where are all the objects?

[Kevin Bourrillion]

On Mon, Jul 25, 2022 at 7:05 AM Brian Goetz <brian.goetz at oracle.com> wrote:

I had another read through your Values document (
> https://docs.google.com/document/d/1J-a_K87P-R3TscD4uW2Qsbt5BlBR_7uX_BekwJ5BLSE/edit#).
> Let me try to summarize.
>
> Values.  You want to use Values to describe "free floating pieces of
> data."  They don't live any place specific, they have no identity, they are
> immutable.  Every value has a type, but values do not necessarily
> incorporate their own typestate; this may live elsewhere (e.g., field
> descriptors.)
>
> Variables.  Variables can hold values.  Variables have types, which
> determine which values may be written to them and what we can assume about
> values read from them.
>
> Containers.  Variables live in containers (classes, instances, arrays,
> stack frames.)
>
> Kinds of values.  Values are primitives, references to objects, or the
> special reference null.
>
> Objects.  Objects have an independent existence, are self-describing
> (e.g., Object::getClass), may have identity, and can only be interacted
> with through references.
>


All correct. Maybe I could have made the doc shorter. :-) Thank you for
rereading it.


I think this is a valid model of where things are today, though I think
> that some of the "essential characteristics" of Objects in your model may
> be more accidental than you give them credit for.  That is, some of these
> characteristics are of "objects that are the target of an object
> *reference*", which happens to be all the objects today.  Similarly, "has
> its own independent existence" may feel more accidental once references are
> optional.
>


Perfect: this is indeed meant to be the part of the document that we haggle
over. The doc doesn't do a good job of portraying it as such, but if I can
get back to working on it at some point I'll try to address that.


Of course something will have to change, and we want that change to feel
> natural and not pulling the rug out from under user's mental models.
>
> The change I'm proposing in this model is:
>
> Instead of values being "primitives and object references", values become
> "value objects and object references".  A Complex.val is a value.  3+2i
> still meets all the requirements of values: free floating, no canonical
> location, no identity, immutable.  It's just a "bigger" value that we could
> have before.  Primitives become value objects.  I think people can
> understand (and will like) this story.
>
> Variables and containers are unchanged.
>
> Objects are instances of classes.  Instances of identity classes remain
> dependent on references to interact with them; instances of value classes
> can also be the target of references, *and* are values on their own.  (This
> is not excessively weird, since "Complex" and "reference to Complex" is
> like `int` and `int *` in C.)
>

It's a familiar aspect of C that Java quite distinctly distanced itself
from!


Let's take a look at your essential characteristics of objects again.
>
>  - Objects are entities, they have their own independent existence.  I
> think this one is a consequence of "objects only can be interacted with
> through references."  That is, there is a kind of value called "reference
> to object", and the reference refers to ... an object, which is a thing
> separate from the reference.
>
> So, *if* an object is the target of a reference, then yes, it must be an
> entity that is somewhere else, with its independent existence.  "Thing that
> is the target of an object reference" is one reasonable definition for
> "Object", but I don't think it is the only one.  What I'm saying is that I
> think its fair to say an instance of Complex is an object (and further,
> that saying "its an instance, but not an object", is likely to be more
> confusing that beneficial.)
>


Two possible directions of that confusion:

1. But why is it not an object? The big problem here, raised by John I
think, is "Java is an object-oriented language". Uh oh. That certainly does
demand the expansive notion of "object"; I have to concede that point. On
the other hand, maybe, Java did then put that theory into practice using
terms like "instance member", not "object member". It also created a class
called "Object" and attached a bunch of specific ideas to that, only 3.5 of
which are really general to all instances.

2. Okay but then why is it still an instance? Here we'd be asking users to
shed some of the baggage they've (incidentally) attached to "instance" in
the past, realizing that that baggage actually belonged with "object". What
remains with "instance" is the essential stuff: instance members, instance
state.


  I think the term for what you are describing is *referent*, and not all
> objects are referents.
>


I agree that "referent" is another reasonable choice of term for what the
doc calls "object". I think it beats "heap object". But I think it has
serious problems (below).



>  - Objects are self-describing.  By this I'll assume you mean
> Object::getClass.
>


Yes, as well as arrays knowing their own length. I mean that if you have
the data itself you have everything you need to know about the layout of
that data in memory.


