8371603: C2: Missing Ideal optimizations for load and store vectors on SVE

[XiaohongGong]

Problem:

This issue occurs on a 256-bit SVE machine, caused by the following problematic pattern in LoadVectorNode::Ideal():

Node* LoadVectorNode::Ideal(PhaseGVN* phase, bool can_reshape) {
  const TypeVect* vt = vect_type();
  if (Matcher::vector_needs_partial_operations(this, vt)) {
    return VectorNode::try_to_gen_masked_vector(phase, this, vt);
  }
  return LoadNode::Ideal(phase, can_reshape);
}

The condition Matcher::vector_needs_partial_operations(this, vt) returns true for LoadVectorNode with 256-bit vector size even when the vector size equals the maximum vector size on SVE. In such cases, when VectorNode::try_to_gen_masked_vector() returns nullptr, the method exits early without calling LoadNode::Ideal(). This results in missing crucial optimizations that would normally be applied by the superclass.

This code was introduced by https://bugs.openjdk.org/browse/JDK-8286941 to generate vector masks for partial vector operations, but it failed to ensure that the superclass Ideal() method is always invoked when no transformation is applied.

Solution:

This patch addresses the issue through two changes:

1. Refine Matcher::vector_needs_partial_operations() to return true only when the vector node genuinely represents a partial vector operation that requires masking.
2. Modify VectorNode::try_to_gen_masked_vector() to never return nullptr, ensuring the superclass Ideal() method is always invoked when no transformation is applied.

Testing:

• Verified on different SVE platforms with different vector sizes (128|256|512 bits).
• Verified on X86 platforms with different avx options (-XX:UseAVX=1|2|3).
• Added two new IR tests to verify 1) previously missing optimizations for LoadVector/StoreVector are now applied, and 2) that mask and the correct IR patterns are generated for partial vector operations.

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue

Issue

• JDK-8371603: C2: Missing Ideal optimizations for load and store vectors on SVE (Bug - P3)(⚠️ The fixVersion in this issue is [26] but the fixVersion in .jcheck/conf is 27, a new backport will be created when this pr is integrated.)

Reviewers

• Emanuel Peter (@eme64 - Reviewer)
• Eric Fang (@erifan - Author) Review applies to 6206e8c0
• Hao Sun (@shqking - Committer) Review applies to 6206e8c0

Contributors

• Emanuel Peter <epeter@openjdk.org>

Reviewing

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[bridgekeeper]

👋 Welcome back xgong! 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]

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

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8371603: C2: Missing Ideal optimizations for load and store vectors on SVE

Co-authored-by: Emanuel Peter <epeter@openjdk.org>
Reviewed-by: epeter, erfang, haosun

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 106 new commits pushed to the master branch:

• cba09cd: 5107379: Component orientation in JOptionPane is not proper in Motif L&F.
• 020e3f9: 8373293: Change the exception handling in TestNestHostErrorWithMultiThread.java
• 35fe0b1: 8372641: [s390x] Test failure TestMergeStores.java
• ... and 103 more: https://git.openjdk.org/jdk/compare/530493fed4066b1efcf3ec22253b110495767eca...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

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[XiaohongGong]

/contributor add @eme64

[openjdk]

@XiaohongGong
Contributor Emanuel Peter <epeter@openjdk.org> successfully added.

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[XiaohongGong]

Hi @eme64 , this is the fixing for the crash issue reported on aws machine. Could you please help take a look? Thanks a lot!

[mlbridge]

Webrevs

• 02: Full - Incremental (94017b66)
• 01: Full - Incremental (6206e8c0)
• 00: Full (ba7592cb)

[eme64]

@XiaohongGong Thanks for taking this over from me and fixing this so quickly, much appreciated!

Thanks for adding my regression tests and for the attribution :)

I only have a minor comment below. Otherwise, the code looks good to me. But since I'm not an SVE specialist, it would be good if someone with deeper knowledge would do a deeper review of the specific SVE parts.

Once an SVE specialist gives the approval for the PR, I'l run some internal testing and approve from my side :)

Ah, and one more thing: you should change the PR title to be more descriptive of the issue. The assert that was hit is only a far removed symptom. I would suggest:

C2 SVE: missing Ideal optimizations for load and store vectors

[XiaohongGong]

Ah, and one more thing: you should change the PR title to be more descriptive of the issue. The assert that was hit is only a far removed symptom. I would suggest:

C2 SVE: missing Ideal optimizations for load and store vectors

The new title looks good to me. Thanks!

[XiaohongGong]

Hi @eme64, I'v updated the patch based on your suggestion. Would you mind taking another look? Thanks!

[XiaohongGong]

Hi @theRealAph , would you mind taking a look at this patch especially the AArch64 part? Thanks a lot!

[XiaohongGong]

Hi @erifan @shqking , could you please help take a look at this PR? Thanks a lot!

[erifan]

LGTM, thanks for the fix.

[shqking]

Thanks for your work. LGTM.

[eme64]

Nice, thanks for the updates. I think this looks cleaner now.

I have one more nit below. But I'll run some sanity testing from my side already.

[eme64]

Tests pass. Thanks for the updates.

And thanks for fixing it so swiftly and for the attribution 😊

[XiaohongGong]

Thanks so much for your review @eme64 , @shqking and @erifan !

[XiaohongGong]

/integrate

[openjdk]

Going to push as commit b6732d6.
Since your change was applied there have been 122 commits pushed to the master branch:

• a262212: 8255463: java/nio/channels/spi/SelectorProvider/inheritedChannel/InheritedChannelTest.java failed with ThreadTimeoutException
• eef9813: 8371446: VectorAPI: Add unit tests for masks from various long values
• 7f9951a: 8373207: Make DeferredStatic class template constant initializable
• ... and 119 more: https://git.openjdk.org/jdk/compare/530493fed4066b1efcf3ec22253b110495767eca...master

Your commit was automatically rebased without conflicts.

[openjdk]

@XiaohongGong Pushed as commit b6732d6.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

[XiaohongGong]

/backport jdk26

[openjdk]

@XiaohongGong The target repository jdk26 is not a valid target for backports.
List of valid target repositories: openjdk/jdk, openjdk/jdk11u, openjdk/jdk11u-dev, openjdk/jdk17u, openjdk/jdk17u-dev, openjdk/jdk21u, openjdk/jdk21u-dev, openjdk/jdk25u, openjdk/jdk25u-dev, openjdk/jdk8u, openjdk/jdk8u-dev, openjdk/jfx, openjdk/jfx17u, openjdk/jfx21u, openjdk/jfx25u, openjdk/lilliput-jdk17u, openjdk/lilliput-jdk21u, openjdk/lilliput-jdk25u, openjdk/shenandoah-jdk21u, openjdk/shenandoah-jdk8u.
Supplying the organization/group prefix is optional.

There is a branch jdk26 in the current repository openjdk/jdk.
To target a backport to this branch in the current repository use:
/backport :jdk26

[XiaohongGong]

/backport :jdk26

[openjdk]

@XiaohongGong the backport was successfully created on the branch backport-XiaohongGong-b6732d60-jdk26 in my personal fork of openjdk/jdk. To create a pull request with this backport targeting openjdk/jdk:jdk26, just click the following link:

➡️ Create pull request

The title of the pull request is automatically filled in correctly and below you find a suggestion for the pull request body:

Hi all,

This pull request contains a backport of commit b6732d60 from the openjdk/jdk repository.

The commit being backported was authored by Xiaohong Gong on 10 Dec 2025 and was reviewed by Emanuel Peter, Eric Fang and Hao Sun.

Thanks!

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