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• The L-World Value Types project is a series of early prototypes for Value Types
  • builds on work of the previous Minimal Values Types prototype (MVT)
    • provides a new type which is: immutable, identity-agnostic, non-nullable, non-synchronizable, final
    • Value Types contained in References, other Value Types or in Arrays are flatten-able
    • Value Types can contain primitives or references
  • Allows value types to inter-operate with Object or "L-Type" descriptors

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The previous MVT prototype maintain maintained completely separate descriptors, "Q-Types", where value type signatures began with "Q" (similar to how Object descriptors begin with "L", i.e. "L-Types"). Data movement and manipulation of value types under the MVT prototype all required separate byte-codes. There were at one point 13 new internal byte codes, known as "v-bytecodes" since they are all started with "v". These are no more. What we have today, is an attempt to incorporate value types into L-type system to maximize backward compatibility with existing Objects and Interfaces, an "L-World" if you will:

• Value Types may be referred to by the same "L-Type" descriptors the VM has always operated on:
  • may pass a vale value type as an objecta java.lang.Object or interface, through existing APIs
  • may implement interfaces with value types
  • identity-less: 
    • synchronization will fail with exception: IllegalMonitorStateException
    • reference equality with "==" (if_acmp<eq|ne>) always returns false, value equality requires using the "equals()" method
• There are only two new byte-codes, otherwise existing byte-codes have been engineered to accept and maintain value type characteristics (identity-less, flattenedflattenable, pass by value):
  • "default" - will create a new default value
  • "withfield"- allows updating value type fields via a copy-on-write semantic, i.e. new value based on the old value combined with new field value.

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• Experimental source support via javac with "__ByValue" class declaration modifier
• Value Types cannot be used as parameterized types, i.e. current generic type declarations do not support primitives, nor value types at this point in time
• Type migration, including partial recompilation is not support supported at this point

How to Try L-World Value Types:

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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

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• Send email to valhalla-dev@openjdk.java.net with bugs or small use cases for optimization

Limitations

• 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 (work-in-progress for an update)
• no support for @Contended value type fields
• static fields are not flattenable
• no AOT, CDS, redefineclassesZGC, serviceability agent
• -Xint and C2 only, no C1, no tiered-compilation, no Graal
• unsafe field accessor APIs are not supported for value types
• interpreter is not optimized, focus is on JIT optimization
• Need additional USE CASES for optimizations
• Working on additional test cases

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