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Project Valhalla is an OpenJDK project started in 2014 in an experimental stage. It is headed by Oracle Java language architect Brian Goetz and supported by the HotSpot group.

The project was created for introducing value-based optimizations to JDK 10 and above. The goal of Project Valhalla is explore and support development of advanced Java VM and language features like, value types, generic specialization, and variable handles.

The Project Valhalla members met last week at Burlington MA to discuss in detail about the current project status and future plans. Goetz notes that it was a very productive meeting with members either attending the venue in person or connecting via calls. After over four years of the project, the members decided to meet as it seemed like a good time to assess the project. Goetz states: “And, like all worthwhile projects, we hadn’t realized quite how much we had bitten off.  (This is good, because if we had, we’d probably have given up.)

This meeting indicates the initiation of Phase III project Valhalla.

Phase I focused on language and libraries. Trying to figure out what exactly a clean migration to value types and specialized generics would look like. This included steps to migrate core APIs like Collections and Streams, and understanding the limitations of the current VM. This enabled a vision for the VM that was needed.

Phase I produced three prototypes, Models 1-3. The exploration areas of these models included specialization mechanics (M1), handling of wildcards (M2) and classfile representations for specialization and erasure (M3). At this point, the list of VM requirements became too long and they had to take a different approach.

Phase II took on the problem from the VM up, with two additional rounds of prototypes namely MVT and LW1. LW1 was a risky experiment; sharing the L-carrier and a* bytecodes between references and values while not losing performance. If this could be achieved, many of the problems from Phase I could go away.  This was successful and now they have a richer base for further work.

The next target is L2, which will capture the choices made so far, provide a useful testbed for doing library experiments, and set the stage for tackle remaining open questions between now and L10.  L10 is the target for a first preview, which eventually should support value types and erased generics over values.

For more information, you can read the mail on Project Valhalla mailing list.

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