[fpc-devel] New feature announcement: constant parameters for generics

[Sven Barth]

Am 26.04.2020 um 14:01 schrieb Benito van der Zander:
> Hi,
>
> perhaps it could be used to merge specializations (if fpc cannot do 
> that on its own):
>
> Like when you have a hashmap THashMap<Key,Value>, and need three 
> specializations:
>
> THashMap<string, pointer>
>
> THashMap<string, TObject>
>
> THashMap<string, sizeint>
>
> It is basically three times the same hashmap, but if fpc does not 
> detect that, it might generate three times the same assembly code, 
> which waste a lot of space.
>
>
> But with constants it can be merged to TBaseHashMap<Key, ValueSize: 
> integer> and then you only have one map in the assembly code, and 
> three wrappers to remove the casting:
>
> THashMap<string, pointer> = TBaseHashMap<string, sizeof(pointer) > = 
> TBaseHashMap<string, 8 >
>
> THashMap<string, TObject> = TBaseHashMap<string, sizeof(TObject) > = 
> TBaseHashMap<string, 8 >
>
> THashMap<string, sizeint> = TBaseHashMap<string, sizeof(sizeint) > = 
> TBaseHashMap<string, 8 >
>

It's not that simple. In principle you're right that the compiler could 
try to merge more implementations, but this does not depend on the 
declaration of the generic, but the use of the parameter types.

Take the following example:

=== code begin ===

{$mode objfpc}

type
   generic TTest<T> = class
     procedure DoSomething;
   end;

   TMyClass1 = class
      procedure Foobar;
   end;

   TMyClass2 = class
     procedure Foobar; virtual;
   end;

procedure TTest.DoSomething;
var
   o: T;
begin
   o.Foobar;
end;

procedure TMyClass1.Foobar;
begin
   Writeln('TMyClass1.Foobar');
end;

procedure TMyClass2.Foobar;
begin
   Writeln('TMyClass2.Foobar');
end;

type
   TTestMyClass1 = specialize TTest<TMyClass1>;
   TTestMyClass2 = specialize TTest<TMyClass2>;

begin
end.

=== code end ===

In case of TMyClass1 this will result in a static call to 
TMyClass1.Foobar, however in case of TMyClass2 this will result in an 
indirect call through the VMT.

The type information needs to be correct as well, even more so once we 
have support for Extended RTTI where one can enumerate non-published 
fields, properties and methods in addition to published.

Not to mention that your TBaseHashMap would not work with managed types...

Regards,
Sven
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