RFC: reference counted Throwable

[Ola Fosheim Grostad via Digitalmars-d]

On Saturday, 20 September 2014 at 08:39:41 UTC, Walter Bright 
wrote:
>> E.g. TSX is coming even if there is
>> a bug in lower end CPUs. Suggest making performance oriented 
>> prototypes on
>> different architectures before concluding.
>
> I'd love to see it.

A 128 bit CAS instruction is at about 19-25 cycles, but a 
transaction on the other hand can by using xbegin/xend cover both 
refcounting, locking and rollback of multiple objects so you need 
cooperation from code gen. Basically all changes between 
xbegin/xend are kept in cache and written to memory upon success. 
On failure you have a slower fallback.

I don't know what that leads to in amortized cost reduction, but 
30-70% might be possible if it is done right.

> As soon as you pass a reference to a function, that all goes 
> out the window. There's a reason why Rust has invested so much 
> effort in the notion of a "borrowed" pointer.

The pure @nogc crowd care less about safety, but you should be 
able to track this using dataflow?

>> 3. True, but you can keep the refcount at a negative offset 
>> for new-based
>> allocations. Besides it only affects those who do @nogc and 
>> they should know
>> what they are doing.
>
> If this is so simple, why doesn't everyone do it?

Never said performance and thread safe RC was easy. It is 
probably difficult to get below 10 cycles for inc/dec pairs even 
with excellent code gen...? And probably closer to 40 cycles for 
regular code gen. Just guessing.