🐛 Preserve structured state in QC-to-QCO conversion#1935
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denialhaag
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Assisted-by: GPT-5.6 via Codex
Assisted-by: GPT-5.6 via Codex
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Signed-off-by: Lukas Burgholzer <burgholzer@me.com>
Signed-off-by: Lukas Burgholzer <burgholzer@me.com>
Keep the stable SCF for/while state preservation and terminator-order fixes while deferring classical conditional results to a dedicated QCO operation change. Assisted-by: GPT-5.6 via Codex
Assisted-by: GPT-5.6 via Codex
Teach QCO mapping to route only qubit-valued SCF state while preserving classical operands and type-changing while signatures. Add mixed-state for and while mapping regressions. Assisted-by: GPT-5.6 via Codex
Assisted-by: GPT-5.6 via Codex
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@MatthiasReumann @denialhaag for awareness. Follow-up for IfOp is coming. |
Opus 4.8 brought up a similar point while I was working on #1927. I still have to understand whether there is really no form of a guaranteed conversion order. If that were the case, the QC-to-QCO conversion at large could fall apart at any moment. We heavily rely on the fact that each program is converted from top to bottom. 🤔 |
Yeah, me too. I was skeptical at first. But the regression tests in this PR did not pass without the changes to the pass, and the changes themselves feel sound. |
🤖 *AI text below* 🤖 ## Summary - Allow `qco.if` and `qco.index_switch` to return ordinary classical SSA values before their existing linear quantum results. - Preserve result-bearing `scf.if` and `scf.index_switch` operations through QC-to-QCO and QCO-to-QC conversion without introducing scratch memory. - Update verification, parsing and printing, RegionBranch interfaces, tied-value helpers, builders, mapping, tensor traversal, module equivalence, and affected optimizations for the segmented result model. - Keep unsupported Jeff lowering explicit without constraining the standard QC → QCO → QC → QIR pipeline. ## Motivation This capability was identified while working on the OpenQASM integration in #1910 and while reducing the structured-control-flow fixes that became #1935. QCO conditionals previously represented only linear quantum results. Preserving classical SCF results therefore required either rejecting otherwise valid programs or lowering classical state through temporary memory. The new representation keeps classical values in ordinary SSA form while retaining QCO's explicit single-use quantum flow: 1. classical result prefix; 2. tied linear quantum result suffix. Conditional region arguments remain linear-only. Classical inputs continue to use normal SSA capture, while `qco.yield` returns the complete classical-plus-linear result signature. ## Implementation notes - Both conditional operations use `AttrSizedResultSegments` and expose explicit classical and linear result accessors. - Custom parsers infer the linear suffix from the `args(...)` assignments and retain quantum-only textual compatibility. - Parent-aware `qco.yield` verification preserves the stricter modifier contracts. - QC↔QCO conversions retain classical def-use chains directly and replace only linear results with QC references on the QC side. - Mapping handles every index-switch case and the default region, preserving classical yield operands while realigning only quantum values. - Module equivalence compares classical yield prefixes positionally while keeping the linear suffix permutation-aware. ## Validation All affected targets build successfully. The focused suites pass 933 tests in total: - 450 QCO IR tests - 134 QC-to-QCO tests - 132 QCO-to-QC tests - 82 QCO utility tests - 3 QTensor utility tests - 15 mapping tests - 117 Jeff round-trip tests Repository lint, changed-source `clang-tidy`, and `git diff --check` pass. Two independent read-only reviews were performed; the final review found no remaining actionable issues.
No no, the changes also look good to me, and the MLIR docs also don't suggest that there is a conversion order. At this point, I'm wondering more why the rest of the QC-to-QCO conversion is working as intended. But yeah, I'll investigate a bit more and then report back! 👍 |
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🤖 AI text below 🤖
Summary
scf.forandscf.while.scf.whileoperations, including reorderedscf.conditionarguments.scf.whileregions with the correct before/after signatures.forandwhile.Context
This conversion gap was found while working on OpenQASM control flow in #1910. The fix is independent of that frontend: any QC producer can create valid
scf.fororscf.whileoperations whose existing classical values must survive conversion while quantum state is made explicit.Scope
This PR is limited to stable
scf.forandscf.whilestate preservation. It intentionally does not add classical results toqco.iforqco.index_switch, and it introduces no scratch allocations, stores, or loads. Classicalqco.ifresults are being designed separately as an SSA-based extension.Validation
uvx nox -s lintgit diff --check