RFR: 8299844: RISC-V: Implement _onSpinWait intrinsic

[Ludovic Henry]

On Tue, 10 Jan 2023 09:52:40 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.

src/hotspot/cpu/riscv/riscv.ad line 1820:

> 1818:   switch (opcode) {
> 1819:     case Op_OnSpinWait:
> 1820:       return VM_Version::supports_on_spin_wait();

You can use `UseZihintpause` directly just like for `UseRVV`, `UsePopCountInstruction`, or `UseZbb` below.

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