[fpc-pascal] Are there any drawbacks to "reference to"?

Sven Barth pascaldragon at googlemail.com
Mon Jun 20 14:11:16 CEST 2022

Michael Van Canneyt <michael at freepascal.org> schrieb am Mo., 20. Juni 2022,

> On Mon, 20 Jun 2022, Sven Barth via fpc-pascal wrote:
> > Am 20.06.2022 um 03:37 schrieb Hairy Pixels:
> >>
> >>> On Jun 19, 2022, at 10:04 PM, Sven Barth <pascaldragon at googlemail.com>
> > wrote:
> >>>
> >>> As you can see the allocation only happens once and not all the time.
> >>> What might be worse however is the optimization behavior as in this
> > example the compiler wouldn't optimize the counter variable into a
> regvar
> > with enabled optimizations (however in case of a nested function the
> compiler
> > wouldn't do this either if the function isn't inlined).
> >>>
> >>
> >> I’m saying if the OUTER method is in a tight loop (Foo in this example)
> > you’re in trouble because it now contains an allocation which is
> unexpected.
> >
> > In that case we're in the same territory as always: ensure that you know
> > what your code does. It's the same reason why adding character by
> > character to a managed String is slower then allocating the string once
> > and then setting the characters. And I think it's very seldom that
> > someone uses a function reference that does not leave the scope of the
> > surrounding function.
> Does this not depend on the callback declaration ?
> Type
>    FuncRef = reference to procedure(a : integer);
> Procedure DoSomething(f : FuncRef);
> begin
> end;
> Procedure UseSomething;
> begin
>    For I:=X to SomeLimit do
>      DoSomething(Whatever)
> end;
> Does the declaration of DoSomething (which uses reference to) not
> ensure that an interface is created in UseSomething ?

Yes, in your example this is the case, but look at mine again: it's
unnecessary to create the capture object here and in theory the compiler
could convert it to an ordinary nested function variable.
But that 's all theoretical, because right now such an optimization doesn't

I tried to find your original mail, to look for the answer myself,
> but I don't find it offhand.

Either in fpc-pascal or fpc-announce from end of May.

> It may be a good idea to add the explanation you gave in the announcement
> mail to the wiki (and link to it in the user changes in trunk page)

There won't be a "User Changes Trunk" entry because nothing existing broke
with this new feature.
There will however be a "New Features Trunk" entry that will point to the
announcement mail. There will be no documentation by me in the wiki.


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