[jdk24u] RFR: 8350483: AArch64: turn on signum intrinsics by default on Ampere CPUs

[duke]

On Tue, 4 Mar 2025 04:12:12 GMT, Patrick Zhang <qpzhang at openjdk.org> wrote:

> Backport the commit to set `-XX:+UseSignumIntrinsic` by default for Ampere CPUs. It is to fix performance problem observed on JMH cases `vm.compiler.Signum|java.lang.*MathBench.sig[nN]um*`. In the worst test cases, run speed is 1~2% of the expected (patched) and functions got severely impacted. So, the fix can be regarded not only a performance fix but also a function problem solving in a manner, which can be a point to support this backport request too.
> 
> The backporting is of low risk as the patch is limited to Ampere CPUs only and well verified on Ampere-1A with related jmh and jtreg tier1 tests.

@cnqpzhang 
Your change (at version 864e9f1edb12656092a653071e2f45fc5ba54a1b) is now ready to be sponsored by a Committer.

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PR Comment: https://git.openjdk.org/jdk24u/pull/101#issuecomment-2705628019