[mlir] Configure mlir so that it can be directly included in a parent cmakelists.txt

[stella.stamenova]

Summary

The goal is to allow a project to directly include the CMakeLists.txt of mlir via something such as add_directory. This currently doesn't work because the exported targets then end up being imported in the same top-level project and cmake is not happy. LLVM works around this by guarding the exports based on the existence of the LLVMSupport library. This change does the same using MLIRSupport. In our experience, no targets need to be added to the export in the case.

If there's a preferred way of doing this or a better lib to use as the main target to check for, please let me know.

[bondhugula]

mlir/cmake/modules/MLIRConfig.cmake.in

25–30

It would be useful to have a comment here along the lines of your commit title.

[stellaraccident]

LGTM modulo comments. I vaguely recall this kind of target sniffing having caused me problems in the past but I can't remember the details. Doing the same as llvm, sgtm.

mlir/cmake/modules/MLIRConfig.cmake.in

 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@")
 set_property(GLOBAL PROPERTY MLIR_TRANSLATION_LIBS "@MLIR_TRANSLATION_LIBS@")
 
 # Provide all our library targets to users.
+# More specifically, configure MLIR so that it can be directly included in a top
+# level CMakeLists.txt, but also so that it can be imported via `find_package`.
+# This is based on how LLVM handles exports.
+if(NOT TARGET MLIRSupport)
   @MLIR_CONFIG_INCLUDE_EXPORTS@
+endif()
 
 # By creating these targets here, subprojects that depend on MLIR's
 # tablegen-generated headers can always depend on these targets whether building
 # in-tree with MLIR or not.
 if(NOT TARGET mlir-tablegen-targets)
   add_custom_target(mlir-tablegen-targets)
 endif()
 if(NOT TARGET mlir-headers)
   add_custom_target(mlir-headers)
 endif()
 if(NOT TARGET mlir-generic-headers)
   add_custom_target(mlir-generic-headers)
 endif()
 if(NOT TARGET mlir-doc)
   add_custom_target(mlir-doc)
 endif()