[mlir][cmake] Do not export MLIR_MAIN_SRC_DIR and MLIR_INCLUDE_DIR (llvm#125842)

MLIR_MAIN_SRC_DIR and MLIR_INCLUDE_DIR point to the source directory,
which is not installed. As such, the installed MLIRConfig.cmake also
should not reference it.

The comment indicates that these are needed for mlir_tablegen(), but I
don't see any related uses.

The motivation for this is the use in flang, where we end up inheriting
a meaningless MLIR_MAIN_SRC_DIR from a previous MLIR build, whose source
directory doesn't exist anymore, and that cannot be overridden with the
correct path, because it's not a cached variable.

Instead do what all the other projects do for LLVM_MAIN_SRC_DIR and
initialize MLIR_MAIN_SRC_DIR to CMAKE_CURRENT_SOURCE_DIR/../mlir.

For MLIR_INCLUDE_DIR there already is an exported MLIR_INCLUDE_DIRS,
which can be used instead.

Author: nikic

flang/CMakeLists.txt

@@ -79,6 +79,8 @@ if(CMAKE_SIZEOF_VOID_P EQUAL 4)
   message(FATAL_ERROR "flang isn't supported on 32 bit CPUs")
 endif()
 
+set(MLIR_MAIN_SRC_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../mlir" CACHE PATH "Path to MLIR source tree")
+
 if (FLANG_STANDALONE_BUILD)
   set(FLANG_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
   set(CMAKE_INCLUDE_CURRENT_DIR ON)
@@ -240,10 +242,9 @@ else()
     set(FLANG_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
   endif()
 
-  set(MLIR_MAIN_SRC_DIR ${LLVM_MAIN_SRC_DIR}/../mlir ) # --src-root
-  set(MLIR_INCLUDE_DIR ${MLIR_MAIN_SRC_DIR}/include ) # --includedir
+  set(MLIR_INCLUDE_DIRS ${MLIR_MAIN_SRC_DIR}/include ) # --includedir
   set(MLIR_TABLEGEN_OUTPUT_DIR ${CMAKE_BINARY_DIR}/tools/mlir/include)
-  include_directories(SYSTEM ${MLIR_INCLUDE_DIR})
+  include_directories(SYSTEM ${MLIR_INCLUDE_DIRS})
   include_directories(SYSTEM ${MLIR_TABLEGEN_OUTPUT_DIR})
 endif()
 

flang/include/flang/Optimizer/Dialect/CMakeLists.txt

@@ -37,7 +37,7 @@ set_target_properties(flang-doc PROPERTIES FOLDER "Flang/Docs")
 set(dialect_doc_filename "FIRLangRef")
 
 set(LLVM_TARGET_DEFINITIONS FIROps.td)
-tablegen(MLIR ${dialect_doc_filename}.md -gen-op-doc "-I${MLIR_INCLUDE_DIR}")
+tablegen(MLIR ${dialect_doc_filename}.md -gen-op-doc)
 set(GEN_DOC_FILE ${FLANG_BINARY_DIR}/docs/Dialect/${dialect_doc_filename}.md)
 add_custom_command(
         OUTPUT ${GEN_DOC_FILE}

mlir/cmake/modules/MLIRConfig.cmake.in

@@ -16,10 +16,6 @@ set(MLIR_INSTALL_AGGREGATE_OBJECTS "@MLIR_INSTALL_AGGREGATE_OBJECTS@")
 set(MLIR_ENABLE_BINDINGS_PYTHON "@MLIR_ENABLE_BINDINGS_PYTHON@")
 set(MLIR_ENABLE_EXECUTION_ENGINE "@MLIR_ENABLE_EXECUTION_ENGINE@")
 
-# For mlir_tablegen()
-set(MLIR_INCLUDE_DIR "@MLIR_INCLUDE_DIR@")
-set(MLIR_MAIN_SRC_DIR "@MLIR_MAIN_SRC_DIR@")
-
 set_property(GLOBAL PROPERTY MLIR_ALL_LIBS "@MLIR_ALL_LIBS@")
 set_property(GLOBAL PROPERTY MLIR_DIALECT_LIBS "@MLIR_DIALECT_LIBS@")
 set_property(GLOBAL PROPERTY MLIR_CONVERSION_LIBS "@MLIR_CONVERSION_LIBS@")