[fpc-devel] Proposal for implementing named address space support

[Christo Crause]

On Sun, Jan 17, 2021 at 3:51 PM Florian Klämpfl via fpc-devel <
fpc-devel at lists.freepascal.org> wrote:

> Am 08.01.21 um 22:00 schrieb Christo Crause via fpc-devel:
> > On Sun, Oct 4, 2020 at 1:49 PM Christo Crause <christo.crause at gmail.com
> > <mailto:christo.crause at gmail.com>> wrote:
> > var
> >    w: word = $BEEF; section '.eeprom';
> >    b: byte;
> >
> > procedure doSomething(constref tmp: word);
> > begin
> >    b := hi(tmp);
> > end;
> >
> > begin
> >    doSomething(w);
> > end.
> >
> > At the moment there doesn't appear to be a way to do this without
> > changing or adding something to the compiler.  One idea could be to
> > define new types identifying types in a specific address space, such as
> > Teepromword. In this case the above example could be written as:
> > var
> >    w: Teepromword = $BEEF;
> >    b: byte;
> >
> > procedure doSomething(constref tmp: word); overload;
> > begin
> >    b := hi(tmp);
> > end;
> >
> > procedure doSomething(constref tmp: Teepromword); overload;
> > begin
> >    b := hi(tmp);
> > end;
> >
> > begin
> >    doSomething(w);
> > end.
> >
> > This way the compiler knows how to generate appropriate access based on
> > the parameter type. It however leads to lots of code duplication.  Which
> > got me thinking about generics - could this be handled internally in the
> > compiler via generics?  It isn't clear to me how to tackle these issues
> > so any advice would be appreciated.
>
> I would simply not allow to use such references as independent pointers.
> This causes too much bloat. They can be used in assignments and
> expressions and that's it.
>

I have given up on that idea.  I'm now busy working on Jonas's suggestion
of section specific pointers (example:
https://github.com/ccrause/freepascal/wiki/Better-support-for-address-spaces-in-FPC#section-pointer-example
).

At the moment it introduces no runtime overhead (other than the required
access instructions of course),
but to make it work across units is problematic.  I suspect an extra field
is required to store the section information associated with a pointer type
(at least the way I've implemented it) in a ppu.  If unit A defining such a
pointer and a procedure using it is as parameter is called from unit B, it
works if both units are compiled together.  If unit A is already compiled
and unit B is recompiled (symbols loaded from ppu) it doesn't carry the
section information across and the compiler cannot determine which
overloaded procedure to call.
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