🔥 Remove legacy MLIR code (#1435)

## Description

This PR removed the legacy MLIR code around the MQTRef and MQTOpt
dialects from the `main` branch of MQT Core in anticipation of the `v4`
release.
The respective functionality still remains around in the stable `v3.x`
branch.
Ongoing contributions targeting the old dialects should target the
`v3.x` branch.
Any new contribution shall target the new dialects from here on out.
We will gradually port over the important bits and pieces from the old
dialect infrastructure.

This PR also updates the RtD documentation to show the new dialects,
which is something that we apparently missed or ignored when merging the
dialect redesign PR.
Since we are using a modern version of MLIR now for generating the docs,
some of the workarounds have been removed as well.

## Checklist:

<!---
This checklist serves as a reminder of a couple of things that ensure
your pull request will be merged swiftly.
-->

- [x] The pull request only contains commits that are focused and
relevant to this change.
- [x] I have added appropriate tests that cover the new/changed
functionality.
- [x] I have updated the documentation to reflect these changes.
- [x] I have added entries to the changelog for any noteworthy
additions, changes, fixes, or removals.
- [x] I have added migration instructions to the upgrade guide (if
needed).
- [x] The changes follow the project's style guidelines and introduce no
new warnings.
- [x] The changes are fully tested and pass the CI checks.
- [x] I have reviewed my own code changes.

Author: burgholzer

cmake/SetupMLIR.cmake

@@ -40,10 +40,6 @@ include_directories(${MQT_MLIR_BUILD_INCLUDE_DIR})
 link_directories(${LLVM_BUILD_LIBRARY_DIR})
 add_definitions(${LLVM_DEFINITIONS})
 
-string(REPLACE "." ";" MLIR_VERSION_COMPONENTS ${MLIR_VERSION})
-list(GET MLIR_VERSION_COMPONENTS 0 MLIR_VERSION_MAJOR)
-add_compile_definitions(MLIR_VERSION_MAJOR=${MLIR_VERSION_MAJOR})
-
 # set the binary directory for the build tree such that, e.g., docs can be generated in the build
 # tree
 set(MLIR_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})

docs/mlir/Conversions.md

@@ -2,20 +2,16 @@
 
 ## Internal Conversions
 
-```{include} Conversions/MLIRMQTOptToMQTRef.md
+```{include} Conversions/MLIRQCToQCO.md
 
 ```
 
-```{include} Conversions/MLIRMQTRefToMQTOpt.md
+```{include} Conversions/MLIRQCOToQC.md
 
 ```
 
 ## QIR Conversions
 
-```{include} Conversions/MLIRMQTRefToQIR.md
-
-```
-
-```{include} Conversions/MLIRQIRToMQTRef.md
+```{include} Conversions/MLIRQCToQIR.md
 
 ```

docs/mlir/MQTOpt.md

@@ -0,0 +1,11 @@
+---
+tocdepth: 3
+---
+
+```{include} Dialects/MLIRQCDialect.md
+
+```
+
+```{include} Dialects/MLIRQCInterfaces.md
+:heading-offset: 1
+```

docs/mlir/MQTRef.md

@@ -0,0 +1,11 @@
+---
+tocdepth: 3
+---
+
+```{include} Dialects/MLIRQCODialect.md
+
+```
+
+```{include} Dialects/MLIRQCOInterfaces.md
+:heading-offset: 1
+```

docs/mlir/QC.md

@@ -4,20 +4,20 @@ This part of the MQT explores the capabilities of the Multi-Level Intermediate R
 
 We define multiple dialects, each with its dedicated purpose:
 
-- The {doc}`MQTRef dialect <MQTRef>` uses reference semantics and is designed as a compatibility dialect that simplifies translations from and to existing languages such as Qiskit, OpenQASM, or QIR.
+- The {doc}`QC dialect <QC>` uses reference semantics and is designed as a compatibility dialect that simplifies translations from and to existing languages such as Qiskit, OpenQASM, or QIR.
 
-- The {doc}`MQTOpt dialect <MQTOpt>` uses value semantics and is mainly designed for running optimizations.
+- The {doc}`QCO dialect <QCO>` uses value semantics and is mainly designed for running optimizations.
 
-Both dialects define various transformation passes.
+Both dialects define various canonicalization and transformations that enable the compilation of quantum programs to native quantum hardware.
 
 For intercompatibility, we provide {doc}`conversions <Conversions>` between dialects.
-So far, this comprises a conversion from MQTOpt to MQTRef and one from MQTRef to MQTOpt.
+So far, this comprises conversions between QC and QCO as well as from QC to QIR.
 
