Optimizing delegates

[Ary Borenszweig]

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?