RFR: 8302780: Add support for vectorized arraycopy GC barriers

[Fei Yang]

On Mon, 20 Feb 2023 15:23:04 GMT, Erik Österlund <eosterlund at openjdk.org> wrote:

> So far, the arraycopy stubs have performed some kind of bulk pre/post barriers for arraycopy, which have been good enough, and allowed the copying itself to be done with plain loads and stores. For generational ZGC, this approach is not good enough, and we need barriers for the actual copying, but instead don't need the pre/post barriers. To prepare the JVM for generational ZGC, we need to add an API for arraycopy barriers.

Hi, I think some riscv-specific changes could be removed or postponed:

  1. The vector version of BarrierSetAssembler::copy_load_at/copy_store_at should be removed as they are not implemented;
  2. Far branches are not needed here for this PR. It's better for these changes to go with the final generational zgc.

And I have prepare a small cleanup for that. Could you please add it if it's good for you? Thanks.
[riscv-changes.txt](https://github.com/openjdk/jdk/files/10800754/riscv-changes.txt)

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PR: https://git.openjdk.org/jdk/pull/12670