✨ Legalize control flow for selected payloads - #2162
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📝 WalkthroughSummary by CodeRabbit
WalkthroughCompiler targets now declare explicit classical-control capabilities through C++ and Python APIs. Target compilation performs fail-closed preflight validation before mutation. Constant ChangesClassical-control compilation
Estimated code review effort: 4 (Complex) | ~60 minutes Merge Risk: 🟡 Moderate · up to The PR adds target classical-control capability checks, but the current head still fails a required lint check and can report a misleading category for dynamic qubit-index diagnostics. These issues should be fixed or explicitly accepted before merging. Sequence Diagram(s)sequenceDiagram
participant Client
participant CompilerTarget
participant TargetCompilationPipeline
participant VerifyTargetClassicalControlPass
participant MLIRModule
Client->>CompilerTarget: create with classical_control
CompilerTarget->>CompilerTarget: validate and canonicalize capabilities
Client->>TargetCompilationPipeline: compile MLIRModule
TargetCompilationPipeline->>VerifyTargetClassicalControlPass: run preflight
VerifyTargetClassicalControlPass->>MLIRModule: inspect reachable control regions
VerifyTargetClassicalControlPass->>CompilerTarget: query capability support
VerifyTargetClassicalControlPass-->>TargetCompilationPipeline: reject with diagnostic or allow lowering
Possibly related PRs
Suggested labels: Suggested reviewers: Poem
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Actionable comments posted: 5
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Inline comments:
In @.agent/plans/classical-control-capabilities.md:
- Around line 269-274: Update the public enum description to use the API-defined
values Iteration and MultiwayBranch instead of Loop and Switch, while retaining
Conditional and ConditionalLoop and the surrounding capability-list behavior.
In `@CHANGELOG.md`:
- Around line 36-38: Update the changelog entry to describe preflight as running
at the start of target compilation, before target-mutating passes such as
cleanup, mapping, and synthesis, rather than before target compilation itself.
In `@mlir/lib/Compiler/TargetCompilation.cpp`:
- Around line 225-230: Update the diagnostic in the hasDynamicQubitIndex
handling to describe the inability to lower the affected operation with a
dynamic qubit index, rather than labeling it a classical-control construct.
Preserve the operation name and the “with a dynamic qubit index” wording so the
existing diagnostic assertion continues to pass.
- Around line 91-94: Remove the redundant const qualifier from the by-value Type
parameter of isQubitTensor, and apply the same change to the lambda parameter
near the same code. Preserve the existing type checks and behavior.
In `@mlir/unittests/Compiler/test_compiler_target.cpp`:
- Around line 179-184: Add a narrowly scoped
clang-analyzer-optin.core.EnumCastOutOfRange suppression around the intentional
ClassicalControl value 255 in RejectsUnknownClassicalControlCapability,
including a brief justification that the out-of-range cast is required to
exercise CompilerTarget::Storage::initialize()’s default branch. Follow the
existing suppression convention in this test file and keep the suppression
limited to this cast.
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.agent/plans/classical-control-capabilities.mdCHANGELOG.mdUPGRADING.mdbindings/mlir/register_mlir.cppbindings/patterns.txtdocs/mlir/target_compilation.mdmlir/include/mlir/Compiler/Target.hmlir/include/mlir/Dialect/QCO/IR/QCOOps.tdmlir/lib/Compiler/Target.cppmlir/lib/Compiler/TargetCompilation.cppmlir/lib/Dialect/QCO/IR/SCF/IndexSwitchOp.cppmlir/unittests/Compiler/test_compiler_pipeline.cppmlir/unittests/Compiler/test_compiler_target.cppmlir/unittests/Dialect/QCO/IR/test_qco_ir.cpppython/mqt/core/mlir.pyitest/python/test_mlir.py
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Inline comments:
In @.agent/plans/classical-control-capabilities.md:
- Around line 269-272: Update the plan’s reference to the capability-list
accessor from CompilerTarget::getClassicalControl() to the actual public API
name, CompilerTarget::classicalControl(); leave
supportsClassicalControl(ClassicalControl) unchanged.
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.agent/plans/classical-control-capabilities.mdCHANGELOG.mdmlir/lib/Compiler/TargetCompilation.cppmlir/unittests/Compiler/test_compiler_target.cpp
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burgholzer
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burgholzer
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This one feels like it will still require a bit of work for simplification and making some assumptions more explicit.
