RFR: 8330266: RISC-V: Restore frm to RoundingMode::rne after JNI [v2]

[Vladimir Kempik]

On Tue, 16 Apr 2024 09:48:48 GMT, Ludovic Henry <luhenry at openjdk.org> wrote:

>> Thanks for the links, however there are no performance claims in that discussion, only few "maybes" for "some hardware", can we check on existing h/w ( c910, u74) ? with jmh test doing dummy jni calls ?
>
> @VladimirKempik https://riscv-optimization-guide.riseproject.dev/#_controlling_rounding_behavior_scalar is written by multiple industry player who know how their hardware is going to behave, and it is confirmed that it will be more performant to have this `frrm`+`beq` than always doing `fsrm`. If you disagree with the wording in that guide, you're welcome to open a discussion on https://gitlab.com/riseproject/riscv-optimization-guide.

@luhenry I've already saw a big criticism of that document. So it makes me take it with a grain of salt. It was written by a limited group of vendors ( not all of them participated) and basically shows  u-arch peculiarities of limited hw group.

I'm gonna check few version of this patch in jni_blank jmh microtest

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PR Review Comment: https://git.openjdk.org/jdk/pull/18785#discussion_r1567100036