RFR: 8302780: Add support for vectorized arraycopy GC barriers [v8]

[Roberto Castañeda Lozano]

On Wed, 1 Mar 2023 10:56:31 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.
>
> Erik Österlund has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add comment about r15 useage

> All this extreme cut-and-paste manual unrolling is very hard to read, maintain, and review.
I wasn't going to say anything, because it's Erik, and what do I know! But I must push back here, this is too much.
Please consider these style changes.

In my opinion, the (very much needed) changes you suggest are outside the scope of this PR, which is about lifting the memory accesses, in their existing form, to a barrier API-level. Conflating this with your suggested changes would make it harder to review this PR, which is sufficiently complex in its current form. I totally agree that your suggestions would improve readability and maintainability, but couldn't we apply them in a follow-up RFE?

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