[Flang] Use find_program() to find clang-tblgen

[nikic]

Summary

Use find_program() to find the clang-tblgen executable. The current code ends up using docs/clang-tblgen for me...

This is the same way that mlir-tblgen is obtained in https://github.com/llvm/llvm-project/blob/27241435515554bc21105713c9d34cf886c5bced/flang/CMakeLists.txt#L95-L96.

[awarzynski]

Makes sense, thanks for doing this!

[nikic]

Okay, this turned out to be a bit less straightforward than I thought. We have to consider two scenarios:

1. Standalone flang build, where we use an installed clang-tblgen binary. This is the case that find_package() fixes.
2. Combined build of clang and flang, in which case the clang-tblgen binary will not yet exist at the time cmake runs.

For the latter case, what we want to do is to use the CLANG_TABLEGEN_EXE variable set by the clang build system. However, it gets set in the wrong scope, so we need to reexport it. This is the same thing that is done for MLIR_TABLEGEN_EXE here: https://github.com/llvm/llvm-project/blob/8d6f61b6ad71290ad5e99a78843e237486cd2488/mlir/CMakeLists.txt#L165

[awarzynski]

Thanks for the quick fix, makes sense! 👍🏻

clang/CMakeLists.txt

 mark_as_advanced(CLANG_EXECUTABLE_VERSION LIBCLANG_LIBRARY_VERSION)
 
 option(CLANG_INCLUDE_TESTS
        "Generate build targets for the Clang unit tests."
        ${LLVM_INCLUDE_TESTS})
 
 add_subdirectory(utils/TableGen)
 
+# Export CLANG_TABLEGEN_EXE for use by flang docs.
+set(CLANG_TABLEGEN_EXE "${CLANG_TABLEGEN_EXE}" CACHE INTERNAL "")
+
 add_subdirectory(include)
 
 # All targets below may depend on all tablegen'd files.
 get_property(CLANG_TABLEGEN_TARGETS GLOBAL PROPERTY CLANG_TABLEGEN_TARGETS)
 add_custom_target(clang-tablegen-targets
   DEPENDS
   omp_gen
   ${CLANG_TABLEGEN_TARGETS})

flang/docs/CMakeLists.txt

if (${SPHINX_OUTPUT_HTML})
 
       # Runs a python script prior to HTML generation to prepend a header to FIRLangRef,
       # Without the header, the page is incorrectly formatted, as it assumes the first entry is the page title.
       add_custom_command(TARGET copy-flang-src-docs
         COMMAND "${Python3_EXECUTABLE}"
         ARGS ${CMAKE_CURRENT_BINARY_DIR}/Source/FIR/CreateFIRLangRef.py)
 
       # CLANG_TABLEGEN_EXE variable needs to be set for clang_tablegen to run without error
-      set(CLANG_TABLEGEN_EXE clang-tblgen)
+      find_program(CLANG_TABLEGEN_EXE "clang-tblgen" ${LLVM_TOOLS_BINARY_DIR} NO_DEFAULT_PATH)
       gen_rst_file_from_td(FlangCommandLineReference.rst -gen-opt-docs FlangOptionsDocs.td docs-flang-html)
     endif()
     if (${SPHINX_OUTPUT_MAN})
       add_sphinx_target(man flang)
     endif()
   endif()
 endif()