RFR: 8321010: RISC-V: C2 RoundVF [v2]

[Hamlin Li]

On Wed, 21 Feb 2024 16:30:24 GMT, Hamlin Li <mli at openjdk.org> wrote:

>> Hamlin Li has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   fix space
>
>> Hello Hamlin, I recall you had licheepi board. I would be nice if you can try to measure rvv performance gain with this https://github.com/syntacore/syntaj21/tree/rvv0.7.1
>> 
>> This PR showed it's not always easy to win perf just by using rvv - #17413
>> 
>> I understand it might not be possible, but would be nice to give it a try (I can share hsdis with support for 0.7.1 if needed)
> 
> Had a internal discussion about your suggestion, seems 0.7.1 is not incompatible with 1.0/2.0, and for this simple intrinsic, we think a better path is to have it first, then re-visit it when we have real hardware to measure the performance later.

> @Hamlin-Li: Thanks for the quick update. Considering that saving/restoring for FRM could be expensive, I do wonder if we could gather some performance numbers before we go. I see people are now testing on RVV-1.0 hardwares [1] and I am also trying to get one (AFAIK, more powerful RVV-1.0 hardwares are also coming later this year, SG2044, SG2380, etc.). Also from discussion on [2], I see there are also other approaches available there without flipping the FP rounding mode. But I am not sure if they make sense for our case or work better without actual testing.
> 
> [1] [#18382 (comment)](https://github.com/openjdk/jdk/pull/18382#issuecomment-2045145255) [2] #8204

Thanks for the information, let me do some investigation on the solution in https://github.com/openjdk/jdk/pull/8204.

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PR Comment: https://git.openjdk.org/jdk/pull/17745#issuecomment-2069741101