8350483: AArch64: turn on signum intrinsics by default on Ampere CPUs

[cnqpzhang]

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 defect fixing in a manner, which can be a point to support this backport request too.

The commit is in jdk mainline and got successfully merged to jdk24u. It is of low risk as the patch is limited to Ampere CPUs only and well tested on Ampere-1A with related jmh and jtreg tier1 tests.

---

Progress

• Change must not contain extraneous whitespace
• JDK-8350483 needs maintainer approval
• Commit message must refer to an issue

Issue

• JDK-8350483: AArch64: turn on signum intrinsics by default on Ampere CPUs (Enhancement - P4 - Approved)

Reviewing

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$ git fetch https://git.openjdk.org/jdk21u-dev.git pull/1606/head:pull/1606
$ git checkout pull/1606

Update a local copy of the PR:
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$ git pull https://git.openjdk.org/jdk21u-dev.git pull/1606/head

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https://git.openjdk.org/jdk21u-dev/pull/1606.diff

Using Webrev

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[bridgekeeper]

👋 Welcome back qpzhang! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

@cnqpzhang This change now passes all automated pre-integration checks.

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8350483: AArch64: turn on signum intrinsics by default on Ampere CPUs

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[openjdk]

This backport pull request has now been updated with issue from the original commit.

[openjdk]

⚠️ @cnqpzhang This change is now ready for you to apply for maintainer approval. This can be done directly in each associated issue or by using the /approval command.

[mlbridge]

Webrevs

• 00: Full (7ee518d6)

[cnqpzhang]

/approval request the backport is straight-forward with the code changes safely confined to AArch64-port Ampere CPUs. These changes have been thoroughly verified on the Ampere-1A system, presenting a very low risk to the target jdk21u-dev branch. The fix is for not only for performance but also can be regarded as a function-wise patch. Because the worst-case performance comparison revealed a significant disparity between the patched and base versions, that the unpatched version taking 80~100x longer than normal cases, severely impacting the signum functions, so important to LTS versions. Thanks for approval.

[openjdk]

@cnqpzhang
8350483: The approval request has been created successfully.

[cnqpzhang]

/integrate

[openjdk]

@cnqpzhang
Your change (at version 7ee518d) is now ready to be sponsored by a Committer.

[mlbridge]

Mailing list message from Patrick Zhang OS on jdk-updates-dev:

Could any committer please help to sponsor this backport? It has been approved just now. Thanks.

Regards, Patrick

-----Original Message-----
From: jdk-updates-dev <jdk-updates-dev-retn at openjdk.org> On Behalf Of Patrick Zhang
Sent: Tuesday, April 8, 2025 17:03
To: jdk-updates-dev at openjdk.org
Subject: [jdk21u-dev] RFR: 8350483: AArch64: turn on signum intrinsics by default on Ampere CPUs

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 **defect** fixing in a manner, which can be a point to support this backport request too.

The commit is in jdk mainline and got successfully merged to jdk24u. It is of low risk as the patch is limited to Ampere CPUs only and well tested on Ampere-1A with related jmh and jtreg tier1 tests.

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

Commit messages:
- Backport 23389d08a2b5c0840d2b18fb2e74d2d112b3192d

Changes: https://git.openjdk.org/jdk21u-dev/pull/1606/files
Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1606&range=00
Issue: https://bugs.openjdk.org/browse/JDK-8350483
Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
Patch: https://git.openjdk.org/jdk21u-dev/pull/1606.diff
Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1606/head:pull/1606

PR: https://git.openjdk.org/jdk21u-dev/pull/1606

[theRealAph]

/sponsor

[openjdk]

Going to push as commit 3c0c998.

[openjdk]

@theRealAph @cnqpzhang Pushed as commit 3c0c998.

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