🐛 Propagate correct mappings to parent of scf::IfOp branches

[MatthiasReumann]

Description

Currently, the mapping pass has the following critical issues:

• After mapping an qco::IfOp, the dispatch function does not properly propagate the updated mappings (layout, wire-infos) to its parent operation.
• The qco::Yield operation in each branch does not guarantee that the same hardware qubits, in the same order, are returned. However, this property must be enforced for proper linear typing. Semantically, any SCF-op must act as a black-box consuming and producing N hardware qubits in the same order. However, contrary to the scf::ForOp where we restore the layout, the qco::IfOp acts like a permutation network, where the return hardware qubits might "house" different program qubits.

This pull request fixes these issues. I've also tried to improve the documentation (the comments) and code quality along the way for both the mapping pass and its tests.

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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[MatthiasReumann]

@coderabbitai full review

[coderabbitai]

✅ Action performed

Full review finished.

[coderabbitai]

📝 Walkthrough

Walkthrough

QCO mapping updates add layout equality, correct additional-qubit IfOp rewriting, return converged routing layouts, restore branch and parent wire ordering, and expand mapping tests for control-flow hardware ordering and GHZ circuits.

Changes

QCO mapping behavior

Layer / File(s)	Summary
Conditional operation result rewriting mlir/lib/Dialect/QCO/IR/SCF/IfOp.cpp	Additional-qubit rewriting creates located blocks and replaces the original IfOp with the matching new results.
Layout convergence and branch routing mlir/include/mlir/Dialect/QCO/Utils/Layout.h, mlir/lib/Dialect/QCO/Transforms/Mapping/Mapping.cpp	Routing convergence returns the shared layout and branch SWAP sequences; dispatch updates parent layouts, wire information, and YieldOp ordering.
Mapping executability tests mlir/unittests/Dialect/QCO/Transforms/Mapping/test_mapping.cpp	Tests validate coupling and control-flow hardware ordering, add GHZ helpers, and rename mapping scenarios.

Estimated code review effort: 4 (Complex) | ~45 minutes

Sequence Diagram(s)

sequenceDiagram
  participant dispatch
  participant converge
  participant IfOp
  participant Layout
  dispatch->>IfOp: Route then and else regions
  dispatch->>converge: Converge branch layouts
  converge->>Layout: Return converged layout
  dispatch->>IfOp: Insert branch SWAPs
  dispatch->>IfOp: Reorder yielded qubits in Hot mode
  dispatch->>Layout: Update parent layout and wire information

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Possibly related PRs

• munich-quantum-toolkit/core#1805: Related scf.for restoration and Layout refactoring in the mapping pass.
• munich-quantum-toolkit/core#1870: Related qco::IfOp convergence and routing logic.
• munich-quantum-toolkit/core#1872: Introduced the IfOp::replaceWithAdditionalQubits helper updated here.

Suggested labels: fix

Suggested reviewers: pre-commit-ci[bot], burgholzer

Poem

I’m a rabbit routing qubits tonight,
Through branching paths kept neat and tight.
Layouts now agree, swaps fall in line,
GHZ loops sparkle, outcomes shine.
Hop, hop—the mapping is right!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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.
Title check	✅ Passed	The title accurately summarizes the main change: propagating correct mappings for qco::IfOp branches.
Description check	✅ Passed	The description includes the change summary, motivation, context, and a completed checklist, which matches the template well.

✨ Finishing Touches

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch bug/wires-after-if-op-map

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[codecov]

Codecov Report

❌ Patch coverage is 97.64706% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...lir/lib/Dialect/QCO/Transforms/Mapping/Mapping.cpp	97.4%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!

[burgholzer]

LGTM 👍🏼