8371881: C2: Fix potential SEGV in VTransformReductionVectorNode tracing

[vish-chan]

This PR fixes a potential SEGV and removes dead code:
• Fix: Prevent potential SEGV in VTransformReductionVectorNode - [from @shipilev] This fixes a crash in diagnostic code when isa_ReductionVector() unexpectedly returns nullptr. While this indicates the graph is already corrupted, the additional crash in TRACE_OPTIMIZE makes debugging harder. The fix adds defensive null checking to prevent the diagnostic crash and improve error handling.

• Cleanup: Remove unused first_red variable

---

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-8371881: C2: Fix potential SEGV in VTransformReductionVectorNode tracing (Enhancement - P5)

Reviewers

• Aleksey Shipilev (@shipilev - Reviewer)
• Emanuel Peter (@eme64 - Reviewer)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/28323/head:pull/28323
$ git checkout pull/28323

Update a local copy of the PR:
$ git checkout pull/28323
$ git pull https://git.openjdk.org/jdk.git pull/28323/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 28323

View PR using the GUI difftool:
$ git pr show -t 28323

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/28323.diff

Using Webrev

Link to Webrev Comment

[bridgekeeper]

👋 Welcome back vish-chan! 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]

@vish-chan 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:

8371881: C2: Fix potential SEGV in VTransformReductionVectorNode tracing

Reviewed-by: shade, epeter

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At the time when this comment was updated there had been 27 new commits pushed to the master branch:

• 09b25cd: 8371465: Parallel: Revise asserts around heap expansion
• 69e3024: 8349157: Unnecessary Hashtable usage in XKeysym.javaKeycode2KeysymHash
• 970533d: 8371779: Replace MemTagBitmap with ResourceBitMap
• ... and 24 more: https://git.openjdk.org/jdk/compare/00f2c38e373f5ae58ad6593cc7b9d53b9596eb17...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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[openjdk]

@vish-chan The following label will be automatically applied to this pull request:

• hotspot-compiler

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[mlbridge]

Webrevs

• 01: Full - Incremental (0824593d)
• 00: Full (f6947173)

[shipilev]

Looks fine, but @eme64 should take a look as well.

/reviewers 2

[openjdk]

@shipilev
The total number of required reviews for this PR (including the jcheck configuration and the last /reviewers command) is now set to 2 (with at least 1 Reviewer, 1 Author).

[eme64]

@vish-chan @shipilev thanks for working on this!

Looks reasonable to me :)

[vish-chan]

@eme64 I'm not sure but your approval might be required.

[eme64]

Sorry, missed the swap from "Comment" to "Approve" 🙈

[vish-chan]

/integrate

[openjdk]

@vish-chan
Your change (at version 0824593) is now ready to be sponsored by a Committer.