RFR: 8299844: RISC-V: Implement _onSpinWait intrinsic [v3]

[Feilong Jiang]

On Tue, 17 Jan 2023 09:45:43 GMT, Yadong Wang <yadongwang at openjdk.org> wrote:

>> Implement _onSpinWait() intrinsic for RISC-V, and it can be implemented by using PAUSE instruction from Zihintpause Extension, which is ratified and a mandatory extension of RVA22U64 (https://github.com/riscv/riscv-profiles/blob/main/profiles.adoc#rva22u64-mandatory-extensions).
>> Software can use the PAUSE instruction to reduce energy consumption while executing spinwait
>> code sequences (https://github.com/riscv/riscv-isa-manual/blob/master/src/zihintpause.tex).
>> 
>> Add a test case of test/hotspot/jtreg/compiler/onSpinWait/TestOnSpinWaitRISCV64.java, passed on QEMU.
>
> Yadong Wang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix copyright year

Marked as reviewed by fjiang (Author).

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PR: https://git.openjdk.org/jdk/pull/11921