[CMake] Use find_package(LLVM) instead of LLVMConfig

[phosek]

Summary

This addresses an issues introduced in r362047.

[phosek]

I've tested this locally and it seems to be working fine, I'd like to land this to unbreak our bots.

[smeenai]

llvm/runtimes/CMakeLists.txt

62

CMAKE_PREFIX_PATH won't work correctly when CMAKE_FIND_ROOT_PATH is used and CMAKE_FIND_ROOT_PATH_MODE_PACKAGE is set to ONLY, which is a fairly common configuration for cross-compilation. Setting LLVM_DIR explicitly to ${LLVM_BINARY_DIR}/lib/cmake/llvm should always work. You could combine that with NO_DEFAULT_PATH to ensure you only search in the specified directory, unless you want to be able to find some other LLVM package that's installed on the system?

[phosek]

llvm/runtimes/CMakeLists.txt

62

I've used find_package(LLVM PATHS ...), is that what you had in mind?

[smeenai]

LGTM with the NO_CMAKE_FIND_ROOT_PATH added.

llvm/runtimes/CMakeLists.txt

62

I believe PATHS is also affected by CMAKE_FIND_ROOT_PATH, though you could explicitly add NO_CMAKE_FIND_ROOT_PATH to avoid that. What I was thinking of was setting LLVM_DIR instead of CMAKE_PREFIX_PATH in your previous patch which was doing the explicit -DCMAKE_PREFIX_PATH=... (the <package>_DIR variable is used by CMake automatically to search for <package>), but I think PATHS + NO_CMAKE_FIND_ROOT_PATH should be equivalent and keeps the path in one place.

Does just using ${LLVM_BINARY_DIR} as the PATHS argument do the right thing, i.e. search inside lib/cmake/llvm itself? If not, you could use ${LLVM_LIBRARY_DIR} as part of the path specification here.

[phosek]

llvm/runtimes/CMakeLists.txt

62

I tested it and just ${LLVM_BINARY_DIR} seems to be sufficient so that's what I went with.

libunwind/CMakeLists.txt

 
 # Add path for custom modules
 set(CMAKE_MODULE_PATH
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake"
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake/Modules"
   ${CMAKE_MODULE_PATH}
   )
 
-if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
+if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR OR LIBUNWIND_STANDALONE_BUILD)
   project(libunwind)
 
   # Rely on llvm-config.
   set(CONFIG_OUTPUT)
   if(NOT LLVM_CONFIG_PATH)
     find_program(LLVM_CONFIG_PATH "llvm-config")
   endif()
   if (DEFINED LLVM_PATH)

llvm/runtimes/CMakeLists.txt

if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_SOURCE_DIR})
 
   function(runtime_register_component name)
     set_property(GLOBAL APPEND PROPERTY SUB_COMPONENTS ${name})
   endfunction()
 
   cmake_minimum_required(VERSION 3.4.3)
   project(Runtimes C CXX ASM)
 
+  find_package(LLVM PATHS "${LLVM_BINARY_DIR}/lib/cmake/llvm" NO_DEFAULT_PATH)
+
   # Add the root project's CMake modules, and the LLVM build's modules to the
   # CMake module path.
   list(INSERT CMAKE_MODULE_PATH 0
     "${CMAKE_CURRENT_SOURCE_DIR}/../cmake"
     "${CMAKE_CURRENT_SOURCE_DIR}/../cmake/modules"
-    "${LLVM_LIBRARY_DIR}/cmake/llvm"
   )
 
   # Some of the runtimes will conditionally use the compiler-rt sanitizers
   # to make this work smoothly we ensure that compiler-rt is added first in
   # the list of sub-projects. This allows other sub-projects to have checks
   # like `if(TARGET asan)` to enable building with asan.
   get_compiler_rt_path(compiler_rt_path)
   if(compiler_rt_path)
     list(REMOVE_ITEM runtimes ${compiler_rt_path})
     if(NOT DEFINED LLVM_BUILD_COMPILER_RT OR LLVM_BUILD_COMPILER_RT)
       list(INSERT runtimes 0 ${compiler_rt_path})
     endif()
   endif()
 
-  # LLVMConfig.cmake contains a bunch of CMake variables from the LLVM build.
-  # This file is installed as part of LLVM distributions, so this can be used
-  # either from a build directory or an installed LLVM.
-  include(LLVMConfig)
-
   # Setting these variables will allow the sub-build to put their outputs into
   # the library and bin directories of the top-level build.
+  set(LLVM_BINARY_DIR ${LLVM_BUILD_BINARY_DIR})
+  set(LLVM_LIBRARY_DIR ${LLVM_BUILD_LIBRARY_DIR})
   set(LLVM_LIBRARY_OUTPUT_INTDIR ${LLVM_LIBRARY_DIR})
   set(LLVM_RUNTIME_OUTPUT_INTDIR ${LLVM_TOOLS_BINARY_DIR})
 
   # This variable makes sure that e.g. llvm-lit is found.
-  set(LLVM_BINARY_DIR ${LLVM_BUILD_BINARY_DIR})
-  set(LLVM_LIBRARY_DIR ${LLVM_BUILD_LIBRARY_DIR})
   set(LLVM_INCLUDE_DIR ${LLVM_BUILD_MAIN_INCLUDE_DIR})
   set(LLVM_MAIN_SRC_DIR ${LLVM_BUILD_MAIN_SRC_DIR})
   set(LLVM_CMAKE_PATH ${LLVM_MAIN_SRC_DIR}/cmake/modules)
 
   # This variable is used by individual runtimes to locate LLVM files.
   set(LLVM_PATH ${LLVM_BUILD_MAIN_SRC_DIR})
 
   if(APPLE)