RFR: 8329134: Reconsider TLAB zapping [v2]

[Roman Kennke]

On Wed, 27 Mar 2024 14:55:35 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> We zap the entire TLAB on initial allocation (`MemAllocator::mem_allocate_inside_tlab_slow`), and then also rezap the object contents when object is allocated from the TLAB (`ThreadLocalAllocBuffer::allocate`). The second part seems excessive, given the TLAB is already fully zapped.
>> 
>> There is also no way to disable this zapping, like you would in other places with the relevant Zap* flags.
>> 
>> Fixing both these issues allows to improve fastdebug tests performance, e.g. in jcstress. 
>> 
>> It also allows to remove the related Zero kludge.
>> 
>> Additional testing:
>>  - [x] Linux AArch64 server fastdebug, `all` tests 
>>  - [x] MacOS AArch64 Zero fastdebug, `bootcycle-images` pass
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Review comments

Looks good to me. Thank you!

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Marked as reviewed by rkennke (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/18500#pullrequestreview-1965810416