RFR: 8310949: RISC-V: Initialize UseUnalignedAccesses [v3]

[Vladimir Kempik]

On Wed, 28 Jun 2023 04:23:59 GMT, Fei Yang <fyang at openjdk.org> wrote:

>> Ludovic Henry has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   fixup! 8310949: RISC-V: Initialize UseUnalignedAccesses
>
> We will need some performance numbers to validate this change. We witnessed some obvious regressions of Unsafe.Put*Unaligned methods when enabling UseUnalignedAccesses on some platforms like HiFive Unmatched with slow unaligned accesses. My guess is that we might want enable UseUnalignedAccesses for platforms like T-Head with fast unaligned accesses (RISCV_HWPROBE_MISALIGNED_FAST). But that will still depend on the measured JMH numbers.
> 
> PS: JMH test for reference: https://cr.openjdk.org/~dzhang/TestUseUnalignedAccesses/

> Yes, that would be my suggestion. But @RealFYang's point still stands: we would still need to confirm that actually works well on Unmatched and other known RISC-V boards.

unmatched has misaligned emulator and slow
thead c910 core has fast misaligned access but it's stuck on linux 5.10 from vendor which don't have new hw_prober syscall

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PR Comment: https://git.openjdk.org/jdk/pull/14676#issuecomment-1611028954