The general direction looks very good though.
| The supported constraints are `max-control-flow-nesting-depth` on all four | ||
| capabilities, `max-iteration-count` on both iteration capabilities, and | ||
| `max-case-count` on multiway branching. Limits are inclusive. The compiler must | ||
| prove a constrained loop's trip count. It currently proves constant `scf.for` | ||
| bounds and rejects a constrained `scf.while` because no general termination | ||
| bound is available. The compiler rejects a constant range when MLIR's native | ||
| trip-count result disagrees with widened arithmetic. A zero, unknown, or | ||
| misapplied constraint makes that capability group unusable. Missing or | ||
| incomplete optional metadata never implies support. |
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It feels like some of these descriptions could/should be upstreamed and included in the QDMI PR that introduces the program capabilities.
Some of these statements feel quite normative and helpful for making sure people export capabilities in a standardized fashion.
| def UnrollUnsupportedPayloadLoops | ||
| : Pass<"unroll-unsupported-payload-loops", "mlir::ModuleOp"> { |
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The naming of this is a bit odd. Can we make this a little more natural? same holds for the other pass. Maybe the passes could even be combined. I like the "legalize-control-flow" part of this. Maybe a combination of both makes sense.
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I just want to add to that there is also the QuantumLoopUnroll pass defined right above that. I think these two could potentially be one.
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| void populateTargetCompilationPipeline(OpPassManager& pm) { | ||
| pm.addPass(createSymbolDCEPass()); | ||
| pm.addPass(createLiftControlFlowToSCFPass()); |
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I might be largely mistaken, but none of the frontends we currently support will produce control flow dialect operations. Is this handling really necessary or could we simplify around that?
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I'd critically question whether the programs being described here could actually reasonably be constructed by one of the frontends we currently consider.
If not, I'd rather make the assumptions a little more explicit and simplify around these new stricter assumptions.
| constexpr llvm::StringLiteral FORWARD_BRANCHING = "forward-branching"; | ||
| constexpr llvm::StringLiteral COUNTED_ITERATION = "counted-iteration"; | ||
| constexpr llvm::StringLiteral CONDITIONAL_LOOP = "conditional-loop"; | ||
| constexpr llvm::StringLiteral MULTIWAY_BRANCHING = "multiway-branching"; | ||
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| constexpr llvm::StringLiteral MAX_NESTING_DEPTH = | ||
| "max-control-flow-nesting-depth"; | ||
| constexpr llvm::StringLiteral MAX_ITERATION_COUNT = "max-iteration-count"; | ||
| constexpr llvm::StringLiteral MAX_CASE_COUNT = "max-case-count"; |
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This should likely be standardized in a more central place.
| /// MLIR 22 computes index differences at their original bit width. Widen the | ||
| /// constants first so an overflowing range cannot appear to have zero trips. |
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This feels like a case where a more explicit assumption on the input could really help to reduce complexity. This feels like a corner case that is merely hypothetical for practical programs.
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this feels way more complicated than it likely needs to be for practical use cases.
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@denialhaag maybe you have some comments on top of my existing ones. |
denialhaag
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denialhaag
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Thanks for getting this started, @simon1hofmann! 😎
Some more comments from my side:
| /// constants first so an overflowing range cannot appear to have zero trips. | ||
| [[nodiscard]] static std::optional<llvm::APInt> | ||
| getExactConstantTripCount(scf::ForOp loop) { | ||
| const auto constant = [](const Value value) -> std::optional<llvm::APInt> { |
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| const auto constant = [](const Value value) -> std::optional<llvm::APInt> { | |
| const auto constant = [](Value value) -> std::optional<llvm::APInt> { |
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We really need to institutionalize this in AGENTS.md. Maybe we should even feed it https://mlir.llvm.org/getting_started/DeveloperGuide/ and its subpages once and distill the essence out of those guidelines. I can try that in a separate PR.
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I am trying to tackle this one systematically first; before kicking off a large scale review and update of this PR stack. Ideally, the entire review process already benefits from the updated guidelines then. I might tag you, @denialhaag and @simon1hofmann, in a review for the updated guidelines once I am done with iterating them out.
