[mlir] NFC: fix typos and improve commentary regarding generation of MLIRConfig.cmake (#127712)

Author: christopherbate

mlir/cmake/modules/CMakeLists.txt

@@ -1,26 +1,45 @@
+# This file configures the CMake package configuration file "MLIRConfig.cmake"
+# (see https://cmake.org/cmake/help/latest/manual/cmake-packages.7.html).
+# Two different versions of the package config file are generated and copied
+# into different locations:
+# 1. One in a canonical location in the build directory,
+#   '<build>/lib/cmake/mlir/MLIRConfig.cmake'. This version may use absolute
+#    paths referring to locations in the build tree.
+# 2. One under '${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles', which is the version
+#    *to be installed* when `cmake --install` command is run or when CPack
+#    is invoked. This is the version that is distributed with LLVM packages.
+#    The key difference between this and (1) is that all paths must be
+#    relative the the installed "MLIRConfig.cmake" file.
+
 include(GNUInstallPackageDir)
 include(ExtendPath)
 include(LLVMDistributionSupport)
 include(FindPrefixFromConfig)
 
-# Generate a list of CMake library targets so that other CMake projects can
-# link against them. LLVM calls its version of this file LLVMExports.cmake, but
-# the usual CMake convention seems to be ${Project}Targets.cmake.
+# This is a relative path to the location of MLIRConfig.cmake within the
+# installation.
 set(MLIR_INSTALL_PACKAGE_DIR "${CMAKE_INSTALL_PACKAGEDIR}/mlir" CACHE STRING
-  "Path for CMake subdirectory for Polly (defaults to '${CMAKE_INSTALL_PACKAGEDIR}/polly')")
-# CMAKE_INSTALL_PACKAGEDIR might be absolute, so don't reuse below.
-set(mlir_cmake_builddir "${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/mlir")
+  "Path for CMake subdirectory for MLIR (defaults to '${CMAKE_INSTALL_PACKAGEDIR}/mlir')")
 
 # Keep this in sync with llvm/cmake/CMakeLists.txt!
 set(LLVM_INSTALL_PACKAGE_DIR "${CMAKE_INSTALL_PACKAGEDIR}/llvm" CACHE STRING
   "Path for CMake subdirectory for LLVM (defaults to '${CMAKE_INSTALL_PACKAGEDIR}/llvm')")
-# CMAKE_INSTALL_PACKAGEDIR might be absolute, so don't reuse below.
 string(REPLACE "${CMAKE_CFG_INTDIR}" "." llvm_cmake_builddir "${LLVM_LIBRARY_DIR}")
+
+# These two variables contain absolute paths, so only use them for the
+# build directory 'MLIRConfig.cmake'.
+set(mlir_cmake_builddir "${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/mlir")
 set(llvm_cmake_builddir "${llvm_cmake_builddir}/cmake/llvm")
 
+# -----------------------------------------------------------------------------
+# Generate `<build>/lib/cmake/mlir/MLIRConfig.cmake'
+# -----------------------------------------------------------------------------
+
+# Generate the 'MLIRTarget.cmake' file.
 get_property(MLIR_EXPORTS GLOBAL PROPERTY MLIR_EXPORTS)
 export(TARGETS ${MLIR_EXPORTS} FILE ${mlir_cmake_builddir}/MLIRTargets.cmake)
 
+# Declare all the variables required by the 'MLIRConfig.cmake.in' template.
 get_property(MLIR_ALL_LIBS GLOBAL PROPERTY MLIR_ALL_LIBS)
 get_property(MLIR_DIALECT_LIBS GLOBAL PROPERTY MLIR_DIALECT_LIBS)
 get_property(MLIR_CONVERSION_LIBS GLOBAL PROPERTY MLIR_CONVERSION_LIBS)
@@ -36,7 +55,7 @@ set(MLIR_CONFIG_INCLUDE_DIRS
   "${MLIR_SOURCE_DIR}/include"
   "${MLIR_BINARY_DIR}/include"
   )
-# Refer to the best host mlir-tbgen, which might be a host-optimized version
+# Refer to the best host mlir-tblgen, which might be a host-optimized version
 set(MLIR_CONFIG_TABLEGEN_EXE "${MLIR_TABLEGEN_EXE}")
 set(MLIR_CONFIG_PDLL_TABLEGEN_EXE "${MLIR_PDLL_TABLEGEN_EXE}")
 set(MLIR_CONFIG_SRC_SHARDER_TABLEGEN_EXE "${MLIR_SRC_SHARDER_TABLEGEN_EXE}")
@@ -49,24 +68,37 @@ configure_file(
   ${CMAKE_CURRENT_SOURCE_DIR}/MLIRConfigVersion.cmake.in
   ${mlir_cmake_builddir}/MLIRConfigVersion.cmake
   @ONLY)
+
+# Reset the variables which are invalid for the installed version of
+# MLIRConfig.cmake.
 set(MLIR_CONFIG_CMAKE_DIR)
 set(MLIR_CONFIG_LLVM_CMAKE_DIR)
 set(MLIR_CONFIG_INCLUDE_DIRS)
 
-# For compatibility with projects that include(MLIRConfig)
-# via CMAKE_MODULE_PATH, place API modules next to it.
+# For compatibility with projects that add '<build>/lib/cmake/mlir' to
+# their CMAKE_MODULE_PATH, place API modules next to it.
 # Copy without source permissions because the source could be read-only,
 # but we need to write into the copied folder.
-# This should be removed in the future.
 file(COPY .
   DESTINATION ${mlir_cmake_builddir}
   NO_SOURCE_PERMISSIONS
   FILES_MATCHING PATTERN *.cmake
   PATTERN CMakeFiles EXCLUDE
   )
 
+# -----------------------------------------------------------------------------
 # Generate MLIRConfig.cmake for the install tree.
+# All path variables used in the template must be relative!
+# -----------------------------------------------------------------------------
+
+# This is a helper that sets 'MLIR_CONFIG_CODE' to some CMake code for
+# insertion at the top of the generated 'MLIRConfig.cmake'. The code calculates
+# the install directory prefix relative to the installed MLIRConfig.cmake (whose
+# prefix is MLIR_INSTALL_PACKAGE_DIR), avoiding any use of absolute paths.
 find_prefix_from_config(MLIR_CONFIG_CODE MLIR_INSTALL_PREFIX "${MLIR_INSTALL_PACKAGE_DIR}")
+
+# Generate the other required paths by setting them relative to the
+# variable 'MLIR_INSTALL_PREFIX'.
 extend_path(MLIR_CONFIG_CMAKE_DIR "\${MLIR_INSTALL_PREFIX}" "${MLIR_INSTALL_PACKAGE_DIR}")
 extend_path(MLIR_CONFIG_LLVM_CMAKE_DIR "\${MLIR_INSTALL_PREFIX}" "${LLVM_INSTALL_PACKAGE_DIR}")
 get_config_exports_includes(MLIR MLIR_CONFIG_INCLUDE_EXPORTS)