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

[Fei Yang]

On Thu, 3 Nov 2022 16:36:49 GMT, Ludovic Henry <luhenry at openjdk.org> wrote:

>> The OpenJDK supports generating prefetch instructions on most platforms. RISC-V supports through the Zicbop extension the use of prefetch instructions. We want to make sure we use these instructions whenever they are available.
>> 
>> It passes `hotspot:tier1` test suite
>
> Ludovic Henry has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - fixup! remove dead code
>  - remove dead code

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).

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