RFR: 8319777: Zero: Support 8-byte cmpxchg

[David Holmes]

On Fri, 10 Nov 2023 14:17:32 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

src/hotspot/cpu/zero/globalDefinitions_zero.hpp line 30:

> 28: 
> 29: // Unconditionally supports 8-byte cmpxchg either with
> 30: // compiler intrinsics or with library/kernel helpers.

That's not what "native support for cx8" was meant to mean though - e.g. see the ARM header - it only sets this when building for ARMv7.

You can just leave this file alone and simply set `_supports_cx8` below to achieve the same goal. And that will fit in cleaner with the changes I am making.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16614#discussion_r1391053336