RFR: 8371643: Remove ThreadLocalAllocBuffer::_reserve_for_allocation_prefetch

[Martin Doerr]

On Tue, 11 Nov 2025 16:18:04 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> Trivial removing obsoleted code for unsupported arch.
>> 
>> Test: tier1
>
> Please ask all OpenJDK platforms supporters to test these changes.
> 
> Note, when this code was introduced we did not have so many platforms.

This change looks incomplete to me. @vnkozlov: Shouldn't we remove `AllocatePrefetchStyle == 3` completely? `PhaseMacroExpand::prefetch_allocation` still mentions "BIS instruction is used on SPARC as prefetch". Please note that PPC64 also still has an implementation for it (nodes with `predicate(AllocatePrefetchStyle == 3)`). I guess that we don't need it any more. Maybe we should check performance again.

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PR Comment: https://git.openjdk.org/jdk/pull/28240#issuecomment-3538765487