[lldb][NFCI] Change logic to find clang resource dir in standalone builds

[bulbazord]

Summary

As of 0beffb854209a41f31beb18f9631258349a99299 there is a CMake
function to actually calculate the relative path to the clang resource
directory. Currently we have some bespoke logic that looks in a few
places, but with this new function we should be able to eliminate some
complexity here.

Also, I moved the functionality from LLDBConfig to LLDBStandalone since
it is only used in standalone builds.

[JDevlieghere]

LGTM

[mib]

LGTM!

[JDevlieghere]

This broke the standalone bot: https://green.lab.llvm.org/green/view/LLDB/job/lldb-cmake-standalone/

I was able to reproduce the issue locally and reverting this commit fixes the issue.

[mgorny]

I'm not sure if it was really intended but the new API is kinda horrible, at least for us in Gentoo.

Our install prefix is /usr/lib/llvm/NN, whereas clang resource dir is /usr/lib/clang/NN.

If I don't override CLANG_RESOURCE_DIR, it infers the wrong directory:
/usr/lib/llvm/18/lib64/cmake/clang/../../..//lib64/clang/18.

To get the correct directory, I need to pass:
-DCLANG_RESOURCE_DIR="../../../clang/${LLVM_MAJOR}"
which is absolutely counterintuitive (the path gets appended to:
/usr/lib/llvm/18/lib64/cmake/clang/../../../bin
).

Not saying it's not workable but I'd hardly call that an improvement over the previous API.

CC @tstellar, @paperchalice.

[bulbazord]

In D156270#4557902, @mgorny wrote:

I'm not sure if it was really intended but the new API is kinda horrible, at least for us in Gentoo.

Our install prefix is /usr/lib/llvm/NN, whereas clang resource dir is /usr/lib/clang/NN.

If I don't override CLANG_RESOURCE_DIR, it infers the wrong directory:
/usr/lib/llvm/18/lib64/cmake/clang/../../..//lib64/clang/18.

To get the correct directory, I need to pass:
-DCLANG_RESOURCE_DIR="../../../clang/${LLVM_MAJOR}"
which is absolutely counterintuitive (the path gets appended to:
/usr/lib/llvm/18/lib64/cmake/clang/../../../bin
).

Not saying it's not workable but I'd hardly call that an improvement over the previous API.

CC @tstellar, @paperchalice.

To give some context for my change, downstream for swift there are further changes that involve finding and/or using the clang resource directory. I wanted to take advantage of the new functionality get_clang_resource_dir in order to replace the bespoke logic that existed before. Downstream, we had some additional logic to find it again in some cases (which was frustratingly different than what was here before).

If the API is unintuitive or doesn't serve you well, I'm of the opinion that we should change the API. It think it would be a lot easier for your use case if we could tell CMake the absolute path to the clang resource dir instead of having to craft some relative directory that works around the implementation instead of with it.

[mgorny]

Yeah, that's why I CC-ed the two people who committed that API, as I don't really understand how it's supposed to work (and don't really have the capacity to figure it out).

[paperchalice]

When I made D141907, I want get_clang_resource_dir in cmake and GetResourcesPath in clang to have the same behavior and it also respects the doc string of CLANG_RESOURCE_DIR, but the behavior of GetResourcesPath itself is unintuitive when custom resource path is empty.

In D156270#4557902, @mgorny wrote:

I'm not sure if it was really intended but the new API is kinda horrible, at least for us in Gentoo.

Our install prefix is /usr/lib/llvm/NN, whereas clang resource dir is /usr/lib/clang/NN.

If I don't override CLANG_RESOURCE_DIR, it infers the wrong directory:
/usr/lib/llvm/18/lib64/cmake/clang/../../..//lib64/clang/18.

IIUC, the resource dir should be /usr/lib/llvm/18/lib64/clang/18 when CLANG_RESOURCE_DIR is empty.

