⚡️ Algorithmic Improvements of A*-search-based Routing

CHANGELOG.md

@@ -11,7 +11,7 @@ This project adheres to [Semantic Versioning], with the exception that minor rel
 
 ### Added
 
-- ✨ Add A\*-search-based routing algorithm to MLIR transpilation routines ([#1237]) ([**@MatthiasReumann**])
+- ✨ Add A\*-search-based routing algorithm to MLIR transpilation routines ([#1237], [#1271]) ([**@MatthiasReumann**])
 
 ### Fixed
 
@@ -221,20 +221,21 @@ _📚 Refer to the [GitHub Release Notes](https://github.com/munich-quantum-tool
 <!-- PR links -->
 
 [#1276]: https://github.com/munich-quantum-toolkit/core/pull/1276
-[#1237]: https://github.com/munich-quantum-toolkit/core/pull/1237
+[#1271]: https://github.com/munich-quantum-toolkit/core/pull/1271
 [#1269]: https://github.com/munich-quantum-toolkit/core/pull/1269
 [#1263]: https://github.com/munich-quantum-toolkit/core/pull/1263
 [#1247]: https://github.com/munich-quantum-toolkit/core/pull/1247
 [#1246]: https://github.com/munich-quantum-toolkit/core/pull/1246
+[#1237]: https://github.com/munich-quantum-toolkit/core/pull/1237
 [#1236]: https://github.com/munich-quantum-toolkit/core/pull/1236
 [#1235]: https://github.com/munich-quantum-toolkit/core/pull/1235
 [#1232]: https://github.com/munich-quantum-toolkit/core/pull/1232
 [#1224]: https://github.com/munich-quantum-toolkit/core/pull/1224
 [#1223]: https://github.com/munich-quantum-toolkit/core/pull/1223
 [#1211]: https://github.com/munich-quantum-toolkit/core/pull/1211
 [#1210]: https://github.com/munich-quantum-toolkit/core/pull/1210
-[#1207]: https://github.com/munich-quantum-toolkit/core/pull/1207
 [#1209]: https://github.com/munich-quantum-toolkit/core/pull/1209
+[#1207]: https://github.com/munich-quantum-toolkit/core/pull/1207
 [#1186]: https://github.com/munich-quantum-toolkit/core/pull/1186
 [#1181]: https://github.com/munich-quantum-toolkit/core/pull/1181
 [#1180]: https://github.com/munich-quantum-toolkit/core/pull/1180

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

@@ -103,9 +103,6 @@ def PlacementPassSC : Pass<"placement-sc", "mlir::ModuleOp"> {
              clEnumValN(PlacementStrategy::Random, "random", "Random placement"),
              clEnumValN(PlacementStrategy::Identity, "identity", "Identity placement"))}]>
   ];
-  let statistics = [
-    Statistic<"tms", "runtime-milliseconds", "The runtime in milliseconds">
-  ];
 }
 
 def RoutingPassSC : Pass<"route-sc", "mlir::ModuleOp"> {
@@ -122,14 +119,11 @@ def RoutingPassSC : Pass<"route-sc", "mlir::ModuleOp"> {
            "astar option: Number of lookahead steps (heuristic horizon)">,
     Option<"alpha", "alpha", "float", "1.0F",
            "astar option: The alpha factor in the cost function">,
-    Option<"beta", "beta", "float", "1.0F",
-           "astar option: The beta factor in the cost function">,
     Option<"lambda", "lambda", "float", "0.5F",
            "astar option: The lambda factor in the cost function">
   ];
   let statistics = [
-    Statistic<"numSwaps", "num-additional-swaps", "The number of additional SWAPs">,
-    Statistic<"msTime", "runtime-milliseconds", "The runtime in milliseconds">
+    Statistic<"numSwaps", "num-additional-swaps", "The number of additional SWAPs">
   ];
 }
 

mlir/include/mlir/Dialect/MQTOpt/Transforms/Transpilation/Architecture.h

@@ -33,15 +33,17 @@ enum class ArchitectureName : std::uint8_t { MQTTest };
  */
 class Architecture {
 public:
-  using CouplingMap = llvm::DenseSet<std::pair<QubitIndex, QubitIndex>>;
+  using CouplingSet = mlir::DenseSet<std::pair<QubitIndex, QubitIndex>>;
+  using NeighbourVector = mlir::SmallVector<mlir::SmallVector<QubitIndex, 4>>;
 
   explicit Architecture(std::string name, std::size_t nqubits,
-                        CouplingMap couplingMap)
+                        CouplingSet couplingSet)
       : name_(std::move(name)), nqubits_(nqubits),
-        couplingMap_(std::move(couplingMap)),
+        couplingSet_(std::move(couplingSet)), neighbours_(nqubits),
         dist_(nqubits, llvm::SmallVector<std::size_t>(nqubits, UINT64_MAX)),
         prev_(nqubits, llvm::SmallVector<std::size_t>(nqubits, UINT64_MAX)) {
     floydWarshallWithPathReconstruction();
+    collectNeighbours();
   }
 
   /**
@@ -58,7 +60,7 @@ class Architecture {
    * @brief Return true if @p u and @p v are adjacent.
    */
   [[nodiscard]] bool areAdjacent(QubitIndex u, QubitIndex v) const {
-    return couplingMap_.contains({u, v});
+    return couplingSet_.contains({u, v});
   }
 
