTED_SET           (1 << 11) /*     |     |  X  |     */
 #define ZEND_ACC_PRIVATE_SET             (1 << 12) /*     |     |  X  |     */
 /*                                                        |     |     |     */
-/* Class Flags (unused: 30,31)                            |     |     |     */
+/* Class Flags (unused: 31)                               |     |     |     */
 /* ===========                                            |     |     |     */
 /*                                                        |     |     |     */
 /* Special class types                                    |     |     |     */
@@ -333,6 +333,9 @@ typedef struct _zend_oparray_context {
 /* Class cannot be serialized or unserialized             |     |     |     */
 #define ZEND_ACC_NOT_SERIALIZABLE        (1 << 29) /*  X  |     |     |     */
 /*                                                        |     |     |     */
+/* Object may be the root of a cycle                      |     |     |     */
+#define ZEND_ACC_MAY_BE_CYCLIC           (1 << 30) /*  X  |     |     |     */
+/*                                                        |     |     |     */
 /* Function Flags (unused: 29-30)                         |     |     |     */
 /* ==============                                         |     |     |     */
 /*                                                        |     |     |     */

Zend/zend_fibers.c

@@ -1107,6 +1107,7 @@ void zend_register_fiber_ce(void)
 	zend_ce_fiber = register_class_Fiber();
 	zend_ce_fiber->create_object = zend_fiber_object_create;
 	zend_ce_fiber->default_object_handlers = &zend_fiber_handlers;
+	zend_ce_fiber->ce_flags |= ZEND_ACC_MAY_BE_CYCLIC;
 
 	zend_fiber_handlers = std_object_handlers;
 	zend_fiber_handlers.dtor_obj = zend_fiber_object_destroy;

Zend/zend_generators.c

@@ -1215,6 +1215,7 @@ void zend_register_generator_ce(void) /* {{{ */
 	/* get_iterator has to be assigned *after* implementing the interface */
 	zend_ce_generator->get_iterator = zend_generator_get_iterator;
 	zend_ce_generator->default_object_handlers = &zend_generator_handlers;
+	zend_ce_generator->ce_flags |= ZEND_ACC_MAY_BE_CYCLIC;
 
 	memcpy(&zend_generator_handlers, &std_object_handlers, sizeof(zend_object_handlers));
 	zend_generator_handlers.free_obj = zend_generator_free_storage;

Zend/zend_inheritance.c

@@ -1810,6 +1810,7 @@ ZEND_API void zend_do_inheritance_ex(zend_class_entry *ce, zend_class_entry *par
 	ce->parent = parent_ce;
 	ce->default_object_handlers = parent_ce->default_object_handlers;
 	ce->ce_flags |= ZEND_ACC_RESOLVED_PARENT;
+	ce->ce_flags |= (parent_ce->ce_flags & ZEND_ACC_MAY_BE_CYCLIC);
 
 	/* Inherit properties */
 	if (parent_ce->default_properties_count) {
@@ -2832,6 +2833,9 @@ static void zend_do_traits_property_binding(zend_class_entry *ce, zend_class_ent
 		if (!traits[i]) {
 			continue;
 		}
+
+		ce->ce_flags |= (traits[i]->ce_flags & ZEND_ACC_MAY_BE_CYCLIC);
+
 		ZEND_HASH_MAP_FOREACH_STR_KEY_PTR(&traits[i]->properties_info, prop_name, property_info) {
 			uint32_t flags = property_info->flags;
 

Zend/zend_object_handlers.c

@@ -70,6 +70,8 @@ ZEND_API HashTable *rebuild_object_properties_internal(zend_object *zobj) /* {{{
 		zend_class_entry *ce = zobj->ce;
 		int i;
 
+		GC_TYPE_INFO(zobj) &= ~(GC_NOT_COLLECTABLE << GC_FLAGS_SHIFT);
+
 		zobj->properties = zend_new_array(ce->default_properties_count);
 		if (ce->default_properties_count) {
 			zend_hash_real_init_mixed(zobj->properties);

Zend/zend_objects.c

@@ -31,6 +31,9 @@ static zend_always_inline void _zend_object_std_init(zend_object *object, zend_c
 {
 	GC_SET_REFCOUNT(object, 1);
 	GC_TYPE_INFO(object) = GC_OBJECT;
+	if (!(ce->ce_flags & ZEND_ACC_MAY_BE_CYCLIC)) {
+		GC_TYPE_INFO(object) |= (GC_NOT_COLLECTABLE << GC_FLAGS_SHIFT);
+	}
 	object->ce = ce;
 	object->extra_flags = 0;
 	object->handlers = ce->default_object_handlers;