Shenandoah support

[rkennke]

WIP

[tkrodriguez]

If you have any questions about how to structure your changes feel free to ping me over slack as I was the primary author of the new LIR support for barriers.

[rkennke]

If you have any questions about how to structure your changes feel free to ping me over slack as I was the primary author of the new LIR support for barriers.

Thanks, Tom! I will do that whenever I get stuck or have questions. So far I'm making progress. Structurally, Shenandoah will be look like a mix of ZGC (for the load-barrier, even though I am modeling it as a Node that consumes the loaded value, instead of replacing the ReadNode altogether) and G1 (for the SATB parts), and likely Serial/Parallel for the card-table parts.

[tkrodriguez]

Sounds good. There's still some more work to finish out the switch to LIR only barriers but I think supporting G1 and ZGC covers the required strategies in a fairly pragmatic way.