Optimize the destructor of WeakMap for large WeakMaps

Postpone calling any destructors of entries within the weak map
itself until after the singleton `EG(weakmap)` is updated to remove all
keys in the weak map.

Before, zend_weakref_unregister would do two hash table lookups when freeing an
entry from a WeakMap

- Free from `EG(weakrefs)` if that was the last reference
- zend_weakref_unref_single to remove the entry from the WeakMap itself

After this change, only the first hash table lookup is done.
The freeing of entries from the weak map itself is done sequentially in
`zend_hash_destroy` without hashing or scanning buckets.

It may be a good idea to prevent modification of a WeakMap after the weak map
starts to get freed.

Closes GH-7672

Author: TysonAndre

Zend/zend_weakrefs.c

@@ -109,7 +109,7 @@ static void zend_weakref_register(zend_object *object, void *payload) {
 		&EG(weakrefs), obj_addr, ZEND_WEAKREF_ENCODE(ht, ZEND_WEAKREF_TAG_HT));
 }
 
-static void zend_weakref_unregister(zend_object *object, void *payload) {
+static void zend_weakref_unregister(zend_object *object, void *payload, bool weakref_free) {
 	zend_ulong obj_addr = (zend_ulong) object;
 	void *tagged_ptr = zend_hash_index_find_ptr(&EG(weakrefs), obj_addr);
 	ZEND_ASSERT(tagged_ptr && "Weakref not registered?");
@@ -122,7 +122,9 @@ static void zend_weakref_unregister(zend_object *object, void *payload) {
 		GC_DEL_FLAGS(object, IS_OBJ_WEAKLY_REFERENCED);
 
 		/* Do this last, as it may destroy the object. */
-		zend_weakref_unref_single(ptr, tag, obj_addr);
+		if (weakref_free) {
+			zend_weakref_unref_single(ptr, tag, obj_addr);
+		}
 		return;
 	}
 
@@ -141,8 +143,10 @@ static void zend_weakref_unregister(zend_object *object, void *payload) {
 	}
 
 	/* Do this last, as it may destroy the object. */
-	zend_weakref_unref_single(
-		ZEND_WEAKREF_GET_PTR(payload), ZEND_WEAKREF_GET_TAG(payload), obj_addr);
+	if (weakref_free)  {
+		zend_weakref_unref_single(
+			ZEND_WEAKREF_GET_PTR(payload), ZEND_WEAKREF_GET_TAG(payload), obj_addr);
+	}
 }
 
 ZEND_API zval *zend_weakrefs_hash_add(HashTable *ht, zend_object *key, zval *pData) {
@@ -156,7 +160,7 @@ ZEND_API zval *zend_weakrefs_hash_add(HashTable *ht, zend_object *key, zval *pDa
 ZEND_API zend_result zend_weakrefs_hash_del(HashTable *ht, zend_object *key) {
 	zval *zv = zend_hash_index_find(ht, (zend_ulong) key);
 	if (zv) {
-		zend_weakref_unregister(key, ZEND_WEAKREF_ENCODE(ht, ZEND_WEAKREF_TAG_MAP));
+		zend_weakref_unregister(key, ZEND_WEAKREF_ENCODE(ht, ZEND_WEAKREF_TAG_MAP), 1);
 		return SUCCESS;
 	}
 	return FAILURE;
@@ -245,7 +249,7 @@ static void zend_weakref_free(zend_object *zo) {
 	zend_weakref *wr = zend_weakref_from(zo);
 
 	if (wr->referent) {
-		zend_weakref_unregister(wr->referent, ZEND_WEAKREF_ENCODE(wr, ZEND_WEAKREF_TAG_REF));
+		zend_weakref_unregister(wr->referent, ZEND_WEAKREF_ENCODE(wr, ZEND_WEAKREF_TAG_REF), 1);
 	}
 
 	zend_object_std_dtor(&wr->std);
@@ -293,8 +297,11 @@ static void zend_weakmap_free_obj(zend_object *object)
 	zend_weakmap *wm = zend_weakmap_from(object);
 	zend_ulong obj_addr;
 	ZEND_HASH_MAP_FOREACH_NUM_KEY(&wm->ht, obj_addr) {
+		/* Optimization: Don't call zend_weakref_unref_single to free individual entries from wm->ht when unregistering (which would do a hash table lookup, call zend_hash_index_del, and skip over any bucket collisions).
+		 * Let freeing the corresponding values for WeakMap entries be done in zend_hash_destroy, freeing objects sequentially.
+		 * The performance difference is notable for larger WeakMaps with worse cache locality. */
 		zend_weakref_unregister(
-			(zend_object *) obj_addr, ZEND_WEAKREF_ENCODE(&wm->ht, ZEND_WEAKREF_TAG_MAP));
+			(zend_object *) obj_addr, ZEND_WEAKREF_ENCODE(&wm->ht, ZEND_WEAKREF_TAG_MAP), 0);
 	} ZEND_HASH_FOREACH_END();
 	zend_hash_destroy(&wm->ht);
 	zend_object_std_dtor(&wm->std);
@@ -395,7 +402,7 @@ static void zend_weakmap_unset_dimension(zend_object *object, zval *offset)
 		return;
 	}
 
-	zend_weakref_unregister(obj_key, ZEND_WEAKREF_ENCODE(&wm->ht, ZEND_WEAKREF_TAG_MAP));
+	zend_weakref_unregister(obj_key, ZEND_WEAKREF_ENCODE(&wm->ht, ZEND_WEAKREF_TAG_MAP), 1);
 }
 
 static int zend_weakmap_count_elements(zend_object *object, zend_long *count)