   /**
@@ -76,7 +78,8 @@ class Architecture {
   /**
    * @brief Collect all neighbours of @p u.
    */
-  [[nodiscard]] llvm::SmallVector<QubitIndex> neighboursOf(QubitIndex u) const;
+  [[nodiscard]] llvm::SmallVector<QubitIndex, 4>
+  neighboursOf(QubitIndex u) const;
 
 private:
   using Matrix = llvm::SmallVector<llvm::SmallVector<std::size_t>>;
@@ -89,9 +92,16 @@ class Architecture {
    */
   void floydWarshallWithPathReconstruction();
 
+  /**
+   * @brief Collect the neighbours of all qubits.
+   * @details Has a time complexity of O(nqubits)
+   */
+  void collectNeighbours();
+
   std::string name_;
   std::size_t nqubits_;
-  CouplingMap couplingMap_;
+  CouplingSet couplingSet_;
+  NeighbourVector neighbours_;
 
   Matrix dist_;
   Matrix prev_;

mlir/include/mlir/Dialect/MQTOpt/Transforms/Transpilation/Common.h

@@ -35,10 +35,16 @@ constexpr std::size_t IF_PARENT_DEPTH = 2UL;
  */
 using QubitIndex = uint32_t;
 
+/**
+ * @brief A pair of SSA Values.
+ */
+using ValuePair = std::pair<mlir::Value, mlir::Value>;
+
 /**
  * @brief Represents a pair of qubit indices.
  */
 using QubitIndexPair = std::pair<QubitIndex, QubitIndex>;
+
 /**
  * @brief Return true if the function contains "entry_point" in the passthrough
  * attribute.
@@ -50,19 +56,28 @@ using QubitIndexPair = std::pair<QubitIndex, QubitIndex>;
  * @param u A unitary.
  * @returns True iff the qubit gate acts on two qubits.
  */
-[[nodiscard]] bool isTwoQubitGate(UnitaryInterface u);
+[[nodiscard]] bool isTwoQubitGate(UnitaryInterface op);
 
 /**
  * @brief Return input qubit pair for a two-qubit unitary.
- * @param u A two-qubit unitary.
+ * @param op A two-qubit unitary.
  * @return Pair of SSA values consisting of the first and second in-qubits.
  */
-[[nodiscard]] std::pair<mlir::Value, mlir::Value> getIns(UnitaryInterface op);
+[[nodiscard]] ValuePair getIns(UnitaryInterface op);
 
 /**
  * @brief Return output qubit pair for a two-qubit unitary.
- * @param u A two-qubit unitary.
+ * @param op A two-qubit unitary.
  * @return Pair of SSA values consisting of the first and second out-qubits.
  */
-[[nodiscard]] std::pair<mlir::Value, mlir::Value> getOuts(UnitaryInterface op);
+[[nodiscard]] ValuePair getOuts(UnitaryInterface op);
+
+/**
+ * @brief Return the first user of a value in a given region.
+ * @param v The value.
+ * @param region The targeted region.
+ * @return A pointer to the user, or nullptr if non exists.
+ */
+[[nodiscard]] mlir::Operation* getUserInRegion(mlir::Value v,
+                                               mlir::Region* region);
 } // namespace mqt::ir::opt

mlir/include/mlir/Dialect/MQTOpt/Transforms/Transpilation/Layout.h

@@ -12,6 +12,7 @@
 
 #include "mlir/Dialect/MQTOpt/Transforms/Transpilation/Common.h"
 
+#include <llvm/ADT/DenseMapInfo.h>
 #include <llvm/ADT/SmallVector.h>
 #include <mlir/IR/Value.h>
 #include <mlir/Support/LLVM.h>
@@ -114,6 +115,9 @@ class [[nodiscard]] ThinLayout {
    * @brief Maps a hardware qubit index to its program index.
    */
   SmallVector<QubitIndex> hardwareToProgram_;
+
+private:
+  friend struct llvm::DenseMapInfo<ThinLayout>;
 };
 
 /**
@@ -260,3 +264,30 @@ class [[nodiscard]] Layout : public ThinLayout {
   SmallVector<Value> qubits_;
 };
 } // namespace mqt::ir::opt
+
+namespace llvm {
+template <> struct DenseMapInfo<mqt::ir::opt::ThinLayout> {
+  using Layout = mqt::ir::opt::ThinLayout;
+  using VectorInfo = DenseMapInfo<SmallVector<mqt::ir::opt::QubitIndex>>;
+
+  static Layout getEmptyKey() {
+    Layout layout(0);
+    layout.programToHardware_ = VectorInfo::getEmptyKey();
+    return layout;
+  }
+
+  static Layout getTombstoneKey() {
+    Layout layout(0);
+    layout.programToHardware_ = VectorInfo::getTombstoneKey();
+    return layout;
+  }
+
+  static unsigned getHashValue(const Layout& layout) {
+    return VectorInfo::getHashValue(layout.programToHardware_);
+  }
+
+  static bool isEqual(const Layout& lhs, const Layout& rhs) {
+    return VectorInfo::isEqual(lhs.programToHardware_, rhs.programToHardware_);
+  }
+};
+} // namespace llvm