8373524: C2: no reachable node should have no use

[rwestrel]

The failure occurs because PhiNode::Ideal uses set_req to update
an input of a Phi. That causes the previous input to be disconnected
but because of the use of set_req, the previous input that has no
use is not enqueued for igvn to be reclaimed. The fix is to use
set_req_X instead. I replaced uses of set_req with set_req_X in
PhiNode::Ideal where I thought it made sense.

---

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-8373524: C2: no reachable node should have no use (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

• Christian Hagedorn (@chhagedorn - Reviewer)
• Manuel Hässig (@mhaessig - Committer)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/28841/head:pull/28841
$ git checkout pull/28841

Update a local copy of the PR:
$ git checkout pull/28841
$ git pull https://git.openjdk.org/jdk.git pull/28841/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 28841

View PR using the GUI difftool:
$ git pr show -t 28841

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/28841.diff

Using Webrev

Link to Webrev Comment

[bridgekeeper]

👋 Welcome back roland! 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]

@rwestrel 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:

8373524: C2: no reachable node should have no use

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 71 new commits pushed to the master branch:

• 360777c: 8371948: TestStackOverflowDuringInit.java fails xss too small on linux-aarch64
• f0add88: 8373837: "assert(thread->is_VM_thread() || thread->is_Java_thread()) failed: Must be VMThread or JavaThread" on macosx-aarch64-26
• 6231646: 8373208: Make Atomic class template constant initializable
• ... and 68 more: https://git.openjdk.org/jdk/compare/ad29642d8f4e8e0fb1223b14b85ab7841d7b1b51...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]

@rwestrel 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

• 02: Full - Incremental (ac789382)
• 01: Full - Incremental (9f51aa5b)
• 00: Full (ab262397)

[chhagedorn]

Otherwise, looks good!

[chhagedorn]

Since this is about IGVN, you could move the test to compiler/igvn.

[rwestrel]

Right. That makes sense. Done in the new commits.

[mhaessig]

Thank you for fixing this, @rwestrel. The changes look good. I also kicked off some testing.

[mhaessig]

Testing passed and the new changes look good as well.

[rwestrel]

@chhagedorn @mhaessig thanks for the reviews and testing.

[rwestrel]

/integrate

[openjdk]

@rwestrel This pull request has not yet been marked as ready for integration.

[chhagedorn]

Update looks good, thanks!

[rwestrel]

@chhagedorn thanks for re-approving.

[rwestrel]

/integrate

[openjdk]

Going to push as commit e72f205.
Since your change was applied there have been 71 commits pushed to the master branch:

• 360777c: 8371948: TestStackOverflowDuringInit.java fails xss too small on linux-aarch64
• f0add88: 8373837: "assert(thread->is_VM_thread() || thread->is_Java_thread()) failed: Must be VMThread or JavaThread" on macosx-aarch64-26
• 6231646: 8373208: Make Atomic class template constant initializable
• ... and 68 more: https://git.openjdk.org/jdk/compare/ad29642d8f4e8e0fb1223b14b85ab7841d7b1b51...master

Your commit was automatically rebased without conflicts.

[openjdk]

@rwestrel Pushed as commit e72f205.

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

[rwestrel]

/backport :jdk26

[openjdk]

@rwestrel the backport was successfully created on the branch backport-rwestrel-e72f205a-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 e72f205a from the openjdk/jdk repository.

The commit being backported was authored by Roland Westrelin on 19 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-rwestrel-e72f205a-jdk26:backport-rwestrel-e72f205a-jdk26
$ git checkout backport-rwestrel-e72f205a-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-rwestrel-e72f205a-jdk26