What exactly shared means?

[John Colvin via Digitalmars-d-learn]

On Friday, 2 January 2015 at 23:26:57 UTC, Jonathan M Davis via 
Digitalmars-d-learn wrote:
> On Friday, January 02, 2015 19:47:50 John Colvin via 
> Digitalmars-d-learn wrote:
>> On Friday, 2 January 2015 at 13:14:14 UTC, Jonathan M Davis via
>> Digitalmars-d-learn wrote:
>> > Objects in D default to being thread-local. __gshared and
>> > shared both make
>> > it so that they're not thread-local. __gshared does it 
>> > without
>> > actually
>> > changing the type, making it easier to use but also dangerous
>> > to use,
>> > because it makes it easy to violate the compiler's 
>> > guarantees,
>> > because it'll
>> > treat it like a thread-local variable with regards to
>> > optimizations and
>> > whatnot.
>>
>> I'm pretty sure that's not true. __gshared corresponds to 
>> C-style
>> globals, which are *not* assumed to be thread-local (see 
>> below).
>
> No, the type system will treat __gshared like a thread-local 
> variable. It
> gets put in shared memory like a C global would be, but 
> __gshared isn't
> actually part of the type, so the compiler has no way of 
> knowing that it's
> anything other than a thread-local variable - which is 
> precisely why it's so
> dangerous to use it instead of shared. For instance,
>
> __gshared int* foo;
>
> void main()
> {
>     foo = new int;
>     int* bar = foo;
> }
>
> will compile just fine, whereas if you used shared, it wouldn't.
>
> - Jonathan M Davis

I understand that. As far as optimisations and codegen go, that 
is not the same as being able to assume that something is 
thread-local, far from it.

The rule (in C(++) at least) is that all data is assumed to be 
visible and mutable from multiple other threads unless proved 
otherwise. However, given that you do not write a race, the 
compiler will provide full sequential consistency. If you do 
write a race though, all bets are off.

Are you telling me that D does not obey the C(++) memory model? 
That would be a fatal hole in our C(++) interoperability.

AFAIK, the only data in D that the compiler is allowed to assume 
to be thread-local is data that it can prove is thread-local. The 
trivial case is TLS, which is thread-local by definition.