[jdk17u-dev] RFR: 8350483: AArch64: turn on signum intrinsics by default on Ampere CPUs

[Patrick Zhang]

On Tue, 1 Apr 2025 07:58:33 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*`.  For example, `vm.compiler.Signum._1_signumFloatTest` thrpt score becomes 30x better on both jdk mainline and jdk17u-dev. The backporting can be very safe as it is limited to Ampere CPUs only and well verified on Ampere-1A with related jmh and jtreg tier1 tests.
>
> Status update: backport to 24 has been done (merged), and the operation to 21 is waiting for a dependent change, and 17 would follow up.

> Hi @cnqpzhang Thanks for backporting to 21 and 24. One question: is this a regression? Can you name the change that cause the performance regression? Or is this a true optimization.

Not a regression, the option was only enabled for some specific CPUs while this commit/backport extends its benefit to more.

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PR Comment: https://git.openjdk.org/jdk17u-dev/pull/3300#issuecomment-2800407206