[riscv-port] RFR: 8278994: riscv: RVC support [v10]

[Fei Yang]

On Fri, 7 Jan 2022 08:36:47 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 with a new target base due to a merge or a rebase. The pull request now contains 14 commits:
> 
>  - Move RVC code to the proper location after rebasing (#42)
>  - Rename misc functions and change the positions of some comments
>  - Remove remaining macros as discussions
>  - Remain an 'minimum_alignment' unchanged
>  - Manually inline all macros into functions as discussions
>  - Remove assembler_riscv_c.hpp as discussions
>  - Remove COMPRESSIBLE & NOT_COMPRESSIBLE macros by adding one layer as discussions
>  - Fix remaining CEXT -> RVC
>  - Remove Alignment-related changes as discussions
>  - Update licenses to the new year
>  - ... and 4 more: https://git.openjdk.java.net/riscv-port/compare/c7944edf...2a6ff151

src/hotspot/cpu/riscv/gc/shared/barrierSetAssembler_riscv.cpp line 245:

> 243:   // RVC: RISCV's amoswap instructions need an alignment for the memory address it swaps
> 244:   //   when we reach here we may get a 2-byte alignment so need to align it
> 245:   __ align(4, nmethod_barrier_guard_offset());

Is it necessary to pass nmethod_barrier_guard_offset() as the "extra_offset" parameter here since this function will return (12 * instruction_size) which is multiple of 4 ?

src/hotspot/cpu/riscv/macroAssembler_riscv.cpp line 2954:

> 2952:   // 8 byte aligned after 3 intructions
> 2953:   // RVC: when we reach here we may get a 2-byte alignment so need to align it
> 2954:   align(wordSize, NativeCallTrampolineStub::data_offset);

Same question: Is there a reason to pass "NativeCallTrampolineStub::data_offset" as the "extra_offset" parameter here since this function will return (3 * NativeInstruction::instruction_size) which is multiple of 4 ?

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