[flang][cmake] Improve how CLANG_DIR is handled

[awarzynski]

Summary

• Added a sanity check with Clang_FOUND to verify that find_package succeeded
• Made sure that find_package won't use any of CMake's standard paths to guarantee that only the path provided with CLANG_DIR is considered (implemented through NO_DEFAULT_PATH)
• Made the call to get_filename_component more explicit (so that it is clear what the base directory is)
• Updated comments to clarify what CLANG_DIR means

[rovka]

LGTM

[stevanradakovic]

One quick question. Otherwise looks good.

flang/CMakeLists.txt

54

Why CLANG_DIR_ABSOLUTEDIR when the var above is declared as CLANG_DIR_ABSOLUTE? Am I missing something?

[awarzynski]

Make the semantics of CLANG_DIR more explicit

[awarzynski]

In D99088#2644491, @stevanradakovic wrote:

One quick question. Otherwise looks good.

That was a typo, cheers for pointing out!

I've updated this patch a bit more. I think that the semantics of CLANG_DIR (and {LLVM|MLIR}_DIR) are a bit too convoluted. I hope that this will help clarifying. If this makes sense, I'll update MLIR_DIR and LLVM_DIR vars in a separate patch.

[stevanradakovic]

Looks good, thanks!

flang/CMakeLists.txt

if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   # We need a pre-built/installed version of LLVM.
   find_package(LLVM REQUIRED HINTS "${LLVM_CMAKE_PATH}")
   # If the user specifies a relative path to LLVM_DIR, the calls to include
   # LLVM modules fail. Append the absolute path to LLVM_DIR instead.
   get_filename_component(LLVM_DIR_ABSOLUTE ${LLVM_DIR} REALPATH)
   list(APPEND CMAKE_MODULE_PATH ${LLVM_DIR_ABSOLUTE})
 
   if(FLANG_BUILD_NEW_DRIVER)
+    # Users might specify a path to CLANG_DIR that's:
+    #   * a full path, or
+    #   * a path relative to the path of this script.
+    # Append the absolute path to CLANG_DIR so that find_package works in both
+    # cases.
+    get_filename_component(
+      CLANG_DIR_ABSOLUTE
+      ${CLANG_DIR}
+      REALPATH
+      CMAKE_CURRENT_SOURCE_DIR)
+    list(APPEND CMAKE_MODULE_PATH ${CLANG_DIR_ABSOLUTE})
+
     # TODO: Remove when libclangDriver is lifted out of Clang
-    list(APPEND CMAKE_MODULE_PATH ${CLANG_DIR})
-    find_package(Clang REQUIRED HINTS "${CLANG_DIR}")
+    find_package(Clang REQUIRED PATHS "${CLANG_DIR_ABSOLUTE}" NO_DEFAULT_PATH)
+    if (NOT Clang_FOUND)
+      message(FATAL_ERROR "Failed to find Clang")
+    endif()
   endif()
 
   # If LLVM links to zlib we need the imported targets so we can too.
   if(LLVM_ENABLE_ZLIB)
     find_package(ZLIB REQUIRED)
   endif()
   option(LLVM_ENABLE_PEDANTIC "Compile with pedantic enabled." ON)
   if(CMAKE_COMPILER_IS_GNUCXX)