8295116: C2: assert(dead->outcnt() == 0 && !dead->is_top()) failed: node must be dead

[chhagedorn]

In IfNode::fold_compares_helper(), we merge two immediately following If nodes with a CmpI that share the left input into a single If with CmpU.

In this case here, a graph is currently dying. The If nodes are not yet removed but data was already folded in such a way that the graph currently looks like this when trying to apply IfNode::fold_compares():

In IfNode::is_ctrl_folds() we check if the dominating If is suited to apply this optimization. Here we check if the left inputs of the CmpI nodes are the same which is indeed the case because both CmpI have top as left input:

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

Lines 734 to 736 in 46cd457

	// Must compare same value
	ctrl->in(0)->in(1)->in(1)->in(1) != NULL &&
	ctrl->in(0)->in(1)->in(1)->in(1) == in(1)->in(1)->in(1);

We start merging 1641 If and 1657 If. During this process, we try to remove dead nodes that are no longer used:

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

Lines 1042 to 1044 in 46cd457

	if (adjusted_val->outcnt() == 0) {
	igvn->remove_dead_node(adjusted_val);
	}

But in this specific setup, adjusted_val is top and we try to remove it because outcnt() for top is zero. This results in the assertion failure.

The fix I propose is to not only check for equality of the left inputs in IfNode::is_ctrl_folds() but also check if one of them is top.

I was only able to reproduce this bug with a replay file and a very specific seed when using -XX:+StressIGVN.

Thanks,
Christian

---

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Issue

• JDK-8295116: C2: assert(dead->outcnt() == 0 && !dead->is_top()) failed: node must be dead

Reviewers

• Tobias Hartmann (@TobiHartmann - Reviewer)
• Roberto Castañeda Lozano (@robcasloz - Reviewer)

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[mlbridge]

Webrevs

• 00: Full (3d6aef94)

[TobiHartmann]

Looks good and trivial.

[robcasloz]

Nice analysis, looks good!

[openjdk]

@chhagedorn 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:

8295116: C2: assert(dead->outcnt() == 0 && !dead->is_top()) failed: node must be dead

Reviewed-by: thartmann, rcastanedalo

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At the time when this comment was updated there had been 18 new commits pushed to the master branch:

• b9346e1: 8298033: Character.codePoint{At|Before}(char[], int, int) doesn't do JavaDoc-specified check
• 297bf6a: 8287397: Print top-level exception when snippet fails to read file
• 073897c: 8294588: Auto vectorize half precision floating point conversion APIs
• 46cd457: 8298341: Ensure heap growth in TestNativeMemoryUsageEvents.java
• 1166c8e: 8296896: Change virtual Thread.yield to use external submit
• 5175965: 8298323: trivial typo in JOptionPane.OK_OPTION
• d5cf18e: 8296198: JFileChooser throws InternalError java.lang.InternalError with Windows shortcuts
• 74f346b: 8298075: RISC-V: Implement post-call NOPs
• 3aa4070: 8294047: HttpResponseInputStream swallows interrupts
• af8fb7e: 8282578: AIOOBE in javax.sound.sampled.Clip
• ... and 8 more: https://git.openjdk.org/jdk/compare/389b8f4b788375821a8bb4b017e50f905abdad2d...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.

[chhagedorn]

Thanks Tobias and Roberto for the quick reviews! I'll integrate this now to get it in before the fork.

[chhagedorn]

/integrate

[openjdk]

Going to push as commit 94575d1.
Since your change was applied there have been 20 commits pushed to the master branch:

• 49b8622: 8290850: C2: create_new_if_for_predicate() does not clone pinned phi input nodes resulting in a broken graph
• 2f426cd: 8298375: Bad copyright header in test/jdk/java/lang/Character/Supplementary.java
• b9346e1: 8298033: Character.codePoint{At|Before}(char[], int, int) doesn't do JavaDoc-specified check
• 297bf6a: 8287397: Print top-level exception when snippet fails to read file
• 073897c: 8294588: Auto vectorize half precision floating point conversion APIs
• 46cd457: 8298341: Ensure heap growth in TestNativeMemoryUsageEvents.java
• 1166c8e: 8296896: Change virtual Thread.yield to use external submit
• 5175965: 8298323: trivial typo in JOptionPane.OK_OPTION
• d5cf18e: 8296198: JFileChooser throws InternalError java.lang.InternalError with Windows shortcuts
• 74f346b: 8298075: RISC-V: Implement post-call NOPs
• ... and 10 more: https://git.openjdk.org/jdk/compare/389b8f4b788375821a8bb4b017e50f905abdad2d...master

Your commit was automatically rebased without conflicts.

[openjdk]

@chhagedorn Pushed as commit 94575d1.

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