forcing weak purity

[Steven Schveighoffer]

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