8271056: C2: "assert(no_dead_loop) failed: dead loop detected" due to cmoving identity

[chhagedorn]

In the testcase, an unsafe cmoving identity is applied in PhiNode::Identity() after parsing which replaces a loop phi in a dead loop creating a dead data loop which triggers the assertion. The problem is that PhiNode::Identity() assumes that a cmoving identity is always safe because PhiNode::Ideal() handles unsafe cases and only leaves safe cases to PhiNode::Identity():

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

Lines 2051 to 2055 in 4c3491b

	// Check for CMove'ing identity. If it would be unsafe,
	// handle it here. In the safe case, let Identity handle it.
	Node* unsafe_id = is_cmove_id(phase, true_path);
	if( unsafe_id != NULL && is_unsafe_data_reference(unsafe_id) )
	opt = unsafe_id;

However, the fix for JDK-8268883 added the following additional condition to wait for the region to be processed:

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

Lines 2047 to 2053 in 4c3491b

	// If one of the diamond's branch is in the process of dying then, the Phi's input for that branch might transform
	// to top. If that happens replacing the Phi with an operation that consumes the Phi's inputs will cause the Phi
	// to be replaced by top. To prevent that, delay the transformation until the branch has a chance to be removed.
	!(can_reshape && wait_for_region_igvn(phase))) {
	// Check for CMove'ing identity. If it would be unsafe,
	// handle it here. In the safe case, let Identity handle it.
	Node* unsafe_id = is_cmove_id(phase, true_path);

This skips the process of an unsafe case in PhiNode::Ideal() in the testcase. Afterwards, the unsafe case is replaced unconditionally in PhiNode::Identity() resulting in a dead data loop.

I therefore propose to add the same check added in JDK-8268883 to PhiNode::Identity() to prevent that.

Thanks,
Christian

---

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Issue

• JDK-8271056: C2: "assert(no_dead_loop) failed: dead loop detected" due to cmoving identity

Reviewers

• Vladimir Kozlov (@vnkozlov - Reviewer)
• Tobias Hartmann (@TobiHartmann - Reviewer)

Reviewing

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[mlbridge]

Webrevs

• 00: Full (fd97ac9d)

[vnkozlov]

Looks good. Thanks!

[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:

8271056: C2: "assert(no_dead_loop) failed: dead loop detected" due to cmoving identity

Reviewed-by: kvn, thartmann

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 278 new commits pushed to the master branch:

• 0395e4e: 8276768: Snippet copy feature should use button instead of link
• d8b0dee: 8276239: Better tables in java.util.random package summary
• fc0fe25: 8273235: tools/launcher/HelpFlagsTest.java Fails on Windows 32bit
• 3934fe5: 8275847: Scheduling fails with "too many D-U pinch points" on small method
• 44047f8: 8274328: C2: Redundant CFG edges fixup in block ordering
• 8849154: 8275846: read_base_archive_name() could read past the end of buffer
• 2653cfb: 8276688: Remove JLinkReproducibleXXXTest from ProblemList.txt
• c7095b2: 8276207: Properties.loadFromXML/storeToXML works incorrectly for supplementary characters
• ed7ecca: 8254739: G1: Optimize evacuation failure for regions with few failed objects
• 59c3dcc: 8276746: Add section on reproducible builds in building.md
• ... and 268 more: https://git.openjdk.java.net/jdk/compare/9d191fce55fa70d6a2affc724fad57b0e20e4bde...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 Vladimir for your review!

[TobiHartmann]

Looks good to me.

[chhagedorn]

Thanks Tobias for your review!

[chhagedorn]

/integrate

[openjdk]

Going to push as commit 5448139.
Since your change was applied there have been 278 commits pushed to the master branch:

• 0395e4e: 8276768: Snippet copy feature should use button instead of link
• d8b0dee: 8276239: Better tables in java.util.random package summary
• fc0fe25: 8273235: tools/launcher/HelpFlagsTest.java Fails on Windows 32bit
• 3934fe5: 8275847: Scheduling fails with "too many D-U pinch points" on small method
• 44047f8: 8274328: C2: Redundant CFG edges fixup in block ordering
• 8849154: 8275846: read_base_archive_name() could read past the end of buffer
• 2653cfb: 8276688: Remove JLinkReproducibleXXXTest from ProblemList.txt
• c7095b2: 8276207: Properties.loadFromXML/storeToXML works incorrectly for supplementary characters
• ed7ecca: 8254739: G1: Optimize evacuation failure for regions with few failed objects
• 59c3dcc: 8276746: Add section on reproducible builds in building.md
• ... and 268 more: https://git.openjdk.java.net/jdk/compare/9d191fce55fa70d6a2affc724fad57b0e20e4bde...master

Your commit was automatically rebased without conflicts.

[openjdk]

@chhagedorn Pushed as commit 5448139.

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