Optimizing delegates

[Ary Borenszweig]

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".