forcing weak purity

[Alex Rønne Petersen]

On 23-05-2012 17:56, Steven Schveighoffer wrote:
> On Wed, 23 May 2012 11:41:00 -0400, Alex Rønne Petersen
> <alex at lycus..org> wrote:
>
>> On 23-05-2012 17:29, Don Clugston wrote:
>
>>> Not so. It's impossible for anything outside of a strongly pure function
>>> to hold a pointer to memory allocated by the pure function.
>>
>> Not sure I follow:
>>
>> immutable(int)* foo() pure
>> {
>> return new int;
>> }
>>
>> void main()
>> {
>> auto ptr = foo();
>> // we now have a pointer to memory allocated by a pure function?
>> }
>
> I think what Don means is this:
>
> 1. upon entry into a strong-pure function, record a GC context that
> remembers what point in the stack it entered (no need to search above
> that stack), and uses its parameters as "context roots".
> 2. Any collection performed while *in* the strong-pure function
> explicitly will simply deal with the contexted GC data. It does not need
> to look at the main heap, except for those original roots.
> 3. upon exiting, you can remove the original roots, and add the return
> value as a root, and run one final GC collection within the context.
> This should deterministically clean up any memory that was temporary
> while inside the pure function.
> 4. Anything that is left in the contexted GC is assimilated into the
> main GC.
>
> Everything can be done without using a GC lock *except* the final
> assimilation (which may not need to lock because there is nothing to add).
>
> Don, why can't gc_collect do the right thing based on whether it's in a
> contexted GC or not? The compiler is going to have to initialize the
> context when first entering a strong-pure function, so we should be able
> to have a hook recording that the thread is using a pure-function
> context, no?
>
> Also, let's assume it's a separate call to do pure function gc_collect,
> i.e. we have a pure gc_pureCollect function.
>
> What if a weak-pure function calls this? What happens when it is not
> called from within a strong-pure function?
>
> I still think gc_collect can be marked pure and do the right thing.
>
> -Steve

I still don't think my concern about pure functions allocating tons of 
memory (and thus requiring global GC to happen) has been addressed, though.

-- 
Alex Rønne Petersen
alex at lycus.org
http://lycus.org