RFR: 8351157: Clean up x86 GC barriers after 32-bit x86 removal [v3]

[Thomas Schatzl]

On Wed, 9 Apr 2025 08:31:44 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Assembler GC barriers have quite a bit of coding to support 32-bit x86. As 32-bit x86 is removed, we can clean up those parts.
>> 
>> We can eliminate `!LP64` blocks quite easily. We can also prune passing around `thread` argument, and just trust that `r15_thread` is always available.
>> 
>> Additional testing:
>>  - [x] Linux x86_64 server fastdebug, `tier1`
>>  - [x] Linux x86_64 server fastdebug, `all`
>
> Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains five commits:
> 
>  - Merge branch 'master' into JDK-8351157-x86-gc-barriers
>  - Merge branch 'master' into JDK-8351157-x86-gc-barriers
>  - Also do tlab_allocate
>  - Rely on R15 to be a thread register
>  - Work

Marked as reviewed by tschatzl (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/24253#pullrequestreview-2755601117