RFR: 8361099: Shenandoah: Improve heap lock contention by using CAS for memory allocation [v20]

[Xiaolong Peng]

On Fri, 9 Jan 2026 19:57:23 GMT, Xiaolong Peng <xpeng at openjdk.org> wrote:

>> I don't have concern to move it to ShenandoahThreadLocalData, the benefit of using THREAD_LOCAL is just better cohesive code because  _alloc_start_index is defined in the same namespace where it is used. Performance wise, I don't think there is much benefits.
>> 
>> I do see ZGC also use THREAD_LOCAL directly, I guess the overhead on all threads is not a huge concern. 
>> But given Shenandoah has ShenandoahThreadLocalData to manage all the thread locals, it make sense to not use THREAD_LOCAL directly, I'll update the PR to move _alloc_start_index to ShenandoahThreadLocalData.
>
> While I am trying to use ShenandoahThreadLocalData, I realized that I need to add 2 alloc start index, one for ShenandoahMutatorAllocator, one for ShenandoahCollectorAllocator. Later when I update ShenandoahOldCollectorAllocator to use CAS allocation, one more will be added.

I have moved all the THREAD_LOCAL to ShenandoahThreadLocalData, also rewrote `alloc_start_index` function in templated impl to avoid virtual table lookup.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26171#discussion_r2678364578