Optimizing delegates

[Ary Borenszweig]

On 12/19/2010 02:17 PM, Andrei Alexandrescu wrote:
> On 12/19/10 11:13 AM, Ary Borenszweig wrote:
>> On 12/19/2010 01:44 PM, Andrei Alexandrescu wrote:
>>> On 12/19/10 10:35 AM, Ary Borenszweig wrote:
>>>> On 12/19/2010 01:21 PM, Andrei Alexandrescu wrote:
>>>>> On 12/19/10 9:32 AM, Ary Borenszweig wrote:
>>>>>> I have this code:
>>>>>>
>>>>>> ---
>>>>>> import std.stdio;
>>>>>>
>>>>>> int foobar(int delegate(int) f) {
>>>>>> return f(1);
>>>>>> }
>>>>>>
>>>>>> int foobar2(string s)() {
>>>>>> int x = 1;
>>>>>> mixin("return " ~ s ~ ";");
>>>>>> }
>>>>>>
>>>>>> void main() {
>>>>>> writefln("%d", foobar((int x) { return 2*x; }));
>>>>>> writefln("%d", foobar2!("9876*x"));
>>>>>> }
>>>>>> ---
>>>>>>
>>>>>> When I compile it with -O -inline I can see with obj2asm that for the
>>>>>> first writefln the delegate is being called. However, for the second
>>>>>> it just passes
>>>>>> 9876 to writefln.
>>>>>>
>>>>>> From this I can say many things:
>>>>>> - It seems that if I want hyper-high performance in my code I must
>>>>>> use
>>>>>> string mixins because delegate calls, even if they are very simple
>>>>>> and
>>>>>> the
>>>>>> functions that uses them are also very simple, are not inlined. This
>>>>>> has the drawback that each call to foobar2 with a different string
>>>>>> will generate a
>>>>>> different method in the object file.
>>>>>
>>>>> You forgot:
>>>>>
>>>>> writefln("%d", foobar2!((x) { return 2*x; })());
>>>>>
>>>>> That's a real delegate, not a string, but it will be inlined.
>>>>>
>>>>>
>>>>> Andrei
>>>>
>>>> Sorry, I don't understand. I tried these:
>>>>
>>>> 1.
>>>> int foobar3(int delegate(int) f)() {
>>>> return f(1);
>>>> }
>>>>
>>>> writefln("%d", foobar3!((int x) { return 2*x; })());
>>>>
>>>> => foo.d(12): Error: arithmetic/string type expected for
>>>> value-parameter, not int delegate(int)
>>>>
>>>> 2.
>>>> int foobar3()(int delegate(int) f) {
>>>> return f(1);
>>>> }
>>>>
>>>> writefln("%d", foobar3!()((int x) { return 2*x; }));
>>>>
>>>> => Works, but it doesn't get inlined.
>>>>
>>>> And I tried that "(x) { ... }" syntax and it doesn't work.
>>>>
>>>> Sorry, it must be my fault I'm doing something wrong. What's the
>>>> correct
>>>> way of writing optimized code in D, code that I'm sure the compiler
>>>> will
>>>> know how to optimize?
>>>
>>> void foobar3(alias fun)() {
>>> return fun(1);
>>> }
>>>
>>>
>>> Andrei
>>
>> This of course has the following problem:
>>
>> int foobar2(int delegate(int x) f) {
>> }
>>
>> foobar2((int x, int y) { ... });
>>
>> Error: function foobar2 (int delegate(int) f) is not callable using
>> argument types (int delegate(int x, int y))
>>
>> ---
>>
>> int foobar3(alias f)() {
>> f(1);
>> }
>>
>> foobar3((x, y) { ... });
>>
>> foo.d(8): Error: template foo.main.__dgliteral1(__T2,__T3) does not
>> match any function template declaration
>> foo.d(8): Error: template foo.main.__dgliteral1(__T2,__T3) cannot deduce
>> template function from argument types !()(int)
>> foo.d(12): Error: template instance foo.main.foobar3!(__dgliteral1)
>> error instantiating
>>
>> So I have to go to foo.d(8) to see what the problem is, understand what
>> is being invoked (in this case it was easy but it get can harder), or
>> otherwise say "Hey, the one that implemented foo, please do a static
>> assert msg if f is not what you expect". Basically "Implement the error
>> message that the compiler would have given you for free if you didn't
>> use a template".
>
> Template constraints are meant to assuage that problem.
>
> Inlining delegates is technically much more difficult than inlining
> aliases. This is because a different function will be generated for each
> alias argument, whereas only one function would be used for all
> delegates. There are techniques to address that in the compiler, but
> they are rather complex.
>
>
> Andrei

I understand.

So why do I have to use a whole different syntax to make something 
accepting a delegate a function or a template?

Why can't this be accepted?

int foobar2(int delegate(int x) f)() {
}

and let the compiler interpret it as:

int foobar2(alias f) if ("the correct constraint which I don't want to 
learn how to write because the above SHOULD work") {
}

?