Dynamic Closure + Lazy Arguments = Performance Killer?

[Denis Koroskin]

On Sat, 25 Oct 2008 18:36:27 +0400, Lars Ivar Igesund  
<larsivar at igesund.net> wrote:

> 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.
>

I believe the default should be the one that is most frequently used, even  
if it is less safe. Otherwise you may end up with a lot of code  
duplication.

I also think that scope and heap-allocated delegates should have different  
types so that no imlicit casting from scope delegate to heap one would be  
possible. In this case callee function that recieves the delegate might  
demand the delegate to be heap-allocated (because it stores it, for  
example).