8315088: C2: assert(wq.size() - before == EMPTY_LOOP_SIZE) failed: expect the EMPTY_LOOP_SIZE nodes of this body if empty

[rwestrel]

The assert fires in code that looks for counted loops that would be
empty if not part of a loop strip mining nest (that is some nodes in
the the loop body are kept alive because of the safepoint in the outer
loop, the loop would otherwise be empty). That logic starts by finding
the set of EMPTY_LOOP_SIZE nodes that are core to all counted loops
(exit test, iv phi etc) and store them in the empty_loop_nodes
list.

Finding the "core" nodes is performed by
IdealLoopTree::collect_loop_core_nodes. It enqueues the backedge of
the loop in empty_loop_nodes and then iteratively pushes inputs of
nodes in empty_loop_nodes that belongs to the loop until it can't
make progress. There should be EMPTY_LOOP_SIZE of those nodes.

It's possible that that 2 or more loops are involved in the process:
a node n in loop 1 could be kept alive by the safepoint of loop 2 so
loop 2 needs to be empty for n to become dead and possibly loop 1 to
be empty. When that happens while the logic is in the process of
determining if loop 1 is empty, it needs to also collect the
EMPTY_LOOP_SIZE nodes that make the "core" of loop 2. They are
enqueued to the empty_loop_nodes list too.

The failure happens when empty_loop_nodes already has the "core"
nodes of loop 1 and "core" nodes for loop 2 are collected. In the
processIdealLoopTree::collect_loop_core_nodes iterates on all
empty_loop_nodes (loop 1 nodes included, not just starting from the
loop 2 backedge). That's inefficient (loop 1 nodes can't help find
nodes that belong to loop 2) and wrong because there could be an edge
from a node in loop 1 to a node in the body of loop 2. That node in
loop 2 is them pushed to the empty_loop_nodes list.

The fix I propose is to only iterate over loop 2 nodes when loop 2 is
processed.

---

Progress

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Issue

• JDK-8315088: C2: assert(wq.size() - before == EMPTY_LOOP_SIZE) failed: expect the EMPTY_LOOP_SIZE nodes of this body if empty (Bug - P3)

Reviewers

• Tobias Hartmann (@TobiHartmann - Reviewer)
• Christian Hagedorn (@chhagedorn - Reviewer) ⚠️ Review applies to cf9f66bb

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• 02: Full - Incremental (5926fd9d)
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[TobiHartmann]

Thanks for the detailed description, the fix looks good to me.

[TobiHartmann]

Please add a @requires vm.compiler2.enabled

[rwestrel]

Done.

[openjdk]

@rwestrel This change now passes all automated pre-integration checks.

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After integration, the commit message for the final commit will be:

8315088: C2: assert(wq.size() - before == EMPTY_LOOP_SIZE) failed: expect the EMPTY_LOOP_SIZE nodes of this body if empty

Reviewed-by: thartmann, chagedorn

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At the time when this comment was updated there had been 165 new commits pushed to the master branch:

• 8bbebbb: 8315644: increase timeout of sun/security/tools/jarsigner/Warning.java
• f292268: 8315454: Add a way to create an immutable snapshot of a BitSet
• 9def453: 8314580: PhaseIdealLoop::transform_long_range_checks fails with assert "was tested before"
• 6c821f5: 8315545: C1: x86 cmove can use short branches
• d7e4087: 8315369: [JVMCI] failure to attach to a libgraal isolate during shutdown should not be fatal
• d1cabe4: 8315566: [JVMCI] deadlock in JVMCI startup when bad option specified
• 94a74a0: 8315534: Incorrect warnings about implicit annotation processing
• 84425a6: 8315452: Erroneous AST missing modifiers for partial input
• 3094fd1: 8314662: jshell shows duplicated signatures of javap
• 0d52c82: 8310220: IGV: dump graph after each IGVN step at level 4
• ... and 155 more: https://git.openjdk.org/jdk/compare/8939d15d92982300f090bc1c51f59550529eaaf3...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]

Looks good to me, too.

[rwestrel]

@TobiHartmann @chhagedorn thanks for the reviews

[rwestrel]

/integrate

[openjdk]

Going to push as commit fe4f900.
Since your change was applied there have been 165 commits pushed to the master branch:

• 8bbebbb: 8315644: increase timeout of sun/security/tools/jarsigner/Warning.java
• f292268: 8315454: Add a way to create an immutable snapshot of a BitSet
• 9def453: 8314580: PhaseIdealLoop::transform_long_range_checks fails with assert "was tested before"
• 6c821f5: 8315545: C1: x86 cmove can use short branches
• d7e4087: 8315369: [JVMCI] failure to attach to a libgraal isolate during shutdown should not be fatal
• d1cabe4: 8315566: [JVMCI] deadlock in JVMCI startup when bad option specified
• 94a74a0: 8315534: Incorrect warnings about implicit annotation processing
• 84425a6: 8315452: Erroneous AST missing modifiers for partial input
• 3094fd1: 8314662: jshell shows duplicated signatures of javap
• 0d52c82: 8310220: IGV: dump graph after each IGVN step at level 4
• ... and 155 more: https://git.openjdk.org/jdk/compare/8939d15d92982300f090bc1c51f59550529eaaf3...master

Your commit was automatically rebased without conflicts.

[openjdk]

@rwestrel Pushed as commit fe4f900.

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