Optimizing delegates

[Andrei Alexandrescu]

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