[mlir][standalone] Enable to build as LLVM external project

[marbre]

Summary

In addition to the component build, this enables the standalone example
to be build as part of a monolithic LLVM build by using the LLVM
external projects mechanism (LLVM_EXTERNAL_PROJECTS).

[marbre]

This was presented as part of my talk How to Build your own MLIR Dialect at FOSDEM 2023.

[stephenneuendorffer]

LGTM

mlir/examples/standalone/CMakeLists.txt

38–39

MLIR_STANDALONE?

mlir/examples/standalone/CMakeLists.txt

 cmake_minimum_required(VERSION 3.13.4)
 project(standalone-dialect LANGUAGES CXX C)
 
 set(CMAKE_BUILD_WITH_INSTALL_NAME_DIR ON)
 
 set(CMAKE_CXX_STANDARD 17 CACHE STRING "C++ standard to conform to")
 
+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   find_package(MLIR REQUIRED CONFIG)
 
   message(STATUS "Using MLIRConfig.cmake in: ${MLIR_DIR}")
   message(STATUS "Using LLVMConfig.cmake in: ${LLVM_DIR}")
 
   set(LLVM_RUNTIME_OUTPUT_INTDIR ${CMAKE_BINARY_DIR}/bin)
   set(LLVM_LIBRARY_OUTPUT_INTDIR ${CMAKE_BINARY_DIR}/lib)
   set(MLIR_BINARY_DIR ${CMAKE_BINARY_DIR})
 
   list(APPEND CMAKE_MODULE_PATH "${MLIR_CMAKE_DIR}")
   list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_DIR}")
+
   include(TableGen)
   include(AddLLVM)
   include(AddMLIR)
   include(HandleLLVMOptions)
+else()
+  # Build via external projects mechanism
+  set(MLIR_MAIN_SRC_DIR ${LLVM_MAIN_SRC_DIR}/../mlir)
+  set(MLIR_INCLUDE_DIR ${MLIR_MAIN_SRC_DIR}/include)
+  set(MLIR_GENERATED_INCLUDE_DIR ${LLVM_BINARY_DIR}/tools/mlir/include)
+  set(MLIR_INCLUDE_DIRS "${MLIR_INCLUDE_DIR};${MLIR_GENERATED_INCLUDE_DIR}")
+endif()
 
 if(MLIR_ENABLE_BINDINGS_PYTHON)
   include(MLIRDetectPythonEnv)
   mlir_configure_python_dev_packages()
 endif()
 
+set(STANDALONE_SOURCE_DIR ${PROJECT_SOURCE_DIR})
+set(STANDALONE_BINARY_DIR ${PROJECT_BINARY_DIR})
 include_directories(${LLVM_INCLUDE_DIRS})
 include_directories(${MLIR_INCLUDE_DIRS})
-include_directories(${PROJECT_SOURCE_DIR}/include)
-include_directories(${PROJECT_BINARY_DIR}/include)
+include_directories(${STANDALONE_SOURCE_DIR}/include)
+include_directories(${STANDALONE_BINARY_DIR}/include)
 link_directories(${LLVM_BUILD_LIBRARY_DIR})
 add_definitions(${LLVM_DEFINITIONS})
 
 add_subdirectory(include)
 add_subdirectory(lib)
 if(MLIR_ENABLE_BINDINGS_PYTHON)
   message(STATUS "Enabling Python API")
   add_subdirectory(python)
 endif()
 add_subdirectory(test)
 add_subdirectory(standalone-opt)
 add_subdirectory(standalone-translate)

mlir/examples/standalone/README.md

 # An out-of-tree MLIR dialect
 
 This is an example of an out-of-tree [MLIR](https://mlir.llvm.org/) dialect along with a standalone `opt`-like tool to operate on that dialect.
 
-## Building
+## Building - Component Build
 
 This setup assumes that you have built LLVM and MLIR in `$BUILD_DIR` and installed them to `$PREFIX`. To build and launch the tests, run
 ```sh
 mkdir build && cd build
 cmake -G Ninja .. -DMLIR_DIR=$PREFIX/lib/cmake/mlir -DLLVM_EXTERNAL_LIT=$BUILD_DIR/bin/llvm-lit
 cmake --build . --target check-standalone
 ```
 To build the documentation from the TableGen description of the dialect operations, run
 ```sh
 cmake --build . --target mlir-doc
 ```
 **Note**: Make sure to pass `-DLLVM_INSTALL_UTILS=ON` when building LLVM with CMake in order to install `FileCheck` to the chosen installation prefix.
 
+## Building - Monolithic Build
+
+This setup assumes that you build the project as part of a monolithic LLVM build via the `LLVM_EXTERNAL_PROJECTS` mechanism.
+To build LLVM, MLIR, the example and launch the tests run
+```sh
+mkdir build && cd build
+cmake -G Ninja `$LLVM_SRC_DIR/llvm` \
+    -DCMAKE_BUILD_TYPE=Release \
+    -DLLVM_TARGETS_TO_BUILD=host
+    -DLLVM_ENABLE_PROJECTS=mlir \
+    -DLLVM_EXTERNAL_PROJECTS=standalone-dialect -DLLVM_EXTERNAL_STANDALONE_DIALECT_SOURCE_DIR=../
+cmake --build . --target check-standalone
+```
+Here, `$LLVM_SRC_DIR` needs to point to the root of the monorepo.

mlir/examples/standalone/test/lit.site.cfg.py.in

 @LIT_SITE_CFG_IN_HEADER@
 
 config.llvm_tools_dir = lit_config.substitute("@LLVM_TOOLS_DIR@")
 config.mlir_obj_dir = "@MLIR_BINARY_DIR@"
 config.enable_bindings_python = @MLIR_ENABLE_BINDINGS_PYTHON@
-config.standalone_obj_root = "@CMAKE_BINARY_DIR@"
+config.standalone_obj_root = "@STANDALONE_BINARY_DIR@"
 
 import lit.llvm
 lit.llvm.initialize(lit_config, config)
 
 # Let the main config do the real work.
-lit_config.load_config(config, "@CMAKE_SOURCE_DIR@/test/lit.cfg.py")
+lit_config.load_config(config, "@STANDALONE_SOURCE_DIR@/test/lit.cfg.py")