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.
|t||type to break down|
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
For 64 bit code, follows Itanium C++ ABI 1.86 Chapter 3