♻️ Use WireIterator in MergeSingleQubitRotationGates

[MatthiasReumann]

Description

While looking at the pass I noticed an opportunity to utilize the wire iterator. Hence, this pull request. Cherry-picks the wire iterator changes introduced in #1600 to support tensors.

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
• I have added migration instructions to the upgrade guide (if needed).
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[coderabbitai]

📝 Walkthrough

Walkthrough

Refactors MergeSingleQubitRotationGates to use pointer-based operation checks and replaces SSA-user traversal with qubit-wire traversal via WireIterator; WireIterator traversal was updated to treat qtensor::InsertOp/qtensor::ExtractOp as traversal sentinels. CHANGELOG entry updated to add PR [#1674].

Changes

Merge pass -> WireIterator integration

Layer / File(s)	Summary
Iterator semantics mlir/lib/Dialect/QCO/Utils/WireIterator.cpp	WireIterator now treats qtensor::InsertOp/qtensor::ExtractOp as traversal sentinels: qubit() returns nullptr for SinkOp and qtensor::InsertOp; forward()/backward() stop/rewind at qtensor::ExtractOp and treat YieldOp/qtensor::InsertOp as end/start markers.
Core pass signature change mlir/lib/Dialect/QCO/Transforms/Optimizations/MergeSingleQubitRotationGates.cpp	areQuaternionMergeable changed to accept Operation* parameters and calls isMergeable on pointers.
Chain collection / traversal mlir/lib/Dialect/QCO/Transforms/Optimizations/MergeSingleQubitRotationGates.cpp	collectChain rewritten to iterate forward along the same qubit wire using WireIterator from the chain head’s output qubit, appending UnitaryOpInterface ops until areQuaternionMergeable fails. isChainStart updated to fetch defining op pointers and use pointer-based mergeability checks.
Changelog CHANGELOG.md	Adds [#1674] reference to the merge-single-qubit-rotation-gates entry and PR links section.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• ✨Add walkProgramGraph driver #1664: Related changes to WireIterator traversal semantics and callers.
• Add WireIterator for the QCO Dialect #1510: Prior refactor and usage updates of WireIterator in passes.
• ✨ Add WireIterator #1310: Introduced the WireIterator implementation relied upon here.

Suggested labels

enhancement

Suggested reviewers

• burgholzer

Poem

🐰 I hop along the qubit line,
Pointers lead where wires entwine,
Insert and extract mark the way,
Gates gather neat, then glide away,
A rabbit's cheer for cleaner play.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 57.14% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately reflects the main change: refactoring the MergeSingleQubitRotationGates pass to use WireIterator, which is the primary objective of this PR.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The PR description is well-structured and comprehensive, following the template with a clear summary, checked checklist items, and AI usage disclosure.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch enh/use-wireiterator-in-merge-1q-rotations

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@mlir/lib/Dialect/QCO/Utils/WireIterator.cpp`:
- Around line 87-95: In backward() guard against op_ being nullptr before the
TypeSwitch<Operation*>(op_) so we don't dereference op in the .Default() path;
check if op_ == nullptr and handle it (e.g., return/stop iteration) prior to the
TypeSwitch, and apply the same nullptr guard for the other similar occurrence
around the code that uses op->getName() (the block using report_fatal_error and
the cases for UnitaryOpInterface, MeasureOp, ResetOp) so report_fatal_error is
never called on a null op_.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: b0f0a221-5b5b-4450-b4d5-449e67738d0e

📥 Commits

Reviewing files that changed from the base of the PR and between d7ced80 and 0b5f825.

📒 Files selected for processing (2)

• mlir/lib/Dialect/QCO/Transforms/Optimizations/MergeSingleQubitRotationGates.cpp
• mlir/lib/Dialect/QCO/Utils/WireIterator.cpp

[burgholzer]

@MatthiasReumann would you mind resolving the conflicts stemming from #1673 before I take a look here?

[denialhaag]

@MatthiasReumann would you mind resolving the conflicts stemming from #1673 before I take a look here?

Should be done. 👍

[burgholzer]

LGTM 👍🏼
It does not save much here, but still feels rather clean.