DIP 1024--Shared Atomics--Community Review Round 1

[Manu]

On Sat, Oct 12, 2019 at 4:40 PM IGotD- via Digitalmars-d
<digitalmars-d at puremagic.com> wrote:
>
> On Saturday, 12 October 2019 at 21:28:36 UTC, Jonathan M Davis
> wrote:
> >
> > Right now, we basically have #2. What this DIP is trying to do
> > is move us to #1. How libraries should work is exactly the same
> > in either case. It's just that with #1, the places where you
> > operate on shared data in a manner which isn't guaranteed to be
> > atomic, the compiler prevents you from doing it unless you  use
> > core.atomic or have @system code with casts. Even if we have #2
> > and thus no such compiler errors, the code should still have
> > been doing what #1 would have required, since if it doesn't,
> > then it isn't thread-safe.
>
> With this DIP, shared integers/small types will be automatically
> atomic.

That's not what this DIP says, as had been clarified by Walter a
number of times now.

> For complex/large types, will you still be able to use
> them as before between threads and you have protect the type
> yourself at least for a transitional period?
>
> "Atomic" here as I get it also mean atomically updating complex
> types. This usually means that you need to guard the operations
> with some kind of mutex. The compiler can of course detect this
> and issue a warning/error to the user which doesn't seem to be
> the scope of this DIP.
>
> Correct me if I'm wrong but we have the following scenarios.
> 1. shared integer/simple type (size dependent?) -> automatically
> HW atomic operations

This is the misunderstanding through most of this thread.

> 2. shared complex type -> write to any member must be protected
> with a mutex.
> 3. shared complex type -> read to any member must be protected
> with a mutex or read/write mutex allowing multiple reads.
>
> The compiler is used the detect these scenarios so that the user
> doesn't forget protecting the shared types.
>
> As I get it this DIP is just a baby step towards this bigger
> scope for the shared keyword.

Useful libraries will follow.