8373525: C2: assert(_base == Long) failed: Not a Long

[dafedafe]

Issue

Olivier's fuzzer found a test that makes C2 crash while running the optimization that collapses the addition with overflow-protection (fold_subI_no_underflow_pattern).

Causes

The crash happens because during fold_subI_no_underflow_pattern the first input of the AddL node (see comment below) becomes top.

jdk/src/hotspot/share/opto/addnode.cpp

Lines 1525 to 1533 in 82b04f0

	// x con2
	// | |
	// AddL clamp2
	// | |
	// Max/MinL con1
	// | |
	// AddL clamp1
	// | |
	// Max/MinL (n)

This happens because of a whole IfFalse subgraph that dies and nodes are being removed. AddL is not removed immediately as it has another input which is still alive but it is put in the IGVN worklist instead.

Unfortunately the fold_subI_no_underflow_pattern optimization runs before the next GVN pass and triggers the assert.

Fix

fold_subI_no_underflow_pattern should actually take into account that we could have the graph in such a state and that x could be top. So, the sensible fix is not to presume x to be of type long and bailout if it is not.

Testing

Tier 1-3+
(also checked for new regression test failure before the change)

---

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-8373525: C2: assert(_base == Long) failed: Not a Long (Bug - P3)

Reviewers

• Manuel Hässig (@mhaessig - Committer) Review applies to 3219ce4f
• Christian Hagedorn (@chhagedorn - Reviewer)

Reviewing

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$ git checkout pull/28920

Update a local copy of the PR:
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$ git pull https://git.openjdk.org/jdk.git pull/28920/head

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$ git pr checkout 28920

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Using Webrev

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[bridgekeeper]

👋 Welcome back dfenacci! 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]

@dafedafe 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:

8373525: C2: assert(_base == Long) failed: Not a Long

Reviewed-by: chagedorn, mhaessig

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 145 new commits pushed to the master branch:

• 551e656: 8374113: Taughtological if check in Reflection::array_set
• e6c3ebe: 8374145: Remove legacy locking remnants from markWord
• 3579c75: 8373876: StackWalkNativeToJava print more output in case of failures
• ... and 142 more: https://git.openjdk.org/jdk/compare/74dca863c2e61c13884c3454b8da7be125235970...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.

➡️ To integrate this PR with the above commit message to the master branch, type /integrate in a new comment.

[openjdk]

@dafedafe The following label will be automatically applied to this pull request:

• hotspot-compiler

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

[mlbridge]

Webrevs

• 03: Full - Incremental (a5e8eb6d)
• 02: Full - Incremental (57f15854)
• 01: Full - Incremental (3219ce4f)
• 00: Full (20fe00fd)

[mhaessig]

Thank you for fixing this, @dafedafe. The fix looks good. I only have some minor comments on the test.

[mhaessig]

Thanks for addressing my comments. Looks good to me.

[chhagedorn]

Otherwise, looks good to me, too, thanks!

[dafedafe]

Otherwise, looks good to me, too, thanks!

Thanks for your suggestions @chhagedorn. I've just addressed them.

[chhagedorn]

Thanks for the update!

[dafedafe]

Thank you for your reviews @mhaessig @chhagedorn.

[dafedafe]

/integrate

[openjdk]

Going to push as commit a61a1d3.
Since your change was applied there have been 146 commits pushed to the master branch:

• 2715f5e: 8374151: Cleanup minor markWord function disorder
• 551e656: 8374113: Taughtological if check in Reflection::array_set
• e6c3ebe: 8374145: Remove legacy locking remnants from markWord
• ... and 143 more: https://git.openjdk.org/jdk/compare/74dca863c2e61c13884c3454b8da7be125235970...master

Your commit was automatically rebased without conflicts.

[openjdk]

@dafedafe Pushed as commit a61a1d3.

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

[dafedafe]

/backport :jdk26

[openjdk]

@dafedafe the backport was successfully created on the branch backport-dafedafe-a61a1d32-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 a61a1d32 from the openjdk/jdk repository.

The commit being backported was authored by Damon Fenacci on 22 Dec 2025 and was reviewed by Christian Hagedorn and Manuel Hässig.

Thanks!

If you need to update the source branch of the pull then run the following commands in a local clone of your personal fork of openjdk/jdk:

$ git fetch https://github.com/openjdk-bots/jdk.git backport-dafedafe-a61a1d32-jdk26:backport-dafedafe-a61a1d32-jdk26
$ git checkout backport-dafedafe-a61a1d32-jdk26
# make changes
$ git add paths/to/changed/files
$ git commit --message 'Describe additional changes made'
$ git push https://github.com/openjdk-bots/jdk.git backport-dafedafe-a61a1d32-jdk26