On Wed, 22 Nov 2023 02:09:38 GMT, David Holmes <dhol...@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 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:
> 
>  - Merge with master and update Zero code accordingly
>  - Merge branch 'master' into 8318776-supports_cx8
>  - Remove unnecessary includes of vm_version.hpp.
>    Fix copyright years.
>  - Remove cx8 comment as no longer relevant (the spinlock is used regardless 
> of cx8)
>  - Remove suports_cx8() checks from gtest
>  - Remove test for VMSupportsCX8
>  - 8318776: Require supports_cx8 to always be true

src/hotspot/share/runtime/vm_version.cpp line 33:

> 31: void VM_Version_init() {
> 32:   VM_Version::initialize();
> 33:   guarantee(VM_Version::supports_cx8(), "Support for 64-bit atomic 
> operations in required in this release");

Typo: "in required in". Also, no need to mention "this release" at all?
Suggestion for message: "JVM requires platform support for 64-bit atomic 
operations"

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/16625#discussion_r1401743607

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