[riscv-port] RFR: 8278994: riscv: RVC support [v2]

[Xiaolin Zheng]

On Fri, 24 Dec 2021 07:55:24 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 three additional commits since the last revision:
> 
>  - Cover most RVC instructions by using CompressibleRegion to cover minimal functions in C2
>  - Revise as proposed comments, including
>    - Fix macros in assembler_riscv_c.hpp
>    - Remove UncompressibleRegion
>    - Modify comments
>    - Change names: C-Ext to RVC
>  - Enable RVC instructions (based on the basic patch)

Rebased and fixed conflicts with RVB(#37)'s initial load. Tested build and a simple spring-boot application with/without RVC.

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