Initial feedback on Minimal Value Types 0.2 for discussion

Karen Kinnear karen.kinnear at
Wed Mar 15 05:20:11 UTC 2017

Summary notes from meeting 2/15/17 - embedded below, starting with #8

Meeting 3/16/17:
— there are a few more already identified topics to discuss
— welcome additional questions/suggestions
— a couple of topics to revisit (or perhaps put on an open issues list until we have more information or feedback)


> On Feb 10, 2017, at 9:47 AM, Karen Kinnear <karen.kinnear at> wrote:
> John - please correct
> Bjorn,
> Good question. This was news to us as well, so John will have the final word.
> My assumption is that he means that:
> If you create an array of Value-Capable-Classes that you would not automagically also create an equivalent
> array of the Derived Value Types.
> That the MethodHandle APIs could explicitly, through new byte codes create arrays of the DVTs by passing a
> Derived Value Type as the “array ref” to anewarray or multianewarray.
> I presume that any explicit byte code generation by power users to go with this code could use the same mechanism.
> thanks,
> Karen
>> On Feb 9, 2017, at 10:56 PM, Bjorn B Vardal <bjornvar at <mailto:bjornvar at>> wrote:
>> Karen / John: Can you clarify this? Do you mean that they will only be flattened when created using the reflection / MethodHandle API?
>> > John: MVT 1.0 will only flatten arrays reflectively
>> --
>> Bjørn Vårdal
>> J9 Java Virtual Machine Developer
>> IBM Runtimes
>> ----- Original message -----
>> From: Karen Kinnear <karen.kinnear at <mailto:karen.kinnear at>>
>> Sent by: "valhalla-spec-experts" <valhalla-spec-experts-bounces at <mailto:valhalla-spec-experts-bounces at>>
>> To: John Rose <john.r.rose at <mailto:john.r.rose at>>
>> Cc: valhalla-spec-experts at <mailto:valhalla-spec-experts at>
>> Subject: Re: Initial feedback on Minimal Value Types 0.2 for discussion
>> Date: Thu, Feb 9, 2017 6:31 PM
>> Notes from discussion on Feb 01,2017. Feedback welcome.
>> John - one question extracted at the top from the embedded notes.
>>>    11. "interfaces (especially with default methods)"
>>>        - please change p.6 to clarify that there are no value type interfaces period.
>> Ed note: There is a distinction here between
>> 11a)  defining an interface as a VCC with a derived DVT and
>> 11b) whether the POJO which defines the VCC can implement interfaces. This discussion was about whether
>> a POJO which defines the VCC can implement interfaces.
>> John: MVT 1.0 : No value capable interfaces for JVMT 1.0.
>> Ed note: was this the answer to 11a or 11b above please?
>>> On Feb 9, 2017, at 5:43 PM, Karen Kinnear <karen.kinnear at <mailto:karen.kinnear at>> wrote:
>>> (This is a resend of an email I sent to valhalla-dev at <mailto:valhalla-dev at> on January 23)
>>> Review of Minimal Value Types August 2016 Shady Edition (v 0.2)
>>> <>
>>> Questions/Comments:
>>> abbreviations used: VCC: value-capable class, DVT: derived value type
>>> 1. Goals
>>> -- might be worth adding to the bullets:
>>>    Allow use of existing toolchain where possible including IDEs and debuggers
>> John: this is just a review comment, no discussion required.
>>> 2. Features:
>>>    "Three" bytecode instructions ->"A small set of”
>> John: just a text edit to encompass extensions in #12 below, not yet discussed as a group.
>>> 3. Typically, value-capable classes will not be exported.
>>>     Is the reason for this to limit exposure since the expectation is that the
>>>     initial APIs and mechanisms will change?
>> John: “yes”. This does not have to be enforced in the implementation.
>> ed. note: perhaps we could remove this from the MVT specification.
>>> 4. Value-capable classes: supporting methods
>>>    p.3 "This design endows both boxes and pure values with a common set of methods; it "lifts" box methods 
>>>       onto the derived values."
>>>    p.5 "The synthetic class has the given fields (unchanged) and has no methods"
>>>    p.5 "Meanwhile, all methods (and other class features) stay on the value-capable class. The value type
>>>         proper is just a "dumb struct" containing the extracted fields"
>>>    - given that in the MVT model we are starting with a POJO box, and instance methods that clearly
