RFR: 8321371: SpinPause() not implemented for bsd_aarch64/macOS

[Fredrik Bredberg]

On Wed, 20 Dec 2023 21:14:47 GMT, Evgeny Astigeevich <eastigeevich at openjdk.org> wrote:

>> @eastig @fisk 
>> 
>> I've now rewritten `SpinPause()` so you can use the `OnSpinWaitInst` option to choose between `none`, `nop`, `isb` and `yield`.
>> I choose not to change the default from `none` to `yield` in this PR. For that I've created [JDK-8322535](https://bugs.openjdk.org/browse/JDK-8322535).
>
> @fbredber 
> The assembly code looks good to me.
> Out of curiosity, why not to use a regular `switch`? Why is the manually written assembly better?
> What if it looked like the following:
> 
> class SpinWait {
> public:
>    static void exec(Inst inst) {
>       switch (inst) {
>         case NOP:
>             asm volatile();
>             break;
>         case ISB:
>             asm volatile();
>             break;
>          case YIELD:
>             asm volatile();
>             break;
>         default:
>       }
>    }
> ...
> };
> 
> extern "C" {
>   int SpinPause() {
>     SpinWait::exec(VM_Version::spin_wait_desc().inst());
>     return 1;
>   }

@eastig
> Out of curiosity, why not to use a regular `switch`? Why is the manually written assembly better?

I just like to keep away from conditional branches in code that is supposed to be in tight loops. :)

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PR Comment: https://git.openjdk.org/jdk/pull/16994#issuecomment-1865955740