Difference between __gshared and shared.

[Jonathan M Davis via Digitalmars-d]

On Thursday, 9 July 2015 at 14:57:56 UTC, Márcio Martins wrote:
> On Thursday, 9 July 2015 at 14:03:18 UTC, Jonathan M Davis 
> wrote:
>> On Thursday, 9 July 2015 at 12:39:00 UTC, Márcio Martins wrote:
>>>[...]
>> Well, the compiler is free to assume that a variable that is 
>> not marked as shared is thread-local. So, it's free to make 
>> optimizations based on that. So, for instance, it can know for 
>> a fact that
>>
>> [...]
>
> But this is what a C/C++ compiler would do, unless you your 
> data is qualified as volatile. I believe __gshared also implies 
> the volatile behavior, right? I wouldn't make sense any other 
> way.
>
> So basically, __gshared is like saying "I want the C/C++ 
> behavior, and I accept I am all on my own as the compiler will 
> not help me".

Sort of, but the assumptions that the D compiler is allowed to 
make aren't the same. Regardless of shared/__gshared itself, D's 
const is very different, and C++ doesn't have const or immutable. 
And the D compiler devs can add whatever optimizations they want 
based on what those features guarantee so long as they can prove 
that they're correct, which changes what what a D compiler is 
allowed to optimize in comparison to a C++ compiler. So, if you 
make assumptions on what's valid based purely on C++, you risk 
shooting yourself in the foot.

__gshared is really only meant for interacting with C APIs, and 
if you're using it for other stuff, you're just begging for 
trouble. You might get away with it at least some of the time, 
but it really isn't a good idea to try.

- Jonathan M Davis