  Here, I say that objects remain self-describing under the "instances are
> objects" model, but something interesting happens under the hood about
> *where* the description lives.  If I have a `Complex` in a variable of type
> `Complex.val`, there is sufficient information *in the container* to know
> the class of the instance, so the instance doesn't have to carry it with
> it.  There is an operation for "take a reference of" that can be applied to
> value objects.  This operation (logically, though this is frequently
> optimized away) takes that information out of the container and puts it in
> an object header.  But regardless of whether the typestate is in the
> container or the object itself, objects are self-describing.
>


I think this paragraph is merely asking for a different term/definition
from the "self-describing" term I'm using and doesn't say anything deeper
than that. Maybe there is a better term. I used "self-describing" partly
because I thought it has a healthy existing connotation that the data
itself is bloated with all that description. e.g. Java serialized forms are
self-describing (setting aside all the ways that description can fail).



>  - Only an object can have identity.  Remains true; new thing is that not
> all objects have identity.
>


I was making a slightly stronger point. There is still a difference here.
In the values-are-not-objects model (VANO or "B" in this email), every
object is *eligible* to have identity -- it only doesn't if the user
declines it. Values are inherently ineligible. That's an intrinsic
difference between the two.


 - An object is always accessed by reference.  This is what I'm saying
> changes; value objects are values. So I think that what you are
> describing as essential characteristics of objects, are really essential
> characteristics of *referents of object references*.  And I would argue
> that while this is a well-defined concept, it's not the most important
> distinction we want to put in the user's face.  Instead, we can say that an
> instance of Complex can be a value, or it can be a referent, but its the
> same Complex either way, and the user gets to decide what packaging it
> wants to put it in.
>


I think the key thing to notice about "referent" is that it's a role noun
(I don't know what grammarians would call it), like "pedestrian" or
"projectile". That is, it doesn't seem to be saying anything at all about
the thing itself, only about the role that thing is currently playing in
some broader relationship or activity.

To my VANO mindset, that just doesn't feel sufficient or appropriate,
because the things feel inherently different.

(I also think "referent" will fail to be a workable term for other reasons,
including but not limited to its very unfortunate plural form. And I
literally just spotted that my fingers had typed "reference" when I'd meant
to say "referent" 2 paragraphs up. I also cannot come up with another term
for this, and that's part of the problem I'm having with VAO.)





