Dynamic Closure + Lazy Arguments = Performance Killer?

[Lars Ivar Igesund]

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.

I agree that D1 behaviour should be the default, since otherwise it'll be
yet another breaking change. However, I do understand that the D1 behaviour
is the unsafe one, and as such the heap allocated version has merit as the
default.

-- 
Lars Ivar Igesund
blog at http://larsivi.net
DSource, #d.tango & #D: larsivi
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