Want reasonable reference counting? Disable automatic sharing of immutable

[Steven Schveighoffer]

On 11/12/21 12:46 PM, deadalnix wrote:
> I really don't think this makes sense.
> 
> Providing people with tool to work on multiple cores (and we are 
> aggressively moving toward heterogeneous compute, so it's getting worse).

`shared(immutable)` is fine for such a task. You only need to atomically 
load any `__mutable` pieces inside.

having immutable implicitly shared is stupid when it's obviously not. 
Like an immutable stack variable, why should I have to worry about 
sharability there? Should I even be able to share it with another 
thread, as the stack is easily destroyed?

> On the other hand, atomic are becoming cheaper and cheaper with each 
> generation of pretty much every platforms. This is mostly due to the 
> fact most machines now do regular loads/stores optimistically and 
> speculate from there, just like they do for branch predictions. If it 
> turns out there was contention then they rewind and revert back to do it 
> in a more expensive fashion.

Removing implicit sharing from immutable doesn't change what shared 
immutable means, or affect the performance of it, it just means you need 
to be explicit about sharing.

-Steve