calling convention optimisation & const/immutable ref

[Jacob Carlborg via Digitalmars-d]

On 2016-11-07 17:48, 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).
>
> The main benefit from this would be:
>
> class C
> {
>   void doSomething(const float[16] m0) const pure
>   {
>     // ...
>   }
>
>   void doSomethingElse(immutable float[16] m1)
>   {
>     // ...
>   }
> }
>
> void foo(C c, float[16] m0, immutable float[16] m1)
> {
>   // ...
>   c.doSomething(m0); // no copying of m0 here
>   c.doSomethingElse(m1); // or m1 here
>   // ...
> }
>
> I presume this has been thought of before, is there a reason why we
> don't do it already? I guess 2) is a bit complicated, but 1) is easy.

Since D is a systems programming language, wouldn't the user want to 
have control over this?

-- 
/Jacob Carlborg