[cmake] Obtain LLVM_CMAKE_PATH from llvm-config if available

[mgorny]

Summary

Use the new --cmakedir option to obtain LLVM_CMAKE_PATH straight from
llvm-config. Fallback to local reconstruction if llvm-config does not
support this option.

[mgorny]

Little ping here. This is quite trivial, and since D26894 has been committed already I'd like to start adding changes like this one to all LLVM projects using llvm-config in stand-alone build.

[EricWF]

Is there a way to write this patch so that old llvm-config's continue to work, at least for a little while until everybody upgrades?

[mgorny]

In D26900#638328, @EricWF wrote:

Is there a way to write this patch so that old llvm-config's continue to work, at least for a little while until everybody upgrades?

Technically it's possible but I think it would add a lot of complexity. So if you believe that to be a major issue, I'd rather just wait a few days before applying the change. However, I don't think we really have a big market for people doing stand-alone builds ;-).

[mgorny]

Updated to support fallback to hardcoded path.

[rnk]

lgtm I figure we've waited a few days at this point.

[mgorny]

As discussed on IRC, I will commit the unconditional versions for projects that require the same version of LLVM (clang, lldb, lld) and the version with fallback for others (runtimes).

cfe/trunk/CMakeLists.txt

if( CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR )
   if(LLVM_CONFIG)
     message(STATUS "Found LLVM_CONFIG as ${LLVM_CONFIG}")
     set(CONFIG_COMMAND ${LLVM_CONFIG}
       "--assertion-mode"
       "--bindir"
       "--libdir"
       "--includedir"
       "--prefix"
-      "--src-root")
+      "--src-root"
+      "--cmakedir")
     execute_process(
       COMMAND ${CONFIG_COMMAND}
       RESULT_VARIABLE HAD_ERROR
       OUTPUT_VARIABLE CONFIG_OUTPUT
     )
     if(NOT HAD_ERROR)
       string(REGEX REPLACE
         "[ \t]*[\r\n]+[ \t]*" ";"
         CONFIG_OUTPUT ${CONFIG_OUTPUT})
     else()
       string(REPLACE ";" " " CONFIG_COMMAND_STR "${CONFIG_COMMAND}")
       message(STATUS "${CONFIG_COMMAND_STR}")
       message(FATAL_ERROR "llvm-config failed with status ${HAD_ERROR}")
     endif()
   else()
     message(FATAL_ERROR "llvm-config not found -- ${LLVM_CONFIG}")
   endif()
 
   list(GET CONFIG_OUTPUT 0 ENABLE_ASSERTIONS)
   list(GET CONFIG_OUTPUT 1 TOOLS_BINARY_DIR)
   list(GET CONFIG_OUTPUT 2 LIBRARY_DIR)
   list(GET CONFIG_OUTPUT 3 INCLUDE_DIR)
   list(GET CONFIG_OUTPUT 4 LLVM_OBJ_ROOT)
   list(GET CONFIG_OUTPUT 5 MAIN_SRC_DIR)
+  list(GET CONFIG_OUTPUT 6 LLVM_CMAKE_PATH)
 
   if(NOT MSVC_IDE)
     set(LLVM_ENABLE_ASSERTIONS ${ENABLE_ASSERTIONS}
       CACHE BOOL "Enable assertions")
     # Assertions should follow llvm-config's.
     mark_as_advanced(LLVM_ENABLE_ASSERTIONS)
   endif()
 
   set(LLVM_TOOLS_BINARY_DIR ${TOOLS_BINARY_DIR} CACHE PATH "Path to llvm/bin")
   set(LLVM_LIBRARY_DIR ${LIBRARY_DIR} CACHE PATH "Path to llvm/lib")
   set(LLVM_MAIN_INCLUDE_DIR ${INCLUDE_DIR} CACHE PATH "Path to llvm/include")
   set(LLVM_BINARY_DIR ${LLVM_OBJ_ROOT} CACHE PATH "Path to LLVM build tree")
   set(LLVM_MAIN_SRC_DIR ${MAIN_SRC_DIR} CACHE PATH "Path to LLVM source tree")
 
   find_program(LLVM_TABLEGEN_EXE "llvm-tblgen" ${LLVM_TOOLS_BINARY_DIR}
     NO_DEFAULT_PATH)
 
-  set(LLVM_CMAKE_PATH "${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
   set(LLVMCONFIG_FILE "${LLVM_CMAKE_PATH}/LLVMConfig.cmake")
   if(EXISTS ${LLVMCONFIG_FILE})
     list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_PATH}")
     include(${LLVMCONFIG_FILE})
   else()
     message(FATAL_ERROR "Not found: ${LLVMCONFIG_FILE}")
   endif()