[dmd-internals] What does POD imply for backends

[Johannes Pfau]

While tracking down a bug in gdc I realized that gdc does nothing 
special for POD/non-POD types yet. That at least caused one bug where 
the runtime va_arg code expected a non-POD parameter to be passed on the 
stack but gcc passes it in registers.

So to implement (non-)POD types correctly in GDC it would be good to know:

* Non-POD types can't be passed in registers for va_arg code. What's the 
reason for this?
* Can Non-PODs be passed in registers to regular, non-variadic functions?
* Can Non-PODs be returned from functions in registers?
* The spec says the backend may create bit copies as convenient. Is this 
true for Non-PODs as well?
* In GCC a non-POD type must always have an address (i.e it can never be 
in registers). Is this true for D as well?

Is there anything more the backend must handle in regard to POD/non-POD 
types?

-- 
Johannes Pfau