RFR: 8318776: Require supports_cx8 to always be true [v6]

[Aleksey Shipilev]

On Thu, 23 Nov 2023 03:14:27 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> As discussed in JBS all platforms (some tweaks to Zero are in progress) actually do support `cx8` i.e. 64-bit compare-and-exchange, so we can strip out the locked-based alternatives to using it and just add a guarantee that it is true at runtime. And all platforms except some ARM variants set `SUPPORTS_NATIVE_CX8`, so we can greatly simplify things. Summary of changes:
>> - `_supports_cx8` field is only needed when `SUPPORTS_NATIVE_CX8` is not defined
>> - Assertions for `supports_cx8()` are removed
>> - Compiler predicates requiring `supports_cx8()` are removed
>> - Access backend is greatly simplified without the need for lock-based alternative
>> - `java.util.concurrent.AtomicLongFieldUpdater` is simplified without the need for a lock-based alternative
>> 
>> I did consider moving all the ARM `kuser_helper` related code to be only defined when `SUPPORTS_NATIVE_CX8` is not defined, but there was a theoretical risk this could change the behaviour if ARMv7 binaries were run on other ARM CPU's. I added a note to that effect in vm_version_linux_arm32.cpp so the ARM port maintainers could clean this up further if desired.
>> 
>> Testing:
>> - All Oracle tiers 1-5 builds (which includes an ARMv7 build)
>> - GHA builds/tests
>> - Oracle tiers 1-3 sanity testing
>> 
>> Zero changes coming in via [JDK-8319777](https://bugs.openjdk.org/browse/JDK-8319777) will be merged when they arrive.
>> 
>> Thanks.
>
> David Holmes has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix typo

Ran full jcstress on linux-arm-zero-release on RPi 4 without problem.

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Marked as reviewed by shade (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/16625#pullrequestreview-1745896266