Dynamic Closure + Lazy Arguments = Performance Killer?

[Jason House]

Bill Baxter Wrote:

> On Sat, Oct 25, 2008 at 5:24 PM, Jason House
> <jason.james.house at gmail.com> wrote:
> > bearophile Wrote:
> >
> >> Jason House:
> >> > The following spends 90% of its time in _d_alloc_memory
> >> > void bar(lazy int i){}
> >> > void foo(int i){ bar(i); }
> >> > void main(){ foreach(int i; 1..1000000) foo(i); }
> >> > Compiling with -O -release reduces it to 88% :)
> >>
> >> I see. So I presume it becomes quite difficult for D2 to compute up to the 25th term of this sequence (the D code is in the middle of the page) (it takes just few seconds to run on D1):
> >> http://en.wikipedia.org/wiki/Man_or_boy_test
> >>
> >> What syntax can we use to avoid heap allocation? Few ideas:
> >>
> >> void bar(lazy int i){} // like D1
> >> void bar(scope lazy int i){} // like D1
> >> void bar(closure int i){} // like current D2
> 
> This makes no sense because the writer of bar has no idea whether the
> caller will need a heap allocation or not.
> 
> > I would assume a fix would be to add scope to input delegates and to require some kind of declaration on the caller's side when the compiler can't prove safety. It's best for ambiguous cases to be a warning (error). It also makes the code easier for readers to follow.
> 
> I think for a language like D,  hidden, hard to find memory
> allocations like the one Andrei didn't know he was doing should be
> eliminated.  By that I mean stack allocation (D1 behavior) should be
> the default.  Then for places where you really want a closure, some
> other syntax should be chosen.  The other reason I say that is that so
> far in D I've only very seldom really wanted an allocated closure.  So
> I think I will have to use the funky no-closure-please syntax way more
> than I would have to use a make-me-a-closure-please syntax.
> 
> But apparently nobody who knows anything about what's actually going
> to happen is involved in this discussion, so I think I'll just pipe
> down for now.
> 
> --bb

While I agree that should be the default, I've already seen plenty of D1 code that incorrectly used stack-based closures. It really depends on your usage patterns. I do a lot of inter-thread communication in D1