https://d.puremagic.com/issues/show_bug.cgi?id=12133

           Summary: Relaxed read-modify-write for shared lvalues
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: druntime
        AssignedTo: nobody@puremagic.com
        ReportedBy: stanislav.blinov@gmail.com


--- Comment #0 from Stanislav Blinov <stanislav.blinov@gmail.com> 2014-02-11 15:31:31 PST ---
TDPL prohibits using operators on shared variables. I.e. this:

shared int i;
++i;

is illegal. It is stated that core.atomic should be used instead:

shared int i;
atomicOp!"+="(i, 1);

However, there are cases when this can introduce unnecessary performance loss: e.g. private shared variables that are only ever accessed under a lock.

I'm proposing to add two functions to core.atomic to allow for operations on shared variables without atomicOp()'s performance penalties:

1) localOp:

performs and acquire-op-release, e.g.:

shared int i;
auto v = localOp!"+="(i, 1);

Useful when there's still need for ordering. Though I don't really like the name "localOp".

2) assumeLocal:

converts shared lvalue to a non-shared lvalue, e.g:

shared int i;
++assumeLocal(i);

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Andrej Mitrovic <andrej.mitrovich@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |andrej.mitrovich@gmail.com


--- Comment #1 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2014-02-11 15:36:23 PST ---
(In reply to comment #0)
> 2) assumeLocal:
> 
> converts shared lvalue to a non-shared lvalue, e.g:
> 
> shared int i;
> ++assumeLocal(i);

shared int i;
++cast()i;

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--- Comment #2 from Stanislav Blinov <stanislav.blinov@gmail.com> 2014-02-11 15:39:09 PST ---
(In reply to comment #1)

> shared int i;
> ++cast()i;

cast()i would cast away everything, including const, silently :)

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--- Comment #3 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2014-02-11 15:49:51 PST ---
(In reply to comment #2)
> (In reply to comment #1)
> 
> > shared int i;
> > ++cast()i;
> 
> cast()i would cast away everything, including const, silently :)

Oops. Maybe something like:

-----
template Unshared(T)
{
         static if (is(T U == shared inout const U)) alias Unshared = U;
    else static if (is(T U == shared inout       U)) alias Unshared = U;
    else static if (is(T U == shared       const U)) alias Unshared = U;
    else static if (is(T U == shared             U)) alias Unshared = U;
    else                                             alias Unshared = T;
}

ref unshared(T)(ref T var)
{
    return *cast(Unshared!T*)&var;
}

void main()
{
    shared int x;
    ++unshared(x);
    assert(x == 1);
}
-----

Unshared code based off of Unqual.

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--- Comment #4 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2014-02-11 15:50:29 PST ---
(In reply to comment #3)
> template Unshared(T)
> {
>          static if (is(T U == shared inout const U)) alias Unshared = U;
>     else static if (is(T U == shared inout       U)) alias Unshared = U;
>     else static if (is(T U == shared       const U)) alias Unshared = U;
>     else static if (is(T U == shared             U)) alias Unshared = U;
>     else                                             alias Unshared = T;
> }

Uhh that should teach me to copy-paste, the RHS should simply strip shared, not strip all the qualifiers.

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--- Comment #5 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2014-02-11 15:51:03 PST ---
(In reply to comment #4)
> Uhh that should teach me to copy-paste, the RHS should simply strip shared, not strip all the qualifiers.

Fixed:

template Unshared(T)
{
         static if (is(T U == shared inout const U)) alias Unshared = inout
const U;
    else static if (is(T U == shared inout       U)) alias Unshared = inout U;
    else static if (is(T U == shared       const U)) alias Unshared = const U;
    else static if (is(T U == shared             U)) alias Unshared = U;
    else                                             alias Unshared = T;
}

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--- Comment #6 from Stanislav Blinov <stanislav.blinov@gmail.com> 2014-02-11 15:57:16 PST ---
Something a bit less involving :)

ref auto assumeLocal(T)(ref shared T var) @trusted pure nothrow
{
    return *cast(T*)&var;
}

I should've probably noted that I already have implementations for both functions in mind, just not in the state of pull request yet.

But comments, suggestions, objections and improvements are key!

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--- Comment #7 from Stanislav Blinov <stanislav.blinov@gmail.com> 2014-02-11 16:01:43 PST ---
See also the discussion thread:

http://forum.dlang.org/thread/moyyibrpnnmrrovylkui@forum.dlang.org

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Stanislav Blinov <stanislav.blinov@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |pull


--- Comment #8 from Stanislav Blinov <stanislav.blinov@gmail.com> 2014-02-14 22:15:42 PST ---
Pull request for assumeLocal():
https://github.com/D-Programming-Language/druntime/pull/724

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Andrei Alexandrescu <andrei@erdani.com> changed:

           What    |Removed                     |Added
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                 CC|                            |andrei@erdani.com


--- Comment #9 from Andrei Alexandrescu <andrei@erdani.com> 2014-03-15 20:38:58 PDT ---
localOp should be atomicOpRelaxed

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