http://d.puremagic.com/issues/show_bug.cgi?id=10165

           Summary: No syntax to create thread-local shared variables
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: pull
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: GenericNPC@gmail.com


--- Comment #0 from IdanArye <GenericNPC@gmail.com> 2013-05-24 14:22:39 PDT ---
Writing

    class Foo
    {
        shared Foo a;
        static Foo b;
        static shared Foo c;
    }

`a` is an instance-local shared reference. `b` is a thread-local not-shared reference. You would expect that `c` would be thread-local shared reference - but instead it's a process-global shared reference.

I do not suggest changing the behaviour of `static shared` - it is the syntax for process-global shared variables by design, and changing it will break too much code. But we do need a way to create thread-local shared variables.

I've implemented a library solution, but I believe this problem requires a compiler solution that's why the component field is "dmd".

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Steven Schveighoffer <schveiguy@yahoo.com> changed:

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--- Comment #1 from Steven Schveighoffer <schveiguy@yahoo.com> 2013-05-24 14:30:04 PDT ---
I'm almost positive this works:

static shared(Foo) a;

That is, if shared is interpreted as a storage class, it puts the variable in global storage, not thread local.  But shared as a type constructor just affects the type.

Don't have the patience to test, so I'll leave the bug open :)

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--- Comment #2 from IdanArye <GenericNPC@gmail.com> 2013-05-24 14:43:55 PDT ---
Here is the pull request: https://github.com/D-Programming-Language/phobos/pull/1302

@Steven Schveighoffer: It doesn't work - `static shared(T)` does the same as
`static shared T`.

I assume the proper solution would be to fix this, but I leave the pull request open anyways. Like I said - there should be a compiler solution - but we don't know how many people use the `static shared(T)` syntax with the intention of declaring a process-global variable, and we can't show a warning message when this syntax is used wrongly - so the dmd solution requires some serious debate. In the meanwhile, the Phobos solution doesn't break anything, and it took me 20 minutes: 1 for the implementation, 2 for the unit test, and the rest for all the "paperwork"(=sending this bug and the pull request) - so I planned from the beginning to send it and let the dev team decide if they want to use my Phobos solution or to fix it in dmd.

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bearophile_hugs@eml.cc changed:

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--- Comment #3 from bearophile_hugs@eml.cc 2013-05-24 15:22:55 PDT ---
(In reply to comment #2)

> and let the dev team decide if they want to use my Phobos solution or to fix it in dmd.

Where possible in D we prefer to fix the language to help all future D programmer.

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Walter Bright <bugzilla@digitalmars.com> changed:

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--- Comment #4 from Walter Bright <bugzilla@digitalmars.com> 2013-05-24 15:42:00 PDT ---
(In reply to comment #0)
>  But we do need a way to create thread-local shared variables.

Why? What is the motivating example?

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--- Comment #5 from IdanArye <GenericNPC@gmail.com> 2013-05-24 16:22:30 PDT ---
(In reply to comment #4)
> (In reply to comment #0)
> >  But we do need a way to create thread-local shared variables.
> 
> Why? What is the motivating example?

I'm implementing the low lock singleton - http://forum.dlang.org/thread/pelhvaxwjzhehdjtpsav@forum.dlang.org - as part of my `std.idioms` library - http://forum.dlang.org/thread/fofbrlqruxbevnxchxdp@forum.dlang.org - in both `shared` and `__gshared` versions(I also have a thread-local(=`static`) version, but it doesn't use the low lock implementation)

A small optimization in the implementation is instead of using a thread-local boolean indicator to determine if the singleton has been initialized yet, to save a thread-local reference to the actual instance object. That way, instead of accessing the memory twice(once to check the indicator and once to fetch the object) we only need to access it once.

Now, doing it in the `__gshared` version was straight-forward, but the `shared` version posed a problem - the need to have a thread-local reference to a shared object - hence this bug report and pull request.

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--- Comment #6 from Walter Bright <bugzilla@digitalmars.com> 2013-05-24 19:35:18 PDT ---
You can have a thread-local reference to a shared object:

    static shared(T)* p;

p is thread local, and it points to a shared instance of T.

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Dmitry Olshansky <dmitry.olsh@gmail.com> changed:

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--- Comment #7 from Dmitry Olshansky <dmitry.olsh@gmail.com> 2013-05-24 22:32:09 PDT ---
(In reply to comment #6)
> You can have a thread-local reference to a shared object:
> 
>     static shared(T)* p;
> 
> p is thread local, and it points to a shared instance of T.

The bug report is about classes. With T* that would be double indirection.

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