> On 7/22/2022 7:16 PM, Brian Goetz wrote:
>
> Now I wonder if these points, at least, might be uncontroversial:
>
> 1. There exist useful well-defined concepts of "value" and "object" that
> are disjoint and that *have been* valid up to now. (I'll hazard a claim
> that my paper still defends at least *this* much well enough.)
> 2. Also, you've had to treat the two quite differently from each other in
> your programs.
> 3. We *are* changing (improving) #2 through this project.
>
>
> I claim we are changing #1 as well, though to a lesser degree.  #2 should
> “mostly go away”; #1 should transform into other terms, such as e.g.
> “object stored directly” vs “reference to object”.  It is those other terms
> that I think we are searching for consensus on, but #1 is moving.
>
> 4. But users may still need #1's disjoint concepts when they are trying to
> reason about the *performance* model (tho they'll also need to understand
> that the VM is empowered to "fake" one as the other when the spirit so
> moves it).
>
>
> Yes, though I think these are concepts that are more _derived from_ the
> distinction in #1.  John’s notion of “placement” is good here; the choice
> of ref vs val constrains the placement, and placement informs the
> performance model.  I think part of what has been missing until today is a
> good attempt to name the intermediate actors, like placement.  I hope that
> if we refine those terms a bit, things will get clearer.
>
> 5. The questions at hand in this thread are not foremost about the
> performance model but about the basic "start-here" user model.
> 6. These miiight be fair descriptions of the 2 camps?
>
> A. Because you'll get to program mostly the same way in both cases, we can
> and should de-emphasize the distinction. There might be a reference sitting
> in between you and the data/"object" or there might not. It's mostly in the
> VM's hands. If you ever think you care about the distinction, you probably
> are dipping down into the performance model. There is a "just don't worry
> about it!" flavor to this option.
> B. It's still helpful to have a solid sense of the distinction, even as we
> benefit from getting to code the same way to each. Even though the VM might
> really fake one as the other; again, that's performance model.
>
>
> Anything controversial about the above?
>
>
> No, and I want to choose both A and B!  I don’t think they are opposed, I
> think they are different angles on the elephant.
>
> (If I had to explain why I've been so dogged about B, maybe it's the sense
> that we simply won't "get away with" A. It feels hard (to me) to tell users
> simultaneously that they should stop caring about a distinction AND that
> we're changing up how all kinds of stuff works across that distinction. It
> feels more solid to firm up the distinction so that we can talk about how
> things are changing, and then let that distinction just slowly matter less
> and less over time.)
>
>
> Agree that we need a good "start here” story, but I think a good one will
> have aspects of A and B.  I think we’re making progress?
>
>
>
> On Fri, Jul 22, 2022 at 12:02 PM John Rose <john.r.rose at oracle.com> wrote:
>
>> On 22 Jul 2022, at 10:55, Brian Goetz wrote:
>>
>> …
>>
>> So then, would we call an instance of `Complex.val` a "non-heap object"
>> or an "inlined object" or what? We need to flesh out a whole lexicon. The
>> phrase "value object" becomes useless for this particular distinction as it
>> will apply to both.
>>
>> Yes, in the taxonomy I’m pushing, a “value object” is one without
>> identity, and is the kind of object you can store directly in variables
>> without going through a reference. But I don’t think that there are
>> instances of Complex.val and instances of Complex.ref; I think there are
>> instances of *Complex*, and multiple ways to describe/store/access them.
>>
>> FTR, I enthusiastically agree with this viewpoint, even though I am also
>> probing for weaknesses and alternatives. (FTR I feel the same about Brian’s
>> summary in his previous short message.)
>>
>> And under this viewpoint, the terms “instance” and “object” have the same
>> denotation, though difference connotations. (When I say “instance” you may
>> well think, “instance of what”? But you don’t ask that question so much if
>> I say “object”.)
>>
>> That `int/Integer` decision you've been making has always been between
>> (1) value and (2) (reference-to) object, and that decision is still exactly
>> between (1) value and (2) (reference-to) object now, and btw the
>> definitions of 'reference' and 'object' remain precisely wedded to each
>> other as always.
>>
>> The "heap object" alternative strikes me (and I am trying to be fair,
>> here) as:
>>
>> Now, that's an object either way, and you're going to apply that old
>> thought process toward which *kind* of object you mean, either a (1)
>> "inline object" or a (2) "(reference-to) heap object". It's now just heap
>> objects and references that are paired together.
>>
>> I think, Kevin, you are going wrong at this point: It’s not a *kind* of
>> object, it is a *placement* of an object. What “kind” of person am I
>> when I am diving to the office? Surely the same “kind” as when I am at
>> home. But when I am driving, I am equipped with a car and a road, much like
>> a heap-placed object is equipped with a header and references.
>>
>> Likewise, an int/Integer is (in Valhalla) the same “kind” of object (if
>> we go all the way to making primitives be honorary objects) whether it is
>> placed in heap or on stack or inside another object.
>>
>> The distinction that comes from the choice of equipping an int with a
>> header in heap storage is a distinction of placement (and corresponding
>> representation). So an int/Integer does not intrinsically have a header
>> because it is an object (because of its “kind”). It *may* have a header
>> if the JVM needs to give it one, because it is stuck in the heap.
>>
>> (My points about int/Integer could partly fail if we fail to align int
