DIP 1024---Shared Atomics---Final Review

RazvanN razvan.nitu1305 at gmail.com
Wed Dec 11 13:28:36 UTC 2019

On Wednesday, 11 December 2019 at 13:12:45 UTC, Petar Kirov 
[ZombineDev] wrote:
> On Wednesday, 11 December 2019 at 11:21:22 UTC, RazvanN wrote:
>> If you think from an implementation point of view, the 
>> compiler will most likely search for specific functions that 
>> can be called on shared variables (a finite list that contains 
>> only the functions defined in druntime). The only way you can 
>> override that is to define functions that have exactly the 
>> same name as the ones in druntime, but that is risky because 
>> the compiler does not have any way of verifying that you are 
>> actually synchronizing the access.
> That's not true. The compiler has no knowledge of the existence 
> of core.atomic (modulo suggesting the user to use it in some 
> error messages). You can pass shared data to any function 
> parameter that is also marked as shared. As such, `shared` 
> works like any other type qualifier.

I see, so basically the compiler simply checks that accesses to 
shared data are made by calling functions that receive shared 
data. The leafs will modify the data via inline assembly or 
external functions.

>> Yes, the function in core atomic are templates, but if there 
>> are internal calls to non-templated functions you will end up 
>> with a link error.
>>> In short, using shared in betterC may be harder, but it's far 
>>> from unusable.
>>> --
>>>   Simen
> As mentioned previously, atomic ops on `shared` data work 100% 
> no matter if you're compiling with -betterC or not. In the 
> worst case, if one would not even want to import core.atomic (I 
> don't see a reason for that), they can still:
> 1. Copy and paste the parts of core.atomic that they need in 
> their code base. The names of the function doesn't need to 
> match at all.
> 2. Use other libraries (e.g. from C and C++) that implement 
> atomic operations. To prevent unnecessary casting from 
> `shared`, the `extern (C)` / `extern (C++)` signatures should 
> use the `shared` type qualifier for parameters.
> 3. Implement the atomic ops that they need themselves.

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