✨ Implement (controlled) swap reconstruction MLIR pass

mlir/include/mlir/Dialect/MQTOpt/Transforms/Passes.h

@@ -28,6 +28,7 @@ namespace mqt::ir::opt {
 void populateGateEliminationPatterns(mlir::RewritePatternSet& patterns);
 void populateMergeRotationGatesPatterns(mlir::RewritePatternSet& patterns);
 void populateElidePermutationsPatterns(mlir::RewritePatternSet& patterns);
+void populateSwapReconstructionPatterns(mlir::RewritePatternSet& patterns);
 void populateQuantumSinkShiftPatterns(mlir::RewritePatternSet& patterns);
 void populateQuantumSinkPushPatterns(mlir::RewritePatternSet& patterns);
 void populateToQuantumComputationPatterns(mlir::RewritePatternSet& patterns,

mlir/include/mlir/Dialect/MQTOpt/Transforms/Passes.td

@@ -64,6 +64,34 @@ def ElidePermutations : Pass<"elide-permutations", "mlir::ModuleOp"> {
   }];
 }
 
+def SwapReconstruction : Pass<"swap-reconstruction", "mlir::ModuleOp"> {
+  let summary = "This pass searches for CNOTs that can be merged into a SWAP.";
+  let description = [{
+    Three CNOTs that are equivalent to a SWAP are directly replaced with a (potentially controlled) SWAP gate.
+
+    For swap reconstructions with just two CNOTs (utilizing an insertion of two self-cancelling CNOTs), only non-controlled SWAP gates are inserted.
+
+    Examples of swap reconstructions:
+
+    ```
+         ┌───┐         ┌───┐
+    ──■──┤ X ├    ──■──┤ X ├──■────■──    ──╳────■──
+    ┌─┴─┐└─┬─┘ => ┌─┴─┐└─┬─┘┌─┴─┐┌─┴─┐ =>   |  ┌─┴─┐
+    ┤ X ├──■──    ┤ X ├──■──┤ X ├┤ X ├    ──╳──┤ X ├
+    └───┘         └───┘     └───┘└───┘         └───┘
+    ```
+
+    ```
+    ──■────■────■──    ──■──
+      |  ┌─┴─┐  |        |
+    ──■──┤ X ├──■── => ──╳──
+    ┌─┴─┐└─┬─┘┌─┴─┐      |
+    ┤ X ├──■──┤ X ├    ──╳──
+    └───┘     └───┘
+    ```
+  }];
+}
+
 def QuantumSinkPass : Pass<"quantum-sink", "mlir::ModuleOp"> {
   let summary = "This pass attempts to push down operations into branches for possible optimizations.";
   let description = [{

mlir/lib/Dialect/MQTOpt/Transforms/SwapReconstruction.cpp

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2023 - 2025 Chair for Design Automation, TUM
+ * Copyright (c) 2025 Munich Quantum Software Company GmbH
+ * All rights reserved.
+ *
+ * SPDX-License-Identifier: MIT
+ *
+ * Licensed under the MIT License
+ */
+
+#include "mlir/Dialect/Common/Compat.h"
+#include "mlir/Dialect/MQTOpt/Transforms/Passes.h"
+
+#include <mlir/IR/PatternMatch.h>
+#include <mlir/Support/LLVM.h>
+#include <utility>
+
+namespace mqt::ir::opt {
+
+#define GEN_PASS_DEF_SWAPRECONSTRUCTION
+#include "mlir/Dialect/MQTOpt/Transforms/Passes.h.inc"
+
+/**
+ * @brief This pass uses the swap reconstruction patterns to replace according
+ * CNOT patterns with SWAP gates.
+ */
+struct SwapReconstruction final
+    : impl::SwapReconstructionBase<SwapReconstruction> {
+
+  void runOnOperation() override {
+    // Get the current operation being operated on.
+    auto op = getOperation();
+    auto* ctx = &getContext();
+
+    // Define the set of patterns to use.
+    mlir::RewritePatternSet patterns(ctx);
+    populateSwapReconstructionPatterns(patterns);
+
+    // Apply patterns in an iterative and greedy manner.
+    if (mlir::failed(APPLY_PATTERNS_GREEDILY(op, std::move(patterns)))) {
+      signalPassFailure();
+    }
+  }
+};
+
+} // namespace mqt::ir::opt