RFR: 8294366: RISC-V: Partially mark out incompressible regions [v2]

[Xiaolin Zheng]

On Sat, 8 Oct 2022 04:57:05 GMT, Fei Yang <fyang at openjdk.org> wrote:

>> Xiaolin Zheng 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 10 additional commits since the last revision:
>> 
>>  - Plural form
>>  - comment polishment
>>  - patchable nop's code style
>>  - Merge remote-tracking branch 'github-openjdk/master' into riscv-rvc-checkin-first-half-part
>>  - Merge remote-tracking branch 'github-openjdk/master' into riscv-rvc-checkin-first-half-part
>>  - [5] RVC: IncompressibleRegions for patchable labels
>>  - [4] RVC: IncompressibleRegions for fixed length code slices
>>  - [3] RVC: IncompressibleRegions for patchable nops
>>  - [2] RVC: Disable auto transformations for branch instructions
>>  - [1] RVC: Add the IncompressibleRegion
>
> Updated change looks good. Thanks.

Thank @RealFYang and @yadongw for taking the time to review this!
Tests on QEMU seem okay.

An integration first so that it won't be blocked by myself.

> src/hotspot/cpu/riscv/assembler_riscv.hpp line 2113:
> 
>> 2111: //
>> 2112: // 4. Using -XX:PrintAssemblyOptions=no-aliases could distinguish RVC instructions from
>> 2113: //    normal ones.
> 
> One more suggestion for this code comment:
> 
> // ========================================
> // RISC-V Compressed Instructions Extension
> // ========================================
> // Note:
> // 1. Assembler functions encoding 16-bit compressed instructions always begin with a 'c_'
> //    prefix, such as 'c_add'. Correspondingly, assembler functions encoding normal 32-bit
> //    instructions with begin with a '_' prefix, such as "_add". Most of time users have no
> //    need to explicitly emit these compressed instructions. Instead, they still use unified
> //    wrappers such as 'add' which do the compressing work through 'c_add' depending on the
> //    the operands of the instruction and availability of the RVC hardware extension.
> //
> // 2. 'CompressibleRegion' and 'IncompressibleRegion' are introduced to mark assembler scopes
> //     within which instructions are qualified or unqualified to be compressed into their 16-bit
> //     versions. An example:
> //
> //      CompressibleRegion cr(_masm);
> //      __ add(...);       // this instruction will be compressed into 'c.and' when possible
> //      {
> //         IncompressibleRegion ir(_masm);
> //         __ add(...);    // this instruction will not be compressed
> //         {
> //            CompressibleRegion cr(_masm);
> //            __ add(...); // this instruction will be compressed into 'c.and' when possible
> //         }
> //      }
> //
> // 3. When printing JIT assembly code, using -XX:PrintAssemblyOptions=no-aliases could help
> //    distinguish compressed 16-bit instructions from normal 32-bit ones.

Thanks for the polish and it looks more concise than before. The other one, the code style of nop is fixed as well.

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PR: https://git.openjdk.org/jdk/pull/10421