RFR: 8350663: AArch64: Enable UseSignumIntrinsic by default

[Patrick Zhang]

On Tue, 4 Mar 2025 11:33:05 GMT, Patrick Zhang <qpzhang at openjdk.org> wrote:

> According to tests on Arm CPUs Neoverse-N1/N2/V1/V2 and Ampere-Altra/AmpereOne, `-XX:+UseSignumIntrinsic` can provide consistent positive performance boost on singnum microbenchmarks (1-4,5,7 in below list) and no obvious regression (ops/s change <0.1%) on other relevant tests (6,9-12). In addition, "_Apple M1 shows no regression with signum intrinsics_" (verified by @theRealAph). So, it can be the time to enable this UseSignumIntrinsic flag by default for aarch64-port. By the way, x86 and riscv ports have already configured it on by default. 
> 
> Tests: passed JTReg tier1 tests on Ampere-1A, no regression found, and particularly checked test results of two signum cases (13,14), both are in good state.
> 
> 
> 1. org.openjdk.bench.java.lang.MathBench.signumDouble
> 2. org.openjdk.bench.java.lang.MathBench.signumFloat
> 3. org.openjdk.bench.java.lang.StrictMathBench.sigNumDouble
> 4. org.openjdk.bench.java.lang.StrictMathBench.signumFloat
> 5. org.openjdk.bench.vm.compiler.Signum._1_signumFloatTest
> 6. org.openjdk.bench.vm.compiler.Signum._2_overheadFloat
> 7. org.openjdk.bench.vm.compiler.Signum._3_signumDoubleTest
> 8. org.openjdk.bench.vm.compiler.Signum._4_overheadDouble
> 9. org.openjdk.bench.vm.compiler.Signum._5_copySignFloatTest
> 10. org.openjdk.bench.vm.compiler.Signum._6_overheadCopySignFloat
> 11. org.openjdk.bench.vm.compiler.Signum._7_copySignDoubleTest
> 12. org.openjdk.bench.vm.compiler.Signum._8_overheadCopySignDouble
> 13. JTReg: compiler/vectorization/TestSignumVector.java
> 14. JTReg: compiler/intrinsics/math/TestSignumIntrinsic.java

Hi @theRealAph Could you please help review this? Thanks.

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PR Comment: https://git.openjdk.org/jdk/pull/23893#issuecomment-2731461554