8347901: C2 should remove unused leaf / pure runtime calls

[tabjy]

Hi all,

This pull request contains a backport of commit ed70910b from the openjdk/jdk repository. This is a clean backport.

In addition to fixing leaf / pure runtime calls not eliminated when the result is not used, this commit is also necessary for backporting JDK-8378713, which is in process backporting to jdk26u and eventually this repo as well.

The commit being backported was authored by Marc Chevalier on 22 Jul 2025 and was reviewed by Tobias Hartmann and Vladimir Ivanov.

Thanks!

---

Progress

• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• JDK-8347901 needs maintainer approval

Issue

• JDK-8347901: C2 should remove unused leaf / pure runtime calls (Enhancement - P3 - Rejected)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk25u-dev.git pull/452/head:pull/452
$ git checkout pull/452

Update a local copy of the PR:
$ git checkout pull/452
$ git pull https://git.openjdk.org/jdk25u-dev.git pull/452/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 452

View PR using the GUI difftool:
$ git pr show -t 452

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk25u-dev/pull/452.diff

Using Webrev

Link to Webrev Comment

[bridgekeeper]

👋 Welcome back kxu! 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]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

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

[openjdk]

⚠️ @tabjy 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 (09853461)

[tabjy]

/approval request Fix Request 25u-dev
Allows removing unused leaf/pure runtime calls during IGVN. This is also a dependency for JDK-8378713 that provides necessary infrastructure for constant-folding call nodes. The latter resolves a performance regression.
Risk is low. The patch modifies C2's call node logic to identify and eliminate pure calls that do not have side effects and whose outputs are unused.
Applies cleanly. tier1 and tier2-ctw pass. Unfortunately I don't have the resources to run all of tier2.

[openjdk]

@tabjy
8347901: The approval request has been created successfully.