RFR: 8295948: Support for Zicbop/prefetch instructions on RISC-V [v4]

[Ludovic Henry]

On Fri, 4 Nov 2022 08:26:51 GMT, Yadong Wang <yadongwang at openjdk.org> wrote:

>> src/hotspot/cpu/riscv/riscv.ad line 5190:
>> 
>>> 5188: 
>>> 5189: instruct prefetchalloc( memory mem ) %{
>>> 5190:   match(PrefetchAllocation mem);
>> 
>> PS: Should we also put this under control of option UseZicbop like you do in Prefetch::read/write? Did you checked whether those (prefetchalloc and Prefetch::read/write) will ever be used/called when AllocatePrefetchStyle is 0 (which is the case when UseZicbop is false).
>
> The generation of PrefetchAllocationNode is controlled by AllocatePrefetchStyle. It should be fine if UseZicbop is associated with this option.

Added `predicate(UseZicbop)` just in case.

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PR: https://git.openjdk.org/jdk/pull/10884