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

[Ludovic Henry]

On Tue, 16 Apr 2024 10:45:47 GMT, Vladimir Kempik <vkempik at openjdk.org> wrote:

>> @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) ?

Great to see there is no performance degradation on the current generation of hardware. That's a great motivator to get this change in with the branch as we already know that the branch is going to bring a performance improvement on next/other generations of hardware.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/18785#discussion_r1567150600