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

[Vladimir Kempik]

On Tue, 16 Apr 2024 10:12:36 GMT, Ludovic Henry <luhenry at openjdk.org> wrote:

>> @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
>
> @VladimirKempik again, happy to get input from more people on that document! :)

Here are results from c910 ( licheePi4a). using:
a) jni_blank as is
b) modified jni_blank where native func() does this:


int x = 2;
    asm
        ("csrw fcsr,%0\n\t"
        :
        : "r" (x)
        );

branchful - exactly this PR
branchless - this PR without csrr&beq

results:

no fcsr change in native code


branchless
Benchmark                       Mode  Cnt    Score   Error  Units
CallOverheadConstant.jni_blank  avgt   30  133.586 ? 1.431  ns/op
CallOverheadVirtual.jni_blank   avgt   30  131.715 ? 0.570  ns/op


branchful
Benchmark                       Mode  Cnt    Score   Error  Units
CallOverheadConstant.jni_blank  avgt   30  133.376 ? 1.491  ns/op
CallOverheadVirtual.jni_blank   avgt   30  133.560 ? 1.782  ns/op


fcsr changed to rdn in native code



branchless
Benchmark                       Mode  Cnt    Score   Error  Units
CallOverheadConstant.jni_blank  avgt   30  153.708 ? 1.191  ns/op
CallOverheadVirtual.jni_blank   avgt   30  150.653 ? 1.617  ns/op

branchful
Benchmark                       Mode  Cnt    Score    Error  Units
CallOverheadConstant.jni_blank  avgt   30  153.595 ?  0.759  ns/op
CallOverheadVirtual.jni_blank   avgt   30  149.992 ?  1.605  ns/op


Basically there are not difference here ( thanks to BranchPredictor), so why would you make code more complex (and require additional registers) ?

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PR Review Comment: https://git.openjdk.org/jdk/pull/18785#discussion_r1567141957