New NullRestricted Attribute in Project Valhalla

[hangshao0]

In the last spec: https://cr.openjdk.org/~dlsmith/jep401/jep401-20230428/specs/flattened-heap-jvms.html, a NullRestricted attribute is introduced.

It indicates that the field cannot be null. putfield and putstatic will throw NPE if there is an attempt to assigned null to such fields. The null restricted field needs to be initialized to the default instance.

[hangshao0]

The null restricted field needs to be initialized to the default instance.

OpenJ9 is already doing this for Q types.

[hangshao0]

FYI @theresa-m

[theresa-m]

@hangshao0 can you point me to where this is happening in the code?

[hangshao0]

can you point me to where this is happening in the code?

The question is regarding #17340 (comment), i.e. where Q type is initialized to the default instance ?

[theresa-m]

can you point me to where this is happening in the code?

The question is regarding #17340 (comment), i.e. where Q type is initialized to the default instance ?

yes.

[hangshao0]

The default instance of a value type is allocated here:

openj9/runtime/vm/ClassInitialization.cpp

Lines 521 to 535 in 5567b8c

	if (J9_IS_J9CLASS_VALUETYPE(clazz)) {
	PUSH_OBJECT_IN_SPECIAL_FRAME(currentThread, initializationLock);
	/* Preparation is the earliest point where the defaultValue would needed.
	* I.e pre-filling static fields. Therefore, the defaultValue must be allocated at
	* the end of verification
	*/
	j9object_t defaultValue = vm->memoryManagerFunctions->J9AllocateObject(currentThread, clazz, J9_GC_ALLOCATE_OBJECT_TENURED | J9_GC_ALLOCATE_OBJECT_NON_INSTRUMENTABLE);
	initializationLock = POP_OBJECT_IN_SPECIAL_FRAME(currentThread);
	clazz = VM_VMHelpers::currentClass(clazz);
	if (NULL == defaultValue) {
	setHeapOutOfMemoryError(currentThread);
	goto done;
	}
	j9object_t *defaultValueAddress = &(clazz->flattenedClassCache->defaultValue);
	J9STATIC_OBJECT_STORE(currentThread, clazz, defaultValueAddress, defaultValue);

The Q type instance fields are set here:

openj9/runtime/gc_modron_startup/mgcalloc.cpp

Line 518 in 5567b8c

vmThread->javaVM->internalVMFunctions->defaultValueWithUnflattenedFlattenables(vmThread, clazz, objectPtr);

openj9/runtime/vm/ValueTypeHelpers.cpp

Lines 78 to 96 in 5567b8c

	defaultValueWithUnflattenedFlattenables(J9VMThread *currentThread, J9Class *clazz, j9object_t instance)
	{
	J9FlattenedClassCacheEntry * entry = NULL;
	J9Class * entryClazz = NULL;
	UDATA length = clazz->flattenedClassCache->numberOfEntries;
	UDATA const objectHeaderSize = J9VMTHREAD_OBJECT_HEADER_SIZE(currentThread);
	MM_ObjectAccessBarrierAPI objectAccessBarrier(currentThread);
	for (UDATA index = 0; index < length; index++) {
	entry = J9_VM_FCC_ENTRY_FROM_CLASS(clazz, index);
	entryClazz = J9_VM_FCC_CLASS_FROM_ENTRY(entry);
	if (!J9_VM_FCC_ENTRY_IS_STATIC_FIELD(entry) && !J9_IS_FIELD_FLATTENED(entryClazz, entry->field)) {
	objectAccessBarrier.inlineMixedObjectStoreObject(currentThread,
	instance,
	entry->offset + objectHeaderSize,
	entryClazz->flattenedClassCache->defaultValue,
	false);
	}
	}
	}

defaultValueWithUnflattenedFlattenables() uses the default instance to set the fields if they are not flattened (inlined). Note the flattenedClassCache struct has all the J9Class of the Q type fields.

[theresa-m]

The remaining tasks for this issue are:

• Class verification + tests:
  JEP 401 NullRestricted attribute part 1 #17781
  NullRestricted attribute cant be in primitive or array field #18006
  NullRestricted: putstatic and putfield throw NPE on null assignment #18028
  NullRestricted attribute field class checks #18030
  NullRestricted field throws NPE on null assignment in withfield #18130
  Flatten non-static NullRestricted fields #18173
• cfdump support
  NullRestricted Valhalla attribute cfdump support #18035
• ddr support:
  ddr support for Valhalla NullRestricted attribute #18042
  Print NullRestricted flag for ddr classAndFlags #18045

Remaining tasks:

• NPE for aastore. For arrays javac does not store a NullRestricted flag with the field but instead inserts java/lang/Class.asNullRestrictedType and java/util/Objects.requireNonNull before calling aastore. asNullRestrictedType will need to be implemented.