[cmake] Hint find_package() to prefer LLVM installed alongside clang

[mgorny]

Summary

Include a path hint for find_package() in ClangConfig.cmake to ensure
that CMake prefers LLVM installed alongside clang over the default
search path.

If two versions of LLVM are installed in the system, and one of them is
in PATH, CMake's find_package() magic prefers the CMake directory
alongside that install by default. Adding a relative hint makes it
possible to prioritize to the install from which find_package() is
called.

If you want to build e.g. LLDB against another install of LLVM, you can
pass LLVM_CONFIG override. In this case, LLDB queries the prefix from
llvm-config and uses the CMake files located there. However, when
including ClangConfig, the implicit find_package() nevertheless prefers
PATH-found LLVM over the one used previously by LLDB, and two versions
of LLVMConfig end up being loaded.

This could be fixed on LLDB end up by explicitly forcing custom package
search location. However, it seems simpler and safer to add a hint to
ClangConfig than to track every usage of ClangConfig.

[rnk]

lgtm

cfe/trunk/cmake/modules/CMakeLists.txt

 # Generate a list of CMake library targets so that other CMake projects can
 # link against them. LLVM calls its version of this file LLVMExports.cmake, but
 # the usual CMake convention seems to be ${Project}Targets.cmake.
 set(CLANG_INSTALL_PACKAGE_DIR lib${LLVM_LIBDIR_SUFFIX}/cmake/clang)
 set(clang_cmake_builddir "${CMAKE_BINARY_DIR}/${CLANG_INSTALL_PACKAGE_DIR}")
 
+# Keep this in sync with llvm/cmake/CMakeLists.txt!
+set(LLVM_INSTALL_PACKAGE_DIR lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm)
+set(llvm_cmake_builddir "${LLVM_BINARY_DIR}/${LLVM_INSTALL_PACKAGE_DIR}")
+
 get_property(CLANG_EXPORTS GLOBAL PROPERTY CLANG_EXPORTS)
 export(TARGETS ${CLANG_EXPORTS} FILE ${clang_cmake_builddir}/ClangTargets.cmake)
 
 # Generate ClangConfig.cmake for the build tree.
 set(CLANG_CONFIG_CMAKE_DIR "${clang_cmake_builddir}")
+set(CLANG_CONFIG_LLVM_CMAKE_DIR "${llvm_cmake_builddir}")
 set(CLANG_CONFIG_EXPORTS_FILE "${clang_cmake_builddir}/ClangTargets.cmake")
 configure_file(
   ${CMAKE_CURRENT_SOURCE_DIR}/ClangConfig.cmake.in
   ${clang_cmake_builddir}/ClangConfig.cmake
   @ONLY)
 set(CLANG_CONFIG_CMAKE_DIR)
+set(CLANG_CONFIG_LLVM_CMAKE_DIR)
 set(CLANG_CONFIG_EXPORTS_FILE)
 
 # Generate ClangConfig.cmake for the install tree.
 set(CLANG_CONFIG_CODE "
 # Compute the installation prefix from this LLVMConfig.cmake file location.
 get_filename_component(CLANG_INSTALL_PREFIX \"\${CMAKE_CURRENT_LIST_FILE}\" PATH)")
 # Construct the proper number of get_filename_component(... PATH)
 # calls to compute the installation prefix.
 string(REGEX REPLACE "/" ";" _count "${CLANG_INSTALL_PACKAGE_DIR}")
 foreach(p ${_count})
   set(CLANG_CONFIG_CODE "${CLANG_CONFIG_CODE}
 get_filename_component(CLANG_INSTALL_PREFIX \"\${CLANG_INSTALL_PREFIX}\" PATH)")
 endforeach(p)
 set(CLANG_CONFIG_CMAKE_DIR "\${CLANG_INSTALL_PREFIX}/${CLANG_INSTALL_PACKAGE_DIR}")
+set(CLANG_CONFIG_LLVM_CMAKE_DIR "\${CLANG_INSTALL_PREFIX}/${LLVM_INSTALL_PACKAGE_DIR}")
 set(CLANG_CONFIG_EXPORTS_FILE "\${CLANG_CMAKE_DIR}/ClangTargets.cmake")
 configure_file(
   ${CMAKE_CURRENT_SOURCE_DIR}/ClangConfig.cmake.in
   ${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles/ClangConfig.cmake
   @ONLY)
 set(CLANG_CONFIG_CODE)
 set(CLANG_CONFIG_CMAKE_DIR)
 set(CLANG_CONFIG_EXPORTS_FILE)

cfe/trunk/cmake/modules/ClangConfig.cmake.in

 # This file allows users to call find_package(Clang) and pick up our targets.
 
-find_package(LLVM REQUIRED CONFIG)
-
 @CLANG_CONFIG_CODE@
 
+find_package(LLVM REQUIRED CONFIG
+             HINTS "@CLANG_CONFIG_LLVM_CMAKE_DIR@")
+
 set(CLANG_EXPORTED_TARGETS "@CLANG_EXPORTS@")
 set(CLANG_CMAKE_DIR "@CLANG_CONFIG_CMAKE_DIR@")
 
 # Provide all our library targets to users.
 include("@CLANG_CONFIG_EXPORTS_FILE@")