tuple of types, each element can be passed in a register. A tuple of zero length means the type cannot be passed/returned in registers. null indicates a void. References: For 64 bit code, follows Itanium C++ ABI 1.86 Chapter 3 http://refspecs.linux-foundation.org/cxxabi-1.86.html#calls
This breaks a type down into 'simpler' types that can be passed to a function in registers, and returned in registers. It's highly platform dependent.