calling convention optimisation & const/immutable ref

[Marc Schütz via Digitalmars-d]

On Monday, 7 November 2016 at 16:48:55 UTC, John Colvin wrote:
> Some people use ref for performance to prevent the copying that 
> must occur when passing by value. I propose a small 
> optimisation to make this unnecessary in a bunch of cases.
>
> At the ABI level (no change in language semantics), if a given 
> parameter would otherwise be copied and passed on the stack:
> 1) immutable parameters to all functions are passed by reference
> 2) const parameters to pure functions are passed by reference, 
> if and only if all other parameters - including any implicit 
> this parameter - contain no mutable references (otherwise 
> "weakly" pure functions could modify the data we are now 
> referring to instead of having copied, changing semantics).

If this is done, it should only apply to parameters larger than a 
certain size, say 2 words. Indirect memory access comes with its 
own cost that is often larger than that of copying.