RFR: 8186670: Implement _onSpinWait() intrinsic for AArch64 [v13]

[Andrew Haley]

On Mon, 18 Oct 2021 16:16:58 GMT, Evgeny Astigeevich <duke at openjdk.java.net> wrote:

>> This PR is a follow-up on the discussion [“RFC: AArch64: Implementing spin pauses with ISB”](https://mail.openjdk.java.net/pipermail/hotspot-dev/2021-August/054033.html).
>> 
>> It adds DIAGNOSTIC options `OnSpinWaitInst=inst`, where `inst` can be:
>> 
>> - `none`: no implementation for spin pauses. This is the default value.
>> - `nop`: use `nop` instruction for spin pauses.
>> - `isb`: use `isb` instruction for spin pauses.
>> - `yield`: use `yield` instruction for spin pauses.
>> 
>> And  `OnSpinWaitInstCount=count`, where `count` specifies a number of `OnSpinWaitInst` and can be in `1..99` range. It is an error to use `OnSpinWaitInstCount` when `OnSpinWaitInst` is `none`.
>> 
>> The code for the `Thread.onSpinWait` intrinsic is generated based on the values of `OnSpinWaitInst` and `OnSpinWaitInstCount`.
>> 
>> Testing:
>> 
>> - `make test TEST="gtest"`: Passed
>> - `make run-test TEST="tier1"`: Passed
>> - `make run-test TEST="tier2"`: Passed
>> - `make run-test TEST=hotspot/jtreg/compiler/onSpinWait`: Passed
>> 
>> CSR: https://bugs.openjdk.java.net/browse/JDK-8274564
>
> Evgeny Astigeevich has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix formatting and spelling

test/micro/org/openjdk/bench/java/lang/ThreadOnSpinWaitProducerConsumer.java line 47:

> 45: @State(Scope.Benchmark)
> 46: public class ThreadOnSpinWaitProducerConsumer {
> 47:     @Param({"100"})

This test seems rather artificial and unrealistic to me. You can get improved performance simply by increasing the `spinNum` so that it waits for longer. The way to get some advantage from `onSpinWait` is to have multiple threads racing to update the same thing, e.g. to acquire a lock.

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PR: https://git.openjdk.java.net/jdk/pull/5562