RFR: 8319777: Zero: Support 8-byte cmpxchg [v2]

[Thomas Stuefe]

On Tue, 14 Nov 2023 13:28:09 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> See related discussion in [JDK-8318776](https://bugs.openjdk.org/browse/JDK-8318776) that targets to require `supports_cx8()` unconditionally.
>> 
>> I think we can claim Zero is `supports_cx8() == true`, because we have enough fallbacks for 8-byte CASes to work. Note that some code already reaches for these without checking for `supports_cx8()`, so the proverbial horses have already left the barn.
>> 
>> I ran tests with [JDK-8319883](https://bugs.openjdk.org/browse/JDK-8319883) applied to fix known problems with x86_32 Zero.
>> 
>> Additional testing:
>>  - [x] Linux x86_32 Zero release; jcstress
>>  - [x] Linux x86_32 Zero fastdebug, `compiler/unsafe java/lang/invoke/VarHandles`
>>  - [x] Linux x86_32 Zero fastdebug, bootcycle-images
>
> Aleksey Shipilev 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 three additional commits since the last revision:
> 
>  - Only do _supports_cx8 = true
>  - Merge branch 'master' into JDK-8319777-zero-64cas
>  - Fix

Good.

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Marked as reviewed by stuefe (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/16614#pullrequestreview-1733667506