After you read that, you might want to look at the LLVM source

https://github.com/llvm-mirror/llvm/blob/master/lib/Target/X...

  /// The C convention as implemented on Windows/x86-64 and
  /// AArch64. This convention differs from the more common
  /// \c X86_64_SysV convention in a number of ways, most notably in
  /// that XMM registers used to pass arguments are shadowed by GPRs,
  /// and vice versa.
  /// On AArch64, this is identical to the normal C (AAPCS) calling
  /// convention for normal functions, but floats are passed in integer
  /// registers to variadic functions.