Moving from VVT to the L-world value types (LWVT)

Frederic Parain frederic.parain at
Wed Jan 24 14:34:30 UTC 2018

I’ve uploaded a quick summary of the potential issues that can be caused
by a mismatch between the expected and the real nature of a class (object
class or value class).

If some cases are missing or incorrect, please let me know.


> On Jan 23, 2018, at 19:05, Frederic Parain <frederic.parain at> wrote:
> Updated JVMS document with a few fixes and the Q-descriptors
> removed (this removal changed only 3 lines!):
> No attribute to list value classes has been added yet, so there’s
> currently some issues for the verification.
> Fred
>> On Jan 23, 2018, at 16:25, Frederic Parain <frederic.parain at> wrote:
>> Hi John,
>> thank you for the detailed feedback.
>> The Q-descriptor is not a fundamental part of the proposal, it is just an unsatisfying
>> way for class files to express their expectations regarding types they think are value
>> class types (to differentiate them from object class types). Q-descriptors provide this
>> information but have drawbacks like the signature matching issue.
>> Remi’s proposal is appealing because it avoids the signature matching issue.
>> An attribute is not the most convenient data structure for the JVM, but we can
>> record the information elsewhere in our meta-data. However, it seems more
>> brittle because the attribute can easily omitted, unless we make it mandatory
>> after a given class file format number, with a slightly different syntax where all
>> classes named in the class files have to be listed, so it can be verified. For
>> older class file format, the attribute would be absent and all classes are assumed
>> to be object classes.
>> We had two brainstorming sessions. yesterday and this morning, trying to figure
>> out what would be the consequences of having only L-descriptors, with class
>> files having different assumptions regarding the real nature of a type (object class
>> or value class), either in the case of VBC migration or simply because of separate
>> compilation. Some issues are related to the calling/returning conventions for the
>> JIT compiled code. Some others issues are related to the class loader constraints,
>> and the fact that a class with the wrong assumption regarding the nature of a class
>> might prevent the real class from being loaded. The case where a class expects
>> a Value Based Class (object class type) and the class is in fact a migrated value
>> class seems to be OK. The case where a class expects a value class, but the
>> class loader loads an object class seems much more problematic to us.
>> Regarding the migration of value based classes, trying to prevent null references
>> from leaking into migrated code seems to be a step to far. We reviewed the issue with
>> Karen this morning, and it doesn’t seems too dangerous to only check for null
>> when the reference is stored in a field or array expecting an instance of a value
>> class.
>> Thank you,
>> Fred
>>> On Jan 19, 2018, at 23:22, John Rose <john.r.rose at> wrote:
>>> On Jan 16, 2018, at 12:56 PM, Frederic Parain <frederic.parain at> wrote:
>>>> Here’s an attempt to bootstrap the L-world exploration, where java.lang.Object
>>>> is the top type of all value classes (as discussed during the November meetings
>>>> in Burlington).
>>> This is excellent work, Frederic; thank you.  I'm really hopeful that we
>>> are on the right track.
>>>> ...
>>>> Here’s a quick summary of the changes with some consequences on the HotSpot code:
>>>> - all v-bytecodes are removed except vdefault and vwithfield
>>> At some point we may want to strip the v-prefix from those survivors.  No hurry.
>>>> - all bytecodes operating on an object receiver are updated to support values as well,
>>>> except putfield and new
>>> Yep.
>>>> - single carrier type for both instances of object classes and instances of value classes
>>>> - this carrier type maps to the T_OBJECT BasicType
>>>> - T_VALUETYPE still exists but its usage is limited (same purpose as T_ARRAY)
>>> T_ARRAY can be a confusing source of bugs.  I've always wondered if it was worth it.
>>>> - qtos TosState is removed
>>>> - JNI: the jobject type can be used to carry either a reference to an object or an
>>>>        array or a value. The type jvaluetype, sub-type of jobject, is used when only
>>>>        a value class instance is expected
>>>> - Q…; remains the way to encode value classes in signature (fields and methods)
>>> I'd like to move towards an ACC_VALUE bit on both fields and classes.
>>> Again, no hurry, but (as in my previous message) I'd like to retire Q-descriptors.
>>>> - In the constant pool, the CONSTANT_CLASS_info entry type is used to store a
>>>> symbolic reference to either an object class or a value class
>>>> - the ;Q escape sequence is not used anymore in value class names
>>>> One important point of this exercise is to ensure that the migration of Value Based Classes
>>>> into Value Classes is possible, and doable with a reasonable complexity and costs. In addition
>>>> to the JVMS update (and consistent with the JVMS modifications), here’s a set of proposals
>>>> on how to deal with the VBC migration. 
>>> I'm glad you are doing this analysis, not only because VBC migration is
>>> a wonderful goal, but also because I think the same analysis is necessary
>>> just to manage separate recompilation, even if we never decided to
