[compiler-rt] Allow finding LLVMConfig if CMAKE_FIND_ROOT_PATH_MODE_PACKAGE is set to ONLY

[kongyi]

Summary

MAKE_FIND_ROOT_PATH_MODE_PACKAGE can be set to ONLY for cross compilation. However we need to find LLVMConfig from the host compiler, which is outside of the root path.

Set CMAKE_FIND_ROOT_PATH_BOTH when finding the LLVMConfig package.

[mgorny]

Do you have a doc link or some precedent? Something to help us understand the implications of this.

[kongyi]

In D146355#4203909, @mgorny wrote:

Do you have a doc link or some precedent? Something to help us understand the implications of this.

We cross compile compiler-rt for Android, and we set CMAKE_FIND_ROOT_PATH_MODE_* to ONLY to force using the correct sysroots.

https://android.googlesource.com/toolchain/llvm_android/+/cd342d2891d684a29cef85378069ddd5d4bc5ea8/base_builders.py#410

[srhines]

LGTM, but @phosek should also be able to confirm this is ok.

[phosek]

I still don't understand the motivation behind this change. You already explicitly set CMAKE_FIND_ROOT_PATH_MODE_PACKAGE to ONLY for Android so why do we need to change the default?

[kongyi]

In D146355#4207160, @phosek wrote:

I still don't understand the motivation behind this change. You already explicitly set CMAKE_FIND_ROOT_PATH_MODE_PACKAGE to ONLY for Android so why do we need to change the default?

We need to pull the build dependencies from the target root path, thus setting CMAKE_FIND_ROOT_PATH_MODE_PACKAGE to ONLY. However LLVMConfig is different that we need the host version instead, as it records the configs of the host toolchain.

compiler-rt/cmake/Modules/CompilerRTUtils.cmake

macro(load_llvm_config)
   get_filename_component(LLVM_MAIN_SRC_DIR_DEFAULT "${COMPILER_RT_ROOT_SRC_PATH}/../llvm" ABSOLUTE)
   if (NOT EXISTS "${LLVM_MAIN_SRC_DIR_DEFAULT}")
     # TODO(dliew): Remove this legacy fallback path.
     message(WARNING
       "LLVM source tree not found at \"${LLVM_MAIN_SRC_DIR_DEFAULT}\". "
       "You are not using the monorepo layout. This configuration is DEPRECATED.")
   endif()
 
-  find_package(LLVM HINTS "${LLVM_CMAKE_DIR}")
+  find_package(LLVM HINTS "${LLVM_CMAKE_DIR}" CMAKE_FIND_ROOT_PATH_BOTH)
   if (NOT LLVM_FOUND)
      message(WARNING "UNSUPPORTED COMPILER-RT CONFIGURATION DETECTED: "
                      "LLVM cmake package not found.\n"
                      "Reconfigure with -DLLVM_CMAKE_DIR=/path/to/llvm.")
   else()
     list(APPEND CMAKE_MODULE_PATH "${LLVM_DIR}")
     # Turn into CACHE PATHs for overwritting
     set(LLVM_BINARY_DIR "${LLVM_BINARY_DIR}" CACHE PATH "Path to LLVM build tree")