RFR: 8362193: Re-work MacOS/AArch64 SpinPause to handle SB [v3]

[Evgeny Astigeevich]

On Tue, 22 Jul 2025 22:35:12 GMT, Evgeny Astigeevich <eastigeevich at openjdk.org> wrote:

>> The original assembly with the current default YIELD: https://godbolt.org/z/McWGM4f44
>> 
>> SpinPause(SpinWait::Inst):
>>         lsl     w8, w0, #3
>>         sxtw    x8, w8
>>         adr     x9, #20
>>         add     x9, x9, x8
>>         br      x9
>>         b       .Ltmp1
>>         nop
>>         nop
>>         b       .Ltmp1
>>         nop
>>         b       .Ltmp1
>>         yield
>> .Ltmp1:
>> 
>>         ret
>> 
>> 
>> 
>> Iterations:        100
>> Instructions:      1300
>> Total Cycles:      254
>
> For the default YIELD we have:
> - Original: ~250 clocks
> - Compiler binary search tree: ~200 clocks (-20%)
> - Optimized with `TBZ`: ~150 clocks (-40%)

I think the differences might be smaller on the real hardware. Maybe everything will be around 200 clocks.
If we need code easy to maintain then this is `switch`. If we need performance then this is `tbz`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26387#discussion_r2223958425