RFR: 8300258: C2: vectorization fails on simple ByteBuffer loop [v2]

[Emanuel Peter]

On Tue, 21 Feb 2023 16:05:27 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> Roland Westrelin has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:
>> 
>>  - comments
>>  - extra test
>>  - more
>>  - Merge branch 'master' into JDK-8300258
>>  - review
>>  - more
>>  - fix & test
>
> Maybe we should first collect all legal packs and then try to find the best alignment, dropping packs that are unaligned on those platforms that support it.

@rwestrel I am wondering if "best alignment" is really about performance? Or is it just the one `mem_ref` that we have to decide to align all other refs to, in case we have `vectors_should_be_aligned() ==  true`? If that were the case, maybe we can rename it. Instead of `best_align_to_mem_ref` it could be something like `alignment_anchor_mem_ref`.

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PR: https://git.openjdk.org/jdk/pull/12440