RFR: 8321010: RISC-V: C2 RoundVF [v3]

[Hamlin Li]

On Wed, 10 Apr 2024 13:07:47 GMT, Hamlin Li <mli at openjdk.org> wrote:

>> Ah, I see. I don't think it's safe for `vsub_fp`, `vdiv_fp`, etc to depend on some uncertain dynamic rounding mode on Java code entry. And it seems that the RISC-V spec doesn't even specify a default dynamic rounding mode in `frm` at the ISA level. This reminds me that we might be lacking some pieces of the puzzle.
>> 
>> If we want to keep `RNE` (Round to Nearest) as a default dynamic rounding mode for Java code (like you do in this PR), we will also need to save and restore the floating-point control state perfering `RNE` when we enter and leave Java code. Something similar for aarch64: https://bugs.openjdk.org/browse/JDK-8319973. This will also help eliminate existing explict settings of dynamic rounding mode to `RNE` in file riscv_v.ad (grep "csrwi(CSR_FRM"), which should be good for performance. I would suggest we fix this in another PR before we continue with this one. Let me know if you are interested : -)
>
> Thanks for discussion.
> Sure. Let me do some investigation and fix it first.

tracked by https://bugs.openjdk.org/browse/JDK-8330094

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PR Review Comment: https://git.openjdk.org/jdk/pull/17745#discussion_r1560820723