>>>      take an Object as a receiver, one proposal for the initial MVT approach  would be to have
>>>      all methods only supported by the box, and require boxing to invoke any methods
>>>       - so I think the first quote would need either removal or modification
>> John: agreed.
>> Karen: See #9 below: Evolved proposal would keep the POJO, which we call the Value Capable Class (VCC) unchanged,
>> and derive a Derived Value Type (DVT) which would only contain a copy of the immutable instance fields, i.e. be
>> a “dumb struct”. 
>> Bjorn: In this model, the source class would be the same as the box class, if we leave the instance fields in it. And we
>> would box to invoke methods for the MVT 1.0 timeframe.
>> John: Agreed.
>> John: Longer-term - will want to invoke methods on values as soon as we can. We will need source support for that.
>> John: Minimal Value Type (MVT) programming models:
>>   1) source — only works for boxes 
>>   2) Method Handle reflection - for early adopters
>>   3) bytecodes
>> The MethodHandle/ValueFactory approach is clearly described.
>> The language and byte code we will use longterm are still uncertain.
>> Bjorn: What would be enabled by having all members in the value vs. just the [instance] fields in the value?
>> Karen: Challenge is instance methods, where the type of the receiver is expected to be a VCC, not a DVT. This same
>> expectation applies to any method called from the instance method, or any field in which the receiver is stored.
>> John: we are using existing javac support, therefore we need to define value types indirectly, box first. This is not the longterm plan.
>>    MethodHandles will provide a direct way of speaking of the values. The MethodHandle runtime will spin byte codes.
>> Maurizio: it is easy to just map the fields
>> Karen: Methods are coded on the box. Static fields we left on the box. So we just lift the instance fields.
>> John: We box the value to run methods. We want to preserve “vagueness”.
>>    Legacy code could misuse identity e.g. equals, hashcode, sync
>>    This only works for early adopters who are aware of value-capable-class identitylessness and implications
>> Maurizio: If you pass the VCC to another method you are passing the box and no mechanical transformation is needed.
>> Karen: You could have a problem if you were to pass the DVT as an argument when an object is expected
>> John: clarify distinction between QType and LType
>>    QType: no identity, not nullable, not shared visible state, no sync, no reference equality
>>    LType: identity, nullable, shared visible state for sync and reference equality
>>    semantic mismatch: nulls, if_acmp_eq/ne
>>    MVT provides a short-term hack - which is ok for early adopters
>>    in future expect explicit boxes for QTypes which are LTypes which actually are identityful unlike the temporary VCCs
>> Bjorn: differences between QTypes and LTypes like int and Integer, where only Integer provides methods?
>> John: short-term there will be no methods on a DVT.
>>     longer-term value types will have methods
>>     verifier will not accept LType for a QType byte code
>> Bjorn: if QType is a subset of an LType is it ok to convert?
>> John: want ability farther future to have different behaviors and different stack representation.
>>     In progress, exploring possibly interpreter implementations, things like allowing the vm to buffer QTypes off the java heap
>>     presumes a universal type must declare whether it has identity or not
>> Bjorn: with boxing and unboxing, if you say you have “no identity” do you still have something?
>> John: must box to call a method, no one can rely on the identity so you can elide it.
>> Ed note: this is only possibly temporarily because the value-capable-class is defined as not being able to rely on identity.
>> This will not carry over to a more general value type approach if we wish to have value types box to identityful LTypes which
>> can be used by existing code that is expecting a subtype of Object or an Interface as defined today.
>> John: after MVT 1.0, exploring a union type, “UType” which is a common type that could contain either a QType or its corresponding
