`in` parameters made useful

[Petar]

On Thursday, 20 August 2020 at 20:29:31 UTC, Araq wrote:
> On Thursday, 20 August 2020 at 17:31:17 UTC, IGotD- wrote:
>> This is interesting on a general level as well and true for 
>> several programming languages. Let the compiler optimize the 
>> parameter passing unless the programmer explicitly ask for a 
>> certain way (copy object, pointer/reference etc.). This is 
>> very unusual and if you have a language that optimizes the 
>> parameter passing by default, please mention it because it 
>> would be interesting.
>>
>
> Nim does this and I took the feature from Ada. You can override 
> the behavior with pragmas but I've only seen that done for C 
> interop, not for optimizations as the compiler always seems to 
> get it right.

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`)?