[riscv-port] RFR: 8278322: riscv: Support RVC: compressed instructions

[Yadong Wang]

On Thu, 9 Dec 2021 11:55:37 GMT, Xiaolin Zheng <xlinzheng at openjdk.org> wrote:

> > > Thank you for your explanation, Felix. :-) Seems we finally reach a consensus now.
> > > > I guess maybe the best places to do this would be C1 and C2.
> > >
> > >
> > > I think I could get your meaning this time.
> > > > I think we can then further consider the "CompressibleRegion" solution suggested by Yadong.
> > >
> > >
> > > Yadong's plan works for me. I am quite in favor of his strategy.
> >
> >
> > I am happy that we are agreed on this solution ;-) We need to be safe at this very early stage, so I would suggest UseRVC disabled by default like UseRVV even if the hardware RVC extension is there. I hope you could understand that. In the future, we will consider auto-enable UseRVC and UseRVV after enough testing and evaluation.
> > > I will implement this approach ASAP. Maybe next week I might open another PR including this implementation and the definition of compressed instructions presented in the `assembler_riscv_cext.hpp` part for a better review. I wonder if this is okay for you.
> > > I am going to close this PR, save it for future reference, and may open another PR and start from scratch. Before that, just one more question: would you mind my keeping definitions of C-Ext instructions into one separate file like `assembler_riscv_cext.hpp`, as the current implementation? I think we might better keep different RISCV extensions into different files.
> >
> >
> > Yes, I think `assembler_riscv_c.hpp` will do here.
>
> Totally agree -- Thanks again for your reviews. Gonna close this PR and the related issue.

@zhengxiaolinX I think a seperate file for rvc is a good choice, and maybe we should do the same thing for rvv later.
By the way, the Open/Closed principle may be suitable for your design. We can use a RVC_Assembler extend the Assembler, override all instructions that may be compressed, and then deliver to the super Assembler if the "compression" failed. Maybe you can swap the context easier when entering or leaving the "CompressedRegion". Just for suggestion, and thank you for your big effort.

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PR: https://git.openjdk.java.net/riscv-port/pull/24