Very limited shared promotion

[Exil]

On Wednesday, 19 June 2019 at 03:02:30 UTC, Manu wrote:
> On Wed, Jun 19, 2019 at 12:15 AM Timon Gehr via Digitalmars-d 
> <digitalmars-d at puremagic.com> wrote:
>>
>> On 18.06.19 15:29, Steven Schveighoffer wrote:
>> > On 6/17/19 7:46 PM, Manu wrote:
>> >> Is this valid?
>> >>
>> >> int x;
>> >> void fun(scope ref shared(int) x) { ... }
>> >> fun(x); // implicit promotion to shared in this case
>> >>
>> >> This appears to promote a thread-local to shared. The 
>> >> problem with such promotion is that it's not valid that a 
>> >> thread-local AND a shared reference to the same thing can 
>> >> exist at the same time.
>> >>
>> >> With scope, we can guarantee that the reference doesn't 
>> >> escape the
>> >> callee.
>> >> Since the argument is local to the calling thread, and 
>> >> since the
>> >> calling thread can not be running other code at the same 
>> >> time as the
>> >> call is executing, there is no way for any code to execute 
>> >> with a
>> >> thread-local assumption while the callee makes shared 
>> >> assumptions.
>> >>
>> >> I think this might be safe?
>> >>
>> >
>> > Seems like it would be safe, but what is the use case for 
>> > it? If you have a scope shared, which can't actually get 
>> > shared with anything else, what does it buy you?
>> >
>> > -Steve
>>
>> He would write @system code that leaks the reference to other 
>> threads temporarily, but ensures those references disappear 
>> once the parallel part of the computation has finished. His 
>> use case is @safe parallel foreach without having to 
>> explicitly qualify the variables in the outer scope as 
>> `shared`:
>>
>> void sum(){ // note: just to illustrate the concept
>>      int result=0;
>>      foreach(i;iota(1000).parallel){
>>          static assert(typeof(result)==shared(int));
>>          result.atomic!"+="(i);
>>      }
>>      static assert(typeof(result)==int);
>>      return result;
>> }
>
> 👍
>
> We must get here.

How is it to know it is supposed to be shared though? opApply is 
already kind of a disaster with attributes.