>> LType
>>> 5. Value bytecodes
>>>     p.11 "Method handles and invokedynamic will always allow bytecode to invoke methods on Q-types".
>>>    - is this still accurate in the context above?  I know the comment says that internally
>>>    the MH might box the Qtype, but do we still want to support MH and indy to appear to invoke methods on Q-types?
>> Ed note: I don’t if we answered this question.
>>> 6. Restrictions on the POJO:
>>> A. It would help to have a bulleted list of restrictions
>>> B. Clarify error/exception to throw - perhaps ClassFormatError for all of these?
>>> C. Request to not support VCC on interfaces at all for MVT 1.0
>>>    I know the restrictions are intermixed in the text today. This is what I extracted:
>>>    (p.3-4, 6)
>>> VCC (and probably going forward)
>>>    1. VCC superclass must be Object (and should be omitted)
>>>    2. the class must be final
>>>    3. all fields must be final
>>>       - please clarify - all instance fields must be final
>> John: yes
>>>    4. all constructors private
>> Maurizio: why?
>> Note that there is NO constructor for the DVT. It can be created via a vunbox or vdefault + vwithfield
>> John: ok to change the spec so the VCC constructor is not limited to being private.
>>>    5. must replace equals, hashcode, toString (with current Object syntax)
>>>    6. may not use any methods provided on Object
>>>       specifically: may not use clone, finalize, wait, notify, notifyAll (directly)
>>>    7. may use getClass
>>> MVT 1.0 additional limitations
>>>    9. may contain primitive instance fields, but no reference instance fields
>>>        - please update document to clarify this restriction is for instance fields only
>> Karen: both IBM and Oracle JVM engineers are interested in an optional extension to support
>> references instance fields.
>> Maurizio: If only primitives allowed, then no support for generics is needed.
>> Clarification for Karen: statics can have generics with erasure today, but can not mention type variables. (thanks :-)
>> Bjorn: With today’s erased generics, this is not a problem
>> John: Ok to explore having references in instance fields, generics are ok. No type variables in instance fields
>> and no “any” generics.
2/15/17: rediscussed:
Maurizio - higher perceived benefit to users to have reference fields in VCC/DVT - e.g. Strings
John concern: hard to do embedded references in values
Mr Simms: almost have it now
— agreed: let’s give it a short
>>>    10. may not contain generic instance fields
>>>        - please update document to clarify this restriction is for instance fields only
>>>        - it is my understanding that you can’t have generic static fields at all
>>>    11. "interfaces (especially with default methods)"
>>>        - please change p.6 to clarify that there are no value type interfaces period.
>> Ed note: There is a distinction here between
>> 11a)  defining an interface as a VCC with a derived DVT and
>> 11b) whether the POJO which defines the VCC can implement interfaces. This discussion was about whether
>> a POJO which defines the VCC can implement interfaces.
>> Karen: concern about setting expectations. Current interfaces assume identity.
>> Maurizio: could always box to call interface methods.
>> John: Question: do early adopters need interfaces?
>> Vladimir Ivanov: Yes
>> note: Vector API has no benefits using MVT 1.0.
>> Ed note: later email clarification from Vlad: 
>> Interface-based Vector API version [1] does not benefit from MVT 1.0. All operations are expressed as interface
>> calls and require vector boxes.
>> That is out of scope for MVT 1.0.
>> Vectors exploring an alternative API, exposing operations as MethodHandles. This is less convenient to use, but
>> allows experimenting to find performance benefits.
>> John: MVT: box to get to methods. Longterm get to call I.defaultmethod without boxing
>> Ed. note: Interface default method will need restrictions.
>> John: MVT 1.0 : No value capable interfaces for JVMT 1.0.
>> Ed note: was this the answer to 11a or 11b above please?
I believe John agreed that we can not use a VCC/DVT to define an interface

