Minimal Value Types

Welcome to the Minimal Value Types early adopter's project !

Target Audience

• Power users 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

Programming Model

• Create a POJO using an experimental javac, with an annotation ValueCapableClass
  • JVM will derive a Derived Value Class (DVC) from this defined "box" which we call the Value Capable Class (VCC)
  • DVC contains an immutable copy of the instance fields from the VCC
• Work with MethodHandles and experimental reflection package ValueType (jdk.experimental.value.ValueType)
• Or spin your own byte codes
  
  • experimental MethodBuilder (jdk/experimental/value.MethodHandleBuilder)
  • future: ASM support is planned

Repository and Build Instructions

To create a new mvt branch:

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

cd valhalla

sh get_source.sh

sh common/bin/hgforest.sh update -r mvt

To update repository: 

cd valhalla

sh common/bin/hgforest.sh pull

sh common/bin/hgforest.sh update -r mvt

To build repository

bash configure

make images

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

Note: Valhalla is a child of the jdk10/hs repository, to keep current with latest hotspot development.

Filing Bugs

• File bugs with synopsis starting with "[MVT]" and labels: mvt 

Feature Subset

• Derived Value Class is a new type which is:
  • immutable, identity-agnostic, non-nullable, non-synchronizable, final
  • does not inherit from java.lang.Object
• Arrays of value types are flattened
• Derived Value Class can contain primitives or references

Experimental JVMS appendix (http://cr.openjdk.java.net/~dlsmith/values.html)

• Derived Value Class derivation rules
  • temporary name mangling: class Foo.class generates Foo$Value
• Classfile extensions
  • temporary CONSTANT_Class: ";QFoo$Value", uses ";Q" as an escape to a derived value class name
  • field and method descriptors: "Q<value name>; in place of "L<reference name>;
• Bytecodes
  • value types byte codes: vdefault, vwithfield, vload, vstore, vreturn, vaload, vastore, vbox, vunbox
  • overloaded bytecodes: getfield, anewarray, multianewarray

Limitations

• platforms: x64 Linux, x64 Mac OS X
• no verifier (work-in-progress)
• no VarHandles
• no support for atomic fields containing value types
• no JNI, unsafe, redefineclasses, reflection
• -Xint and C2 only, no C1, no tiered-compilation
• interpreter is not optimized, focus is on JIT optimization
• Need additional USE CASES for optimizations
• Working on additional test cases

References

• Experimental Appendix for Java Virtual Machine Specification for Value Classes
• http://cr.openjdk.java.net/~chegar/docs/sandbox.html  // instructions for working with branch repositories

• https://wiki.openjdk.java.net/display/valhalla/Minimal+Value+Types+and+Valhalla+value+types+command+line+options // MVT and post-MVT command-line options

(TODO - attach slide presentations from JVMLS ?)