8386482: C2 CountedLoopConverter::filtered_type_from_dominators: assert(_base == Int) failed: Not an Int

[eme64]

This fixes another bug in the CountedLoopConverter::has_truncation_wrap optimization.

This issue was discovered by the template fuzzer: #31501

The issue here: we did not expect an unreachable state. But if we traverse several if conditions, their join type can become top, which means the phi type is impossible, the loop unreachable.

Fix: just return nullptr if we get an empty type.

Looks like this bug was created by #17508. Before that change, we could have TypeInt with lo > hi, and that would be considered an empty type, TypeInt::empty() == true. So it used to make sense to return TypeInt* from these kinds of optimizations, and they could return empty type that way. Now that's not possible any more. We would have to return top, and change the signatures to Type*. That would require a lot of effort, and I don't think that makes sense for a bug-fix. Returning nullptr just means we did not narrow the type, and that should always be safe.

---

• I confirm that I make this contribution in accordance with the OpenJDK Interim AI Policy.

---

Progress

• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• Change must be properly reviewed (2 reviews required, with at least 1 Reviewer, 1 Author)

Issue

• JDK-8386482: C2 CountedLoopConverter::filtered_type_from_dominators: assert(_base == Int) failed: Not an Int (Bug - P4)

Reviewers

• Vladimir Kozlov (@vnkozlov - Reviewer)
• Quan Anh Mai (@merykitty - Reviewer)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/31481/head:pull/31481
$ git checkout pull/31481

Update a local copy of the PR:
$ git checkout pull/31481
$ git pull https://git.openjdk.org/jdk.git pull/31481/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 31481

View PR using the GUI difftool:
$ git pr show -t 31481

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/31481.diff

Using Webrev

Link to Webrev Comment

[bridgekeeper]

👋 Welcome back epeter! 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]

@eme64 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:

8386482: C2 CountedLoopConverter::filtered_type_from_dominators: assert(_base == Int) failed: Not an Int

Reviewed-by: kvn, qamai

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 11 new commits pushed to the master branch:

• d3d67ec: 8385892: TestResidentSetSizeEvent fails with RuntimeException: Should be non-zero: expected 0 > 0
• c01b025: 8385899: G1: G1ConcurrentMarkThread::_state should be Atomic
• 3046dc2: 8386344: runtime/StackGuardPages/TestStackGuardPages build failure after JDK-8303612
• ... and 8 more: https://git.openjdk.org/jdk/compare/d46298d681f78d8f16ee4ed8031520dcd76d3c4c...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]

@eme64 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.

[openjdk]

The total number of required reviews for this PR has been set to 2 based on the presence of this label: hotspot-compiler. This can be overridden with the /reviewers command.

[mlbridge]

Webrevs

• 00: Full (c18bb658)

[vnkozlov]

Good.

[merykitty]

Looks good to me, thanks for making the change.

[eme64]

@merykitty @vnkozlov Thanks for the reviews and approvals!

/integrate

[openjdk]

Going to push as commit b9f7bd2.
Since your change was applied there have been 20 commits pushed to the master branch:

• d4acb51: 8386589: Permitting a preview subclass should not produce a preview note
• a13dd29: 8380390: Shenandoah: Missing store barrier when resetting bitmaps
• 0c81774: 8385732: Shenandoah: Penalize triggering heuristics for out-of-cycle degenerated
• ... and 17 more: https://git.openjdk.org/jdk/compare/d46298d681f78d8f16ee4ed8031520dcd76d3c4c...master

Your commit was automatically rebased without conflicts.

[openjdk]

@eme64 Pushed as commit b9f7bd2.

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