Open Question:
Do we need VCC/DVT to support interfaces and require boxing to invoke default methods?
Perhaps try early access without support for interfaces and get feedback?

Concerns: existing interfaces with default methods - assume identity
Longer-term interfaces that can be implemented by valhalla value types will need to be
special interfaces which support UTypes - union of LType and QType - so that they
can be implemented by references or by value types.
   UType - must not assume identity, must retain identity if it has it
   QType - not assume identity
   LType - must preserve identity
John/Brian - outline of road ahead - post MVT 1.0 - looking at a potential carrier or wrapper for
an “any” type
Dan: If a local is a UType - will enforce single type through program?
John: simple answer - not fixed type slots
Dan: verification issues
John: like a cast - must be explicit in byte codes and it might fail
   LType-> UType always valid, UType-> LType might fail
>> John: longer-term:
>>    L-Type: always identity
>>   Q- Type: never identity
>>    U-Type: do not assume identity, must preserve identity
>>>    12. 0.2 version states: may not contain a value class as an instance field
>>>        - see below for further discussion
>>> 7. potential extensions:
>>>    12. 0.2 version states: value class may not contain a value class as an instance field
>>>        - we would like to propose supporting this - perhaps as an optional extension?
>>>        - we would need to add an exception for handling circularity
>>>        - note: no way to express this in java, but you could express in a classfile
>> John: NO for MVT 1.0. Potential ambiguity whether the field contains a value capable class or a derived value type
>> javac just deals with boxes, so no flattening here. Wants same layouts whether boxed or not.
>> John: MVT 1.0 will only flatten arrays reflectively
>> End of discussion 
>> thanks,
>> Karen
>> =======
>>> 8. Splitting the value type from the object type
>>>   Propose not using the nested class approach, to not tie us into this relationship longer term
>>>   - so remove example and the "looks like an inner class"
>>>   - note: a key point here is how the user generating bytecodes would know the generated name of the DVT
>>>   This will need further discussion.
>>>   In the constant pool, references can use Qpackage.Class; rather than Lpackage.Class;
>>>   Are there requirements for java sources to be able to refer to the derived value type by name?
>>>   e.g. Class.forName()? If not, then perhaps the temporary naming convention could be kept internal?
#8: not use nested class relationship between VCC and DVT:
Bjorn: not an issue either way
*** remove from spec: Does not belong in the spec - this is an implementation detail
We don’t want to expose this as a nested class
   - reflection might expose differently
   - private static member access needs special handling, but not via javac trampolines
   - Minimal Value Types will to have nest mate support
Note: box must be in the same package & module
Need a way to mark the DVT - e.g. ACC_VALUE (other options ok)

Note : DVT name should not be exposed. Ok to use unsafe.defineAnonymousClass
to generate the DVT. The VM generates the DVT.
API ValueType.valueclass() returns, but doesn’t need to know how generated

Goal: VM’s problem to resolve QPoint descriptor reference to DVT for Point
*** add to spec: requirement: not want byte code to resolve LPoint$12345 (i.e.
a temporary name) - want ClassNotFound here
>>> 9. Splitting the value type from the object type
>>>   p.5 "The original class is given a new synthetic field of the new value type, to hold the state for the original class".
>>>   - to simplify implementation, and allow experiments which go beyond the initial MVT plans, we propose
>>>      * that the VCC is left untouched
>>>      * the DVT has a copy of the immutable instance fields
>>>   - We think this qualifies as "any equivalent technique" on p.5
>>>   - the quote above would need modifying or removing
>>> 10. Scoping of these features
>>>    - exploring adding class file capability bits  for experimental features, as a versioning approach
>>>      - we will want to pin this down later in the cycle
Brainstormed possibility of adding capability bits in minor version. Would need to go through JCP.
editor’s note: may need to revisit this - turns out minor version 45.3 was already used and set a precedent
that changes in a minor version would all be present in follow-on major versions. So probably not in scope for MVT.
>>> 11. JVM changes to support Q-types
>>>    - "So when the class loader loads an object whose fields are Q_types, it must resolve (and perhaps load)
>>>       the classes of those Q_types, ..."
>>>    - for instance fields that are Q-types, I believe we need to explicitly specify temporary JVMS load/link/init rules
>>>    (I will propose an early draft in a later email).
>>>     - e.g. Specifically, for instance fields that are Q-types, we would propose requiring eager loading of the Q-types,
>>>       modifying JVMS 5.3.5 Deriving a Class from a class File Representation. Bullets 3 and 4 described eager resolution
>>>       of the direct superclass and direct superinterfaces. The expectation is that an additional bullet would be added
>>>       for direct instance fields that are Q-types.
>>>      - note that this change would make it the JVM's responsibility, not the class loaders' responsibility,
>>>      to resolve the classes of those Q-types.
>>>     Note: in the JVMS load/link/init rules I will also propose VCC/DVT load/link/init requirements.
Note: there are two discussions here relative to load/link/init rules.
One is for the DVT/VCC relationship.
Karen: always load VCC first
Bjorn: if MH API: always create VCC first
AI: Karen - send load/link/init proposal for MVT 

Another is for flattening. Note VCC and DVT have the same field reference types, which means no
QType fields, so no flattened fields in a DVT.

Note: VCC can not mention QTypes in source, since javac can not translate.

Bjorn: Can we use QTypes in descriptors of methods and fields if we spin byte codes?
John: MethodHandles work with QTypes, therefore need an internal translation, therefore need to
handle QTypes in descriptors, therefore ok to spin byte codes like MethodHandles can.

