[CMake] Support for cross-compilation when build runtimes

[phosek]

Summary

When cross-compiling, we cannot use the just built toolchain, instead
we need to use the host toolchain which we assume has a support for
targeting the selected target platform. We also need to pass the path
to the native version of llvm-config to external projects.

[beanz]

LGTM

llvm/trunk/cmake/modules/CrossCompile.cmake

function(llvm_create_cross_target_internal target_name toolchain buildtype)
 
   add_custom_command(OUTPUT ${LLVM_${target_name}_BUILD}/CMakeCache.txt
     COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}"
         -DCMAKE_MAKE_PROGRAM="${CMAKE_MAKE_PROGRAM}"
         ${CROSS_TOOLCHAIN_FLAGS_${target_name}} ${CMAKE_SOURCE_DIR}
         -DLLVM_TARGET_IS_CROSSCOMPILE_HOST=TRUE
         -DLLVM_TARGETS_TO_BUILD="${targets_to_build_arg}"
         -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD="${experimental_targets_to_build_arg}"
+        -DLLVM_DEFAULT_TARGET_TRIPLE="${TARGET_TRIPLE}"
+        -DLLVM_TARGET_ARCH="${LLVM_TARGET_ARCH}"
         ${build_type_flags} ${linker_flag} ${external_clang_dir}
     WORKING_DIRECTORY ${LLVM_${target_name}_BUILD}
     DEPENDS CREATE_LLVM_${target_name}
     COMMENT "Configuring ${target_name} LLVM...")
 
   add_custom_target(CONFIGURE_LLVM_${target_name}
                     DEPENDS ${LLVM_${target_name}_BUILD}/CMakeCache.txt)
 
 endfunction()
 
 function(llvm_create_cross_target target_name sysroot)
   llvm_create_cross_target_internal(${target_name} ${sysroot} ${CMAKE_BUILD_TYPE})
 endfunction()
 
 llvm_create_cross_target_internal(NATIVE "" Release)

llvm/trunk/cmake/modules/LLVMExternalProjectUtils.cmake

foreach(variableName ${variableNames})
       if(variableName MATCHES "^${prefix}")
         string(REPLACE ";" "|" value "${${variableName}}")
         list(APPEND PASSTHROUGH_VARIABLES
           -D${variableName}=${value})
       endif()
     endforeach()
   endforeach()
 
-  if(ARG_USE_TOOLCHAIN)
+  if(ARG_USE_TOOLCHAIN AND NOT CMAKE_CROSSCOMPILING)
     if(CLANG_IN_TOOLCHAIN)
       set(compiler_args -DCMAKE_C_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang
                         -DCMAKE_CXX_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang++)
     endif()
     if(llvm-ar IN_LIST TOOLCHAIN_TOOLS)
       list(APPEND compiler_args -DCMAKE_AR=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-ar)
     endif()
     if(llvm-ranlib IN_LIST TOOLCHAIN_TOOLS)
function(llvm_ExternalProject_Add name source_dir)
   if(ARG_EXCLUDE_FROM_ALL)
     set(exclude EXCLUDE_FROM_ALL 1)
   endif()
 
   if(CMAKE_SYSROOT)
     set(sysroot_arg -DCMAKE_SYSROOT=${CMAKE_SYSROOT})
   endif()
 
+  if(CMAKE_CROSSCOMPILING)
+    set(compiler_args -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
+                      -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
+                      -DCMAKE_AR=${CMAKE_AR}
+                      -DCMAKE_RANLIB=${CMAKE_RANLIB})
+    set(llvm_config_path "${LLVM_NATIVE_BUILD}/bin/llvm-config")
+  else()
+    set(llvm_config_path "$<TARGET_FILE:llvm-config>")
+  endif()
+
   ExternalProject_Add(${name}
     DEPENDS ${ARG_DEPENDS} llvm-config
     ${name}-clobber
     PREFIX ${CMAKE_BINARY_DIR}/projects/${name}
     SOURCE_DIR ${source_dir}
     STAMP_DIR ${STAMP_DIR}
     BINARY_DIR ${BINARY_DIR}
     ${exclude}
     CMAKE_ARGS ${${nameCanon}_CMAKE_ARGS}
                ${compiler_args}
                -DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}
                ${sysroot_arg}
                -DLLVM_BINARY_DIR=${PROJECT_BINARY_DIR}
-               -DLLVM_CONFIG_PATH=$<TARGET_FILE:llvm-config>
+               -DLLVM_CONFIG_PATH=${llvm_config_path}
                -DLLVM_ENABLE_WERROR=${LLVM_ENABLE_WERROR}
                -DLLVM_HOST_TRIPLE=${LLVM_HOST_TRIPLE}
                -DLLVM_HAVE_LINK_VERSION_SCRIPT=${LLVM_HAVE_LINK_VERSION_SCRIPT}
                -DPACKAGE_VERSION=${PACKAGE_VERSION}
                -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
                -DCMAKE_MAKE_PROGRAM=${CMAKE_MAKE_PROGRAM}
                -DCMAKE_EXPORT_COMPILE_COMMANDS=1
                ${ARG_CMAKE_ARGS}

llvm/trunk/runtimes/CMakeLists.txt

function(runtime_default_target)
 
     llvm_ExternalProject_Add(runtimes
                              ${CMAKE_CURRENT_SOURCE_DIR}
                              DEPENDS ${ARG_DEPS}
                              # Builtins were built separately above
                              CMAKE_ARGS -DCOMPILER_RT_BUILD_BUILTINS=Off
                                         -DLLVM_INCLUDE_TESTS=${LLVM_INCLUDE_TESTS}
                                         -DLLVM_LIBRARY_DIR=${LLVM_LIBRARY_DIR}
+                                        -DCMAKE_C_COMPILER_TARGET=${TARGET_TRIPLE}
+                                        -DCMAKE_CXX_COMPILER_TARGET=${TARGET_TRIPLE}
+                                        -DCMAKE_ASM_COMPILER_TARGET=${TARGET_TRIPLE}
                                         -DCMAKE_C_COMPILER_WORKS=ON
                                         -DCMAKE_CXX_COMPILER_WORKS=ON
                                         -DCMAKE_ASM_COMPILER_WORKS=ON
                              PASSTHROUGH_PREFIXES LLVM_ENABLE_RUNTIMES
                                                   ${ARG_PREFIXES}
                              EXTRA_TARGETS ${extra_targets}
                                            ${test_targets}
                                            ${SUB_COMPONENTS}