[M3devel] loophole/copysign

[Jay K]

Tony, et. al.. in m3front/src/exprs/CastExpr.m3..what's the difference between a "designator" and a "value"?


http://www-plan.cs.colorado.edu/diwan/modula3/designators.html

An identifier is a writable designator
if it is declared as a variable, 
is a VAR or VALUE parameter,
is a local of a TYPECASE 
or TRY EXCEPT statement,
or is a WITH local that is bound to a writable designator.
An identifier is a readonly designator if it is
a READONLY parameter,
a local of a FOR statement,
or a WITH local bound to a non-designator or 
readonly designator.



I guess a designator is what I would think of a "variable" or "read only variable"?
Something that either is "in memory" or can "reasonably" be put there?


1 + 2 is not a designator.


Or, generally, a "variable", but that includes such similar things as parameters, "with variables", "for variables", "TYPECASE vairables", "TRY variables"


Anything with a name??? (not functions/modules/generics -- "named data")


Anyway, the next questions include:


In CastExpr.m3 would it be terrible and/or wrong to treat "designators" the same as "values"?
 I realize, probably a deoptimization.
 I think this lets the backend work.
 

And really, more to the point...shouldn't CastExpr.m3 use cg.loophole far more?
I haven't had much luck with that. I always get the cg stack out of balance or with the wrong types, even though it seems like it should be easy.


I have more testing to do, but classifying the loophole as V_to_S (value to structure) in place of D_to_S (designator to structure), at least if either side is one of the three float types, seems reasonable and correct, albeit slight deoptimization -- in unsafe code dealing with floating point..should be rare..


 - Jay



----------------------------------------
> From: jay.krell at cornell.edu
> To: m3devel at elegosoft.com
> Date: Mon, 5 Jul 2010 09:24:20 +0000
> Subject: [M3devel] loophole/copysign
>
>
> Our codegen is remarkably low level. That is, lower level earlier than C.
>
>
> gcc/m3cg -ftree-dump-all
>
>
> As early as LongFloat.mc.003t.original, the first file dumped, we have:
>
> LongFloat__CopySign (M3_CtKayy_x, M3_CtKayy_y)
> {
>   xreel M3_CtKayy__result;
>   xreel M3_CtKayy_res;
>
>     xreel M3_CtKayy__result;
>     xreel M3_CtKayy_res;
>   M3_CtKayy_res = M3_CtKayy_x;
>   BIT_FIELD_REF  = (word_8) ((int_64)
>   BIT_FIELD_REF  & -129 | (word_64) BIT_FIELD_REF <(int_64) BIT_FIELD_REF , 1, 7> << 7 & 255);
>    = M3_CtKayy_res;
>   return ;
> }
>
> compared to C where as test_copysign.c.t69.copyrename3, the last file dumped, we have:
>
> copy_sign_f (from, to)
> {
>   float res;
>   float D.1918;
>    D.1917;
>   struct float_t * from.1;
>   struct float_t * res.0;
>
> :
>   res = to_1;
>   res.0_4 = (struct float_t *) &res;
>   from.1_5 = (struct float_t *) &from;
>   D.1917_6 = from.1_5->sign;
>   res.0_4->sign = D.1917_6;
>   D.1918_7 = res;
>   return D.1918_7;
>
> }
>
>
> See, you know, from gcc's point of view, we don't have any records/structs/unions.
> Just integers and offsets from them mostly.
>
>
> The right fix is to build up types.
> That way also debugging with gdb will have a chance.
> Perhaps not a small amount of work. But maybe not too bad.
>
>
> For now my inclination is in m3front to insert a barrier between the store and the load associated with loopholes.
> At least if one type but not the other is floating point.
> I don't know if that will work, but maybe.
>
> Or maybe have m3front actually call loophole for this case and again, either a barrier or make the load and/or
> store volatile.
>
>  - Jay
>