This is not a gating goal for MVT, but is not forbidden.

Is it ok to flatten a QType field?

VCC->DVT can NOT change descriptor, therefore can not contain a QType.

John: For MethodHandles and classes spun from byte codes with QType fields,
it is ok to flatten the QType.
     - implementation choice.
     - not required, but ok if not hard

(ed. note: I have other notes on flattening which I can not translate - if others have notes
on non-array flattening please let me know)

We will however support flattening for arrays.
Ed note: thanks to Mr. Simms - he realized we need to initialize (and therefore link) any value type that
is the element type for anewarray/multianewarray.

Array flattening:
   this will not be done automagically
   flatten array only if explicitly an array of value types, i.e. anewarray/multianewarray with a DVT as the

TODO: open questions - need to investigate:
    - Need to check how core reflection would deal with seeing an object that is a DVTarray
    - What does core reflection do if handed the j.l.Class for a DVT?
    - Class.forName should not be able to find a j.l.Class for a DVT

>>> 12.  value bytecodes
>>>    - the following are useful in the MethodHandle implementation, and likely to be useful for direct bytecode access
>>>    - we would like to propose the following as the minimal bytecode set:
>>>     in addition to vload, vstore, vreturn (and slot-specific variants)
>>>      - vdefault/vwithfield
open issue
Maurizio: nice on paper
In practice: challenges - vwithfield write to final field
note: all fields in DVT are final

If we do not provide vdefault/vwithfield, but only provide vbox/vunbox
MethodHandles will use unsafe to set fields and then unbox
ed. note - need to revisit 
>>>      - vbox/vunbox
Dan: what about a2b vs. vbox/vunbox?
ed note: we initially investigated a general a2b and we don’t yet have a good way to define a CONSTANT_Type
or equivalent which we would need, so we found it easier to explicitly do vbox/vunbox. (need to bring up in next meeting)
>>>      - vaload/vastore
>>>      - vgetfield (fetch a field from a value type)
>>>      - NOT vcmp_eq/ne (equality can be implemented as component-wise comparison)
>>>     clarify that for MVT 1.0, statics are only available through the box. (TODO: where does this go in shady?)
>>> 13. value bytecodes
>>>     - open issue
>>>       - typed prefix vs. vbytecodes for initial prototype
>>> 14.  Value bytecodes
>>>    use of Qtype as class component:
>>>    "Initially the only valid use of a Q-type [is] as the class component of a CONSTANT_Methodref or CONSTANT_Fieldref
>>>     is as a CONSTANT_MethodHandle constant."
>>>    - if we extend the bytecodes as above, and we disallow anyone (MethodHandles, bytecodes) from invoking methods on Qtypes, we
>>>      could modify this to disallow Q-types for CONSTANT_Methodref or CONSTANT_InterfaceMethodRef completely.
>>>    - but perhaps you want the MethodHandles to be able to invoke methods on DVTs by dynamically boxing them. This works as long
>>>      as the methods don't assume identity.
>>> 15. Q-types and bytecodes
>>>    We propose modifying anewarray and multianewarray to allow operands that are Q-types.
>>> 16. Value Type Reflection
>>>    With the proposed modifications in #8 above: i.e. leaving the VCC untouched and copying the
>>>    instance fields to the DVT, the VCC now matches the source file.
>>>    So Class.forName() would return the VCC which is the original POJO which fits the backward compatibility model.
>>>    So we don't need a separate SourceClass, but leaving it in the proposal provides implementation flexibility.
>>> 17. Q-type method handles & behaviors "possible bytecode"
>>>    might want to change vnew to vdefault
>>>    these are samples and evolving, so maybe not worth changing
>>> I did not do this level of detailed review for the Future Work yet.
>>> thanks,
>>> Karen
>>>> On Sep 1, 2016, at 8:08 PM, John Rose <john.r.rose at <mailto:john.r.rose at>> wrote:
>>>> On Aug 31, 2016, at 11:59 PM, John Rose <john.r.rose at <mailto:john.r.rose at>> wrote:
>>>>> I have updated of this document to reflect comments so far.
>>>>> It is stored to CR (in place) and enclosed here.
>>>>> — John
>>>>> Link: <>
>>>>> <shady-values.html>
>>>> I have updated the document again with small corrections and clarifications.

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