[swift-dev] Implementation of Type Erasers

[Chris Eidhof]

Hm. I'm not sure if the performance difference is a hard limitation because
of implementation decisions, or if it would be possible to make the current
implementation as fast as the closure-based implementation. I don't
understand enough about the codegen (and about possible optimizations) to
make that decision. Anyone who's willing to help me there?

On Thu, Sep 15, 2016 at 5:01 PM, Dave Abrahams via swift-dev <
swift-dev at swift.org> wrote:

>
> on Tue Sep 06 2016, Chris Eidhof <swift-dev-AT-swift.org> wrote:
>
> > Thanks! That explains a lot.
> >
> > In my version, performance only is improved by 10%, so I'm not sure if
> the
> > tradeoff is worth it:
> >
> > In the isolated case of AnyIterator, there's only one method, so the
> sizeof
> > is the same (and the performance is worth it). In the case of
> AnySequence,
> > it would be a lot more work to write the implementation (and the size
> would
> > change a lot). Having two different mechanisms for type erasure seems
> like
> > it might confuse things even more.
>
> We'll happily take a litte more implementation complexity in exchange
> for performance gains.  10% is nothing to sneeze at.
>
> > On Mon, Sep 5, 2016 at 9:50 PM, Dmitri Gribenko <gribozavr at gmail.com>
> wrote:
> >
> >> On Mon, Sep 5, 2016 at 11:13 AM, Chris Eidhof via swift-dev
> >> <swift-dev at swift.org> wrote:
> >> > Hello swift-dev,
> >> >
> >> > I was wondering why type erasers (e.g. for AnyIterator) are
> implemented
> >> the
> >> > way they are. I would implement AnyIterator like this:
> >> >
> >> > final class MyAnyIterator<A>: IteratorProtocol {
> >> >     var nextImpl: () -> A?
> >> >
> >> >     init<I: IteratorProtocol>(iterator: I) where I.Element == A {
> >> >         var copy = iterator
> >> >         nextImpl = { copy.next() }
> >> >     }
> >> >
> >> >     func next() -> A? {
> >> >         return nextImpl()
> >> >     }
> >> > }
> >> >
> >> > Instead, it is implemented in a class which contains a box.
> >>
> >> Your approach requires adding a new closure property for every method
> >> that is forwarded.  Subclassing does not require that.
> >>
> >> If you see a case where performance is not good, we would appreciate a
> >> bugreport at https://bugs.swift.org/ .  Please attach a self-contained
> >> reproducer.  Thank you.
> >>
> >> Dmitri
> >>
> >> --
> >> main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
> >> (j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/
> >>
>
> --
> -Dave
>
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-- 
Chris Eidhof
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