DIP74 updated with new protocol for function calls

[Manu via Digitalmars-d]

On 1 March 2015 at 18:57, Matthias Bentrup via Digitalmars-d
<digitalmars-d at puremagic.com> wrote:
> On Sunday, 1 March 2015 at 07:04:09 UTC, Zach the Mystic wrote:
>>
>> On Saturday, 28 February 2015 at 21:12:54 UTC, Andrei Alexandrescu wrote:
>>>
>>> Defines a significantly better function call protocol:
>>>
>>> http://wiki.dlang.org/DIP74
>>>
>>> Andrei
>>
>>
>> This is obviously much better, Andrei.
>>
>> I think an alternative solution (I know -- another idea -- against my own
>> first idea!) is to keep track of this from the caller's side. The compiler,
>> in this case, when copying a ref-counted type (or derivative) into a
>> parameter, would actually check to see if it's splitting the variable in
>> two. Look at this:
>>
>> class RcType {...}
>>
>> void fun(RcType1 c, RcType1 d);
>>
>> auto x = new RcType;
>>
>> fun(x, x);
>>
>> If the compiler were smart, it would realize that by splitting parameters
>> this way, it's actually adding an additional reference to x. The function
>> should get one x for free, and then force an opAdd/opRelease, for every
>> additional x (or x derivative) it detects in the same call.
>>
>> This might be even better than the improved current proposal. The real key
>> is realizing that duplicating an lvalue into the same function call is
>> subtly adding a new reference to it.
>>
>> Eh??
>
>
> Note that you can get the same issue without duplicate parameters, if you
> pass an alias to a global variable.
>
>
> static A a;
>
> void fun(A x) {
>   a = null; // Releases x
>   x.func();
> }
>
> void main() {
>   a = new A();
>   fun(a);
> }

Thing here is that 'a' does not belong to main().
There must be a reference on the caller's stack for it to be passed to
a non-pure function.
If a function X passes a value it does not own to some other function,
then X needs to increment the refcount such that it holds one
reference for itself.
For the callee, it received via an argument, so it has confidence a
stack reference exists, and it doesn't need to bump anymore down the
call tree.