"LW1" - Minimal L-World

LW1 initial prototype is intended to get this into your hands as early as possible so you can provide us with feedback. 

Javac source support:

• Requires source level >= JDK11
• A class declaration may be tagged as being a value type by using the "__ByValue" modifier
  
  • Interfaces, annotation types, enums can not be value types
  • Top level, inner, nested, local classes may be value types. Value types may declare inner, nested, local types
  • Value Types are implicitly final, so cannot be abstract)
• Value Types may not declare an explicit super class. They implicitly extend java.lang.Object akin to enums, annotation types and interfaces
• Value Types may declare explicit interfaces
• Value Types constructors may not pass the "this" handle until all instance fields are definitely assigned.
• All instance fields of a value class are implicitly final
• Value types may not to declare fields of its own type directly or indirectly
• java.lang.Object methods:
  • javac automatically generates hashCode, equals, longHashCode and toString computed solely from the instance's state and not from its identity
  • javac does not clone(), finalize(), wait*), notify*() on value receivers
• javac does not allow comparison of values using ==, != 
• Value Types can not be assigned null, null can not be cast to or compared with value types
• Value Types cannot be type arguments in generic type parameterizations, type witnesses in generic method invocations, wildcard bounds
• Type migration, including partial recompilation is not supported

Java APIs:

• Support for java.lang.Object methods:
  • TODO
• Class.isValue() new API
• Class.newInstance() on a value type throws IllegalAccessException
• SetAccessible() on a value type throws InaccessibleObjectException

Runtime Exceptions:

• Generated class files require ValueTypes attribute, and will throw IncompatibleClassChangeError if value types are used which are not in the attribute
• IllegalMonitorException thrown on attempts to synchronize or call wait(*) or notify*() on a value type
• ClassCircularityError thrown if loading an instance field of a value type which declares its own type either directly or indirectly

Limitations for LW1

• platforms: x64 Linux, x64 Mac OS X, Windows
• no support for value class with no instance fields
• no support for atomic fields containing value types
• no support for @Contended value type fields
• static fields are not flattenable
• no AOT, CDS, ZGC, serviceability agent
• -Xint and C2 only, no C1, no tiered-compilation, no Graal
• unsafe field and array accessor APIs are not supported for value types
  • VarHandles support value types
    • however: they do not enforce immutability  or non-nullability for value types
      • note that immutable types can be copied, so updates may not be seen
  • low-level unsafe APIs are UNSAFE and will not be changed to support value types
  • risks: If a value type has been flattened in a container, unsafe does not know the layout
    • getObject could return the first flattened element rather than the expected reference
• interpreter is not optimized, focus is on JIT optimization
• Please provide additional USE CASES for optimizations
• Working on additional test cases

Future Possibilities

• LW1 updates: 
  • fix bugs, add optimizations and support for additional minor features
• LWX:
  • Addition of significant feature support
  • Addition of language level syntax and semantics
  • Eventually a preview with an openjdk release
  • Expect specialized generics and support for primitives as value types will have their own early access cycles 

How to Try L-World Value Types

Target Audience

• Power users  - Java/JVM Language, Framework, Library authors/experts
  • who are comfortable with early experimental software
  • who recognize that everything in the experiment - the model, the classfile extensions, the byte codes is likely to change
  • who want to contribute to early exploration of Value Types
  • who will not build any products based on these prototypes
• Who are willing to provide feedback to the developers on a subset of Value Type features
• Who will provide use cases for the development team to experiment with optimizations

Early Access Binaries

http://jdk.java.net/valhalla/

Repository and Build Instructions

To create a new local repository, based on "lworld" branch:

hg clone http://hg.openjdk.java.net/valhalla/valhalla valhalla-lworld

cd valhalla-lworld

hg defpath du <openjdkname> 

hg update -r lw1 // name of branch

To update repository: 

cd valhalla-lworld

hg pull

hg update -r lw1  // name of branch

To build repository

bash configure

make images

Instructions for working with branch repositories: http://cr.openjdk.java.net/~chegar/docs/sandbox.html

Note: Valhalla is a child of the jdk/jdk repository, to keep current with latest OpenJDK development.

Programming Model

CMH: Example code

Sample JMH benchmarks: http://mail.openjdk.java.net/pipermail/valhalla-dev/2018-June/004380.html

Run Experimental L-World

• java -XX:+EnableValhalla <Test>
• Further experimental flags can be found here

Helpful Feedback Please

This is intended as an early prototype to give you a chance to experiment and provide feedback on the currently supported features.

Please ensure you are on the latest EA binaries before reporting a problem.

Bugs are tracked in JIRA: Start summary with /[lworld/]. You can search for known problems or already reported bugs.

Send email to valhalla-dev@openjdk.java.net 

• Use cases that worked for you, including any positive performance or footprint information
• Bugs or questions about surprise behavior
• Use cases that you would like to see optimized PLEASE

Too Early for Feedback

This is intended as an early prototype to give you a chance to experiment and provide feedback on the currently supported features.

At this time it would premature to provide feedback on

• Language syntax
• Problems already listed under Limitations

References

• Draft Java Virtual Machine Specification Value Type changes
• http://cr.openjdk.java.net/~chegar/docs/sandbox.html  (instructions for working with branch repositories)