`in` parameters made useful

[Araq]

On Friday, 21 August 2020 at 13:59:30 UTC, Petar Kirov 
[ZombineDev] wrote:
> Interesting, how does it work? As mentioned previously, the 
> issue with choosing by-val vs by-ref passing solely based on 
> the type is that it doesn't take into consideration the ABI and 
> how many registers are available. E.g. if you have more than X 
> parameters some will need to be spilled on the stack anyway 
> (even if they can fit in a GPR).
>
> Can the Nim compiler (I'm guessing `extccomp`) query the C/C++ 
> compiler backend for e.g. the number of registers available for 
> parameters? Or you have this logic built into the frontend (say 
> `ccgtypes`)?

It's built into cctypes indeed. The logic is mostly "pass by 
pointer if sizeof(T) > 3 machine words". ABI and registers are 
not relevant all that much, what you want to prevent are copies 
of large sizes.