>> and Integer in the end. So transfer the argument to C.val/C.ref if you
>> prefer. It is the same argument.)
>>
>> And I would say the *placement* of an object is in three broad cases
>> which are worth teaching even to beginners:
>>
>>    -
>>
>>    “in the heap”: therefore referred to by a machine word address, and
>>    presumably equipped with a header and maybe surrounded by some alignment
>>    waste; a JVM might have multiple heaps but at this level of discourse we
>>    say “the heap”
>>    -
>>
>>    “on the stack”: therefore manipulated directly by its components,
>>    which are effectively separated into scalars (it is “scalarized”, we
>>    sometimes say); we might sometimes wish to say “JVM stack or locals”
>>    instead of “stack”, or, with increasing detail, “on stack, in locals,
>>    and/or in registers, and/or as immediates in the machine code”
>>    -
>>
>>    “contained in another object”: in a field or array element, therefore
>>    piggy-backing on the other object’s placement; and note that even arrays
>>    are scalarized sometimes, lifting their elements into registers etc.
>>
>> To summarize: Placement = Heap | Stack | Contained[Placement].
>>
>> One might use the term “inline” somewhere in there, either to mean
>> Contained or Stack|Contained[*].
>>
>> Static field values are a special case, but they can be classified in one
>> of the above ways. HotSpot places static fields inside a special per-class
>> object (the mirror, in fact), so their values are either contained or
>> separate in the heap (JVM’s choice again).
>>
>> One might be pedantic and say that an instance can be contained “in
>> static memory” (neither heap nor stack) if the JVM implements storage for
>> static fields outside of the heap. But in that case I’d rather say that
>> they are in a funny corner of the heap, where perhaps headers are not
>> needed, because some static metadata somewhere dictates what is stored.
>>
>> (Hence I like to be cagey about whether a heap-object actually has a
>> physical header. It might not in some JVM implementations.)
>>
>> Starting to prefer the first way (as I did) did not feel like going
>> rogue: after all, did we not gravitate toward ".ref" and ".val" as our
>> placeholder syntaxes, not ".inline" and ".heap" or anything else?
>>
>> With you on this. I think asking users to reason about “heap objects” vs
>> “inline objects” is pushing them towards the implementation, not the
>> concepts. They may have to reason about this to understand the performance
>> model, but that’s already advanced material.
>>
>> Yes. And even more specifically in the implementation, users who think
>> about “heap objects” are really (IMO) trying to predict the *placement*
>> of the objects, *where* the JVM will choose to place their bits in
>> physical memory.
>>
>> This question of placement is very interesting to the “alert”
>> performance-minded programmer. Not every programmer is in that state; for
>> me I try to practice “first make it work then make it fast”. I get “alert”
>> to performance only in the “make it fast phase”, a phase which many of my
>> codes never reach.
>>
>> As a sort of “siren song” the question of placement is * also*
>> interesting to the beginning student who is struggling to build a mental
>> image of Java data, and is reaching for visualizations in terms of memory
>> and addresses, or (what is about the same) boxes and arrows. But the JVM
>> will make a hash of all that, if it is doing a good job. So the student
>> must be told to hold those mental models lightly.
>>
>> Kevin is insisting (for his own good reasons) on his answer to “where are
>> the objects”: They are always “in the heap” and thus “with headers,
>> accessed by pointers”. I suspect (but haven’t seen from Kevin himself yet)
>> that this is in part due to a desire to work with, rather than work
>> against, the student’s desire to make simple visual models of Java data.
>>
>> Crucially, in a literal “boxes and arrows” model, an arrow (perhaps a
>> C.ref reference to an instance) looks very different from a nested box
>> (perhaps a C.val instance), and the naive user might insist that such
>> differences are part of the contract between the user and the JVM. But they
>> are not. The JVM might introduce invisible “arrows” (because of heap
>> buffering) and it might remove arrows (because of scalarization for a
>> number of possible reasons).
>>
>> So if the student is told that the arrows and boxes are “what’s really
>> going on” the student using that assurance to predict performance and
>> footprint will feel cheated in the end.
>>
>> To summarize: Any given instance/object has logically independent
>> properties of class and placement.
>>
>> And thus: The choice of companion type does not affect class but may
>> (may!) affect placement.
>>
>> Circling back to the language design, it might seem odd that there are
>> three ways to place an object but just two companion types. But this
>> oddness goes away if you realize that C.val and C.ref are not placement
>> directives. The choice between the two is a net-binary selection from a
>> sizeable menu of “affordances” that the user might be expecting or
>> disavowing at any given point in the code. (See my lists of “affordances”
>> and “alternative affordances” in encapsulating-val
>> <http://cr.openjdk.java.net/~jrose/values/encapsulating-val.html#affordances-of-c.ref>
>> .)
>>
>> The user is given this simplified switch to influence the JVM’s decisions
>> about placement (and therefore representation). It is useful because the
>> JVM can employ different implementation tactics depending on the
>> differences between the user-visible contracts of C.ref and of C.val. In
>> the choice of implementation tactics, the JVM has the final say.
>>
>
>
> --
> Kevin Bourrillion | Java Librarian | Google, Inc. | kevinb at google.com
>
>
>
>

-- 
Kevin Bourrillion | Java Librarian | Google, Inc. | kevinb at google.com
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