[M3devel] small set comparisons understood, now just to understand the front end code..

[Tony Hosking]

I would hesitate to implement small sets as LONGINT instead of  
INTEGER, since there is no guarantee that LONGINT is necessarily  
efficient, whereas INTEGER is intended to be the same as the natural  
word size of the target.

I could take a look at this but not anytime soon, since I have several  
other things I need to work on.

On Apr 14, 2008, at 10:51 AM, Jay wrote:

> currently set <,>,<=,>= are implemented merely as
> integer <,>,<=,>=

Yes, that seems quite wrong.  I can't imagine things ever worked  
properly if that is how they are implemented.

>
>
> This is wrong.
>
> I believe it should be:
>
> a < b => (a & b) == a
> a <= b => (a & b) == a (same as <=)
> a > b => (a & b) == b
> a >= b => (a & b) == b (same as >)

Probably should be:

a <= b => (a & b) == a
a < b => (a # b) && ((a & b) == a)
a >= b => (a & b) == b
a > b => (a # b) && ((a & b) == b)

>
>
> The bug is in the frontend.
>
> m3-sys\m3front\src\misc\CG.m3.
>
> The code should be /something/ like:
>
> PROCEDURE Set_compare (s: Size; op: Cmp) =
> VAR tmp: Val;
>   swap := FALSE;
> BEGIN
>   (* a op b => BOOLEAN *)
>   IF Force_pair (commute := TRUE) THEN
>     op := M3CG.SwappedCompare [op];
>   END;
>   IF (s <= Target.Integer.size) THEN
>     IF (op = Cmp.EQ) OR (op = Cmp.NE) THEN
>       cg.compare (Target.Word.cg_type, Target.Integer.cg_type, op);
>     ELSE
>       (* set a is less than or equal to set b, if all of set a's  
> members are in set b. *)
>       IF (op = Cmp.LT) OR (op = Cmp.LE) THEN
>         swap := TRUE;
>         Swap ();
>       END;
>       tmp := Pop ();
>       Push (tmp);
>       IF swap THEN
>         Swap ();
>       END;
>        cg.and (Target.Integer.cg_type);
>        Push (tmp);
>       SimpleIndirectLoad (tmp^, Target.Word.cg_type);
>       EVAL Force_pair (commute := TRUE);
>        cg.compare (Target.Word.cg_type, Target.Integer.cg_type,  
> Cmp.EQ);
>       SPop (1, "Set_compare");
>       Free (tmp);
>     END;
>   ELSE
>     cg.set_compare (AsBytes (s), op, Target.Integer.cg_type);
>   END;
>   SPop (2, "Set_compare");
>   SPush (Type.Int32);
> END Set_compare;
>
> though this doesn't quite drive all the machinery correctly, since  
> it yields assertion failures in the compiler due to an unbalanced  
> software stack.
> upgrade works, but the test case (p155) fails assertions in the  
> integrated backend.
>
> I'd love to figure this out but have to do other stuff for now.
> Anyone (if there is anyone) familiar with what all is being pushed  
> and popped around here should be able to figure it out easily from  
> this mail.
> Otherwise I'll stare at more later.
>
> ps: "small" sets should probably be anything up to the number of  
> bits in longint, rather than int or pointer, since that is probably  
> efficient enough at the next level down. This is tangential.
>
> - Jay

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