[Dlang-study] [lifetime] Local garbage collector for pure functions

[Andrei Alexandrescu]

Very nice idea, and realizable regardless of other work on lifetime 
management. Could you please submit it as a bugzilla issue?

Thanks much, awesome idea!


Andrei

On 12/23/2015 07:49 AM, Ralph Tandetzky wrote:
> Some pure functions use the garbage collector, but the allocated memory
> cannot escape the function due to its signature. Therefore all the
> created garbage could be locally collected at function exit.
>
>      int f( const int[] arr ) pure; // @localgc ?
>
> The only requirements on the function would be, that they are pure and
> that return types and argument types do not allow allocated memory to
> escape.
>
> And here's how I imagine it to work: At function entry the GC could be
> instructed to use a region allocator. At function exit the whole
> allocated memory is discarded all at once. The only problem I see with
> this, is that internally functions could be called which allocate memory
> in a tight loop producing to much garbage. Hence, collecting the garbage
> may be necessary in extreme cases. Therefore, the region allocator
> should not allow to much memory to be allocated, but fall back to
> garbage collection, if a lot of memory is requested. The good news is
> though, that this garbage collection can be local to our pure function.
>
> Therefore, it is not necessary to freeze the whole application or to
> scan the whole stack and global memory. Instead, only the running thread
> needs to scan the stack from our function upwards and the local pool of
> allocated memory and collect there.
>
> Pushing it all a little further: In templated code, where the all
> function definitions are visible to the compiler and no allocations are
> done in loops, heap allocations could be transformed into stack
> allocations by the compiler and the functions could even be marked
> @nogc. Standard library algorithms could use lambdas with captures like
> crazy again and do things idiomatically and functional in D while still
> ensuring @nogc.
>
> What do you think?
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