RFR: 8334396: RISC-V: verify perf of ReverseBytesI/L

[Fei Yang]

On Mon, 17 Jun 2024 14:38:37 GMT, Hamlin Li <mli at openjdk.org> wrote:

> Hi,
> Can you help to review the patch?
> 
> The test data below shows that, if zbb is not supported on current hardware, then intrinsic brings perf regression rather than benefit.
> So, ReverseBytesI/L should only be enabled when zbb is supported.
> 
> ## Test data
> existing benchmark:
> test/micro/org/openjdk/bench/java/lang/Longs.java, reverseBytes()
> test/micro/org/openjdk/bench/java/lang/Integers.java, reverseBytes()
> 
> all tests running below with rvv disabled.
> tested on K230-CanMV
> 
> 1. -XX:-UseZbb, intrinsic enabled by default
> o.o.b.j.lang.Integers.reverseBytes N/A 500 avgt 6128.125 ns/op
> o.o.b.j.lang.Longs.reverseBytes N/A 500 avgt 10807.930 ns/op
> 
> 2. -XX:+UseZbb, intrinsic enabled by default
> o.o.b.j.lang.Integers.reverseBytes N/A 500 avgt 1788.990 ns/op
> o.o.b.j.lang.Longs.reverseBytes N/A 500 avgt 1113.734 ns/op
> 
> 3. -XX:-UseZbb, and disable ReverseBytesI/L instrinsic
> o.o.b.j.lang.Integers.reverseBytes N/A 500 avgt 3552.902 ns/op
> o.o.b.j.lang.Longs.reverseBytes N/A 500 avgt 4586.980 ns/op

Good catch! Did you check the friends ReverseBytesS/ReverseBytesUS? It's very likely that they bear a similar peformance issue.

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PR Review: https://git.openjdk.org/jdk/pull/19750#pullrequestreview-2124645434