 ```{toctree}
 :maxdepth: 2
 
-MQTRef
-MQTOpt
+QC
+QCO
 Conversions
 ```
 
@@ -27,100 +27,3 @@ The content is not yet complete and subject to change.
 Contributions are welcome.
 See the {doc}`contribution guide <../contributing>` for more information.
 :::
-
-## Register Handling
-
-In MQT's MLIR dialects, quantum and classical registers are represented by MLIR-native `memref` operations rather than custom types.
-This design choice offers several advantages:
-
-- **MLIR Integration**: Seamless compatibility with existing MLIR infrastructure, enabling reuse of memory handling patterns and optimization passes
-- **Implementation Efficiency**: No need to define and maintain custom register operations, significantly reducing implementation complexity
-- **Enhanced Interoperability**: Easier integration with other MLIR dialects and passes, allowing for more flexible compilation pipelines
-- **Sustainable Evolution**: Standard memory operations can handle transformations while allowing new features without changing the fundamental register model
-
-### Quantum Register Representation
-
-A quantum register is represented by a `memref` of type `!mqtref.Qubit` or `!mqtopt.Qubit`, depending on the dialect:
-
-```mlir
-// A quantum register with 2 qubits
-%qreg = memref.alloc() : memref<2x!mqtref.Qubit>
-```
-
-Individual qubits are accessed through standard memory operations:
-
-```mlir
-// Load qubits from the register
-%q0 = memref.load %qreg[%i0] : memref<2x!mqtref.Qubit>
-%q1 = memref.load %qreg[%i1] : memref<2x!mqtref.Qubit>
-```
-
-Here's a complete example of quantum register allocation, qubit access, and deallocation:
-
-```mlir
-module {
-  func.func @main() attributes {passthrough = ["entry_point"]} {
-    %i1 = arith.constant 1 : index
-    %i0 = arith.constant 0 : index
-    %qreg = memref.alloc() : memref<2x!mqtref.Qubit>
-    %q0 = memref.load %qreg[%i0] : memref<2x!mqtref.Qubit>
-    %q1 = memref.load %qreg[%i1] : memref<2x!mqtref.Qubit>
-    memref.dealloc %qreg : memref<2x!mqtref.Qubit>
-    return
-  }
-}
-```
-
-### Classical Register Representation
-
-Classical registers follow the same pattern as quantum registers but use the `i1` type for boolean measurement results:
-
-```mlir
-// A classical register with 1 bit
-%creg = memref.alloc() : memref<1xi1>
-```
-
-Measurement operations produce `i1` values that can be stored in classical registers:
-
-```mlir
-// Measure a qubit and store the result
-%c = mqtref.measure %q
-memref.store %c, %creg[%i0] : memref<1xi1>
-```
-
-### Example: Full Quantum Program with Measurement
-
-Consider the following quantum computation represented in OpenQASM 3.0 code:
-
-```qasm3
-OPENQASM 3.0;
-include "stdgates.inc";
-qubit[1] q;
-bit[1] c;
-x q[0];
-c[0] = measure q[0];
-```
-
-In the MQTRef dialect, this corresponds to:
-
-```mlir
-module {
-  func.func @main() attributes {passthrough = ["entry_point"]} {
-    %i0 = arith.constant 0 : index
-    %qreg = memref.alloc() : memref<1x!mqtref.Qubit>
-    %q = memref.load %qreg[%i0] : memref<1x!mqtref.Qubit>
-    %creg = memref.alloca() : memref<1xi1>
-    mqtref.x() %q
-    %c = mqtref.measure %q
-    memref.store %c, %creg[%i0] : memref<1xi1>
-    memref.dealloc %qreg : memref<1x!mqtref.Qubit>
-    return
-  }
-}
-```
-
-## Development
-
-Building the MLIR library requires LLVM version 21.1 or later.
-Our CI pipeline on GitHub continuously builds and tests the MLIR library on Linux, macOS, and Windows.
-To access the latest build logs, visit the [GitHub Actions page](https://github.com/munich-quantum-toolkit/core/actions/workflows/ci.yml).

docs/mlir/QCO.md

@@ -26,6 +26,5 @@ add_subdirectory(tools)
 
 # add test code
 if(BUILD_MQT_CORE_TESTS)
-  add_subdirectory(test)
   add_subdirectory(unittests)
 endif()

docs/mlir/index.md

@@ -6,10 +6,6 @@
 #
 # Licensed under the MIT License
 
-add_subdirectory(MQTRefToMQTOpt)
-add_subdirectory(MQTOptToMQTRef)
-add_subdirectory(MQTRefToQIR)
-add_subdirectory(QIRToMQTRef)
 add_subdirectory(QCOToQC)
 add_subdirectory(QCToQCO)
 add_subdirectory(QCToQIR)