[M3devel] layout of objects?

[Jay K]

Do you understand hereabouts m3front/src/exprs/MethodExpr.m3?


PROCEDURE Compile (p: P) =
  VAR
    x := ObjectType.MethodOffset (p.holder);
    method: Method.Info;
  BEGIN
    Type.Compile (p.object);
    Method.SplitX (p.method, method);

    Type.LoadInfo (p.object, M3RT.OTC_defaultMethods, addr := TRUE);
    IF (x >= 0) THEN
      INC (method.offset, x);
    ELSE (* runtime offset to methods *)
      Type.LoadInfo (p.holder, M3RT.OTC_methodOffset);
      CG.Index_bytes (Target.Byte);
    END;
    CG.Boost_alignment (Target.Address.align);
    CG.Load_indirect (CG.Type.Addr, method.offset, Target.Address.size);
    CG.Boost_alignment (Target.Address.align);
  END Compile;


"runtime offset to methods"?


ObjectType.MethodOffset:

PROCEDURE MethodOffset (t: Type.T): INTEGER =
  VAR p := Confirm (t);
  BEGIN
    IF (p = NIL) THEN RETURN Unknown_w_magic END;
    GetOffsets (p, use_magic := TRUE);
    RETURN p.methodOffset;
  END MethodOffset;


PROCEDURE Confirm (t: Type.T): P =
  VAR info: Type.Info;
  BEGIN
    LOOP
      t := Type.CheckInfo (t, info);
      IF (info.class = Type.Class.Object) THEN
        RETURN t;
      ELSIF (info.class = Type.Class.Opaque) THEN
        t := Revelation.LookUp (t);
      ELSE
        RETURN NIL;
      END;
    END;
  END Confirm;


...



 - Jay





Subject: Re: [M3devel] layout of objects?
From: hosking at cs.purdue.edu
Date: Fri, 22 Mar 2013 01:44:37 -0500
CC: jay.krell at cornell.edu; m3devel at elegosoft.com
To: dragisha at m3w.org

Not quite.  I just checked RTAllocator.m3 and it is pointer to methods at offset 0, followed by fields.Heap header is at -ADRSIZE(Header).The type information in RT0 is used to initialize the object instances.


On Mar 22, 2013, at 1:34 AM, Dragiša Durić <dragisha at m3w.org> wrote:It's not m3front, it's RT0.
First data field is at offset 0, and pointer to type information is at -BYTESIZE(POINTER). Type information records are specified in RT0.

On Mar 22, 2013, at 5:56 AM, Jay K wrote:layout of objects?



How are Modula-3 objects layed out?
i.e. "OBJECT"/"METHODS"/"OVERRIDES"
I skimmed m3front and it wasn't obvious.



A common way for C++ "objects" to be layed out,
in the face of no RTTI and only single inheritance,
and virtual functions, is that a pointer to a record
of function pointers is first in the record.


Like this:


class Type
{
virtual void F1();
virtual void F2();
int data1;
int data2;
};


ends up lik more this:


struct TypeFunctions
{
 void (*F1)(Type*);
 void (*F2)(Type*);
};


struct Type
{
TypeFunctions* Functions; /* always first,
or at least a fixed offset, and located independent
of the size of the data; could also be at "-1" or such */.
int data1;
int data2;
};


Type* x;
x->F1();


=>
x->Functions->F1(x);


Functions added in more derived types go at the end.
Ditto for data.
In the absence of multiple-inheritance and RTTI, it is simple and predictable.
(RTTI makes only small modifications.)


Looking through m3front, it wasn't at all obvious if it works this way.


I would like to declare something in C (or possibly C++, but not likely),
such that I might actually recognize the various low level operations
and "uncompile" it back to a typeful/typesafe form, like the above C++
to C transform.



I can't likely uncompile to C++ with virtual functions,
because the actual layout in C++ is not guaranteed.



Granted, I am being lazy.
I should/could compile some small samples.
But I might not get the entire story that way.



Thanks,
- Jay






Antony Hosking | Associate Professor | Computer Science | Purdue University305 N. University Street | West Lafayette | IN 47907 | USAMobile +1 765 427 5484

 		 	   		  
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