⚡ Avoid redundant target-compilation liveness analysis

[simon1hofmann]

🤖 AI text below 🤖

Summary

Remove the final module-wide remove-dead-values pass from target compilation. The second QCO cleanup immediately before native synthesis already establishes global liveness, while native synthesis replaces one- and two-qubit unitary operations with native equivalents and reconnects their results.

Post-synthesis CSE and final target-conformance verification remain unchanged.

Motivation

The production-shaped profile in #2134 measured the final RemoveDeadValues pass at 7.77 seconds of a 29.93-second target-compilation run (about 26%), including 4.09 seconds in liveness analysis, for hwb10 targeting FakeTorino.

This PR removes that measured cleanup stage without changing placement, routing, native-synthesis choices, or target-conformance checks. It addresses the cleanup portion of #2134.

Safety and testing

The added end-to-end regression compiles a program containing a private helper call, dynamic qco.if, and one- and two-qubit synthesis. It then reruns RemoveDeadValues and requires the IR to remain byte-for-byte unchanged, protecting the intended fixed point.

• mqt-core-mlir-unittests-compiler: 140/140 passed.
• uvx nox -s lint: passed.
• uvx nox -s cpp-lint with LLVM/Clang 22: 0 findings.
• git diff --check: passed.

Stack

• Base: main (includes ✨ Preserve classical control through target mapping #2179)
• Follow-ups: ✨ Canonicalize classical register comparisons across frontends #2181 → ✨ Support fixed-width classical registers across formats #2297

AI assistance: Codex assisted with diagnosis, implementation, review, testing, stack maintenance, and this description.

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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[simon1hofmann]

@coderabbitai full review

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Team

Run ID: e6c3f789-42c4-44a6-8869-d8caf20c707e

📥 Commits

Reviewing files that changed from the base of the PR and between 79c7688 and 7d85391.

📒 Files selected for processing (2)

• mlir/lib/Compiler/TargetCompilation.cpp
• mlir/unittests/Compiler/test_compiler_pipeline.cpp

Included review availability: Your plan provides up to 2 included reviews per hour; 0 remain after this review.

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📝 Summary

Summary by CodeRabbit

• Bug Fixes

  • Improved target compilation cleanup to ensure generated operations are fully eliminated when no longer needed.
  • Prevented unnecessary changes to compiled program output during dead-value cleanup.

• Tests

  • Added coverage verifying that target compilation reaches a stable state for programs with conditional control flow and private functions.

Walkthrough

Changes

Target compilation cleanup

Layer / File(s)	Summary
Native synthesis cleanup pipeline mlir/lib/Compiler/TargetCompilation.cpp	The pipeline no longer runs createRemoveDeadValuesPass() after native synthesis. CSE remains responsible for removing trivially dead local operations.
Dead-value cleanup fixed-point test mlir/unittests/Compiler/test_compiler_pipeline.cpp	A test compiles a QCO program and verifies that a subsequent RemoveDeadValues pass leaves the IR unchanged.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 7d853

This change removes a redundant compiler cleanup pass while preserving the remaining pipeline checks and adding fixed-point regression coverage; no actionable merge-blocking risk remains after normal checks and review.

Suggested reviewers: burgholzer, denialhaag

Poem

A rabbit hops through native code,
CSE clears the unused load,
The IR rests, unchanged and bright,
Fixed-point checks confirm it right,
Then carrots mark the clean-up road.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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 clearly and concisely describes the main change: removing redundant liveness analysis during target compilation.
Description check	✅ Passed	The description explains the change, motivation, safety impact, testing, issue context, stack, AI assistance, and checklist status. It does not explicitly list dependencies or use the template's `Fixe…

Full details: Description check

Explanation

The description explains the change, motivation, safety impact, testing, issue context, stack, AI assistance, and checklist status. It does not explicitly list dependencies or use the template's Fixes #(issue) line, but the required information is otherwise sufficiently complete.

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[simon1hofmann]

@coderabbitai full review

[coderabbitai]

✅ Action performed

Full review finished.

[burgholzer]

LGTM 👍🏻 it's likely that we are running these passes too often.