Dynamic Closure + Lazy Arguments = Performance Killer?

[Lars Ivar Igesund]

Denis Koroskin wrote:

> 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).

I definately agree with this.

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