[bug] Force CMAKE_OSX_ARCHITECTURES in sync with host processor's arc…

…hitecture to avoid ABI issues on Mac m1 (taichi-dev#5952)

CMakeLists.txt

@@ -6,6 +6,20 @@ cmake_minimum_required(VERSION 3.17)
 
 project(taichi)
 
+# Taichi does not set target architecture explicitly,
+# but rather rely on CMake to detect the host arch.
+#
+# However on Mac m1, there are two available architectures namely x86_64 and arm64.
+# On some combination of "OSX version" and "CMake version", CMake will use x86_64 as default architecture even if it's on m1 chip.
+# This causes conflicts with the precompiled LLVM/Clang binaries downloaded from Taichi's repo (pre-built for arm64)
+#
+# Therefore we force CMake to choose arm64 architecture on arm64 chips.
+if (APPLE)
+  if( "${CMAKE_HOST_SYSTEM_PROCESSOR}" STREQUAL "arm64" )
+    set(CMAKE_OSX_ARCHITECTURES ${CMAKE_HOST_SYSTEM_PROCESSOR})
+  endif()
+endif()
+
 if (NOT DEFINED TI_VERSION_MAJOR)
     message(WARNING "It seems that you are running cmake manually, which may cause issues. Please use setup.py to build taichi from source, see https://docs.taichi-lang.org/docs/dev_install for more details.")
     set(TI_VERSION_MAJOR 0)