Null-restricted types: Why so complicated?

[John Bossons]

Thanks for your comments. I was not sufficiently explicit.

Let me focus on implicit. I guess my dislike is of introducing a 'fake'
constructor into the definition of a class. I say 'fake' because, as I
understand it, the only purpose of the implicit constructor is to indicate
to the JVM/compiler that a never-null instance can be created. But in Java
idiom that means that a developer can invoke the public implicit
constructor, which will cause confusion.

Maybe it would be better to require a potentially null-restricted class to
extend a marker interface ('extends NeverNullPossible'? Or maybe, looking
ahead to my next comment, 'extends AllZerosIsNull'?). That would enable the
compiler to catch an invalid use of the ! marker in a declaration, just as
the proposed implicit constructor does, while conforming better to common
Java idiom.

My further suggestion is that appending ! to a type should mean that the
default initialized value of an instance (all fields zero) is equivalent to
null, so that
      Range![] a = new Range![100];  // allocated with zero values
      System.out.println(a[5]);      // throws NullPointerException (zero
fields)
This better conforms to current idiom, where the initial initialization is
with nulls and the println invocation on a null array element or field
throws a NPE.

As you say, my suggestion means runtime testing to determine if all fields
are zero, which has a performance cost. This will only occur if the JVM
implements the ! specification, which it presumably will only do if the
object is small. And the cost will be small (I am presuming) relative to
savings from allowing the memory footprint to match that of primitives. Am
I wrong? There is value in conforming to current idiom.

Turning to the LooselyConsistentValue, I withdraw my comments. I mistakenly
presumed that its use would be required, which is false. It simply enables
a single-threaded (or volatile-protected) application to allow additional
inlining, which is harmless.

John

On Thu, Jan 18, 2024 at 4:56 PM - <liangchenblue at gmail.com> wrote:

> Hi John,
>
> On Thu, Jan 18, 2024 at 2:30 PM John Bossons <jbossons at gmail.com> wrote:
>
>> Hi all,
>>
>> Maybe I am missing something, but the proposal seems to be trying to do
>> too much.
>>
>> Specifically: Why not simply provide that appending ! to a type
>> specification for an object (field, array element, or parameter) means that
>> that the object is not only null-restricted but also never zero and
>> necessarily non-atomic unless small?
>>
> First, a reminder that some objects cannot be non-atomic, mostly when
> fields have dependencies/constraints on each other: if you have a range,
> you cannot allow its lower bound to be larger than its upper bound.
> Non-atomic representations cannot avoid this pitfall. Also you seem
> to misunderstand non-atomic: if an object is non-atomic, each of its fields
> can update independently from each other, so a 3-d position can be
> non-atomic, but not so for a range. Non-atomicity is dangerous, and it
> should not be the default. However, if an atomic class is small enough,
> like OptionalInt (as now many architecture has like atomic handling of 16
> bytes etc.) JVM may choose to apply non-atomic optimizations to them for
> better performance without violating their object constraints.
>
>>
>> Why complicate the specification with an implicit constructor that a
>> developer will never explicitly invoke? Why permit a developer to 'opt in'
>> to non-atomic?
>>
> The implicit constructor can always be called; its existence asks
> programmers to affirm that the zero-filled inlined instance is a valid
> instance. And this instance is different from a null, as null is a pointer,
> yet the zero-instance has a different size defined by the class layout in
> the stack/heap.
>
>>
>> Sure, that means trying to read a zero value triggers a NPE. That just
>> means that a type that can legitimately have a zero value cannot be
>> specified as null-restricted, since a zero value (e.g. a {null, null} Name)
>> is the equivalent of a null unrestricted value object. Why go beyond that?
>> If a non-null zero value is possible, the type cannot be null-restricted
>> and so can only be an unrestricted JEP 401 value type. End of story.
>>
> You see the inlined zero instance and the null pointer have different
> sizes, and thus they are not exchangeable. Converting the inlined zero
> instance to null to throw NPE is complex and hurtful to performance as you
> will scan unrelated bits for almost every field access.
>
> And for unrestricted value type, yes, they exist and can possibly be
> inlined as well if the restricted type is small enough (i.e. has space for
> extra bit indicating nullity) But reminder, the nullity bit itself isn't
> even non-atomic with (depends on) the rest of the object! You don't want
> the nullity to indicate null while the rest of the object indicate some
> sort of non-null value, which can happen in a non-atomic context.
>
>>
>> With respect to non-atomic, what is new? Yes, unexpected  instances may
>> occur without synchronization if the object is larger than the word size of
>> the implementation. Why do we need to extend a LooselyConsistentValue
>> interface to know/permit that?
>>
> Unexpected instances don't occur without synchronization if you use
> finals, such as in Java's String or immutable List.of(). These APIs may
> capture any "permitted value" from the arrays passed in, but once
> constructed, the captured value remains constant no matter which thread
> observes the String/List object reference. (Technically, JVM implements
> this with a store-store fence between end of field writes in the
> constructor and object reference is shared anywhere, and a load-load fence
> between object reference read and field read) Value classes is about the
> safety of final fields in programming instead of the close encounter of
> third kinds of synchronization, volatiles, and fences.
>
>>
>> Can we not keep this 'simple' (if that word has meaning in this context)?
>> What am I missing?
>>
> I think you are missing a bit about how the layout (inlining is
> represented in memory) and value classes (the thread safety its final
> offers) work, and what "non-atomic" means. Feel free to question more.
>
>>
>> John
>>
>>
>> --
>> Phone:  (416) 450-3584 (cell)
>>
>

-- 
Phone:  (416) 450-3584 (cell)
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