where are all the objects?

[Kevin Bourrillion]

The contours of our discussions maybe 9 months ago are finally coming back
to me, and I'm suddenly realizing they haven't even changed much. I hadn't
properly swapped that context back in and so I guess I've been wasting
y'all's time just repeating my "camp's" position louder and more slowly.
Sorry!

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.
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).
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?

(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.)


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
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