RFR: 8330094: RISC-V: Save and restore FCSR in the call stub

[Hamlin Li]

On Mon, 15 Apr 2024 11:54:04 GMT, Fei Yang <fyang at openjdk.org> wrote:

>> It's fair to claim that `csrw` has side effects which may be more detrimental to performance, side effects that `csrr` that doesn't have (and can't as it's only reading).
>> 
>>> I didn't measure the difference.
>> 
>> I don't know how that affects current hardware, but remember that current hardware are in-order CPUs which may not be impacted by many of these performance problems.
>
> Another thing that is worth considering is that the `fflags` (exception flags) contained in `fcsr` is very likely to change after the Java call which does floating point calculations.

Should we replace read/write/comp of fcsr with frm only?

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PR Review Comment: https://git.openjdk.org/jdk/pull/18758#discussion_r1565753690