To get the correct directory, I need to pass:
-DCLANG_RESOURCE_DIR="../../../clang/${LLVM_MAJOR}"
which is absolutely counterintuitive (the path gets appended to:
/usr/lib/llvm/18/lib64/cmake/clang/../../../bin
).

Not saying it's not workable but I'd hardly call that an improvement over the previous API.

CC @tstellar, @paperchalice.

The API here is indeed user unfriendly, a possible improvement is to let CLANG_RESOURCE_DIR aslo accept absolute path.
Also, CLANG_RESOURCE_DIR doesn't seem to work in standalone mode, some extra variables should be exported to ClangConfig.cmake e.g. the install prefix and bin dir of clang.

lldb/cmake/modules/LLDBConfig.cmake

if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
   set_target_properties(lldb-headers PROPERTIES FOLDER "lldb misc")
 
   if (NOT CMAKE_CONFIGURATION_TYPES)
     add_llvm_install_targets(install-lldb-headers
                              COMPONENT lldb-headers)
   endif()
 endif()
 
-
-# If LLDB is building against a prebuilt Clang, then the Clang resource
-# directory that LLDB is using for its embedded Clang instance needs to point
-# to the resource directory of the used Clang installation.
-if (NOT TARGET clang-resource-headers)
-  set(LLDB_CLANG_RESOURCE_DIR_NAME "${LLVM_VERSION_MAJOR}")
-  # Iterate over the possible places where the external resource directory
-  # could be and pick the first that exists.
-  foreach(CANDIDATE "${Clang_DIR}/../.." "${LLVM_DIR}" "${LLVM_LIBRARY_DIRS}"
-                    "${LLVM_BUILD_LIBRARY_DIR}"
-                    "${LLVM_LIBRARY_DIR}")
-    # Build the resource directory path by appending 'clang/<version number>'.
-    set(CANDIDATE_RESOURCE_DIR "${CANDIDATE}/clang/${LLDB_CLANG_RESOURCE_DIR_NAME}")
-    if (IS_DIRECTORY "${CANDIDATE_RESOURCE_DIR}")
-      set(LLDB_EXTERNAL_CLANG_RESOURCE_DIR "${CANDIDATE_RESOURCE_DIR}")
-      break()
-    endif()
-  endforeach()
-
-  if (NOT LLDB_EXTERNAL_CLANG_RESOURCE_DIR)
-    message(FATAL_ERROR "Expected directory for clang-resource headers not found: ${LLDB_EXTERNAL_CLANG_RESOURCE_DIR}")
-  endif()
-endif()
-
 # Find Apple-specific libraries or frameworks that may be needed.
 if (APPLE)
   if(NOT APPLE_EMBEDDED)
     find_library(CARBON_LIBRARY Carbon)
     find_library(CORE_SERVICES_LIBRARY CoreServices)
   endif()
   find_library(FOUNDATION_LIBRARY Foundation)
   find_library(CORE_FOUNDATION_LIBRARY CoreFoundation)

lldb/cmake/modules/LLDBStandalone.cmake

 
 if(NOT DEFINED LLVM_COMMON_CMAKE_UTILS)
   set(LLVM_COMMON_CMAKE_UTILS ${CMAKE_CURRENT_SOURCE_DIR}/../cmake)
 endif()
 
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
 set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX})
 set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX})
+
+# If LLDB is building against a prebuilt Clang, then the Clang resource
+# directory that LLDB is using for its embedded Clang instance needs to point to
+# the resource directory of the used Clang installation.
+if (NOT TARGET clang-resource-headers)
+  include(GetClangResourceDir)
+  get_clang_resource_dir(LLDB_EXTERNAL_CLANG_RESOURCE_DIR
+    PREFIX "${Clang_DIR}/../../../")
+
+  if (NOT EXISTS ${LLDB_EXTERNAL_CLANG_RESOURCE_DIR})
+    message(FATAL_ERROR "Expected directory for clang-resource-headers not found: ${LLDB_EXTERNAL_CLANG_RESOURCE_DIR}")
+  endif()
+endif()