RFR: 8343249: [Windows] Implement SpinPause

[Stuart Monteith]

On Wed, 30 Oct 2024 06:29:29 GMT, Julian Waters <jwaters at openjdk.org> wrote:

> SpinPause is currently not implemented on any Windows platforms, due to a lack of access to assembly in the Microsoft compiler. The YieldProcessor macro can act as a stand in for this purpose, as it compiles down to a single pause instruction on x64 (Which seems to be all that one needs to implement SpinPause in HotSpot). I am less certain about the Windows/ARM64 implementation. There, YieldProcessor compiles down to dmb ishst; yield and I am unsure whether that is a correct SpinPause implementation. If need be, I can retract the ARM64 implementation and only have this for x86

The linux aarch64 code implements this as a call to a stub generated at runtime:

https://github.com/openjdk/jdk/blob/5995786dbd69ed11dd1cacb2a3ac86e3e6f43ab7/src/hotspot/os_cpu/linux_aarch64/os_linux_aarch64.cpp#L411

As this is using the macroassembler, etc, and not static inline assembly, I presume this could be ported to Windows.
I don't believe "dmb ishst" gives us the result we would like. While the YIELD instruction is targeted primarily for SMT CPUs, in practice it will effectively be a NOP .

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PR Comment: https://git.openjdk.org/jdk/pull/21781#issuecomment-2452102583