ARM64EC is a new ARM64 ABI designed by Microsoft to support x64 application emulation on ARM64 CPUs. When compiling for ARM64EC, both the _M_X64 and _M_ARM64EC macros are defined. However, that causes problem in compiling this file, because the __movsb intrinsic, which is lowered to rep movsb, is not supported on ARM64, so the optimization for native x64 should be disabled for ARM64EC.