RFR: 8326306: RISC-V: Re-structure MASM calls and jumps [v10]

[Robbin Ehn]

On Fri, 10 May 2024 08:00:49 GMT, Fei Yang <fyang at openjdk.org> wrote:

>> Robbin Ehn has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 12 additional commits since the last revision:
>> 
>>  - Merge branch 'master' into jal-fixes
>>  - Revert JNI field, call()->li()
>>  - Use li instead of movptr for call
>>  - REVERT: Use li instead of movptr
>>  - Use li instead of movptr
>>  - VM leaf should use li
>>  - Merge branch 'master' into jal-fixes
>>  - Merge branch 'master' into jal-fixes
>>  - Merge branch 'master' into jal-fixes
>>  - Corrected method name
>>  - ... and 2 more: https://git.openjdk.org/jdk/compare/3438e4d1...d53e9694
>
> src/hotspot/cpu/riscv/assembler_riscv.hpp line 2836:
> 
>> 2834:         Rd == x0 &&
>> 2835:         is_simm12(offset) && ((offset % 2) == 0)) {
>> 2836:       c_j(offset);
> 
> Is RV32C-only instructions usable for RV64C which is our case? Or will this if block be test covered?
> 
> The spec says:
> 
> In addition, RV32C includes a compressed jump and link instruction to compress
> short-range subroutine calls, where the same opcode is used to compress ADDIW for RV64C and
> RV128C.

There are two instructions using the CJ format: c.j and c.jal.
- c.j is already in our assembler.
- c.jal is not in our assembler and is RV32C only.


000000000000064e <c_j>:
     64e: 11 a0        	j	0x652 <c_j+0x4>  // 0xa011 is c.j 4
     650: 2a 85        	mv	a0, a0
     652: 82 80        	ret


Works fine on both VF2 and qemu-rv64.

The comment was just saying that we can't try to map to c.jal since it's RV32C.

The instruction c.j have the same test coverage as before (as we already have it in assembler).
The toogle to C I think is untested as it only can be generated for short backwards branches in non-relocated code.

As the comment is obvious confusing, suggestions to change it?

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/18942#discussion_r1597965012