RFR: 8344306: RISC-V: Add zicond

[Hamlin Li]

On Tue, 26 Nov 2024 13:19:29 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

>> Not, as we are only removing one in a thousand branches, the different would be so little that I don't think I can measure it in a reasoanble amount of iterations. I think you need something very synthetic, e.g. write a hot loop where we can remove a branch in it to measure it.
>
>> Seems the critical path when `+UseZicond` is longer than `-UseZicond`, I'm not sure how much impact the branch code will bring when `-UseZicond`.
> 
> As I said, those 4 instructions saves ~1 cycle compared branch in out-order cpu models.

Thanks for the discussion!
Overall speaking, I think the new patch with cmov is more promising in terms of performance (in terms of pipeline), although we can not enumerate all the use cases.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22386#discussion_r1858670339