About ref used for performance reasons with struct

[Namespace]

On Monday, 11 February 2013 at 10:53:40 UTC, deadalnix wrote:
> On Monday, 11 February 2013 at 10:04:55 UTC, Namespace wrote:
>> But what if we want to pass a struct with intent as value?
>> With this solution, we don't have much control.
>> In general, I like the idea, but we should mark such parameter
>> with '&' or whatever, so at least we can still take some 
>> influence.
>
> That is the whole point, everything appear as if it is passed 
> by value.
>
> struct A {
>     uint member;
>     this(this) {
>         // Very long but pure operation.
>     }
> }
>
> void main() {
>     A a;
>     foo(a);
>
>     assert(a.member = 0); // Pass.
> }
>
> void foo(A a) { // Compiler can choose to pass by ref here.
>     bar(a);
> }
>
> void bar(A a) { // If compiler choose to pass by ref, then it 
> is bar responsibility to create a copy. A smarter move from the 
> compiler is to mark bar as non electable for such optimization.
>     a.member = 5;
> }
>
> In the example above, the compiler is allowed to pass a by ref 
> to foo. As a consequence, the very long operation within the 
> postblit can only be executed once, instead of 2.

I see. Nice idea. But what is the criterion for the compiler to 
pass 'a' by ref?
I'm still therefor, to mark such paramters with '&' or something 
but that's maybe my C++ background.
I'm curious to see if Walter or Andrei say something to the idea.