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| [[nodiscard]] static bool canUseFullUnroll(scf::ForOp loop, | ||
| const uint64_t iterations) { | ||
| const auto constant = [&](const Value value) -> std::optional<int64_t> { |
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| const auto constant = [&](const Value value) -> std::optional<int64_t> { | |
| const auto constant = [&](Value value) -> std::optional<int64_t> { |
| const auto build = [&](auto&& self, const size_t index) -> scf::IfOp { | ||
| auto constant = arith::ConstantIndexOp::create( | ||
| rewriter, operation.getLoc(), operation.getCases()[index]); | ||
| auto condition = arith::CmpIOp::create( | ||
| rewriter, operation.getLoc(), arith::CmpIPredicate::eq, | ||
| adaptor.getArg(), constant.getResult()); | ||
| auto ifOp = | ||
| scf::IfOp::create(rewriter, operation.getLoc(), | ||
| operation.getResultTypes(), condition, true); | ||
| rewriter.eraseBlock(&ifOp.getThenRegion().front()); | ||
| rewriter.eraseBlock(&ifOp.getElseRegion().front()); | ||
| rewriter.inlineRegionBefore(cases[index], ifOp.getThenRegion(), | ||
| ifOp.getThenRegion().end()); | ||
| if (index + 1U == cases.size()) { | ||
| rewriter.inlineRegionBefore(*defaultRegion, ifOp.getElseRegion(), | ||
| ifOp.getElseRegion().end()); | ||
| return ifOp; | ||
| } | ||
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| Block& elseBlock = ifOp.getElseRegion().emplaceBlock(); | ||
| const OpBuilder::InsertionGuard guard(rewriter); | ||
| rewriter.setInsertionPointToEnd(&elseBlock); | ||
| scf::IfOp nested = self(self, index + 1U); | ||
| scf::YieldOp::create(rewriter, operation.getLoc(), nested.getResults()); | ||
| return ifOp; | ||
| }; |
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Not sure if this is really worth it, but maybe this function and its counterpart in LowerQCOIndexSwitch can become a shared helper. 🤔
| if (hasLinearCapture(loop)) { | ||
| loop.emitError( | ||
| "SCF loop captures QCO linear values; pass them as " | ||
| "iteration arguments"); | ||
| return WalkResult::interrupt(); | ||
| } |
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This feels like one of the cases that could be excluded by construction. If we believe a linearity check is necessary regardless, maybe we can run the verifyLinearity function added in #2220 once at the beginning. That should pretty much guarantee that this check (and similar ones) cannot ever fail.
| continue; | ||
| } | ||
| if (llvm::hasSingleElement(loop.getBody()->getOperations())) { | ||
| const ValueRange yielded = loop.getYieldedValues(); |
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| const ValueRange yielded = loop.getYieldedValues(); | |
| auto yielded = loop.getYieldedValues(); |
| } | ||
| if (llvm::hasSingleElement(loop.getBody()->getOperations())) { | ||
| const ValueRange yielded = loop.getYieldedValues(); | ||
| if (llvm::all_of(yielded, [&](const Value value) { |
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| if (llvm::all_of(yielded, [&](const Value value) { | |
| if (llvm::all_of(yielded, [&](Value value) { |
| if (isa<scf::ExecuteRegionOp, CtrlOp, InvOp, PowOp>(operation)) { | ||
| return true; | ||
| } |
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I might be missing something, but is there a reason we need to explicitly name these? I don't think any of our modifiers implement the RegionBranchOpInterface. 🤔
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Thanks a lot for the review @burgholzer and @denialhaag, will work on them asap. |
Before you start working on these, let's wait a bit. I planned to kick off a large-scale run on all the PR feedback in the stack, which should hopefully produce coherent edits across the board. I am almost done with the entire PR stack with feedback, so I should be able to kick that off later today. |
Sounds good, then let me know when it's ready 👍 |
Use stock MLIR normalization, bounded static loop unrolling, and dialect conversion to match residual structured control flow to the selected payload specification. Co-authored-by: Simon Hofmann <simon.t.hofmann@tum.de> Assisted-by: GPT-5.6 Sol via Codex
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Description
Legalize structural control flow against the payload specification stored in
mqt.target_env.This pull request:
The pass assumes valid linear IR; its SCF checks concern runtime transport and repeated loop execution, not recovery from malformed multi-user SSA.
Closes #2131.
Dependencies
This PR is stacked directly on #2227. The preceding stack is #2218, #2215, #2219, and #2226.
Validation
Hosted CI is pending for the rebased revision.
AI assistance: OpenAI Codex assisted with implementation, review, testing, and this description.
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