>>> migrate a single class.
>>> In short, I see you are leaning hard on Q-descriptors, but I don't think
>>> you are getting enough value out of them, and they cause serious
>>> problems.  More comments below… 
>>>> Migration of Value Based Classes into Value Classes:
>>>> - challenges:
>>>>   - signature mismatch
>>> Goes away when/if we retire Q-descriptors!
>>>>   - null
>>> Can be dealt with by assuming non-null and throwing dynamic NPEs
>>> as needed where Q types are in play.  Also, we tolerate "polluting nulls"
>>> along paths where the Q/R distinction is not available, even if (at some
>>> point later on) we realize that it was a Q all along.  Eventually, the
>>> polluting null will cause an NPE.
>>> (In my view, the NPE should happen later than one might prefer if it were
>>> a true coding error rather than a recompilation artifact.  Catching polluting
>>> nulls early in the presence of recompilation requires too many heroics.)
>>>>   - change in behavior
>>> Yes, that's the tricky part.
>>>> - proposal for signature mismatch:
>>>>    - with LWVT, value class types in signatures are using the Q…; format
>>>>    - legacy code is using signature with L…; format (because VBC are object classes)
>>>>    - methods will have two signatures:
>>>>      - true signature, which could include Q…; elements 
>>>>      - a L-ified signature where all Q…; elements are re-written with the L…; format
>>>>      - method lookup still works by signature string comparisons
>>>>      - the signature of the method being looked up will compared against both the
>>>>        true and the L-ified signatures, if the looked up signature matches the L-ified
>>>>        signature but not the true signature, it means a situation where legacy code
>>>>        is trying to invoke migrated code has been detected, and additional work might
>>>>        be required for the invocation (actions to be taken have to be defined)
>>>>     - signature mismatch can also occur for fields, this is still being investigating, the
>>>>       proposal will be updated as soon as we have a solution ready to be published
>>> This sort of thing is, for me, a rich argument against keeping Q-descriptors.
>>>> - proposal for null references leaking to migrated code
>>>>   - having a null reference for a Value Based Class variable or field is valid in legacy code
>>>>     but it becomes invalid when the Value Based Class has been migrated to a Value Class
>>>>   - trying to prevent all references with a value class type to get a null value would be very
>>>>     expensive (it would require to look at the stackmap for each assignment to a local variable)
>>> Yes.  We have to tolerate polluting nulls where the Q/R distinction is unavailable.
>>>>  -  the proposed solution is to allow null references for local variable and expression stack slots,
>>>>     but forbid them for fields or array elements (bytecodes operating on fields and array have to
>>>>     be updated to throw a NPE whenever a null reference is provided instead of a value class
>>>>     instance)
>>> Yes, I think this is on the right track.  On paths where a Q-type is needed
>>> we do a null check.  That's the Java way.
>>>>  - null references are likely to be an issue for JIT optimizations like passing values in registers
>>>>    when a method is invoked. The proposed solution is to only allow null references for value classes
>>>>    in legacy code, by detecting them and blocking them when leaking to migrated code. The
>>>>    detection can be done at invocation time, when a mismatch between the signature expected
>>>>   by the caller and the real signature of the callee is detected (see signature mismatch proposal above)
>>> At some point, a polluting null might reach code that "knows" there is a Q type
>>> (and may even "know" that it goes in an xmm register).  That's the point where
>>> an NPE should be thrown.  In some cases, a deopt might be appropriate, to
>>> correctly order the NPE by executing interpreter code.
>>> Note that this combination of techniques does not Q-descriptors.  The lack
>>> of Q-descriptors doesn't totally destroy the Q/R distinction; it just means you
>>> have to execute a little further before you get to code which "knows" that
>>> the null is illegal.
>>>> - the null reference should also be detected and blocked when it is used as a return value and the
>>>>   type of the value to be returned is a value class type 
>>> Doing this requires (a) Q-descriptors in method returns, (b) Remi's
>>> ValueTypes table, or (c) toleration of nulls in the interpreter.  (The JIT
>>> doesn't have to tolerate nulls:  It can deopt if it hits a surprise null,
>>> or perhaps throw an early NPE.)  So, I am arguing for (c).
>>>> In addition to the JVMS update, here’s a chart trying to summarize the new checks that will have to
>>>> be added to existing bytecode when moving the vbytecodes semantic in to a* bytecodes. The categories
>>>> in the chart are not very precise, but we can use it as a starting point for our discussions. The chart
>>>> can also help defining which experiments could be done to estimate the costs of the different additional
>>>> checks needed to be added to existing bytecodes.
>>> The chart is really helpful, thanks.  More comments later.
>>> Onward!
>>> — John

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