✨ Add Rotation Gate Merging using Quaternions

mlir/include/mlir/Compiler/CompilerPipeline.h

@@ -39,6 +39,9 @@ struct QuantumCompilerConfig {
 
   /// Print IR after each stage
   bool printIRAfterAllStages = false;
+
+  /// Enable quaternion-based rotation gate merging
+  bool mergeRotationGates = false;
 };
 
 /**

mlir/include/mlir/Dialect/QCO/CMakeLists.txt

@@ -7,3 +7,4 @@
 # Licensed under the MIT License
 
 add_subdirectory(IR)
+add_subdirectory(Transforms)

mlir/include/mlir/Dialect/QCO/Transforms/CMakeLists.txt

@@ -0,0 +1,12 @@
+# Copyright (c) 2023 - 2026 Chair for Design Automation, TUM
+# Copyright (c) 2025 - 2026 Munich Quantum Software Company GmbH
+# All rights reserved.
+#
+# SPDX-License-Identifier: MIT
+#
+# Licensed under the MIT License
+
+set(LLVM_TARGET_DEFINITIONS Passes.td)
+mlir_tablegen(Passes.h.inc -gen-pass-decls -name QCO)
+add_public_tablegen_target(MLIRQCOTransformsIncGen)
+add_mlir_doc(Passes QCOPasses Passes/ -gen-pass-doc)

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

@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) 2023 - 2026 Chair for Design Automation, TUM
+ * Copyright (c) 2025 - 2026 Munich Quantum Software Company GmbH
+ * All rights reserved.
+ *
+ * SPDX-License-Identifier: MIT
+ *
+ * Licensed under the MIT License
+ */
+
+#pragma once
+
+#include <mlir/Pass/Pass.h> // from @llvm-project
+
+namespace mlir::qco {
+#define GEN_PASS_DECL
+#include "mlir/Dialect/QCO/Transforms/Passes.h.inc" // IWYU pragma: export
+
+#define GEN_PASS_REGISTRATION
+#include "mlir/Dialect/QCO/Transforms/Passes.h.inc" // IWYU pragma: export
+} // namespace mlir::qco

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

@@ -0,0 +1,20 @@
+// Copyright (c) 2023 - 2026 Chair for Design Automation, TUM
+// Copyright (c) 2025 - 2026 Munich Quantum Software Company GmbH
+// All rights reserved.
+//
+// SPDX-License-Identifier: MIT
+//
+// Licensed under the MIT License
+
+include "mlir/Pass/PassBase.td"
+
+def MergeRotationGates : Pass<"merge-rotation-gates",
+"mlir::ModuleOp"> {
+  let summary = "Merge rotation gates using quaternion-based fusion";
+  let description = [{
+    Merges consecutive rotation gates of different types (rx, ry, rz, u)
+    using quaternion representation and the Hamilton product.
+  }];
+
+  let dependentDialects = ["::mlir::math::MathDialect"];
+}

mlir/lib/Compiler/CMakeLists.txt

@@ -20,6 +20,7 @@ add_mlir_library(
   QCToQCO
   QCOToQC
   QCToQIR
+  MLIRQCOTransforms
   MQT::MLIRSupport
   MQT::ProjectOptions
   DISABLE_INSTALL)

mlir/lib/Compiler/CompilerPipeline.cpp

@@ -13,6 +13,7 @@
 #include "mlir/Conversion/QCOToQC/QCOToQC.h"
 #include "mlir/Conversion/QCToQCO/QCToQCO.h"
 #include "mlir/Conversion/QCToQIR/QCToQIR.h"
+#include "mlir/Dialect/QCO/Transforms/Passes.h"
 #include "mlir/Support/PrettyPrinting.h"
 
 #include <llvm/ADT/StringRef.h>
@@ -161,18 +162,23 @@ QuantumCompilerPipeline::runPipeline(ModuleOp module,
 
   // Stage 5: Optimization passes
   // TODO: Add optimization passes
-  addCleanupPasses(pm);
-  if (failed(pm.run(module))) {
-    return failure();
+
+  // quaternion gate merging pass
+  if (config_.mergeRotationGates) {
+    pm.addPass(mlir::qco::createMergeRotationGates());
+    if (failed(pm.run(module))) {
+      return failure();
+    }
+    pm.clear();
   }
+
   if (record != nullptr && config_.recordIntermediates) {
     record->afterOptimization = captureIR(module);
     if (config_.printIRAfterAllStages) {
       prettyPrintStage(module, "Optimization Passes", ++currentStage,
                        totalStages);
     }
   }
-  pm.clear();
 
   // Stage 6: QCO canonicalization
   addCleanupPasses(pm);

mlir/lib/Dialect/QCO/CMakeLists.txt

@@ -8,3 +8,4 @@
 
 add_subdirectory(Builder)
 add_subdirectory(IR)
+add_subdirectory(Transforms)

mlir/lib/Dialect/QCO/Transforms/CMakeLists.txt

@@ -0,0 +1,40 @@
+# Copyright (c) 2023 - 2026 Chair for Design Automation, TUM
+# Copyright (c) 2025 - 2026 Munich Quantum Software Company GmbH
+# All rights reserved.
+#
+# SPDX-License-Identifier: MIT
+#
+# Licensed under the MIT License
+
+get_property(dialect_libs GLOBAL PROPERTY MLIR_DIALECT_LIBS)
+set(LIBRARIES ${dialect_libs} MQT::CoreIR)
+add_compile_options(-fexceptions)
+
+message(STATUS "MLIR_DIALECT_LIBS contains: ${dialect_libs}")
+
+file(GLOB_RECURSE TRANSFORMS_SOURCES *.cpp)
+
+add_mlir_library(MLIRQCOTransforms ${TRANSFORMS_SOURCES} LINK_LIBS ${LIBRARIES} DEPENDS
+                 MLIRQCOTransformsIncGen)
+
+# collect header files
+file(GLOB_RECURSE TRANSFORMS_HEADERS_SOURCE
+     ${MQT_MLIR_SOURCE_INCLUDE_DIR}/mlir/Dialect/QCO/Transforms/*.h)
+file(GLOB_RECURSE TRANSFORMS_HEADERS_BUILD
+     ${MQT_MLIR_BUILD_INCLUDE_DIR}/mlir/Dialect/QCO/Transforms/*.inc)
+
+# add public headers using file sets
+target_sources(
+  MLIRQCOTransforms
+  PUBLIC FILE_SET
+         HEADERS
+         BASE_DIRS
+         ${MQT_MLIR_SOURCE_INCLUDE_DIR}
+         FILES
+         ${TRANSFORMS_HEADERS_SOURCE}
+         FILE_SET
+         HEADERS
+         BASE_DIRS
+         ${MQT_MLIR_BUILD_INCLUDE_DIR}
+         FILES
+         ${TRANSFORMS_HEADERS_BUILD})