[cmake] Support host architectures other than x64

[smeenai]

Summary

Allow building for other architectures when cross-compiling for Windows.

[zturner]

cmake/platforms/WinMsvc.cmake

177–181

Can you make these two branches also handle the relatively common spellings "x86" and "x64"?

[smeenai]

Support alternate architecture spellings

cmake/platforms/WinMsvc.cmake

 # Cross toolchain configuration for using clang-cl on non-Windows hosts to
 # target MSVC.
 #
 # Usage:
 # cmake -G Ninja
 #    -DCMAKE_TOOLCHAIN_FILE=/path/to/this/file
 #    -DLLVM_NATIVE_TOOLCHAIN=/path/to/llvm/installation
+#    -DHOST_ARCH=[aarch64|armv7|i686|x86_64]
 #    -DMSVC_BASE=/path/to/MSVC/system/libraries/and/includes
 #    -DWINSDK_BASE=/path/to/windows-sdk
 #    -DWINSDK_VER=windows sdk version folder name
 #    -DWINSDK_CASE_SENSITIVE=[YES|NO]
 #
 # LLVM_NATIVE_TOOLCHAIN:
 #   *Absolute path* to a folder containing the toolchain which will be used to
 #   build.  At a minimum, this folder should have a bin directory with a
 #   copy of clang-cl, clang, clang++, and lld-link, as well as a lib directory
 #   containing clang's system resource directory.
 #
+# HOST_ARCH:
+#    The architecture to build for.
+#
 # MSVC_BASE:
 #   *Absolute path* to the folder containing MSVC headers and system libraries.
 #   The layout of the folder matches that which is intalled by MSVC 2017 on
 #   Windows, and should look like this:
 #
 # ${MSVC_BASE}
 #   include
 #     vector
foreach(library ${libraries})
     string(TOLOWER "${library}" symlink_name)
     execute_process(COMMAND "${CMAKE_COMMAND}"
                             -E create_symlink
                             "${winsdk_um_lib_dir}/${library}"
                             "${output_dir}/${symlink_name}")
   endforeach()
 endfunction()
 
-# FIXME: We should support target architectures other than x64
 set(CMAKE_SYSTEM_NAME Windows)
 set(CMAKE_SYSTEM_VERSION 10.0)
 set(CMAKE_SYSTEM_PROCESSOR AMD64)
 
 init_user_prop(LLVM_NATIVE_TOOLCHAIN)
+init_user_prop(HOST_ARCH)
 init_user_prop(MSVC_BASE)
 init_user_prop(WINSDK_BASE)
 init_user_prop(WINSDK_VER)
 init_user_prop(WINSDK_CASE_SENSITIVE)
 
+if(NOT HOST_ARCH)
+  set(HOST_ARCH x86_64)
+endif()
+if(HOST_ARCH STREQUAL "aarch64")
+  set(WINSDK_ARCH "arm64")
+elseif(HOST_ARCH STREQUAL "armv7")
+  set(WINSDK_ARCH "arm")
+elseif(HOST_ARCH STREQUAL "i686")
+  set(WINSDK_ARCH "x86")
+elseif(HOST_ARCH STREQUAL "x86_64")
+  set(WINSDK_ARCH "x64")
+else()
+  message(SEND_ERROR "Unknown host architecture ${HOST_ARCH}. Must be aarch64, armv7, i686, or x86_64.")
+endif()
+
 set(MSVC_INCLUDE "${MSVC_BASE}/include")
 set(MSVC_LIB "${MSVC_BASE}/lib")
 set(WINSDK_INCLUDE "${WINSDK_BASE}/Include/${WINSDK_VER}")
 set(WINSDK_LIB "${WINSDK_BASE}/Lib/${WINSDK_VER}")
 
 # Do some sanity checking to make sure we can find a native toolchain and
 # that the Windows SDK / MSVC STL directories look kosher.
 if(NOT EXISTS "${LLVM_NATIVE_TOOLCHAIN}/bin/clang-cl" OR

 list(APPEND _CTF_NATIVE_DEFAULT "-DCMAKE_ASM_COMPILER=${LLVM_NATIVE_TOOLCHAIN}/bin/clang")
 list(APPEND _CTF_NATIVE_DEFAULT "-DCMAKE_C_COMPILER=${LLVM_NATIVE_TOOLCHAIN}/bin/clang")
 list(APPEND _CTF_NATIVE_DEFAULT "-DCMAKE_CXX_COMPILER=${LLVM_NATIVE_TOOLCHAIN}/bin/clang++")
 
 set(CROSS_TOOLCHAIN_FLAGS_NATIVE "${_CTF_NATIVE_DEFAULT}" CACHE STRING "")
 
 set(COMPILE_FLAGS
     -D_CRT_SECURE_NO_WARNINGS
+    -Xclang -triple -Xclang ${HOST_ARCH}-pc-windows-msvc19.11.0
     -fms-compatibility-version=19.11
     -imsvc "${MSVC_INCLUDE}"
     -imsvc "${WINSDK_INCLUDE}/ucrt"
     -imsvc "${WINSDK_INCLUDE}/shared"
     -imsvc "${WINSDK_INCLUDE}/um"
     -imsvc "${WINSDK_INCLUDE}/winrt")
 
 if (WINSDK_CASE_SENSITIVE)

 
 set(_CMAKE_CXX_FLAGS_INITIAL "${CMAKE_CXX_FLAGS}" CACHE STRING "")
 set(CMAKE_CXX_FLAGS "${_CMAKE_CXX_FLAGS_INITIAL} ${COMPILE_FLAGS}" CACHE STRING "" FORCE)
 
 set(LINK_FLAGS
     # Prevent CMake from attempting to invoke mt.exe. It only recognizes the slashed form and not the dashed form.
     /manifest:no
 
-    # FIXME: We should support target architectures other than x64.
-    -libpath:"${MSVC_LIB}/x64"
-    -libpath:"${WINSDK_LIB}/ucrt/x64"
-    -libpath:"${WINSDK_LIB}/um/x64")
+    -libpath:"${MSVC_LIB}/${WINSDK_ARCH}"
+    -libpath:"${WINSDK_LIB}/ucrt/${WINSDK_ARCH}"
+    -libpath:"${WINSDK_LIB}/um/${WINSDK_ARCH}")
 
 if(WINSDK_CASE_SENSITIVE)
   # Ensure all sub-configures use the top-level symlinks dir instead of generating their own.
   init_user_prop(winsdk_lib_symlinks_dir)
   if(NOT winsdk_lib_symlinks_dir)
     set(winsdk_lib_symlinks_dir "${CMAKE_BINARY_DIR}/winsdk_lib_symlinks")
-    generate_winsdk_lib_symlinks("${WINSDK_BASE}/Lib/${WINSDK_VER}/um/x64" "${winsdk_lib_symlinks_dir}")
+    generate_winsdk_lib_symlinks("${WINSDK_BASE}/Lib/${WINSDK_VER}/um/${WINSDK_ARCH}" "${winsdk_lib_symlinks_dir}")
     init_user_prop(winsdk_lib_symlinks_dir)
   endif()
   list(APPEND LINK_FLAGS
        -libpath:"${winsdk_lib_symlinks_dir}")
 endif()
 
 string(REPLACE ";" " " LINK_FLAGS "${LINK_FLAGS}")