RFR: 8320069: RISC-V: Add Zcb instructions

[Robbin Ehn]

On Mon, 18 Dec 2023 15:10:13 GMT, Vladimir Kempik <vkempik at openjdk.org> wrote:

> We already have "macroses" for load and stores in macroAssembler_riscv.hpp, what's the reason to do compression decision in assembler_riscv.hpp instead ( not saying it's wrong) ?
> 
> https://github.com/openjdk/jdk/blob/38d94725a1a85156e30b72b325886b0e25d4db03/src/hotspot/cpu/riscv/macroAssembler_riscv.hpp#L880

No, you are correct I also think this is not optimal. I don't know the background, but it seems like this is the easiest way to add compressed transparently. But to fully utilize C instruction we should favor the x8->x15, we often don't get C due to e.g. BCP is in x22. I think to be able to better utilize C we can't have it so transparent.

So here I just try to follow the current code, see how lw is changed to c_lw.

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PR Comment: https://git.openjdk.org/jdk/pull/17122#issuecomment-1862283903