[riscv-port] RFR: 8278994: riscv: RVC support [v14]

[Xiaolin Zheng]

On Mon, 10 Jan 2022 07:30:28 GMT, Xiaolin Zheng <xlinzheng at openjdk.org> wrote:

>> Hi team,
>> 
>> This patch includes the basic definition of the RVC instruction set and some cleanups. Tested a simple `test/hotspot/jtreg/compiler/` folder on qemu.
>> 
>> Using `<JAVA_HOME>/bin/java -XX:+UnlockExperimentalVMOptions -XX:+UseRVC -XX:+UnlockDiagnosticVMOptions -XX:+PrintAssembly -XX:PrintAssemblyOptions=no-aliases,numeric -XX:+PrintStubCode -XX:-TieredCompilation` could show RVC instructions.
>> 
>> Thanks,
>> Xiaolin
>
> Xiaolin Zheng has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Polish `nmethod_entry_barrier` and RISC-V CAS related comments

> 

I have tested this patch under whole `test/jdk` and `test/hotspot/jtreg` folders without Monday's change through last weekend on a Hifive Unleashed and only revealed [JDK-8279664](https://bugs.openjdk.java.net/browse/JDK-8279664), which has nothing to do with this patch, for the [RISC-V backend initial-load patch](https://github.com/openjdk/riscv-port/commit/7148c39832fe522bad91df77ef4b5c9371193c35) could also reveal this problem on the Unleashed board I'm currently using. This one is a bit weird and I need more time to dig deep into that.

About Monday's incremental changesets I tested `test/hotspot/jtreg/compiler`, `test/hotspot/jtreg/gc/z`, and `test/hotspot/jtreg/gc/shenandoah` with or without RVC and revealed no errors. Also SPECjbb2015 is tested and the result seems to have no regression found under RVC - the result on Unleashed remains a `max-jOPs` level of score 370 with minor fluctuations.

I think maybe this patch could safely move forward -- except I need to take a look at JDK-8279664 to find whether it is a board-related issue because I cannot reproduce it on C910 and qemu. And thanks for the scrupulous reviews and the delicious suggestions, Felix and Yadong. :-)

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PR: https://git.openjdk.java.net/riscv-port/pull/34